Marie-Sonia Buhour

Franck Doidy

Audrey Mondou

Alice Pélerin

Laurence Carluer

Francis Eustache

Fausto Viader

Béatrice Desgranges
email: beatrice.desgranges@unicaen.fr

M-S Buhour

Voxel-based mapping of grey matter volume and glucose metabolism profiles in amyotrophic lateral sclerosis

Keywords: amyotrophic lateral sclerosis, magnetic resonance imaging, positron emission tomography, voxel-based morphometry, hypermetabolism

BACKGROUND

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that primarily affects motor function, but also concerns extramotor systems. The degeneration of the motor system typically involves both upper motor neurons located in the primary motor cortex, and lower motor neurons from the brainstem nuclei and anterior horns of the spinal cord. The disease has a uniformly fatal outcome as a result of muscle weakness, with median survival of 2-4 years [START_REF] Chiò | Phenotypic heterogeneity of amyotrophic lateral sclerosis: a population based study[END_REF]. The underlying pathophysiology is poorly understood and effective treatments are still needed for this neurodegenerative disease.

There is increasing awareness that ALS is a clinically heterogeneous disease [START_REF] Turner | Controversies and priorities in amyotrophic lateral sclerosis[END_REF]. There is also a general recognition that ALS patients commonly present deficits in executive functions but also memory [START_REF] Machts | Memory deficits in amyotrophic lateral sclerosis are not exclusively caused by executive dysfunction: a comparative neuropsychological study of amnestic mild cognitive impairment[END_REF][START_REF] Ringholz | Prevalence and patterns of cognitive impairment in sporadic ALS[END_REF] and social cognition with impairment of both cognitive and affective theory of mind [START_REF] Carluer | Neural substrate of cognitive theory of mind impairment in amyotrophic lateral sclerosis[END_REF][START_REF] Girardi | Deficits in emotional and social cognition in amyotrophic lateral sclerosis[END_REF][START_REF] Cerami | Emotional empathy in amyotrophic lateral sclerosis: a behavioural and voxel-based morphometry study[END_REF].

Magnetic resonance imaging (MRI) of the brain and spinal cord is routinely used in the diagnostic work-up of ALS, to rule out various pathological conditions that may masquerade as motor neuron disease, but rarely gives specific clues to the positive diagnosis. Automated techniques for analysing MRI images have been developed notably with statistical parametric mapping (SPM) software to carry out voxel-byvoxel analysis of the whole brain (voxel-based morphometry, VBM). Most of the studies, have reported extensive GM atrophy, not confined to motor areas [START_REF] Shen | Voxel-Wise Meta-Analysis of Gray Matter Changes in Amyotrophic Lateral Sclerosis[END_REF].

Positron emission tomography (PET) combined with 18 F-fluorodeoxyglucose (FDG), a specific radiotracer for glucose metabolism, indicates glucose uptake by astrocytes and neurons [START_REF] Nehlig | Cellular pathways of energy metabolism in the brain: is glucose used by neurons or astrocytes?[END_REF] and reveals local brain activity. Early studies found reduced regional cerebral glucose utilization in patients with ALS, [START_REF] Dalakas | Lowered cerebral glucose utilization in amyotrophic lateral sclerosis[END_REF] mostly in the frontal cortex but also in other cortical territories such as the superior occipital cortex [START_REF] Ludolph | Frontal lobe function in amyotrophic lateral sclerosis: a neuropsychologic and positron emission tomography study[END_REF].

More recently, other authors, adopting a voxel-by-voxel approach [START_REF] Cistaro | Brain hypermetabolism in amyotrophic lateral sclerosis: a FDG PET study in ALS of spinal and bulbar onset[END_REF][START_REF] Pagani | Functional pattern of brain FDG-PET in amyotrophic lateral sclerosis[END_REF][START_REF] Van Laere | Value of 18fluorodeoxyglucose-positron-emission tomography in amyotrophic lateral sclerosis: a prospective study[END_REF][START_REF] Canosa | 18F-FDG-PET correlates of cognitive impairment in ALS[END_REF][START_REF] Van Weehaeghe | Prospective Validation of 18F-FDG Brain PET Discriminant Analysis Methods in the Diagnosis of Amyotrophic Lateral Sclerosis[END_REF], have observed severe hypometabolism in the premotor cortex, postcentral gyrus, prefrontal cortex, lingual gyrus, fusiform gyrus and thalamus. They also observed an increased cerebral glucose metabolism or hypermetabolism in the medial temporal lobe, the cerebellum, the occipital cortex and brainstem. This hypermetabolism has been proposed as a possible biomarker for ALS by Pagani and colleagues [START_REF] Pagani | Functional pattern of brain FDG-PET in amyotrophic lateral sclerosis[END_REF]. It is not yet known, however, whether it reflects a compensatory mechanism in the brain or deleterious metabolic activity. Nevertheless, it is important to note that the limited PET camera resolution can give rise to partial volume effects (PVEs) that result in blurring and, therefore, an underestimation of regional activity, particularly in small structures or those with volume loss. Since previous FDG-PET studies did not correct for PVEs, the metabolic abnormalities observed in patients with ALS have to be interpreted cautiously.

The above-mentioned studies investigated different groups of nondemented patients with ALS using either brain MRI or FDG-PET (without correction for PVEs). Only one combined these two techniques in a single group of 18 patients with ALS who met the Neary criteria for frontotemporal dementia (FTD) [START_REF] Rajagopalan | Comparing brain structural MRI and metabolic FDG-PET changes in patients with ALS-FTD: "the chicken or the egg?" question[END_REF].

The authors assessed brain GM structural changes using VBM, and metabolic changes using FDG-PET.

They concluded that the metabolic changes corresponded to the structural changes, with a few exceptions.

To our knowledge, no combined assessment of GM volume and regional cerebral metabolism has yet been carried out in nondemented patients with ALS. The goals of the present study were threefold: 1) determine both GM volume and glucose metabolism changes in a sample of patients with ALS by adopting a voxel-based approach; 2) further study the clinical significance of hypermetabolism, by assessing its relationship with selected cognitive scores; and 3) carry out a direct voxelwise comparison of the degrees of local GM atrophy and hypometabolism throughout the brain, by using a specific processing technique developed in our laboratory that has already been applied in early Alzheimer's disease [START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF], alcoholism [START_REF] Ritz | Direct voxel-based comparisons between grey matter shrinkage and glucose hypometabolism in chronic alcoholism[END_REF], and the behavioural variant of FTD [START_REF] Buhour | Pathophysiology of the behavioral variant of frontotemporal lobar degeneration: A study combining MRI and FDG-PET[END_REF]. speakers and had a minimum level of education equivalent to a now obsolete school-leaving certificate that was generally taken at 14 years.. All the patients met the modified El Escorial criteria for probable or definite ALS [START_REF] Brooks | El Escorial revisited: revised criteria for the diagnosis of amyotrophic lateral sclerosis[END_REF]. Exclusion criteria were the additional presence of severe and chronic illness, alcohol or drug abuse, traumatic brain injury, or an extremely severe communication problem that could seriously compromise the administration of cognitive tests. None of the patients fulfilled the criteria for a diagnosis of FTD according to the core and supportive diagnostic features of FTD detailed in Lund and Manchester's consensus statement [START_REF] Neary | Frontotemporal lobar degeneration: a consensus on clinical diagnostic criteria[END_REF]. None of the patients with ALS met the criteria for a possible and/or probable behavioural variant of FTD [START_REF] Rascovsky | Sensitivity of revised diagnostic criteria for the behavioural variant of frontotemporal dementia[END_REF]. Behavioural disorders were explored in 32 patients via the short form of the Neuropsychiatric Inventory [START_REF] Cummings | The Neuropsychiatric Inventory: comprehensive assessment of psychopathology in dementia[END_REF]. None had behavioural disorders of the frontal type sufficiently severe to meet the criteria for ALS with behavioural impairment. All the patients underwent a neurological assessment that included the ALS Functional Rating Scale Revised (ALS-FRS-R) (25), Norris scale (26), and Medical Research Council (MRC, 1976) Muscle Strength Scale. They were able to speak or to write intelligibly. All the patients gave their written informed consent, and the independent regional ethics committee approved the study. Some of these patients had already been included in a previous study [START_REF] Carluer | Neural substrate of cognitive theory of mind impairment in amyotrophic lateral sclerosis[END_REF].

METHODS

The ALS patients underwent a neuropsychological assessment covering a range of cognitive functions (for details, see [START_REF] Carluer | Neural substrate of cognitive theory of mind impairment in amyotrophic lateral sclerosis[END_REF]).

Control groups.

Neuroimaging

To compare the neuroimaging data, we included a group of 37 healthy controls (21 men and 16 women; mean age = 61.11 years, standard deviation = 11.11; mean level of education = 11.45 years, standard deviation = 2.97). Healthy controls were recruited from the community and performed in the normal range on a neuropsychological examination assessing multiple domains of cognition including episodic and semantic memory, executive and visuo-spatial functions, language and praxis. Within a few days from recruitment, they also underwent a structural MRI and a PET using [18F] fluoro-2-deoxy-D-glucose (18FDG). These controls were closely matched for sex, age, and level of education with the group of patients with ALS.

Cognition

As most of our neuropsychological tests were original, with no available normative values, to compare the cognitive scores, we included another group of 37 healthy subjects (21 men and 16 women; mean age = 61.81 years, standard deviation = 8.84; mean level of education = 11.19 years, standard deviation = 2.60).

The Mattis cut-off score was set at 130, to avoid the possible inclusion of controls with mild cognitive impairment (for details, see Table 1).

Neuropsychological assessment

Patients and controls underwent a neuropsychological assessment that evaluated a set of cognitive functions that were already described and published in a previous study (for details see [START_REF] Carluer | Neural substrate of cognitive theory of mind impairment in amyotrophic lateral sclerosis[END_REF]). In order to understand the neurobehavioral significance of cerebral hypermetabolism, we only worked on cognitive scores that could be related to brain regions showing hypermetabolism (see infra). This is why we chose to explore the affective theory of mind abilities and the episodic memory of ALS patients. Affective theory of mind was assessed with the Reading the Mind in the Eyes test derived from the procedure used by Baron-Cohen et al.

(2001) [START_REF] Baron-Cohen | The "Reading the Mind in the Eyes" Test Revised Version: A Study with Normal Adults, and Adults with Asperger Syndrome or High-functioning Autism[END_REF]. The participants had to make inferences about the affective and motivational states of others on the basis of a picture of their eyes (for more details see [START_REF] Laisney | Cognitive and affective Theory of Mind in mild to moderate Alzheimer's disease[END_REF]). We then explored episodic memory, with a task in which participants had to intentionally encode a list of eighteen unrelated words. Every three words, they were subjected to an immediate free recall task. Following the study phase, there was a 20-second interval during which participants were engaged in an unrelated distractor task. Participants then had to recall as many words as possible, with no time-limit. After a 20-min interval, they were finally subjected to a delayed free recall task, again with no time-limit.

