Baptiste Véron

Arnaud Hubert

Joël Abadie

Nicolas Andreff

Hybrid electromagnet model for multiple mobile coil magnetic manipulation

In gastroenterology, endoscopic capsules have demonstrated their usefulness for diagnosis in the small bowel. However, they also show limitations due to the impossibility to control their progress in the small bowel. Magnetic actuation is often considered to control these capsules, but current systems are very bulky and some have stability concerns. To improve the manipulability of the capsule and to reduce the size of the magnetic system, it is interesting to consider a system composed of mobile electromagnets having a ferromagnetic core. These electromagnets allow to create large efforts at lower currents, but produce a magnetic field highly non-linear. This paper proposes a hybrid model (analytical adjusted on measures) to compute the magnetic field created by the electromagnets, and discusses the usefulness of this model.

I. INTRODUCTION

There exist several paths towards small bowel exploration. The double-balloon enteroscopy allows the visualization of some parts of the small bowel (up to the beginning of the ileum) in real time, but not the whole small bowel. The capsule endoscope is a swallowable capsule embedding a camera which allows to take pictures all along the gastrointestinal tract, but has the major drawback of only moving thanks to peristalsis.

To cope with this lack of means for small bowel diagnosis several studies tried to enhance the capabilities of the capsule endoscopes, especially to actuate the capsule and allow the control of its position. Magnetic steering is quite appropriate for that, because magnetism is a contactless effect for which body tissues are transparent. Some studies have already been led on that topic using external permanent magnets (EPM) moving around the patient to steer the capsule embedding a small permanent magnet [START_REF] Ciuti | Robotic magnetic steering and locomotion of capsule endoscope for diagnostic and surgical endolumial procedures[END_REF]. With the same objective, the Niobe II system from Stereotaxis has also been used [START_REF] Carpi | Magnetic maneuvering of endoscopic capsules by means of a robotic navigation system[END_REF]. These approaches showed critical limitations. The impossibility of turning off the magnetic field leads to a lack of stability, and the use of an industrial robot to move the EPM, can be dangerous for the patient and the medical staff. Other studies have led to the use of static electromagnets placed around the patient similarly to an MRI system [START_REF] Rey | Blinded nonrandomized comparative study of gastric examination with a magnetically guided capsule endoscope and standard videoendoscope[END_REF]. These systems are extremely bulky, they do not let the staff access to the patient, and they use a large amount of energy.

Thus, these two approches can be combined to study a light weight system composed of several mobile electromagnets. This kind of system presents many advantages, as shown in [START_REF] Véron | Advocacy for multi mobile coil magnetic manipulation in active digestiveendoscopy[END_REF]. It allows full dexterity with a shorter distance between the electromagnet and the manipulated object, what reduces the heat and the energy consumption. More, it allows to easily design redundant systems to enhance the manipulability of the magnetic object and the stability. Furthermore, the bulkiness of such a system is even more reduced by the use of electromagnets composed of a winding and a ferromagnetic core. These electromagnets permit to get large effort at lower currents, but the magnetic field created is highly non-linear, what makes its precise real-time computation difficult. To overcome this, an analytical model is proposed, and its usability is discussed.

II. ELECTROMAGNETIC MODELING

The magnetic efforts are computed via the magnetic field B produced by a magnetic source :

F m = V • ∇ (M • B(P )) C m = V • M ∧ B(P )
where V is the volume of the permanent magnet, ∇ is the gradient operator and ∧ is the cross-product.

For electromagnets, three methods are commonly used to compute this magnetic field. The first method is to measure and to map the magnetic field over the whole workspace. The accuracy of this method relies on the precision and the number of measures. Its implementation is long and requires a large amount of memory to store the measures.

The second method, usually used for the design, consists in finite element modeling. This method is based on a mesh of the space, thus, it requires long computation time and a large amount of memory to store the data.

The third method is to use the well known dipole approximation :

B = µ 0 4 I a 2 r 3 2 cos θ sin θ 0 (1)
where µ 0 is the magnetic permeability of free space, a is the radius of a loop, I the current flowing in this turn, and (r, θ, ϕ) are the spherical coordinates where B is computed.

This analytical formula is only valid on some parts of the space around the electromagnet (for r a) and does not take the ferromagnetic core into account. This greatly limits its use.

None of the existing models for computing B (mapping, finite elements, dipole approximation) satisfy the contradictory requirements of accuracy and real-time computation.

Actually, B is the curl of the vector potential A : B = ∇ ∧ A. In spherical coordinates, A = (A r A θ A ϕ ) T of one current loop has only one component :

A ϕ = µ 0 4π 4Ia √ a 2 + r 2 + 2ar sin θ (2 -k 2 )K(k) -2E(k) k 2
(2) where K(k) and E(k) are respectively the complete elliptic integrals of first and second order, for which k is defined as :

k 2 =
4ar sin θ a 2 + r 2 + 2ar sin θ This allows to compute the magnetic field created by the winding by summing the contribution of each turn. To be taken into account, the ferromagnetic core is considered as a magnetic dipole located at the center of the coil, its magnetization depending on the current flowing in the coil. Thus, we propose to model the field generated by the coil and the core as a linear combination of (2) and the dipole approximation (1) :

B = G 1 • amax amin L/2 -L/2 ∇ ∧ A + G 2 • µ 0 4 I a 2 r 3 2 cos θ sin θ 0 (3)
where G 1 and G 2 are introduced to adjust the relative contribution of the winding and the core. These gains depend on parameters such as the core material, its temperature, etc.

The gains are identified by magnetic measures and robust linear regression, what makes (3) a hybrid model. TABLE I shows the relevance of this model compared to the others. Because it is analytical, it does not require memory for data storage, and it alows real-time computation. More, has a good accuracy, and it is setted up quickly, with few measures to identify the gains.

Measures

III. USABILITY OF THE MODEL TO CONTROL A SYSTEM

The first way to use (3) is to compute the efforts produced by an electromagnet at a given point, for a given current. Thereby, it is easy to verify wether a system is able to produce the expected forces over the whole workspace. For instance, a system consisting of three electromagnets rotating about a vertical axis (Fig. 1) was simulated and shown to be capable of producing an effort of 0.3N , needed to drive a capsule in the small bowel [START_REF] Wang | Physiological factors of the small intestine in design of active capsule endoscopy[END_REF], throughout its workspace with a current not exceeding 2A in each coil.

The second way to use (3) relies on its analytical nature, and invert it to compute the current and the position of each electromagnet to achieve a given effort. This allows to implement the control system. The system in Fig. 1 permits the manipulation of a capsule in the plane (3 DOF) and has 6 inputs (3 currents and 3 orientations). This redundant system allows to develop commands to optimize the manipulation of the capsule while optimally positioning the coils to achieve the efforts needed. This control method has been tested in simulations [START_REF] Véron | Geometric analysis of the singularities of a magnetic manipulation system with several mobile coils[END_REF], and is now being implemented on a prototype.

An example of results obtained with such a simulation is shown on Fig. 2 and Fig. 3. In this simulation, the expected trajectory is perfectly realized with an error in position less than a tenth of a millimeter (Fig. 3(a)), and an error in orientation less than one tenth of a degree (Fig. 3(b)). Note that these simulation results were obtained by including disturbances on the detection of the capsule, the positions of the coils, and the currents in the coils. Furthermore, this method can be extended to more complex systems. One can imagine a system such as that shown in Fig. 4. Its configuration can be adapted to the part of the body wherein the object is manipulated, to realize the best control.

IV. USABILITY OF THE MODEL FOR DESIGN

OPTIMISATION

From another point of view, this model can also be used to optimize the design of a system. Thanks to their high precision, finite element models are usually used for this purpose, but these calculations are time consuming, requiring to select a limited number of parameters for the optimization. Model (3) allows faster computation while maintaining good accuracy. For example, for a single electromagnet the finite element model is extremely simple since it is reduced to an axisymmetric simulation. In this case, model (3) needs 80% less computation time. In 3D systems, computation time exponentially increases in finite element simulations. This highlights the usefulness of an analytical model for optimization of complex systems.