Neuroimaging data acquisition

All participants were scanned using the same MRI and PET cameras at the Cyceron centre (Caen, France): a Philipps Achieva 3.0 T scanner and a discovery RX VCT 64 PET-CT device (General Electric Healthcare, respectively.

MRI data.

For each participant, a high-resolutionT1-weighted anatomical image was acquired on a Philips Achieva 3T scanner using a three-dimensional fast field echo sequence (sagittal; repetition time = 20 ms, echo time = 4.6 ms, flip angle = 10°, 180 slices, slice thickness = 1 mm, field of view = 256 x 256 mm 2 , matrix = 256 x 256).

PET data.

Each participant underwent a PET examination the day after MRI examination. FDG scans were acquired on a Discovery RX VCT 64 PET-CT device (GE Healthcare) with a resolution of 3.76 x 3.76 x 4.9 mm 3 FWHM (axial field of view = 157 mm). Forty-seven planes were obtained with septa out (3D acquisition); with a voxel size of 2.7 x 2.7 x 3.27 mm 3 . A CT transmission scan was performed for attenuation correction before the PET acquisition. Participants were fasted for at least 6 hours before scanning. After a 30-minute resting period in a quiet and dark environment, ~180 MBq of FDG were intravenously injected as a bolus. A 10-minute PET acquisition scan began 50 minutes after injection. During data acquisition, head motion was continuously monitored with laser beams projected onto ink marks drawn on the forehead, which also served to make any necessary corrections.

Neuroimaging data handling and transformation

Preprocessing Using the VBM5.1 toolbox (http://dbm.neuro.uni-jena.de/vbm/vbm5-for-spm5/), implemented in SPM5 software (Wellcome Trust Centre for Neuroimaging, London, UK), the raw MRI data were spatially normalized to Montreal Neurological Institute (MNI) space (voxel size = 1 mm 3 , matrix = 156 x 189 x 157) and segmented into GM, white matter (WM) and cerebrospinal fluid (CSF). The normalized GM images were modulated by the Jacobian determinants to correct for nonlinear warping only, to obtain maps of local GM volumes corrected for brain size.

FDG-PET data were first corrected for CSF and WM PVEs, using the voxel-by-voxel modified Müller-Gartner method, [START_REF] Muller-Gartner | Measurement of Radiotracer Concentration in Brain Gray Matter Using Positron Emission Tomography: MRI-Based Correction for Partial Volume Effects[END_REF][START_REF] Rousset | Correction for partial volume effects in PET: principle and validation[END_REF] described in detail elsewhere [START_REF] Quarantelli | Integrated software for the analysis of brain PET/SPECT studies with partial-volume-effect correction[END_REF] and already used in our laboratory. [START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF][START_REF] Ritz | Direct voxel-based comparisons between grey matter shrinkage and glucose hypometabolism in chronic alcoholism[END_REF][START_REF] Buhour | Pathophysiology of the behavioral variant of frontotemporal lobar degeneration: A study combining MRI and FDG-PET[END_REF] Using SPM5, the PVE-corrected PET data were then coregistered (rigid-body coregistration) to their respective native MRIs and normalised to MNI space by reapplying the normalization parameters estimated from the VBM protocol described above. The resulting images underwent quantitative scaling using mean GM as a reference.

Smoothing

For the between-group comparison, and in order to blur individual variations in gyral anatomy and increase the signal-to-noise ratio, the registered MRI data were smoothed using an 8 x 8 x 8 (x, y, z) mm Gaussian kernel. PET datasets were also smoothed, using a 12 x 12 x 12 mm Gaussian kernel.

Differential Smoothing

For the direct comparison between GM atrophy and glucose hypometabolism, since the MRI and PET data had different spatial resolutions, differential smoothing was applied to equalize the effective smoothness [START_REF] Richardson | Cortical grey matter and benzodiazepine receptors in malformations of cortical development. A voxel-based comparison of structural and functional imaging data[END_REF][START_REF] Agosta | Voxel-based morphometry study of brain volumetry and diffusivity in amyotrophic lateral sclerosis patients with mild disability[END_REF][START_REF] Chang | A voxel-based morphometry study of patterns of brain atrophy in ALS and ALS/FTLD[END_REF]. A Gaussian kernel of 8 x 8 x 8 (x,y,z) mm was used for the MRI data, and 7.1 x 7.1 x 6.4 for the PET data. Finally, images were masked to exclude non-GM voxels from the analysis.

W-score maps

To obtain measurements of atrophy and hypometabolism expressed in the same units so that we could undertake direct comparisons of the different modalities, we computed W-score maps for each patient and each imaging modality, using the healthy control group as a reference. W scores are analogous to z scores, but are adjusted for specific covariates [START_REF] Boccardi | The MRI pattern of frontal and temporal brain atrophy in fronto-temporal dementia[END_REF]-age in the present case. The smoothed MRI and PET data were used to create W-score maps [(raw value for each patient) -(value expected in the control group for the patient's age) / (SD for each patient and each modality); see [START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF] for more details].

As the two datasets had different original spatial resolutions, the MRI data were coregistered and resliced into PET space to obtain images with the same dimensions.

Anatomical localization

GM anatomical localization was carried out using AAL automated labelling software implemented in SPM5. [START_REF] Tzourio-Mazoyer | Automated anatomical labeling of activations in SPM using a macroscopic anatomical parcellation of the MNI MRI single-subject brain[END_REF] For further information about thalamic subdivision, we used the Talairach atlas. [START_REF] Lancaster | Automated Talairach atlas labels for functional brain mapping[END_REF] We focused on five major regions of the thalamus: medial dorsal nucleus, anterior and lateral ventral nuclei, posterior ventral nuclei, lateral posterior-pulvinar complex, and lateral dorsal and anterior nuclei.

Statistical Analyses

Between-group comparison of GM volume and metabolism

To determine the pattern of GM atrophy in the ALS group as well as glucose hypometabolism and hypermetabolism, compared to the control group, we used a two-sample t test in SPM5. This yielded three maps of statistically significant GM atrophy and metabolic abnormalities in patients relative to controls. To address the issue of multiple comparisons, for each analysis, the significant cluster size was determined using a Monte Carlo simulation program (AlphaSim) as previously done in our laboratory [START_REF] Mutlu | Connectivity Disruption, Atrophy, and Hypometabolism within Posterior Cingulate Networks in Alzheimer's Disease[END_REF][START_REF] Tomadesso | Brain structural, functional, and cognitive correlates of recent versus remote autobiographical memories in amnestic Mild Cognitive Impairment[END_REF], with a cumulative proportion criterion of less than .05 [START_REF] Cox | AFNI: software for analysis and visualization of functional magnetic resonance neuroimages[END_REF]. For the GM volume analysis, this equated to a cluster volume of 475 voxels and for the metabolism analyses a k= 113 voxels.

Correlations between hypermetabolism and selected cognitive scores

Statistical analyses were performed using Statistica 10.0 (StatSoft, Tulsa, OK, USA). The threshold of significance was set at p = .05 (one-tailed). We assessed correlations between the metabolism of significant clusters identified in the previous analysis showing hypermetabolism and selected cognitive tests.

To this end, we extracted the mean metabolic values of the regions presenting hypermetabolism using the "Binary ROIs analysis" in SPM5 and then entered them into Statistica to assess correlations with selected scores of the patients with ALS.

Within-group comparison between morphological and metabolic alterations

Individual GM MRI W scores maps and PET W scores maps were entered in a one-paired t test analysis with one group (patients with AL) and two images per participant (i.e., MRI and PET W score maps), using SPM5. We put years of education and Mattis (total score) as confounding covariates in this analysis. Both contrasts were assessed (W-PET > W-MRI and W-MRI > W-PET) to generate statistical maps reflecting predominant structural abnormalities or glucose hypometabolism. To address the issue of multiple comparisons, significant cluster size was determined using a Monte Carlo simulation program (AlphaSim), with a cumulative proportion criterion of less than .05 [START_REF] Cox | AFNI: software for analysis and visualization of functional magnetic resonance neuroimages[END_REF]. This equated to a cluster volume of 31 voxels.

RESULTS

Behavioral results

Regarding executive functions, compared with the control group, the patients exhibited significant impairment of executive functions on the Trail-Making Test B-A (p < 0.001), letter verbal fluency score (p < 0.001), and letter verbal fluency index (p = 0.017). The same was true for both episodic memory, measured with a classic word learning test (p < 0.001), and cognitive theory of mind, assessed with a false-belief task (p = 0.009), with a trend for affective theory of mind, assessed with the Face/Eyes test (p = 0.062) for details see Table 1). When comparing ALS patients with bulbar onset to those with spinal onset, we did not find any difference in their cognitive status (data not shown).

Patterns of grey matter atrophy and hypometabolism in ALS (see Fig. 1)

The comparison of GM volume between patients and controls revealed significant GM atrophy in the left postcentral gyrus, the left inferior temporal gyrus, the left paracentral lobule and the left supramarginal gyrus. We also found GM atrophy within the right precentral gyrus and the right putamen.

Hypometabolism mainly concerned the right paracentral lobule, the left inferior parietal gyrus, the thalamus bilaterally as well as the left superior medial frontal gyrus.

Patients also showed hypermetabolism mainly in the cerebellar vermis (IV, V and VII), cerebellar lobules IV and V, bilaterally within the medial temporal cortex (both hippocampus and parahippocampal gyri) and, to a lesser extent, the fusiform gyrus. All the results are listed in Table 2.

In our quest to better understand this metabolic dysfunction and investigate its neurobehavioural significance, we assessed the relationships between hypermetabolism and selected cognitive functions.

Given that the medial temporal lobe is involved in episodic memory, the cerebellum is part of a motor network, and fusiform gyrus is involved in social cognition [START_REF] Carrington | Are there theory of mind regions in the brain? A review of the neuroimaging literature[END_REF][START_REF] Schultz | The role of the fusiform face area in social cognition: implications for the pathobiology of autism[END_REF], we looked for positive or negative correlations between the metabolism of each of these regions and the relevant (and available) cognitive performances. All the significant correlations we found were negative. Thus, we observed significant negative correlations between episodic memory (immediate recall) and the metabolic value of the right hippocampus (r = -0.36; p = 0.04), the left hippocampus (r = -0.36; p = 0.04) and the left parahippocampal gyrus (r= -0.46; p=0.007). Concerning the delayed recall, we found significant negative correlations with the metabolic values of the left parahippocampal gyrus (r=-0.45; p=0.01). We also found negative correlations between metabolic activity within the left fusiform gyrus and TOM-15 (cognitive theory of mind) (r=-0.33; p= 0.04) (Table 3).