V. CONCLUSION

We have shown the importance of efforts for non-contact manipulation. Better control of these efforts ensures the manipulability of the object, which benefits both the patient whose safety is ensured, and the medician for whom the manipulation of the capsule is simplified, more reliable, and more accurate.

Electromagnets make sense when it comes to producing magnetic efforts by taking into account access to the patient by medical staff. Indeed, the system must be lightweight and not bulky, while enabling to produce efforts powerful enough to overcome any blocquage of the object (for endoscopic capsule, the passage of a sphincter for instance). The hybrid model developed here allows easy calculation of the magnetic efforts in real time, as well as the control system composed of such electromagnets. This promotes the automation of some procedures, such as navigation in the stomach before a physician performs the passing of the pylorus.

We also showed that this model allows to optimize the design of the system, ensuring that all the movements of the capsule are possible at any point of the workspace.

Fig. 1 .

 1 Fig. 1. (a) System studied, and (b) of this system (top-view).

-

  (a) Simulator.-0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06

Fig. 2 .

 2 Fig.2. Simulation of the system studied on a planned trajectory.

Fig. 3 .

 3 Fig. 3. Errors on the position and orientation of the capsule obtained along the trajectory Fig. 2.

Fig. 4 .

 4 Fig. 4. Artistic view of a reconfigurable system with mobile electromagnets.

TABLE I .

 I ELECTROMAGNET MODELING METHODS


Yann Chaussy

Lorédane Vieille

Elise Lacroix

Marion Lenoir

Florent Marie

Lisa Corbat

Julien Henriet

Frédéric Auber

3D reconstruction of Wilms' tumor and kidneys in children: Variability, usefulness and constraints

Keywords: Wilms' tumor, segmentation, 3D reconstruction, surgical planning, nephronsparing surgery

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Wilms' tumor (WT), or nephroblastoma, is the most common type of malignant kidney tumor in children, affecting 1 in 10,000 children [START_REF] Owens | Bilateral disease and new trends in Wilms tumour[END_REF][START_REF] Han | Clinical features, treatment, and outcomes of bilateral Wilms' tumor: A systematic review and meta-analysis[END_REF]. Its peak incidence occurs between 2 and 5 years of age, with 95% of diagnoses being made before 10 years of age [START_REF] Kaste | Wilms tumour: prognostic factors, staging, therapy and late effects[END_REF]. WT develops from the cortex or the medulla, which can then invade the urinary cavities and, in 6% of cases, the tumor can invade the renal vein and the inferior vena cava as a thrombus [START_REF] Kaste | Wilms tumour: prognostic factors, staging, therapy and late effects[END_REF]. Therapeutic management of this tumor varies depending on the recommendations of learned societies, notably the SIOP (Société Internationale d'Oncologie Pédiatrique) in Europe and the COG (Children's Oncology Group) in North America. Irrespective of the protocol followed, the surgical procedure remains a fundamental component of the management of these tumors, as the final prognosis is dependent on its successful implementation.

Total uretero-nephrectomy is the recommended surgical procedure for unilateral WT that occur in a non-syndromic setting. A nephron-sparing surgery (NSS) can be undertaken in specific cases, namely bilateral WT, single-kidney WT, WT occurring in a context of genetic predisposition to tumors, and in a syndromic context where there is an increased risk of renal failure (such as Denys-Drash syndrome or WAGR syndrome). In light of the good prognosis with this type of tumor, there is ample merit in decreasing treatment-related morbidity without increasing the risk of local or remote tumor recurrence [START_REF] Owens | Bilateral disease and new trends in Wilms tumour[END_REF][START_REF] Han | Clinical features, treatment, and outcomes of bilateral Wilms' tumor: A systematic review and meta-analysis[END_REF][START_REF] Kaste | Wilms tumour: prognostic factors, staging, therapy and late effects[END_REF][START_REF] Dome | Advances in Wilms Tumor Treatment and Biology: Progress Through International Collaboration[END_REF]. In line with this notion of therapeutic de-escalation, carrying out conservative surgery is considered to be appropriate in certain well-selected cases of unilateral tumors. The SIOP has, therefore, defined criteria in order to select the patients most likely to benefit from NSS (Umbrella protocol, SIOP-RTSG 2016, version 1 2018-A02878-47, 27/11/2018).