Direct comparison between GM atrophy and hypometabolism in ALS patients

GM atrophy was greater than hypometabolism in the temporal lobes (left and right middle temporal poles, left hippocampus and left parahippocampus as well as left inferior temporal gyrus). This was also true in the left calcarine (Table 2).

The reverse contrast (hypometabolism > GM atrophy) only concerned the left superior medial frontal cortex.

DISCUSSION

To our knowledge, this was the first neuroimaging study to closely examine the relationship between cerebral GM volume and glucose metabolism via a direct comparison of both parameters, as well as the relationship between hypermetabolism and cognitive performance in patients with ALS. We first compared GM volume and cerebral glucose metabolism in patients versus controls, via a two-sample t test. We found 1) marked GM atrophy in the right premotor cortex, but also in extramotor regions, 2) glucose hypometabolism mainly in both thalamus and the parietal lobe, and 3) glucose hypermetabolism located bilaterally in the cerebellar vermis, medial temporal lobe, and fusiform gyrus. We then extracted the metabolic values of the main hypermetabolic clusters and correlated them with cognitive scores that were expected to depend on these brain regions. We found only negative correlations, first, between the hippocampus and left parahippocampus and episodic memory, and second between the left fusiform gyrus and cognitive theory of mind. We also conducted a voxelwise comparison of the degrees of local atrophy and hypometabolism, using a method specially designed for this purpose, and we observed that GM atrophy predominated in the bilateral middle temporal pole and the left hippocampus, while hypometabolism predominated in a single cluster located in the left superior medial frontal cortex.

Between-group comparisons

The profile of GM atrophy in our patients was in agreement with previous studies that had reported motor and extramotor GM atrophy in a parieto-temporal network [START_REF] Cosottini | Structural and functional evaluation of cortical motor areas in Amyotrophic Lateral Sclerosis[END_REF][START_REF] Grosskreutz | Widespread sensorimotor and frontal cortical atrophy in Amyotrophic Lateral Sclerosis[END_REF][START_REF] Mioshi | Cortical atrophy in ALS is critically associated with neuropsychiatric and cognitive changes[END_REF][START_REF] Agosta | Voxel-based morphometry study of brain volumetry and diffusivity in amyotrophic lateral sclerosis patients with mild disability[END_REF][START_REF] Chang | A voxel-based morphometry study of patterns of brain atrophy in ALS and ALS/FTLD[END_REF][START_REF] Filippini | Corpus callosum involvement is a consistent feature of amyotrophic lateral sclerosis[END_REF][START_REF] Grossman | What's in a name: voxelbased morphometric analyses of MRI and naming difficulty in Alzheimer's disease, frontotemporal dementia and corticobasal degeneration[END_REF][START_REF] Kassubek | Global brain atrophy and corticospinal tract alterations in ALS, as investigated by voxel-based morphometry of 3-D MRI[END_REF], as well as the putamen [START_REF] Mioshi | Cortical atrophy in ALS is critically associated with neuropsychiatric and cognitive changes[END_REF][START_REF] Agosta | Longitudinal assessment of grey matter contraction in amyotrophic lateral sclerosis: A tensor based morphometry study[END_REF][START_REF] Machts | Basal ganglia pathology in ALS is associated with neuropsychological deficits[END_REF][START_REF] Kim | Structural explanation of poor prognosis of amyotrophic lateral sclerosis in the non-demented state[END_REF] .

The pathological involvement of the putamen in ALS may seem surprising, as patients do not typically exhibit extrapyramidal signs. However, ubiquitin immunoreactive intracytoplasmic inclusions and neuronal loss have been found in the striatonigral system in half of patients with classic motor neuron disease [START_REF] Mackenzie | Ubiquitin immunohistochemistry suggests classic motor neuron disease, motor neuron disease with dementia, and frontotemporal dementia of the motor neuron disease type represent a clinicopathologic spectrum[END_REF]. Interestingly, in the study of Kim et al., the putamen was among the most atrophied brain regions in cognitively impaired ALS patients [START_REF] Kim | Structural explanation of poor prognosis of amyotrophic lateral sclerosis in the non-demented state[END_REF]. Furthermore, the putamen has a major role in verbal fluency, working memory and speech production [START_REF] Sefcsik | The role of the putamen in cognitive functionsa case study[END_REF], cognitive domains frequently impaired in ALS [START_REF] Beeldman | The cognitive profile of ALS: a systematic review and meta-analysis update[END_REF].

Finally, putaminal regions receive afferences from the motor, premotor cortices (53) all of which being heavily affected in this study.

Concerning the hypometabolism, our results are in line with most previous studies [START_REF] Cistaro | Brain hypermetabolism in amyotrophic lateral sclerosis: a FDG PET study in ALS of spinal and bulbar onset[END_REF][START_REF] Pagani | Functional pattern of brain FDG-PET in amyotrophic lateral sclerosis[END_REF][START_REF] Van Laere | Value of 18fluorodeoxyglucose-positron-emission tomography in amyotrophic lateral sclerosis: a prospective study[END_REF][START_REF] Van Weehaeghe | Prospective Validation of 18F-FDG Brain PET Discriminant Analysis Methods in the Diagnosis of Amyotrophic Lateral Sclerosis[END_REF].

Concerning the hypometabolism in the paracentral lobule, this result has rarely been found with 18 FDG-PET. Indeed, Kew et al., using C 15 O 2 in 12 patients with ALS, observed a significant reduction of cerebral blood flow at rest within this structure [START_REF] Kew | Cortical function in amyotrophic lateral sclerosis. A positron emission tomography study[END_REF]. Concerning the decreased uptake of 18 FDG within the left inferior parietal gyrus found in our sample of patients, PET studies using [ 11 C]-flumazenil, or C 15 O 2 , (58-60), also found reduced regional cerebral blood flow within the parietal lobe. We also found significantly lower frontal metabolism (superior medial) in our patient group that is line with the fact that they present frontal dysfunction. A study went further, showing that prefrontal hypometabolism was associated with reduced clinical functioning in ALS patients [START_REF] Van Laere | Value of 18fluorodeoxyglucose-positron-emission tomography in amyotrophic lateral sclerosis: a prospective study[END_REF]. Decreased glucose uptake in the thalamus has also been reported by Pagani et al., and is thought to be a metabolic signature of C9orf72-related ALS [START_REF] Cistaro | The metabolic signature of C9ORF72-related ALS: FDG PET comparison with nonmutated patients[END_REF]. Up to 10% of patients with ALS have a mutated gene, the most common of which is C9orf72. In patients with this mutation, the thalamic hypometabolism is slightly more marked than it is in sporadic ALS cases [START_REF] Van Laere | Value of 18fluorodeoxyglucose-positron-emission tomography in amyotrophic lateral sclerosis: a prospective study[END_REF].

Our results confirm the brain hypermetabolism in ALS already demonstrated in previous studies [START_REF] Cistaro | Brain hypermetabolism in amyotrophic lateral sclerosis: a FDG PET study in ALS of spinal and bulbar onset[END_REF][START_REF] Pagani | Functional pattern of brain FDG-PET in amyotrophic lateral sclerosis[END_REF][START_REF] Van Laere | Value of 18fluorodeoxyglucose-positron-emission tomography in amyotrophic lateral sclerosis: a prospective study[END_REF][START_REF] Van Weehaeghe | Prospective Validation of 18F-FDG Brain PET Discriminant Analysis Methods in the Diagnosis of Amyotrophic Lateral Sclerosis[END_REF]. Concerning the brain regions that were found to exhibit increased glucose metabolism, results were mainly in agreement with previous studies that also observed increased glucose metabolism in the medial temporal lobe and cerebellum [START_REF] Pagani | Functional pattern of brain FDG-PET in amyotrophic lateral sclerosis[END_REF][START_REF] Van Laere | Value of 18fluorodeoxyglucose-positron-emission tomography in amyotrophic lateral sclerosis: a prospective study[END_REF].

Negative correlations with episodic memory and affective theory of mind

We observed significant negative correlations between episodic memory (immediate recall) and the metabolic value of the right hippocampus, the left hippocampus and the left parahippocampal gyrus. We also found negative correlations between metabolic activity within the left fusiform gyrus and cognitive theory of mind. The negative correlation between metabolism and cognitive scores means that hypermetabolism is associated with a functional deficiency of the involved brain area. Indeed, given that our patients with ALS scored more poorly than controls for episodic memory as well as for cognitive theory of mind, this result indicates that the abnormal pattern of glucose metabolism within these regions was not associated with preserved performances. This result was not expected because generally, when studies found hypermetabolism or hyperactivation, it is usually considered as the reflect of compensatory mechanisms [START_REF] Ashraf | Cortical hypermetabolism in MCI subjects: a compensatory mechanism?[END_REF][START_REF] Hu | Distinct cerebral perfusion patterns in FTLD and AD[END_REF]. However, within the framework of Huntington's disease, a previous work has shown a link between hyperactivation of the precuneus and impaired performances of motor sequence learning [START_REF] Feigin | Preclinical Huntington's disease: Compensatory brain responses during learning[END_REF]. In ALS, it has been suggested that cerebral hypermetabolism reflects neuroinflammation, characterized by activated astrocytes and microglia [START_REF] Van Weehaeghe | Prospective Validation of 18F-FDG Brain PET Discriminant Analysis Methods in the Diagnosis of Amyotrophic Lateral Sclerosis[END_REF], rather than compensatory neuronal activity. Neuroinflammation has a deleterious effect, and is therefore more consistent with the negative correlations we found in our patients between metabolic data and relevant cognitive scores.

This finding further highlights the potential role of cerebral hypermetabolism as a functional imaging marker for ALS.

Voxelwise comparisons between alterations

This analysis revealed differences in the relative degrees of GM atrophy and cerebral glucose hypometabolism.

Greater atrophy than hypometabolism

Atrophy was greater than hypometabolism in three main regions: the temporal lobe (anterior, lateral and medial), and the calcarine sulcus. Based on previous studies carried out in Alzheimer's disease, frontotemporal dementia or chronic alcoholism [START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF][START_REF] Buhour | Pathophysiology of the behavioral variant of frontotemporal lobar degeneration: A study combining MRI and FDG-PET[END_REF][START_REF] Pitel | Morphological and glucose metabolism abnormalities in alcoholic Korsakoff's syndrome: group comparisons and individual analyses[END_REF][START_REF] Shimizu | Concordance and discordance between brain perfusion and atrophy in frontotemporal dementia[END_REF], it is generally thought that loss of brain cells other than neurons might result in atrophy without associated hypometabolism. In ALS, an increasing number of studies have suggested that astrocytosis and/or microglia activation could be involved in the pathophysiology of the disease [START_REF] Blackburn | Astrocyte function and role in motor neuron disease: A future therapeutic target?[END_REF][START_REF] Brettschneider | Microglial Activation Correlates with Disease Progression and Upper Motor Neuron Clinical Symptoms in Amyotrophic Lateral Sclerosis[END_REF] The finding of a normal, i.e. higher than expected, level of glucose metabolism in atrophic cerebral zones may also suggests that compensatory mechanisms are at work in these structures, helping to maintain a moderately high metabolic level relative to structural alterations-a hypothesis previously developed for Alzheimer's disease [START_REF] Joie | Region-Specific Hierarchy between Atrophy, Hypometabolism, and -Amyloid (A ) Load in Alzheimer's Disease Dementia[END_REF]. In this disease, according to the authors, the presence of abnormally phosphorylated tau proteins that aggregates to form neurofibrillary tangles, may be one of the processes underlying severe atrophy and moderate hypometabolism in the hippocampus [START_REF] Joie | Region-Specific Hierarchy between Atrophy, Hypometabolism, and -Amyloid (A ) Load in Alzheimer's Disease Dementia[END_REF]. In ALS, TDP-43 inclusions are located in neurons and astrocytes of ALS patients, not only in the motor regions, but also in the temporal lobe [START_REF] Geser | Evidence of Multisystem Disorder in Whole-Brain Map of Pathological TDP-43 in Amyotrophic Lateral Sclerosis[END_REF]. Following the same reasoning, the presence of TDP-43 could lead to a massive neuronal loss whereas moderate hypometabolism could be explained by a compensation of the remaining neurons.

Greater hypometabolism than GM atrophy

We found that hypometabolism was more severe than atrophy in the left superior medial frontal cortex. This result suggests that ALS is characterized by genuine functional alterations (metabolic, chemical or molecular) on top of the neuronal loss, heightening the functional consequences of local GM atrophy. Supporting the notion of metabolic alterations in ALS, a study used the benzodiazepine GABA A marker 11 Cflumazenil to study brain dysfunction in 17 patients [START_REF] Lloyd | Extramotor involvement in ALS: PET studies with the GABA(A) ligand [(11)C]flumazenil[END_REF]. Flumazenil is an antagonist at the benzodiazepine subunit of the GABA A receptor. These authors found reduced binding of 11 C-flumazenil in the dorsomedial prefrontal cortex notably. This may reflect the downregulation of postsynaptic GABA A receptor expression [START_REF] Lloyd | Extramotor involvement in ALS: PET studies with the GABA(A) ligand [(11)C]flumazenil[END_REF]. Given that glucose metabolism reflects synaptic activity [START_REF] Rocher | Resting-state brain glucose utilization as measured by PET is directly related to regional synaptophysin levels: a study in baboons[END_REF], the detrimental effects of ALS on neurotransmission systems could explain why metabolic dysfunction precedes GM atrophy (e.g., within the frontal lobe). Greater hypometabolism than GM atrophy has also been observed in the behavioural variant of FTD [START_REF] Buhour | Pathophysiology of the behavioral variant of frontotemporal lobar degeneration: A study combining MRI and FDG-PET[END_REF], suggesting that this could be a remote effect of GM atrophy on metabolism, or a diaschisis.

Diaschisis refers to a change in the metabolic activity of neurons that are anatomically or functionally connected to a damaged area. The medial prefrontal cortex is connected to limbic structures such as the medial temporal lobe and putamen [START_REF] Ongur | The Organization of Networks within the Orbital and Medial Prefrontal Cortex of Rats, Monkeys and Humans[END_REF]. The neuronal loss reflected by GM atrophy within the temporal lobe and putamen (see above) may remotely affect the metabolism of the medial prefrontal cortex, though possibly only for a limited period of time, as disconnected neurons eventually die, giving rise to different patterns of brain volume and metabolic impairment.

The study has some limitations. The major one is the existence of two groups of healthy subjects. Indeed, as our protocol did not include the neuroimaging examinations for healthy subjects, we used the scans of healthy subjects of another protocol of our laboratory. Then, the threshold that we used in this article is relatively liberal even if it has been employed in several MRI [START_REF] Agosta | Voxel-based morphometry study of brain volumetry and diffusivity in amyotrophic lateral sclerosis patients with mild disability[END_REF][START_REF] Kassubek | Global brain atrophy and corticospinal tract alterations in ALS, as investigated by voxel-based morphometry of 3-D MRI[END_REF][START_REF] Lillo | Grey and White Matter Changes across the Amyotrophic Lateral Sclerosis-Frontotemporal Dementia Continuum[END_REF][START_REF] Mezzapesa | Whole-brain and regional brain atrophy in amyotrophic lateral sclerosis[END_REF][START_REF] Senda | Progressive and widespread brain damage in ALS: MRI voxel-based morphometry and diffusion tensor imaging study[END_REF][START_REF] Tavazzi | Grey matter damage in progressive multiple sclerosis versus amyotrophic lateral sclerosis: a voxel-based morphometry MRI study[END_REF][START_REF] Tsujimoto | Behavioral changes in early ALS correlate with voxel-based morphometry and diffusion tensor imaging[END_REF][START_REF] Ellis | Volumetric analysis reveals corticospinal tract degeneration and extramotor involvement in ALS[END_REF] or PET studies [START_REF] Van Laere | Value of 18fluorodeoxyglucose-positron-emission tomography in amyotrophic lateral sclerosis: a prospective study[END_REF][START_REF] Van Weehaeghe | Prospective Validation of 18F-FDG Brain PET Discriminant Analysis Methods in the Diagnosis of Amyotrophic Lateral Sclerosis[END_REF] in ALS. However, with a more stringent threshold we could have missed some interesting findings. Finally, this study should be replicated in a bigger group of ALS patients.

CONCLUSIONS

Taken together, our results confirm the existence of structural and metabolic changes in the brains of patients with ALS without dementia. We found that regional cerebral hypermetabolism is associated to impaired cognitive performance, which suggests that it reflects a local deleterious neuronal and/or astrocytic process. Our findings also emphasize the complex relationships between GM atrophy and hypometabolism, and the regional heterogeneity in their hierarchy. Greater GM atrophy than hypometabolism might mean either that brain tissue loss does not involve metabolically active cells, or that the metabolism of the remaining cells is higher than expected. Greater hypometabolism than GM atrophy could reflect either a disconnection mechanism, or an early stage of metabolic neuronal failure preceding cell death. Longitudinal studies are warranted to take account of the potential lapse between the different pathological processes.

Participants

  Participants with ALS. Participants were 37 patients with ALS (21 men and 16 women; mean age = 61.18 years, standard deviation = 11.11; mean level of education = 10.03 years, standard deviation = 2.96) with either bulbar (n = 10) or spinal (n = 27) onset. Patients with ALS were recruited between November 2009 and June 2014 via the ALS clinic of the neurology department of Caen University Hospital (France). All participants were native French

Table 1

 1 Demographic, medical data, and executive function as well as episodic memory and affective theory of mind performances of controls and patients with ALS.

		Controls (cognition)	Controls (imaging)	ALS patients	P
	Main features of the participants
	Age (years)	61.81	59	61.83	0.83 (cognition)/0.91 (imaging)
	Sex (F/M)	16/21	16/21	16/21
	Years of education	11.11	11.43	9.92	0.07 (cognition)/ 0.09 (imaging)
	Mattis (total)	139.95	141	136.47	0.005 (cognition)/<0.001 (imaging)
	ALSFRS-R score	/		37.31
	Cognitive profile
	TMT(B-A)	15.94		62.97	<0.001
	Letter verbal fluency score	22.24		15.03	<0.001
	letter verbal fluency index	4.71		7.68	0.017
	episodic memory	12.14		7.08	<0.001
	cognitive Theory of Mind	13.22		11.36	0.009

Table 2

 2 Labelization. MNI coordinates. Cluster size in number of voxels and T value of the significant peaks for the SPM analyze of GM atrophy, hypometabolism, hypermetabolism and GM atrophy > Hypometabolisme and Hypometabolisme > GM atrophy.

	Labels	K	MNI coordinates	p corrected at the level cluster	p FWE voxel-level corrected at	T (voxel level)
			x y z
	GM atrophy
	L postcentral	569	-53 -12 29	0.518	0.036	5.18
	L inferior temporal gyrus	3146	-27 9 -33	0.002	0.102	4.86
	L paracentral lobule	1331	-4 -27 64	0.084	0.133	4.77
	L supramarginal gyrus	484	-52 -22 31	0.621	0.159	4.71
	R precentral gyrus	1115	16 -26 67	0.14	0.541	4.23
	R putamen	771	35 0 4	0.323	0.785	3.99
	Hypometabolism
	R paracentral lobule	218	4 -22 70	0.217	0.032	5.21
	L inferior parietal gyrus	142	-50 -24 34	0.398	0.099	4.26
	R thalamus	244	8 -16 4	0.176	0.31	3.92
	L superior medial frontal gyrus	142	-8 40 36	0.398	0.314	3.91
	L thalamus	144	-8 -18 6	0.392	0.564	3.87
	Hypermetabolism
	L Hippocampus	868	-22 -18 -18	0.003	0.008	5.33
	L parahippocampus
	L fusiform
	Vermis VII	941	-2 -72 -32	0.002	0.044	4.82
	Vermis IV_V
	R fusiform	557	22 -32 -20	0.019	0.163	4.39
	R cerebellum IV_V
	R parahippocampal gyrus
	R Hippocampus
	Atrophy > Hypometabolism
	L parahippocampus	147	-26 4 -30	0	0.709	4.85
	L middle temporal pole
	L calcarine	36	-4 -96 8	0.109	0.784	4.76
	R middle temporal pole	58	46 12 -26	0.013	0.936	4.52
	L inferior temporal gyrus	87	-50 -6 -32	0.001	0.997	4.2
	L hippocampus	56	-30 -12 -22	0.016	1	4.05
	Hypometabolism > Atrophy
	L superior medial frontal gyrus		48 -6 34 40	0,033	1	4,06

Table 3 .

 3 Correlations between the metabolism of each region presenting hypermetabolism and the relevant cognitive performances in ALS patients. R = right. L= left.

		Episodic Memory (immediate recall) Episodic memory (delayed recall)	TOM-15
	L hippocampus	r= -0,3575; p= 0,045	r=-0,2921; p=0,105
	R hippocampus	r=-0,3585; p=0,044	r=-0,3171; p=0,077
	L parahippocampus	r= -0,4641; p=0,007	r=-0,4501; p=0,010
	R parahippocampus	r=-0,1932; p=0,289	r=-0,1059; p=0,564
	L fusiform			r=-0,3327; p= 0,044
	R fusiform			r=-0,1324; p=0,435

Figure 1. Patterns of grey matter atrophy, hypometabolism and hypermetabolism in ALS

Acknowledgments

The authors are grateful to B. Landeau and R. La Joie for their contribution and the Cyceron MRI-PET staff members (C. Lebouleux, M.H. Noel, and M.C. Onfroy) for their help with patients and imaging examination acquisition. The authors also wanted to thank S.Segobin for his valuable advices during the revision phase.