Concomitant with these developments, medical image processing techniques have also benefited from significant advances in recent years. Segmentation of anatomical structures on 2D images combined with 3D reconstruction allows the three-dimensional surgical field to be recreated. Such 3D reconstruction could greatly assist surgeons in the surgical planning phase with regard to anticipation of the preoperative risks and especially the vascular risks.

Additionally, these 3D models could be an aid in surgical decisions by assisting with the selection of patients who are likely to be able to benefit from reconstructive surgery. The primary objective of this work was to determine the feasibility of segmentation and 3D reconstruction of a specific type of pediatric kidney tumor. The secondary objectives were to assess its usefulness in the surgical planning phase and in the selection of patients for NSS.

Materials and Methods

We retrospectively selected 12 patients treated for nephroblastoma at our center between 2005 and 2018. Patients whose imaging was performed in another center or whose images were of low quality were excluded from this study. The CT-scan of these patients were downloaded from the Picture Archiving and Communication System (PACS) in DICOM (Digital Imaging and Communications in Medicine) format. These scans were anonymized using PACS anonymization software. For each scan, we selected a vascular acquisition phase and a delayed acquisition phase (when available). In total, 14 scans were selected for this study. The pre-chemotherapy scan (at diagnosis) and the post-chemotherapy scan (before surgery) were segmented for 2 patients in order to measure tumor volume reduction induced by chemotherapy.

The segmentation process (assigning a pixel to a region) was performed for each scan by 2 operators (a pediatric surgeon and a pediatric radiologist) to calculate inter-individual variability. For each CT scan, the different structures were segmented manually or semiautomatically, section by section: the Wilms' tumor, the pathological kidney, the healthy kidney, the arterial vascularization (aorta, renal arteries, superior mesenteric artery), the venous vascularization (inferior vena cava, renal veins), and the urinary tract (when a delayed acquisition phase was available).

The segmentation and 3D reconstruction process was carried out with 3D Slicer software version 4.8.1 (https://www.slicer.org/). This is an open-source software, used primarily for medical image processing. The various structures of interest were segmented manually (by a contouring method) or semi-automatically (using a thresholding method). Each structure was assigned a label (Figure 1). The 3D reconstruction of the various anatomical structures was then implemented using the "merge and build" module of 3D Slicer (triangular mesh method).

Finally, a smoothing filter was applied to the 3D reconstruction (Figure 2). The time required to complete this 3D reconstruction and segmentation process was recorded for each scan.

The Dice similarity index was used to calculate the inter-individual variability of the reconstructions performed by the two operators. The utility of 3D reconstructions for the surgical planning was evaluated by four pediatric surgeons (three senior surgeons and 1 junior surgeon) using a 5-point Likert scale. Three questions were asked:

-Are the 3D reconstructions realistic representations? -Are 3D reconstructions useful for the surgical planning? -Are 3D reconstructions useful for evaluating the possibility of performing nephronsparing surgery?

To assess the possibility of performing NSS, the criteria defined in the Umbrella protocol (SIOP RTSG 2016 Umbrella) were evaluated on the 3D reconstructions; namely:

• Tumor restricted to one pole of the kidney or peripheral at mid-kidney

•

Figure 1: Segmentation of kidneys (A) and vessels (C) using a thresholding method. Segmentation of the renal tumor (B) using a contouring method. A merge of the different segmentations (D). Figure 2: 3D reconstruction of kidneys (brown), Wilms' tumor (green), and their vascularization (arteries in red and veins in blue) with 3D Slicer software (version 4.8.1).

Results

Fourteen scans from 12 patients were selected. For 2 patients (patients 2 and 12), the 3D reconstruction was performed on 2 scans that had been performed at 1-month intervals (before chemotherapy and after 4 chemotherapy cycles), thereby allowing measurement of the reduction in tumor volume and thus the effectiveness of the chemotherapy. All 14 scans studied had a vascular acquisition phase but only 5 had a delayed acquisition phase that allowed segmentation of the urinary cavities.

Of these 12 patients, 5 were boys and 7 were girls, with a mean age at diagnosis of 67 months

Feasibility

Segmentation of the WT, the kidneys, and the arterial and venous vascularization could be performed for all of the patients (Figure 3). Urinary cavities segmentation could only be performed for 5 scans with a delayed acquisition phase (Figure 4).

Segmentation and 3D reconstruction time

The time required to complete these manual segmentations was recorded for each scan. The mean time was 8.6 hours [3-15 hours].

Inter-individual variability

The segmentations carried out by the two operators were compared using the Dice similarity index. The mean Dice index for all of the scans was 0.

Usefulness of 3D reconstructions for the surgical planning

All of the interviewed surgeons fully agreed or largely agreed that the 3D reconstructions were realistic representations of the reality and that they were useful for the surgical planning.

Three of the four surgeons thought that the 3D reconstructions were useful to evaluate the possibility of performing NSS. One surgeon thought that the 3D reconstructions did not always provide additional information about the possibility of performing NSS compared to 2D scan images.

Usefulness of 3D reconstructions for the selection of patients for NSS

The reconstructed 3D images allowed most of the criteria defined by the Umbrella Protocol (SIOP RTSG 2016 Umbrella) to be evaluated regarding the selection of patients who are likely to be able to benefit from conservative surgery. The last criterion, which requires functional imaging (a DMSA renal scan), could not be met. These 3D reconstructions were of particular relevance for the following criteria: the tumor location, a volume less than 300 ml at diagnosis, the relationship with the urinary cavities, the relationship with nearby organs, and the possibility of removal with adequate resection margins.

Discussion

WT is one of pediatric oncology's greatest success stories, with dramatic improvement in the prognosis as a result of advances in oncology management (chemotherapy, radiation therapy) and surgery [START_REF] Han | Clinical features, treatment, and outcomes of bilateral Wilms' tumor: A systematic review and meta-analysis[END_REF][START_REF] Dome | Advances in Wilms Tumor Treatment and Biology: Progress Through International Collaboration[END_REF]. Indeed, the 5-year survival rate for patients treated for WT has increased from 30% a few decades ago to almost 90% (at all stages combined) at present (3). This extraordinary improvement in survival rates was achieved thanks to the studies and clinical trials carried out over the years by large multi-institutional organizations, including the SIOP (Société Internationale d'Oncologie Pédiatrique) in Europe and the COG (Children's Oncology Group) in North America (3). Currently, long-term survival exceeds 90% for localized WT (Stage I to III) and it is greater than 70% for metastatic WT (Stage IV) (1).

Surgery, chemotherapy, and radiotherapy are used to treat nephroblastomas. Their methods of use vary between countries, with a therapeutic strategy defined by the learned societies [START_REF] Owens | Bilateral disease and new trends in Wilms tumour[END_REF]. Irrespective of the protocol that is used, surgery continues to have a central role in the treatment of nephroblastoma as it allows resection of the primary tumor, thereby providing control of loco-regional (staging) and metastatic invasion.

The Contribution of 3D Reconstruction to Surgical Planning

Medical image processing techniques have evolved considerably in recent years.

Segmentation is a process that assigns a label to a pixel, thereby delineating anatomical structures that are relative to each other. This is an essential step before undertaking threedimensional reconstruction of these anatomical structures.