Funding

This work was supported by the French Ministry of Health (PHRC, 2008, n° ID-RCB A01150-55) and Fondation pour la Recherche Médicale (FRM)

Abbreviations

ALS: Amyotrophic lateral Sclerosis; ALS-FRS-R: ALS functional rating scale revised; CSF: cerebrospinal fluid; FDG: 18

Competing of interests

The authors declare that they have no competing interests

Authors' contributions

MSB was actively involved in this study from design to drafting. She conducted all the statistical analyses and played a central role in interpreting the results and writing the article. . FD greatly contributed to the neuroimaging part of this study. AM and AP participated in the acquisition of the cognitive data, especially in the careful screening of our cohort. FD, LC, and FE provided their critical revision of the manuscript. FV and BD supervised and coordinated the teamwork from start to finish. Their knowledge and expertise in neuropsychology, and ALS pathology were crucial for the design, analysis, and interpretation of the results of this project. They were also particularly involved in the revising of the manuscript. All authors read and approved the final manuscript.

Ethical approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Author details

Normandie Univ, UNICAEN, EPHE, INSERM, U1077, CHU de Caen, Neuropsychologie et Imagerie de la Mémoire Humaine, 14000 Caen, France


M-S Buhour

; F Doidy

; M Laisney

; Al Pitel

; V De

La Sayette

; F Viader

; F Eustache

B Desgranges
email: beatrice.desgranges@unicaen.fr

Pathophysiology of the behavioral variant of frontotemporal lobar degeneration: a study combining MRI and FDG-PET

Keywords: bv-FTD, structural MRI, FDG-PET, voxel-based Morphometry, individual analysis

Gray matter (GM) lobar atrophy and glucose hypometabolism are well-described hallmarks of frontotemporal lobar degeneration (FTLD), but the relationships between them are still poorly understood. In this study, we aimed to show the patterns of GM atrophy and hypometabolism in a sample of 15 patients with the behavioral variant of FTLD (bv-FTD), compared to 15 healthy controls, then to provide a direct comparison between GM atrophy and hypometabolism, using a voxel-based method specially designed to statistically compare the two imaging modalities. The participants underwent structural magnetic resonance imaging and 18 F-fluorodeoxyglucose (FDG) positron emission tomography examinations. First, between-group comparisons of GM volume and metabolism were performed. Then, in the patient group, correlations between regional alterations and direct between-modality voxelwise comparison were performed. Finally, we examined individual patterns of brain abnormalities for each imaging modality and each patient. The observed patterns of GM atrophy and hypometabolism were consistent with previous studies. We found significant voxelwise correlations between changes in GM and FDG uptake, mainly in the frontal cortex, corresponding to the typical profile of alterations in bv-FTD. The direct comparison revealed regional variability in the relationship between hypometabolism and atrophy. This analysis revealed greater atrophy than hypometabolism in the right putamen and amygdala, and left insula and superior temporal gyrus, whereas hypometabolism was more severe than GM atrophy in the left caudate nucleus and anterior cingulate cortex. Finally, GM atrophy affected the right amygdala/hippocampus and left insula in 95% of the patients. These findings provide evidence for regional variations in the hierarchy of hypometabolism and GM atrophy and the relationships between them, and enhance our understanding of the pathophysiology of bv-FTD.

Introduction

Frontotemporal lobar degeneration (FTLD) is a neurodegenerative disease with three variants: nonfluent primary progressive aphasia, semantic dementia (SD), and the frontal variant of FTLD (bv-FTD). The incidence of FTLD is similar to that of Alzheimer's disease (AD) in 45-to 64-year-olds, and bv-FTD represents approximately 75% of these cases [START_REF] Mercy | Incidence of earlyonset dementias in Cambridgeshire, United Kingdom[END_REF]. Patients with bv-FTD show marked changes in behavior and personality (Bathgate, Snowden, Varma, Blackshaw, & Neary, 2001;[START_REF] Bozeat | Which neuropsychiatric and behavioural features distinguish frontal and temporal variants of frontotemporal dementia from Alzheimer's disease?[END_REF][START_REF] Ikeda | Changes in appetite, food preference, and eating habits in frontotemporal dementia and Alzheimer's disease[END_REF][START_REF] Nyatsanza | A study of stereotypic behaviours in Alzheimer's disease and frontal and temporal variant frontotemporal dementia[END_REF][START_REF] Piguet | Eating disturbance in behavioural-variant frontotemporal dementia[END_REF][START_REF] Snowden | Distinct behavioural profiles in frontotemporal dementia and semantic dementia[END_REF]. A decline in social interpersonal conduct is among the most striking features of bv-FTD: patients may become socially disinhibited, exhibiting facetiousness and inappropriate jocularity. Apathy, reduced empathy, and a complete lack of concern for others are often observed. Impaired insight also translates into a lack of consciousness of mental symptoms, with patients being unaware of the changes in their personality and behavior [START_REF] Piguet | Behavioural-variant frontotemporal dementia: diagnosis, clinical staging, and management[END_REF]. Even though some recent findings have pointed to genuine episodic memory deficits in bv-FTD [START_REF] Mendez | Accuracy of the clinical evaluation for frontotemporal dementia[END_REF][START_REF] Piguet | Sensitivity of current criteria for the diagnosis of behavioral variant frontotemporal dementia[END_REF], severe amnesia is still considered to be a neuropsychological exclusion criterion for the diagnosis of bv-FTD [START_REF] Rascovsky | Sensitivity of revised diagnostic criteria for the behavioural variant of frontotemporal dementia[END_REF].

Bv-FTD is characterized by predominant lesions in the frontal and/or anterior temporal brain regions of variable distribution [START_REF] Hodges | The classification, genetics and neuropathology of frontotemporal dementia. Introduction to the special topic papers: Part I[END_REF][START_REF] Neary | Frontotemporal lobar degeneration: a consensus on clinical diagnostic criteria[END_REF][START_REF] Rascovsky | Diagnostic criteria for the behavioral variant of frontotemporal dementia (bvFTD): current limitations and future directions[END_REF]. A number of magnetic resonance imaging (MRI) studies have sought to map in vivo structural brain changes associated with bv-FTD, mostly using statistical parametric mapping (SPM) software, which allows for voxel-by-voxel analysis of the whole brain (voxel-based morphometry, VBM, [START_REF] Good | A voxel-based morphometric study of ageing in 465 normal adult human brains[END_REF]). Numerous studies have attempted to investigate the impact of bv-FTD on cerebral gray matter (GM). Some of them had a less than ideal design (e.g., comparison between patients with bv-FTD and a group composed of both healthy individuals and patients with SD [START_REF] Rosen | Patterns of brain atrophy in frontotemporal dementia and semantic dementia[END_REF] or a mixture of patients with bv-FTD and SD [START_REF] Grossman | What's in a name: voxel-based morphometric analyses of MRI and naming difficulty in Alzheimer's disease, frontotemporal dementia and corticobasal degeneration[END_REF][START_REF] Rabinovici | Distinct MRI atrophy patterns in autopsy-proven Alzheimer's disease and frontotemporal lobar degeneration[END_REF][START_REF] Williams | Neural correlates of semantic and behavioural deficits in frontotemporal dementia[END_REF], making it impossible to say which brain regions were specifically involved in bv-FTD. More recent studies, comparing patients with bv-FTD and healthy agematched controls, have highlighted abnormalities in the frontal dorsomedial, orbital and insular cortices, as well as in the striatum and temporal cortex [START_REF] Boccardi | The MRI pattern of frontal and temporal brain atrophy in frontotemporal dementia[END_REF][START_REF] Couto | Structural neuroimaging of social cognition in progressive non-fluent aphasia and behavioral variant of frontotemporal dementia[END_REF][START_REF] Irish | Common and unique gray matter correlates of episodic memory dysfunction in frontotemporal dementia and Alzheimer's disease[END_REF][START_REF] Lillo | Grey and white matter changes across the amyotrophic lateral sclerosis-frontotemporal dementia continuum[END_REF][START_REF] Möller | More atrophy of deep gray matter structures in frontotemporal dementia compared to Alzheimer's disease[END_REF][START_REF] Perry | Voxel-based morphometry in patients with obsessivecompulsive behaviors in behavioral variant frontotemporal dementia[END_REF][START_REF] Rohrer | Distinct profiles of brain atrophy in frontotemporal lobar degeneration caused by progranulin and tau mutations[END_REF][START_REF] Schroeter | Conceptualizing neuropsychiatric diseases with multimodal datadriven meta-analyses -the case of behavioral variant frontotemporal dementia[END_REF][START_REF] Seeley | Frontal paralimbic network atrophy in very mild behavioral variant frontotemporal dementia[END_REF][START_REF] Whitwell | Gray and white matter water diffusion in the syndromic variants of frontotemporal dementia[END_REF][START_REF] Zhang | MRI signatures of brain macrostructural atrophy and microstructural degradation in frontotemporal lobar degeneration subtypes[END_REF] Metabolic imaging analyses using 18 fluorodeoxyglucose-PET (FDG-PET) have also been performed in bv-FTD. These investigations revealed metabolic deficits not only in frontal and/or temporal regions [START_REF] Diehl | Cerebral metabolic patterns at early stages of frontotemporal dementia and semantic dementia. A PET study[END_REF][START_REF] Garraux | Comparison of impaired subcortico-frontal metabolic networks in normal aging, subcortico-frontal dementia, and cortical frontal dementia[END_REF][START_REF] Jeong | 18F-FDG PET findings in frontotemporal dementia: an SPM analysis of 29 patients[END_REF][START_REF] Franceschi | Glucose metabolism and serotonin receptors in the frontotemporal lobe degeneration[END_REF][START_REF] Salmon | Predominant ventromedial frontopolar metabolic impairment in frontotemporal dementia[END_REF], but also in other brain structures, such as the basal ganglia [START_REF] Diehl | Cerebral metabolic patterns at early stages of frontotemporal dementia and semantic dementia. A PET study[END_REF][START_REF] Franceschi | Glucose metabolism and serotonin receptors in the frontotemporal lobe degeneration[END_REF][START_REF] Garraux | Comparison of impaired subcortico-frontal metabolic networks in normal aging, subcortico-frontal dementia, and cortical frontal dementia[END_REF][START_REF] Jeong | 18F-FDG PET findings in frontotemporal dementia: an SPM analysis of 29 patients[END_REF][START_REF] Salmon | Predominant ventromedial frontopolar metabolic impairment in frontotemporal dementia[END_REF], anterior cingulate gyrus [START_REF] Franceschi | Glucose metabolism and serotonin receptors in the frontotemporal lobe degeneration[END_REF][START_REF] Garraux | Comparison of impaired subcortico-frontal metabolic networks in normal aging, subcortico-frontal dementia, and cortical frontal dementia[END_REF][START_REF] Ishii | Cerebral glucose metabolism in patients with frontotemporal dementia[END_REF] , hippocampus [START_REF] Franceschi | Glucose metabolism and serotonin receptors in the frontotemporal lobe degeneration[END_REF][START_REF] Ishii | Cerebral glucose metabolism in patients with frontotemporal dementia[END_REF], and parietal cortex [START_REF] Diehl-Schmid | Decline of cerebral glucose metabolism in frontotemporal dementia: a longitudinal 18F-FDG-PET-study[END_REF]Garraux, Salmon, Degueldre, Lemaire, & Franck, 1999;[START_REF] Ishii | Cerebral glucose metabolism in patients with frontotemporal dementia[END_REF][START_REF] Kipps | Behavioural variant frontotemporal dementia: not all it seems?[END_REF]. These findings have to be interpreted cautiously, as measures of glucose metabolism may be biased in patients with degenerative diseases studied by PET, owing to partial volume effects (PVEs). Despite technological advances, PET continues to suffer from relatively low spatial resolution. As a result, in structures with dimensions similar to the scanner's spatial resolution and bordered by cerebrospinal fluid or WM, the apparent radiotracer concentration is underestimated. PVEs are also particularly marked when the brain tissue is atrophic, as it is in bv-FTD [START_REF] Quarantelli | Integrated software for the analysis of brain PET/SPECT studies with partialvolume-effect correction[END_REF]. While it is essential to correct for PVEs in order to improve spatial resolution, to our knowledge, none of the above-mentioned PET studies did so.