3D reconstruction of the tumor and its vascularization assists surgeons with the surgical planning phase. This preoperative visualization of the renal tumor in three dimensions makes it possible to anticipate the risks of surgery, especially vascular risks. In the absence of preoperative chemotherapy, when surgery is the first line of treatment, the tumor is most often of considerable size. This results in a change in the normal anatomical relationships as well as compression of vascular structures, especially venous structures (inferior vena cava, renal veins), which can make their identification difficult during the surgical procedure. 3D

analysis of the kidney and its vascularization in the preoperative phase of the surgery could thus facilitate recognition of these structures when the surgical procedures are being carried out. Surgeons consider that analysis of a 3D model is more "natural" than analysis of the 2D CT (or MRI) images since it allows them to preview the operating field as they will see it when undertaking the surgical procedures.

The Contribution of 3D Reconstruction in the Selection of Patients for NSS

The very good prognosis with this tumor allows the morbidity of treatments to be reduced without increasing the oncological risk. One of the possible consequences of this pathology and its treatments is the development of end-stage renal failure (ESRD) in Wilms' tumor survivors, which results in the patient requiring dialysis and/or renal transplantation [START_REF] Green | Evaluation of renal function after successful treatment for unilateral, nonsyndromic wilms tumor: Renal Function After Unilateral Renal Tumors[END_REF]. The risk of ESRD is a particular concern in patients with bilateral WT (5-14% of patients), a single kidney WT, or a WT occurring in a syndromic context. For example, the risk of ESRD occurring after 20 years of follow-up is estimated to be 43.3% in patients with WAGR syndrome (Wilms' tumor, Aniridia, Genitourinary malformations, and mental Retardation) and 82.7% in patients with Denys-Drash syndrome [START_REF] Green | Evaluation of renal function after successful treatment for unilateral, nonsyndromic wilms tumor: Renal Function After Unilateral Renal Tumors[END_REF][START_REF] Auber | Management of Wilms tumors in Drash and Frasier syndromes[END_REF]. Conservative surgery (nephron-sparing surgery) is, therefore, recommended for these patients, whenever possible, in order to preserve the maximum amount of renal parenchyma and thus limit the risk of progression to ESRD.

The indication of conservative surgery is less clear for patients with unilateral WT in the absence of an exacerbating syndrome. The risk of progression to ESRD in these patients is extremely low, in the range of 0.7% at 20 years of age [START_REF] Green | Evaluation of renal function after successful treatment for unilateral, nonsyndromic wilms tumor: Renal Function After Unilateral Renal Tumors[END_REF][START_REF] Green | The evolution of treatment for Wilms tumor[END_REF]. However, some series have reported a risk of renal damage in relation to a glomerular hyperfiltration phenomenon occurring after total nephrectomy [START_REF] Green | Evaluation of renal function after successful treatment for unilateral, nonsyndromic wilms tumor: Renal Function After Unilateral Renal Tumors[END_REF][START_REF] Green | The evolution of treatment for Wilms tumor[END_REF][START_REF] Wright | Late effects of treatment for Wilms tumor[END_REF]. It is essential to select the patients who can benefit from this conservative surgery, as otherwise the rate of local recurrence will increase. The SIOP has, therefore, issued recommendations (SIOP RTSG 2016 Umbrella) to accurately identify the patients with unilateral WT who can benefit from this conservative surgery.

3D reconstruction of renal tumor and kidneys can, therefore, be a valuable aid in the identification of these patients. 3D reconstruction also appears to be particularly useful for accurate calculation of the diagnostic tumor volume as well as the volume of the renal parenchyma that can be retained. Indeed, 3D reconstruction allows for more accurate calculation of the tumor volume than 2D imaging (whereby the tumor volume is usually estimated from 3 measurements taken in 3 orthogonal axes). This hypothesis has been confirmed by Durso (9), who found that three-dimensional volume reconstructions were more precise than traditional 2D methods for estimating renal volumes and tumor volumes. He concluded that three-dimensional volume reconstructions are more accurate, reproducible, and more clinically useful in estimating volumes for urologists managing patients with renal tumors. Therefore, evaluation of a chemotherapy's effectiveness is more reliable when the tumor volume is calculated based on three-dimensional reconstruction.