Even though morphological and functional abnormalities seem to concern broadly the same areas, it is important to look more closely for possible differences or similarities in the regions with brain volume loss and hypometabolism, if we want to improve our understanding of the disease's pathophysiology. No study has yet attempted to do this in the same group of patients. Some studies have combined anatomical MRI and FDG-PET within the same sample of patients, but not to probe the relationship between those two abnormalities [START_REF] Frisch | Dissociating memory networks in early Alzheimer's disease and frontotemporal lobar degeneration -a combined study of hypometabolism and atrophy[END_REF][START_REF] Kanda | Comparison of grey matter and metabolic reductions in frontotemporal dementia using FDG-PET and voxel-based morphometric MR studies[END_REF][START_REF] Kipps | Combined magnetic resonance imaging and positron emission tomography brain imaging in behavioural variant frontotemporal degeneration: refining the clinical phenotype[END_REF][START_REF] Woost | Neural correlates of the DemTect in Alzheimer's disease and frontotemporal lobar degeneration -A combined MRI & FDG-PET study[END_REF].

Moreover, most previous imaging investigations have involved group comparisons between bv-FTD patients and controls, with only one examining individual patterns of metabolic abnormalities [START_REF] Salmon | Predominant ventromedial frontopolar metabolic impairment in frontotemporal dementia[END_REF]. This study identified the ventromedial frontopolar cortex as the one region to be affected in every single patient in the sample.

The first goal of the present study was thus to provide a comprehensive assessment of GM volumes, as well as glucose metabolism, in a group of patients with bv-FTD, adopting a voxel-based approach. After assessing each of them separately, we carried out voxelwise correlations between GM atrophy and hypometabolism, using biological parametric mapping (BPM) software. This method allowed us to improve our understanding of the disease's pathophysiological mechanisms by probing neuroimaging data in both the functional and structural imaging modalities. We then used a specific processing technique to carry out direct voxelwise comparisons of the degree of local GM atrophy and hypometabolism, using a method that had previously been developed in our laboratory and successfully implemented in early AD [START_REF] Chételat | Direct voxel-based comparison between grey matter hypometabolism and atrophy in Alzheimer's disease[END_REF][START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF] and alcoholism [START_REF] Pitel | Morphological and glucose metabolism abnormalities in alcoholic Korsakoff's syndrome: group comparisons and individual analyses[END_REF]. The second goal of the present study was to identify the brain regions affected by GM atrophy or hypometabolism in each of the patients in our sample, looking for heterogeneity in the individual patterns of brain abnormalities. We conducted separate analyses of the MRI and FDG-PET data.

Materials and Methods

Participants

We included 15 patients with probable bv-FTD (10 men and 5 women, mean age = 67 ± 8.2 years) in the present study, selected according to the revised criteria established by [START_REF] Rascovsky | Sensitivity of revised diagnostic criteria for the behavioural variant of frontotemporal dementia[END_REF]. These patients underwent a neurological examination and a standard neuropsychological assessment (including the Free and Cued Selective Reminding Test [START_REF] Eustache | Entorhinal cortex disruption causes memory deficit in early Alzheimer's disease as shown by PET[END_REF]. The data of some of these patients had already been included in previous publications [START_REF] Laisney | The underlying mechanisms of verbal fluency deficit in frontotemporal dementia and semantic dementia[END_REF][START_REF] Matuszewski | Retrieval mechanisms for autobiographical memories: insights from the frontal variant of frontotemporal dementia[END_REF][START_REF] Piolino | In search of autobiographical memories: A PET study in the frontal variant of frontotemporal dementia[END_REF]. The diagnosis of bv-FTD was confirmed by following the patients up for several years. To analyze the neuroimaging data, we included a group of 15 healthy controls (6 men and 9 women), matched with the patient group for age (mean age = 66.5 ± 8.3 years), sex and years of education. The participants had a minimum level of education equivalent to the French primary school certificate, obtained after 8 years of primary education. None of the participants had a history of alcoholism or psychiatric or neurological problems (head injury, trauma, epilepsy, depression, etc.). All the participants gave their written informed consent prior to their inclusion in the study, which was approved by the local ethics committee.

Imaging data acquisition

Each participant underwent a high-resolution, T1 weighted MRI scan, consisting of a set of 128 adjacent axial cuts parallel to the AC-PC line with a slice thickness of 1.5 mm and a 0.94 x 0.94-mm pixel size, using a spoiled gradient echo sequence (TR = 10.3 ms, TE = 2.1 ms, FOV = 240x180 mm² matrix = 256x192). All the MRI datasets were acquired on the same scanner (1.5T Signa Advantage Echospeed; General Electric) using the same acquisition protocol.

Participants also underwent a PET scan within days of the MRI examination. Data were acquired using an ECAT Exact HR+ PET device with an isotropic resolution of 4.6 x 4.2 x 4.2 mm and axial field of view of 158 mm. Participants were fasted for at least 4 hours before the scan. To minimize anxiety, the PET procedure was explained in detail beforehand.

The head was positioned on a headrest relative to the canthomeatal line and gently restrained with straps. 18 FDG uptake was measured in the resting condition with eyes closed, in a quiet and dark environment. A catheter was inserted into a vein of the arm to inject the radiotracer.

Following 68 gallium transmission scans, 3-5 mCi of 18 FDG were injected as a bolus at Time 0 and a 10-min data acquisition period started 50 min post injection. Sixty-three planes were acquired with no septa (3D acquisition), using a voxel size of 2.2 x 2.2 x 2.43 mm (x, y, z).

During PET data acquisition, head motion was continuously monitored with, and whenever necessary corrected according to, laser beams projected onto ink marks on the forehead.

Image processing

The data processing procedure was especially designed to compare morphological and metabolic data, as described in detail elsewhere [START_REF] Chételat | Direct voxel-based comparison between grey matter hypometabolism and atrophy in Alzheimer's disease[END_REF][START_REF] Pitel | Morphological and glucose metabolism abnormalities in alcoholic Korsakoff's syndrome: group comparisons and individual analyses[END_REF] and illustrated in Figure 1. The MRI datasets were analyzed using VBM in SPM5, as recommended by Good et al. This procedure included segmentation, spatial normalization of the original raw MRI data using the MNI template of SPM5 as a prior, and modulation.

Finally, the registered MRI data were smoothed (see below). The procedure yielded maps of local GM volume corrected for brain size.

The PET data were first corrected for PVEs arising from both cerebrospinal fluid and white matter, using the modified Müller-Gärtner method [START_REF] Müller-Gärtner | Measurement of radiotracer concentration in brain gray matter using positron emission tomography: MRI-based correction for partial volume effects[END_REF][START_REF] Rousset | Correction for partial volume effects in PET: principle and validation[END_REF] described in detail in [START_REF] Quarantelli | Integrated software for the analysis of brain PET/SPECT studies with partialvolume-effect correction[END_REF]. All the image-processing steps for PVE correction were carried out using PMOD software (PMOD Technologies Ltd., Adliswil, Switzerland). The PVE-corrected PET data were then coregistered onto their corresponding MRI scans and normalized using the deformation parameters defined by the VBM procedure described above. The resulting images were scaled using the metabolism of the cerebellum as a reference. This meant that the PET images were divided by their mean cerebellar FDG uptake value to control for individual variations in the overall PET value [START_REF] Mevel | Detecting hippocampal hypometabolism in Mild Cognitive Impairment using automatic voxel-based approaches[END_REF]. The cerebellum was selected as a reference because it had the best preserved FDG uptake value in our sample of patients relative to controls (data not shown).

Finally, the corrected and spatially normalized PET data were smoothed.

For the between-group comparisons, and in order to blur individual variations in gyral anatomy and increase the signal-to-noise ratio, the registered MRI data were smoothed using a 12-mm Gaussian kernel, and the PET datasets were smoothed using a 14-mm Gaussian kernel. For the direct comparison between structural and metabolic GM abnormalities, a different Gaussian kernel was applied to the registered unsmoothed MRI datasets, to compensate for the difference in the original spatial resolution between the two modalities [START_REF] Chételat | Direct voxel-based comparison between grey matter hypometabolism and atrophy in Alzheimer's disease[END_REF][START_REF] Villain | Relationships between hippocampal atrophy, white matter disruption, and gray matter hypometabolism in Alzheimer's disease[END_REF]. A 14.6-mm Gaussian kernel was used for the MRI GM data, resulting in an effective smoothness identical to that of the PET images smoothed at 14 mm.

The resulting MRI and PET datasets were masked to include only GM voxels of interest and prevent contamination of the analysis by misclassified voxels as much as possible (see [START_REF] Villain | Relationships between hippocampal atrophy, white matter disruption, and gray matter hypometabolism in Alzheimer's disease[END_REF] for details). Briefly, we created mean images of GM and white matter partitions for the whole sample (N = 30 images). The white matter mask was obtained by thresholding the mean white matter image at a value of 0.4. A preliminary GM mask was first created by thresholding the mean GM image at a value of 0.3. The final GM mask was obtained by subtracting the white matter mask from the preliminary GM mask. The resulting binary GM mask was applied to both the MRI GM and PET datasets.

Our main objective was to perform a direct whole brain comparison of the degrees and location of atrophy and hypometabolism within the bv-FTD group, using a method especially designed for this purpose. Atrophy and hypometabolism do not have the same units, making their direct comparison impossible. Therefore, the smoothed and masked GM and PET images were used to create z-score maps (

) for each voxel of each patient and for each modality.

Statistical analysis

Between-group comparisons of GM volumes and metabolism First, we compared the two groups within each imaging modality to obtain maps of significant GM atrophy, as well as hypometabolism in the patients with bv-FTD compared with controls. Two independent analyses were performed on the MRI and PET data of the two groups (bv-FTD and controls), using the SPM5 two-sample t test routine. For GM volumes, results were deemed to be significant at the familywise error (FWE) threshold of p = 0.05, with a minimum cluster size of 160 voxels (160 mm 3 ). Concerning the PET data, results were deemed to be significant at the FWE threshold of p = 0.05, with a minimum cluster size of 20 voxels (160 mm 3 ). In addition, we also reported the results of the between group comparisons using a less restrictive threshold of p uncorrected <0.001 in order to have a global view of the pathology.

Correlation between regional alterations in patients

To assess the relationship between regional GM atrophy and hypometabolism, z-score maps were entered pairwise in a voxelwise correlation analysis using the BPM toolbox implemented in SPM, which is especially designed to analyze brain images in different modalities [START_REF] Casanova | Biological parametric mapping: A statistical toolbox for multimodality brain image analysis[END_REF]. This analysis shows all the voxels that evolve in the same manner. More specifically, the voxels presenting a correlation between GM atrophy and glucose hypometabolism are highlighted. Results were deemed to be significant below an uncorrected threshold of p = 0.001, with a minimum cluster size of 160 voxels.

Comparisons between structural and metabolic abnormalities in patients

Direct comparisons between the degree of structural and metabolic abnormalities were only performed for voxels with significant GM atrophy or hypometabolism. For this purpose, we averaged the GM volume and FDG uptake z-score maps for all participants, to obtain a mean z-score map for GM volume and a mean z-score map for FDG uptake. We then thresholded these two maps by only including voxels with a mean MRI or PET z score < -1.77

(corresponding to the t value for one-tailed p = 0.05 with 13 degrees of freedom). These two new masked images were applied to all the patients' z-score maps. The individual masked atrophy and hypometabolism z-score maps were finally entered in an SPM one-tailed t test analysis, with one group (bv-FTD) and two images per participant. The two contrasts (z PET < z MRI and z MRI < z PET) yielded statistical maps reflecting any predominance of atrophy over hypometabolism and vice versa. Results were deemed to be significant below the FWE threshold of p = 0.05, with a minimum cluster size of 100 voxels for both contrasts.

Variability of the individual patterns of GM atrophy and hypometabolism

To examine between-participants variability in the patterns of GM atrophy and hypometabolism in the bv-FTD group, we first thresholded each individual GM and PET zscore map at -1.77 to obtain a value of 1 for voxels in each patient's map with significant atrophy or hypometabolism, and a value of 0 everywhere else. We then summed the 15 individual binary images for GM and PET. The two resulting maps reflected the frequency in each voxel of significant GM atrophy or hypometabolism in our sample, ranging from 0 (i.e., present in none of the patients) to 15 (i.e., present in all the patients).

RESULTS

Between-group comparisons GM volume loss was located within the frontal lobe (right ventrolateral prefrontal cortex, superior frontal gyrus and rectus gyrus on both sides, right orbitofrontal gyrus and left superior medial frontal cortex), left insula, both caudate nuclei, right thalamus, right anterior cingulate cortex, temporal lobe (right superior temporal gyus, left inferior temporal gyrus), and parietal lobe (right supramarginal gyrus and right angular gyrus) (Table 1 and Fig. 2).

Significant hypometabolism was present in the caudate nuclei, frontal cortex (right inferior orbital gyrus, right middle orbital gyrus, right supplementary motor area and superior frontal gyrus and medial superior frontal gyrus on both sides), cingulate cortex (middle cingulate cortex on both sides, left anterior cingulate cortex) and right inferior temporal gyrus (Table 2 and Fig. 2).

Voxelwise correlations between GM atrophy and hypometabolism

The pairwise intermodality correlation analyses revealed that significant correlations between GM atrophy and hypometabolism in the patient group mainly involved 1) the frontal cortex, including the superior medial frontal cortex on both sides, right middle frontal gyrus and right inferior frontal cortex, 2) the right temporal pole, and 3) the left anterior cingulate cortex (Fig. 3).

Within-group comparisons between morphological and metabolic abnormalities

The GM atrophy > hypometabolism contrast was significant in the right putamen and amygdala, as well as the left insula and superior temporal gyrus (Fig. 4). The reverse contrast (hypometabolism > GM atrophy) was only significant in the left caudate and anterior cingulate cortex.

Variability of the individual patterns of GM atrophy and hypometabolism

We sought to identify brain regions that were consistently atrophied (for GM and WM) or hypometabolic in our patients. GM volume loss was found in the right amygdala, right hippocampus and left insula of 14/15 (95%) of them. No brain region consistently presenting glucose hypometabolism was observed in the patients. However, in 12/15 (85%) of patients, we observed glucose hypometabolism in both caudate nuclei and right orbital frontal and t superior frontal gyri. (Table 3).

DISCUSSION

To our knowledge, this is the first neuroimaging study to have closely examined the matches and mismatches between the profiles of GM volume loss and hypometabolism in a single group of 15 patients with bv-FTD, compared with a group of 15 strictly matched controls. We also sought to identify the brain regions affected by GM atrophy or hypometabolism in each individual patient. The patterns and magnitude of these brain abnormalities and the relationships between them were assessed voxelwise.

We found 1) marked GM atrophy in the frontal lobe, insula and caudate nuclei and, to a lesser extent, the parietal and temporal cortices, and 2) glucose hypometabolism mainly in the caudate nuclei and several parts of the frontal cortex the superior medial frontal cortex and superior frontal gyrus on both sides, and right middle frontal gyrus). We also found correlations between GM atrophy and hypometabolism within a large proportion of the prefrontal regions. We further observed that GM atrophy predominated over hypometabolism in the right putamen and amygdala, as well as in the left insula and superior temporal gyrus, while hypometabolism predominated over GM atrophy in the left caudate nucleus and anterior cingulate cortex. Finally, we found that GM atrophy affected the right amygdala/hippocampus and left insula in 95% of patients with bv-FTD, while the caudate nuclei and prefrontal cortex were hypometabolic in 85% of patients.

Our analyses of GM volume and metabolism, considered separately, highlighted the involvement of a large frontotemporal-insular network, as well as the caudate nuclei, thalamus and anterior cingulate cortex, in accordance with the literature (see introduction, [START_REF] Schroeter | Conceptualizing neuropsychiatric diseases with multimodal datadriven meta-analyses -the case of behavioral variant frontotemporal dementia[END_REF]. Abnormalities in these structures are thought to be responsible for the main clinical features of this pathology (e.g., apathy, disinhibition, dietary changes, emotion and reward processing deficit, loss of empathy and dysexecutive syndrome; see, for instance [START_REF] Schroeter | Dissociating behavioral disorders in early dementia-An FDG-PET study[END_REF].

The voxelwise correlations between GM volume deficits and changes in FDG uptake, established here for the first time in bv-FTD, showed that atrophy and hypometabolism were correlated within the dorsolateral (to a greater extent on the right side) and median frontal cortices. This voxelwise correlation found within the prefrontal cortex may indicate that changes in this area share common mechanisms, and more specifically that the loss of GM volume is primarily dependent on neuronal death, with neither compensatory nor disconnection mechanisms at work. Interestingly, the dorsolateral and median frontal areas where metabolism and brain volume were correlated appear to be highly clinically significant in the setting of bv-FTD, just as the temporoparietal cortex is in AD, as shown by La [START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF] using a similar methodology. This result is also in accordance with [START_REF] Shimizu | Concordance and discordance between brain perfusion and atrophy in frontotemporal dementia[END_REF], who used perfusion imaging and structural MRI. These authors revealed concordant GM atrophy and hypoperfusion changes in the right prefrontal cortex and both sides of the medial frontal lobe.

In an attempt to provide a more detailed description of brain changes in our group of patients with bv-FTD, we undertook a direct comparison between both modalities, namely anatomical MRI and FDG-PET. Analyses revealed that cortical atrophy predominated over hypometabolism in the right putamen and amygdala, as well as in the left insula and superior temporal gyrus. [START_REF] Frisch | Dissociating memory networks in early Alzheimer's disease and frontotemporal lobar degeneration -a combined study of hypometabolism and atrophy[END_REF] found that GM atrophy was located in similar regions as hypometabolism, as well as in the left hippocampus and the thalamus. These authors hypothesized that GM atrophy and cerebral glucose hypometabolism can be relatively independent of each other. With our specific methodology, allowing the direct comparison of MRI and FDG-PET data, we were able to confirm this hypothesis.

A discrepancy between brain volume and metabolism measurements had previously been found by [START_REF] Shimizu | Concordance and discordance between brain perfusion and atrophy in frontotemporal dementia[END_REF] in a series of 28 patients, and confirmed by [START_REF] Schroeter | Conceptualizing neuropsychiatric diseases with multimodal datadriven meta-analyses -the case of behavioral variant frontotemporal dementia[END_REF] in their meta-analysis of more than 400 cases, but its relevance to the disease's pathophysiology remains to be elucidated. It seems surprising that atrophy can exceed hypometabolism, as GM loss is usually synonymous with neuronal/synaptic loss, so we would not normally expect to see atrophy exceeding hypometabolism. However, as Shimizu et al.

suggested, the loss of brain cells other than neurons may result in atrophy without regional hypometabolism. This is supported by two neuropathological studies showing that the earliest cellular changes in FTLD occur in astrocytes [START_REF] Broe | Staging disease severity in pathologically confirmed cases of frontotemporal dementia[END_REF][START_REF] Kersaitis | Regional and cellular pathology in frontotemporal dementia: relationship to stage of disease in cases with and without Pick bodies[END_REF]. The finding of less hypometabolism than atrophy also suggests that compensatory mechanisms are at work in these structures, helping to maintain a moderately high metabolic level relative to structural alterations-a hypothesis previously developed by Chetelat et al., (2003) and La [START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF] for AD.