Three-dimensional representation also appears to be more useful than 2D imaging to visualize a tumor's relationship with the intrarenal urinary cavities, provided that delayed acquisition scans are available.

The authors of the Umbrella protocol claim that, for bilateral WT, MRI appears to be the best choice for staging and assessment of the treatment response. If renal MR Angiography is not feasible, a CT scan with 3D reconstruction can be an option for the preoperative planning in order to visualize the vascular pedicles and to assess the feasibility of NSS.

The usefulness of 3D reconstruction has already been demonstrated in adults. For example, Wang [START_REF] Wang | Application of Three-Dimensional Visualization Technology in Laparoscopic Partial Nephrectomy of Renal Tumor: A Comparative Study[END_REF] concluded that 3D visualization assists with the surgical planning phase for laparoscopic partial nephrectomies in complicated adult renal tumors. Chen [START_REF] Chen | Surgical planning and manual image fusion based on 3D model facilitate laparoscopic partial nephrectomy for intrarenal tumors[END_REF] stated that 3D reconstructions are not only useful during the surgical planning phase but also during surgery, as they allow segmented models to be superimposed on the surgical field as part of augmented reality. Porpiglia (12), in a review of the literature, confirmed that 3D technology (3D printing and 3D digital models) is not only useful for the surgical planning but also for teaching and training of doctors as well as to generate preoperative information that can be presented to the patient and their family.

Limitations/Constraints

This study identified the constraints and limitations for performing these three-dimensional reconstructions in daily practice. First, the scans of these patients were collected retrospectively. There was hence a degree of heterogeneity regarding the image quality with different image acquisition protocols. Some of the scans were of poor quality. This can be explained by the use of low-dose protocols in children to limit irradiation as well as by the low level of body fat in children that limits the contrast differences between anatomical structures on scans.

On the other hand, only 5 scans (out of 14) had a delayed acquisition phase that allowed 3D reconstruction of the urinary tract. It is, therefore, essential to standardize image acquisition protocols in order to obtain operable and good quality images, which is something that has been implemented in our center as a result of this study.

The main impediment to performing these routine segmentations and 3D reconstructions is the time it takes to complete them. Indeed, in our study, the average time to perform these segmentations and 3D reconstructions was 8.6 hours, using either a manual or semiautomatic method. Although the time required for segmentation tended to decrease the more the software was used, developing techniques to automate this segmentation process as much as possible appears to be paramount. To the best of our knowledge, there is presently no fully automated method for segmentation of renal tumor in children.

Finally, performing these segmentations manually (or semi-automatically) is subject to the risk of human error. Due to the aforementioned reason regarding the assessments' implementation, our segmentations were carried out by two operators. The inter-individual variability appears to be quite acceptable in our series, with a mean Dice index close to 90%.

However, when analyzing the Dice index for specific anatomical structures, it was found to be less than 80% for the venous structures. This is due to the poor image quality and the choice of the vascular acquisition time used for the segmentation. Again, standardization of the CTscan protocols and automation of the segmentation process should improve these results.

Overview

The future will likely entail reliance on MR images, as this non-irradiating imaging technique is increasingly used in this indication, provided that good quality images are available, which currently remains a challenge for children in this age group. The segmentation techniques used in this study with CT images (notably thresholding and contouring) can equally be used with MR images.

In order to reduce the time required to complete these three-dimensional reconstructions, our team is currently working on the use of artificial intelligence-based tools (case-based reasoning, deep learning) to automate the segmentation process [START_REF] Marie | Segmentation of deformed kidneys and nephroblastoma using Case-Based Reasoning and Convolutional Neural Network[END_REF].

Standardization of image acquisition protocols is also essential in this context. This will make it possible to use images of homogeneous quality, as well as to have all the acquisition phases (especially delayed acquisition phase) to be able to more readily identify the different anatomical structures that are to be segmented (urinary tract, vessels). This standardization of protocols is a fundamental step to automation of the segmentation process.

Conclusion

Segmentation with 3D reconstruction of renal tumors, kidneys, and their vascularization in children is a feasible process that has several advantages. Indeed, three-dimensional representation assists surgeons with the surgical planning by allowing them to better anticipate the risks of the surgery, especially vascular risks. 3D reconstructions also appear to help when it comes to selecting patients for conservative surgery. However, the manual or semi-automatic method is very time-consuming, making routine use of such reconstruction difficult. The development of techniques to automate this segmentation process, therefore, appears to be essential if surgeons and radiologists are going to be able to use it in daily practice.

  Using X as the segmentation performed by operator 1 and Y the segmentation performed by number of pixels present in class X and in class Y B is the number of pixels present in class Y and absent in class X C is the number of pixels present in class X and absent in class Y

  Volume < 300 ml at diagnosis • No preoperative rupture • No intraluminal tumor on preoperative imaging in the renal pelvis • No invasion of the surrounding organs • No thrombus in the renal vein or the vena cava • No multifocal tumor • Excision can be performed with oncological safe margins • The kidney remnant is expected to have a degree of remaining function • At least 66% of the renal tissue should be spared after the tumor resection with a margin of healthy tissue, to provide sufficient protection against hyperperfusion. If this is in doubt, preoperative DMSA may be able to define the expected postoperative function.

[ 13 -

 13 179 months].

Figure 3 :

 3 Figure 3: 3D reconstruction of the 14 scans: Wilms' tumor (green), renal parenchyma (brown), arterial vascularization (red), and venous vascularization (blue).

Figure 4 :

 4 Figure 4: Segmentation of kidneys (brown), urinary cavities (yellow), and Wilms' tumor (green) for delayed acquisition phase scans.

Figure 5 :

 5 Figure 5: 3D reconstruction of the scans for patients 2 and 12 before (A and C) and after (B and D) chemotherapy.

Table 1 :

 1 87 [0.83-0.91]. The mean Dice indices calculated for each of the anatomical structures were as follows: 0.95 [0.91-0.97] for the The Dice similarity indices according to the anatomical structure for each patient Evaluation of response to chemotherapy (tumor volume reduction) For two of the patients, the tumor volume was measured before and after chemotherapy. The tumor volume decreased by 88% in the 1 st patient and by 93.3% in the 2 nd patient. The results are shown in Table 2 and Figure 5.

	Patients	Wilms'	Pathological	Healthy	Arteries	Veins	Average
		Tumor	Kidney	Kidney
	1	0.94	0.83	0.96	0.87	0.75	0.87
	2	0.91	0.96	0.94	0.82	0.70	0.87
	3	0.95	0.92	0.95	0.91	0.58	0.86
	4	0.92	0.88	0.94	0.89	0.77	0.88
	5	0.96	0.89	0.96	0.84	0.69	0.87
	6	0.96	0.82	0.93	0.83	0.84	0.88
	7	0.94	0.92	0.96	0.85	0.86	0.91
	8	0.97	0.79	0.94	0.82	0.72	0.85
	9	0.95	0.69	0.95	0.74	0.82	0.83
	10	0.96	0.92	0.94	0.81	0.79	0.88
	11	0.96	0.88	0.95	0.82	0.84	0.89
	12	0.95	0.88	0.95	0.83	0.83	0.88
	Average	0.95	0.87	0.95	0.84	0.77	0.87

Wilms' tumor, 0.87 [0.69-0.96] for the pathological kidney, 0.95 [0.93-0.96] for the healthy kidney, 0.84 [0.74-0.91] for the arterial vascularization, and 0.77 [0.58-0.86] for the venous vascularization. The results are shown in Table 1.

Table 2 :

 2 The response to chemotherapy (CT) based on calculation of the tumor volume before and after chemotherapy.