We also found the reverse dissociation, namely greater hypometabolism than atrophy, in the left caudate nucleus and anterior cingulate cortex. This points to a genuine functional alteration (metabolic, chemical or molecular) over and above neuronal loss, heightening the functional consequence of local atrophy. This phenomenon has also been found in a recent meta-analysis [START_REF] Schroeter | Conceptualizing neuropsychiatric diseases with multimodal datadriven meta-analyses -the case of behavioral variant frontotemporal dementia[END_REF] covering a total of 417 patients with bv-FTD where 20 studies measured GM atrophy and 11 others that examined hypometabolism. Superimposition of the corresponding maps (though with no statistical comparisons) and visual analysis of the two imaging methods revealed that hypometabolism exceeded atrophy in the gyrus rectus and middle thalamus on both sides, and in the left posterior inferior frontal sulcus. In AD, some authors also found greater hypometabolism than GM atrophy and putatively considered it to be a remote effect of atrophy on metabolism though the functional disruption of connected areas [START_REF] Chételat | Direct voxel-based comparison between grey matter hypometabolism and atrophy in Alzheimer's disease[END_REF]. This is suggestive of diaschisis, namely a decrease in the metabolic activity of neurons that are anatomically or functionally connected to the damaged area. The GM atrophy found in the medial temporal and frontal lobes of our patients may thus correspond to neuronal loss that remotely affected the metabolism of the anterior cingulate cortex and caudate nucleus, respectively. This situation may be limited in time, as disconnected neurons eventually die as a result of transsynaptic degeneration, giving rise to different patterns of brain volume and metabolic impairment, depending on the stage of the disease.

Interindividual variability in the patterns of GM atrophy and hypometabolism

When we analyzed the variability in the structural and metabolic brain abnormalities in our sample of 15 patients with bv-FTD, we found that 12 of them had cerebral glucose hypometabolism within the frontal lobe (right orbitofrontal gyrus and right superior frontal gyrus) and both caudate nuclei.

Regarding GM volume, we observed atrophy in the right amygdala and hippocampus, as well as the left insula, in 14 of the patients. Atrophy of the amygdala has already been observed in several volumetric studies [START_REF] Boccardi | The MRI pattern of frontal and temporal brain atrophy in frontotemporal dementia[END_REF][START_REF] Irish | Common and unique gray matter correlates of episodic memory dysfunction in frontotemporal dementia and Alzheimer's disease[END_REF][START_REF] Möller | More atrophy of deep gray matter structures in frontotemporal dementia compared to Alzheimer's disease[END_REF][START_REF] Seeley | Frontal paralimbic network atrophy in very mild behavioral variant frontotemporal dementia[END_REF]. The amygdala's involvement has been confirmed by neuropathological studies showing that this structure degenerates at an early stage in bv-FTD (Kril & Halliday, 2004;[START_REF] Kril | Distribution of brain atrophy in behavioral variant frontotemporal dementia, 83-90[END_REF]. The insula has also already been found to be atrophic in several VBM [START_REF] Boccardi | Frontotemporal dementia as a neural system disease[END_REF][START_REF] Lillo | Grey and white matter changes across the amyotrophic lateral sclerosis-frontotemporal dementia continuum[END_REF][START_REF] Perry | Voxel-based morphometry in patients with obsessivecompulsive behaviors in behavioral variant frontotemporal dementia[END_REF][START_REF] Zhang | MRI signatures of brain macrostructural atrophy and microstructural degradation in frontotemporal lobar degeneration subtypes[END_REF] and cortical thickness studies [START_REF] Richards | Patterns of cortical thinning in Alzheimer's disease and frontotemporal dementia[END_REF]. Furthermore, our finding is in line with neuropathological studies suggesting that the insula is one of the sites where bv-FTD begins [START_REF] Broe | Staging disease severity in pathologically confirmed cases of frontotemporal dementia[END_REF]Kril et al., 2005).

Regarding the involvement of the hippocampus, our result fits several recent studies that found that this structure was atrophied in bv-FTD [START_REF] De Souza | Is hippocampal volume a good marker to differentiate Alzheimer's disease from frontotemporal dementia?[END_REF][START_REF] Lindberg | Hippocampal shape analysis in Alzheimer's disease and frontotemporal lobar degeneration subtypes[END_REF][START_REF] Muñoz-Ruiz | Structural MRI in frontotemporal dementia: comparisons between hippocampal volumetry, tensor-based morphometry and voxel-based morphometry[END_REF][START_REF] Tan | Retrosplenial cortex (BA 29) volumes in behavioral variant frontotemporal dementia and Alzheimer's disease[END_REF]. These studies featured group analyses; whereas we showed that 14/15 patients with bv-FTD had hippocampal volume deficits. Moreover, it has been increasingly recognized that bv-FTD patients can also exhibit amnesia in addition to their frontal behavioral disturbances [START_REF] Bertoux | Two distinct amnesic profiles in behavioral variant frontotemporal dementia[END_REF][START_REF] Hou | Patterns of autobiographical memory loss in dementia[END_REF][START_REF] Irish | Profiles of recent autobiographical memory retrieval in semantic dementia, behaviouralvariant frontotemporal dementia, and Alzheimer's disease[END_REF][START_REF] Matuszewski | Retrieval mechanisms for autobiographical memories: insights from the frontal variant of frontotemporal dementia[END_REF][START_REF] Pasquier | Memory impairment differs in frontotemporal dementia and Alzheimer's disease[END_REF][START_REF] Pijnenburg | Initial complaints in frontotemporal lobar degeneration[END_REF][START_REF] Piolino | In search of autobiographical memories: A PET study in the frontal variant of frontotemporal dementia[END_REF], even at initial stage. In addition, Hodges et al. [START_REF] Hodges | Clinicopathological correlates in frontotemporal dementia[END_REF] showed that 5 of 26 patients with pathologically proven FTLD exhibited marked amnesia. [START_REF] Graham | Pathologically proven frontotemporal dementia presenting with severe amnesia[END_REF] made the same observation when they examined the records of a neuropathological series of 71 patients with bv-FTD GM atrophy of the hippocampus is not, however, specific to this pathology. It has also been found in SD (e.g., [START_REF] Desgranges | Anatomical and functional alterations in semantic dementia: a voxel-based MRI and PET study[END_REF]), as well as in AD, of course (La [START_REF] Joie | Region-specific hierarchy between atrophy, hypometabolism, and β-amyloid (Aβ) load in Alzheimer's disease dementia[END_REF][START_REF] Villain | Relationships between hippocampal atrophy, white matter disruption, and gray matter hypometabolism in Alzheimer's disease[END_REF]. La [START_REF] Joie | Intrinsic connectivity identifies the hippocampus as a main crossroad between Alzheimer's and semantic dementia-targeted networks[END_REF] even identified the hippocampus as the main intersection between the networks targeted by AD and SD. According to these authors, regions within the anterior temporal system are connected to the anterior hippocampus (impaired in SD), whereas the posterior medial system is primarily linked to the posterior hippocampus (impaired in AD). In the case of bv-FTD, the impairment may concern the hippocampus and its connection to other brain structures, such as the frontal cortex or the anterior cingulate cortex, which may account for the cognitive pattern specific to this pathology. Further studies are needed to explore the clinical impact of hippocampal atrophy in bv-FTD.

Several limitations in our study deserve to be mentioned. First, the patients were diagnosed clinically, with no post mortem confirmation of bv-FTD, and above all we had no information about the type of molecular pathology. Although bv-FTD can occur in the case of either tau, TDP-43 or FUS protein deposits [START_REF] Josephs | Frontotemporal lobar degeneration with ubiquitin-positive, but TDP-43-negative inclusions[END_REF], these proteinopathies are not characterized by specific neuroimaging patterns [START_REF] Whitwell | Neuroimaging in frontotemporal lobar degeneration--predicting molecular pathology[END_REF]. It is important to note that the patients with bv-FTD were followed up over several years, undergoing regular neuropsychological assessments that confirmed their diagnosis. Second, the small number of patients with bv-FTD (15) limited the power of the final analysis, which was designed to probe the variability of the morphological and metabolic brain abnormalities.

It is, however, challenging to find larger numbers of carefully selected patients with both MRI and PET data.

CONCLUSION

Taken together, our results highlighted structural and metabolic changes in the brains of patients with bv-FTD. Frontal regions cannot be regarded as the only brain target of bv-FTD. Furthermore, there is a relative overlap of the profiles of metabolic and structural change. However, we also found that atrophy was greater than hypometabolism in some brain regions, suggesting that cellular changes in bv-FTD may occur in brain cells other than neurons. By contrast, we also found that hypometabolism was more severe than atrophy in some brain areas-a finding that can be explained by a disconnection mechanism. Finally, GM volume loss was found in 14/15 of patients with bv-FTD, located in the right amygdala and right hippocampus, as well as in the left insula-abnormalities that could be regarded as potential brain biomarkers for bv-FTD. Informed consent: Informed consent was obtained from all individual participants included in the study.

COMPLIANCE WITH ETHICAL STANDARDS

LEGENDS OF FIGURES

Fig 1

 1 Fig 1 Schematic representation of the procedures for analyzing MRI and PET data after registration. A: Procedure for analyses of GM atrophy and GM hypometabolism; B: Procedure for the direct voxel-based comparison between morphological and metabolism abnormalities; C: Procedure for the analysis of the variability in the individual patterns of atrophy and hypometabolism. See Pitel et al. (2009) for more details.

Fig 2

 2 Fig 2 GM (in red) and metabolic (in blue) abnormalities in patients with bv-FTD compared with healthy controls. Between-group comparisons were conducted with the SPM5 twosample t test routine. We used a p value cutoff of p <0.05 Family-Wise Error correction but we also displayed the results using a more permissive threshold p<0.001 uncorrected in order to have a global view of the pathology. Cluster size: k > 160 mm3. On the axial slices, the right is displayed on the right. The colored bar represents the t value of significant voxels.

Fig 3 Fig 4

 34 Fig 3 Voxelwise correlations between regional GM volume loss and glucose hypometabolism in the 15 patients with bv-FTD. P < 0.001uncorrected. Cluster size: k > 160 mm3. On the slices, the right is displayed on the right. The colored bar represents the r value of significant voxels.

Figure 1 Figure 2 Figure 3 Figure 4

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  Conflict of Interests: Author Marie-Sonia BUHOUR declares that she has no conflict of interest. Author Franck Doidy declares he has no conflict of interest. Author Mickaël Laisney declares he has no conflict of interest. Author Anne-Lise Pitel declares she has no conflict of interest. Author Vincent de la Sayette declares he has no conflict of interest. Author Fausto Viader declares he has no conflict of interest. Author Francis Eustache declares he has no conflict of interest. Author Béatrice Desgranges declares she has no conflict of interest. All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

	Ethical approval: