Catherine Perpignan
email: catherine.perpignan@utc.fr

Yacine Baouch

Vincent Robin

Benoît Eynard

Engineering education perspective for sustainable development: A maturity assessment of cross-disciplinary and advanced technical skills in eco-design

Keywords: eco-design, cross-disciplinary competencies, sustainable engineering

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

Introduction

General context

For several years, the question of sustainable development integration in higher education is discussed. First it appeared in 1972 during the Stockholm conference where it was recognized the necessity of environment education. Then in 1983, World Commission on the Environment and Development report started to encourage a more global approach in curricula with Sustainable Development (SD) concepts. It was confirmed by the launch of the United Nations Decade for Education for Sustainable Development from 2005 to 2014 which allowed the development of a lot of SD initiatives in higher education. However, even if many researchers agree to say that SD training is a great challenge and is essential [START_REF] Abdul-Wahab | The need for inclusion of environmental education in undergraduate engineering curricula[END_REF], there is a gap between what is preconized and what is really done in academic courses [START_REF] Perpignan | From Ecodesign to DFS in engineering education[END_REF]. Moreover, industrial practitioners wait for higher knowledge level on SD [START_REF] Hanning | Are we educating engineers for sustainability[END_REF]. So, we can no longer afford to stay in this situation. Future scenarii are more and more disturbing about global warming if people do not change their modes of consumption and production. Thus, future generations are becoming more involved in these issues and do not hesitate to stimulate a reaction. One of their requests is to be correctly train to SD questions in order to have a global vision and to be able to make evolve the situation. In France a draft national law is under review to make compulsory in higher education a training which will take into account, in all academic pathways, the issues related to the preservation of the environment and the biological diversity, and the climatic changes within the framework of the limits of the planet.

Engineering education context

Our research work focuses on engineering education in France and the main issue comes from nature of competencies and skills to achieve. On the one hand SD education is based on environmental, social and economic knowledges and cross-disciplinary skills such as ethics, systemic thinking, critical thinking and so on [START_REF] Dahlin | Attitudes towards curriculum integration of sustainable development among program directors in engineering education[END_REF]. On the other hand, engineering education is primarily focused on technical knowledges [START_REF] Crofton | Educating for sustainability: Opportunities in undergraduate engineering[END_REF]. Moreover, we have to pay attention that « Instead of adding SD to an unsustainable curriculum, we should rebuild curricula by taking the contribution of a field of expertise to SD as the leading principle for curricula » [START_REF] Svanström | How to educate engineers for/in sustainable development: Ten years of discussion, remaining challenges[END_REF]. In such a context, we have chosen to analyse ecodesign trainings which are in engineering education the most involved with SD questions. Research questions are: a. Are these competencies addressed in ecodesign training in France? b. How can we really combine cross-disciplinary skills, sustainable skills and technical skills in an ecodesign training?

Our objective is to propose guidelines which can help teachers to develop curricula and courses and cross technical knowledge with more transversal skills in their training pathways. First, we have identified by a literature review which cross-disciplinary skills are necessary in ecodesign trainings. Then we proposed a survey to academics in order to define which competencies are targeted when they offer ecodesign trainings to their students. This first step allows us to understand the gap to fill between what it is expecting and what it is really doing on academic ground. The following section details skills need to be developed and presents a first proposition of links that can be made between these 2 kinds of skills. Finally, we will present a first proposition of a skills assessment tool which embedded cross-disciplinary competencies and technical knowledges.

Literature review

Cross-disciplinary competencies in Engineering Education

Many researchers have written about the necessity to acquire key cross-disciplinary competencies to answer to the global issue of sustainability. All these authors have in common to highlight communicative competence, interdisciplinarity and a more global vision of the issues with system or systemic thinking methodology (Table 1). Researchers continue to question the identification of nondisciplinary skills in engineering training and in particular for sustainability. Some of these competencies can be acquire during courses which do not deal with sustainable development or ecodesign.

In 2019, Quelhas [START_REF] Quelhas | Engineering education and the development of competencies for sustainability[END_REF] published a paper focusing on competencies for sustainability in engineering education. This literature review from 22 papers highlighted 8 key competencies and skills (Table .2).

• Critical thinking • Working in interdisciplinarity • Ability to solve problems • Systemic thinking • Normative competence • Self-knowledge • Anticipatory
These 8 competencies structure a synthesis between the proposal of Segalas [START_REF] Segalàs | What do EESD "experts" think sustainability is? Which pedagogy is suitable to learn it?: Results from interviews and Cmaps analysis gathered at EESD 2008[END_REF] and Wiek [START_REF] Wiek | Key competencies in sustainability: A reference framework for academic program development[END_REF] and they are confirmed by other researchers who have recently published on the subject. In this paper, authors have defined each competency. For example, critical thinking: «can be understood as the ability of questioning standards, practices and opinions». This work can be considered as basics to help teachers in order to clearly define skills to achieve with their students. Certainly, more detailed descriptors should be added to specify the level of competency acquisition. Futhermore, Thürer [START_REF] Thürer | A systematic review of the literature on integrating sustainability into engineering curricula[END_REF] recently presented a research review on integration of sustainability perspective into engineering curricula. This review demonstrates that it exists an increasing attention for the integration of sustainability into curricula since 2006 but it also highlights new questions. Among these questions, some are about competencies to develop: what kind of competencies? What kind of teaching strategies? How evaluate competencies acquisition? All of these questions are the core of our research work. Thürer [START_REF] Thürer | A systematic review of the literature on integrating sustainability into engineering curricula[END_REF] also concludes that "while there have been several recent literature reviews, these reviews did not focus on engineering, have been limited in scope and did not explicitly discuss implementation".

So, reviewed papers do not deal with concrete lementation but remain more positioned on a macro level. It's often very abstract that do not allow readers to project into a real training situation for integrating the SD in engineering education. Technicians and engineers must acquire this crossdisciplinary skills but they also have to integrate technical skills and knowledges. So, we need to identify a list of engineer technical skills and knowledge. ABET defined 11 competencies without categorization and 4 of them can be directly linked to sustainability: • (c) an ability to design a system, component, or process to meet desired needs within realistic constraints such as economic, environmental, social, political, ethical, health and safety, manufacturability, and sustainability • (f) an understanding of professional and ethical responsibility • (h) the broad education necessary to understand the impact of engineering solutions in a global, economic, environmental, and societal context • (j) a knowledge of contemporary issues CTI defined 14 competencies for engineer's certification classified into 3 categories: Scientific and technical knowledge; Adaptation to the specific requirements of the company and society; Taking into account the organizational, personal and cultural dimension.

Engineer specific skills

In the second category, 4 competencies are directly linked to sustainability: • Ability to take into account the company's challenges: financial dimension, respect for quality, competitiveness and productivity, business requirements, economic intelligence • Ability to take into account the issues of workplace relations, ethics, responsibility, safety and health at work • Ability to take into account the environmental issues, in particular by applying the principles of sustainable development • Ability to take into account the issues and needs of society And finally, ENAEE, which have the most detailed framework, defined 22 competencies for the bachelor level and 27 for the master degree level. These competencies are ranked into 8 categories: • Knowledge and understanding;

○ critical awareness of the wider multidisciplinary context of engineering and of knowledge issues at the interface between different fields.

• Engineering Analysis ○ ability to identify, formulate and solve unfamiliar complex engineering problems that are incompletely defined, have competing specifications, may involve considerations from outside their field of study and nontechnical -societal, health and safety, environmental, economic and industrial -constraints… • Engineering Design ○ ability to develop, to design new and complex products (devices, artefacts, etc.), processes and systems, with specifications incompletely defined and/or competing, that require integration of knowledge from different fields and non-technical -societal, health and safety, environmental, economic and industrial commercial -constraints… ○ critical awareness of economic, organizational and managerial issues (such as project management, risk and change management) • Making Judgements ○ ability to integrate knowledge and handle complexity, to formulate judgements with incomplete or limited information, that include reflecting on social and ethical responsibilities linked to the application of their knowledge and judgement;

• Communication and Team-working • Lifelong Learning Among these competencies, 6 are linked directly to sustainability, but in this framework others transversal skills are essential in sustainable project. For example, in ENAEE framework Communication and Team working competency is defined as: "ability to use diverse methods to communicate clearly and unambiguously their conclusions, and the knowledge and rationale underpinning these, to specialist and non-specialist audiences in national and international contexts [15]". In SD project, it is necessary to obtain to agreement and support from all stakeholders. This means that in engineering framework specific technical skills, sustainable technical skills and cross-disciplinary skills are already present. Now, a first step of diagnosis can be made to assess if cross-disciplinary are achieved when engineering curricula develop their program based on engineer's specific skills. The three above-mentioned frameworks highlight that different kind of competencies are necessary in engineer training. It means that academics recognize the importance of technical specific skills, cross-disciplinary skills and sustainable skills. They should try to set up these skills and competencies in their curricula but often it looks like a laundry list and the integration is more complex in the real context. « The engineering community lacks consensus on established methods for infusing sustainability into the curriculum and on verified approaches to assess engineers' sustainability knowledge » [START_REF] Duarte | Engineering Education for Sustainable Development: The European Project Semester Approach[END_REF]. Teachers often develop curricula that are addition of courses and obtain a patchwork of not linked competencies and knowledge. To avoid this kind of curricula, we have tried to evaluate if links exist between engineering skills and cross-disciplinary skills. Objective was to use these possible links and to propose structured training modules in a global curriculum allowing at the same time the development of competencies and knowledge. For this work, we have decided to use the European framework edited by the ENAEE.

In table 3, we can see that all cross-disciplinary skills are not targeted in an engineering training. Some are addressed but rarely in a global way. This table gives a global vision of skills crossing. That is why, after identifying skills targeted for engineering training as well as the cross competencies deemed necessary to solve problems that take into account issues related to sustainable development, we chose to limit our study to ecodesign training.

Technical skills, advanced technical knowledge and cross-disciplinary for ecodesign

Indeed, according to the ISO 14006:2011: "the goal of ecodesign is to integrate environmental aspects into product design and development so as to reduce the adverse environmental impacts of products throughout their life cycles". It means that engineers must solve a technological problem while taking into account parameters t for which they initially did not receive specific training [START_REF] Perpignan | Ecodesign from High School to Bachelor Level: A French Case Study[END_REF] like economic or environmental issues.

However, this activity allows to acquire all engineering skills and also relies on transversal skills [START_REF] Luttikhuis | Effective integration of life cycle engineering in education[END_REF]. ISO 14006 gives us the 6 steps to following order to ecodesign. These 6 steps are: When a teacher develops an ecodesign course, he/she has to evaluate what kind of engineering competencies and crossdisciplinary are essential to transmit. He/she has to identify the level of skill's acquirement by using 6 cognitive processes of the Bloom's taxonomy for instance [START_REF] Krathwohl | A revision of Bloom's taxonomy: An overview[END_REF]. With students in bachelor, he/she could target the 3 first processes: remember (level 1), understand (2) and apply (3). Then, these competencies could be re-evaluated during another step of eco-design process in different level of acquirement according to Bloom's revised Taxonomy which is often use in educational system. For example, we propose a possible representation of engineer/cross-disciplinary skills concerning the first step of the ecodesign processes -Life cycle thinking (Fig. 1). Due to the short length of this paper, the way that the metrics (Fig. 1) are obtained is not detailed. To enrich our proposition and check consistency of our representation, we submitted a survey to a group of teachers (80 and we receive about 30 answers) involved in this kind of engineering training. Survey was also a way to question the current education practices in ecodesign in Higher Education in France and then in order to analyze training content.

•

Survey on ecodesign teaching

The survey was submitted in June 2019 to teachers of the higher education in France at the bachelor's and master's degree level. Ecodesign perception evolves over last years. Some of researchers think that ISO 14006 definition does not take into account enough parameters. They are closer from Charter's definition which define sustainable solutions like: "products, services, hybrids or system changes that minimize negative and maximize positive sustainability impacts economic, environmental, social and ethical throughout and beyond the life-cycle of existing products or solutions, while fulfilling acceptable societal demands/needs'' [START_REF] Charter | Sustainable solutions[END_REF]. So, to ensure that we "talk about the same thing", the first question in the survey was: "Is the definition of ISO 14006:2001 sufficient according to you?". It is interesting to note that 33% of teachers agree with the considered definition, another 33% have no opinion. It means that either they are not really concerned or something miss but they can not say exactly what. The last 33% consider ISO ecodesign definition is not sufficient. They think that in addition to the environmental impact, ecodesign should also considers aspects such as: economic, marketing, innovation and ethic. So, it seems that ecodesign must evolve toward a more sustainable and global vision of design [START_REF] Ceschin | Evolution of design for sustainability: From product design to design for system innovations and transitions[END_REF].

Next, we wanted to analyze what kind of knowledge or competencies related to sustainability, teachers target during their training. Fig. 2 highlights that teachers mainly work on cleaner production, waste management or pollution prevention. But they not really work on economic and social aspects of sustainable development. There also exits a big place granted to Life Cycle Assessment tool. But companies are looking for new engineering profiles which are able to have a systemic way of thinking for product design and to analyze all the scenarii and the external impacts that could have their decision-making during each step of product development process. So, keeping this teaching way will increase gap between academic curricula and industrial training needs.

Last, even if teachers do not first mention the needs of cross-disciplinary skills, in anyway they take them into account during their training. Teachers agree that cross disciplinary are necessary to have a more global vision of ecodesign. We have interviewed them on six different transversal skills that they think work with their students. These skills are: ethic, complex thinking, collaboration, critical thinking, systemic thinking and agility. In this panel of teachers, critical thinking is the most necessary skill according to teachers. But they highlight a real difficulty to clearly define what it is and how to assess it during teaching activity.

Discussion

The survey allows to identify that ecodesign training are always focus on technical aspects such as choice of materials or energy consumption to reduce environmental impact by using tools (LCA tools for instance) in order to assess product's environmental performance and to propose curative solution. The survey also underlines that teachers are aware their training must include cross-disciplinary skills. Their main difficulties are to correctly define these competencies, to find some detailed descriptors and to cross-check these competencies with specific technical skills. Another challenge is to define training actions allowing students to gradually acquire all the skills related to ecodesign. Then, teachers need guidelines for helping them to create training sessions including these skills but also to quickly visualize the level of skills of their students.

Conclusion and future work

The aim of this paper was to define cross-disciplinary skills necessary to include sustainability in engineering curricula with a specific focus on ecodesign expertises and to propose a methodology to help teachers to deal with these skills in such technical context. Literature review allows us to clearly identify competencies and we have crossed-check them with specific technical skills. Our matrix (Fig. 1) must to be refine and to be consolidated. Then we could propose a first version of guidelines which will be available at the end of our work.

In future work, we have to continue our survey nearby teachers and students. Some groups, such as the SEFI's working group on sustainability (European Society for Engineering Education), are also resources which could help us to strengthen our research work at an international level.

  Engineering skills were established by some organizations like ABET [14] (Accreditation Board for Engineering and Technology) in USA, ENAEE [15] (European Network for Accreditation of Engineering Education) or CTI [16] (Commission des Titres d'Ingénieur) in France. All of these organizations have defined some engineering competencies more or less precisely.

•○

  Investigations ability to conduct bibliographic research, to consult and use scientific databases with a critical thinking ○ ability to design and conduct experimentations • Engineering Practice ○ knowledge and understanding of the non-technicalsocietal, health and safety, environmental, economic and industrial -implications of engineering practice;

ScienceDirect

  Procedia CIRP 00 (2020) 000-000 www.elsevier.com/locate/procedia 2212-8271 © 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of the 27th CIRP Life Cycle Engineering (LCE) Conference. doi:10.1016/j.procir.2020.xx.xxx 3. Maturity assessment of sustainability in engineering curriculum: a focus on ecodesign teaching

Step 1 ,

 1 Life cycle thinking • Step 2, Ecodesign process • Step 3, Environmental assessment of product • Step 4, Analysis of interested parties' environmental requirements • Step 5, Analysis of interested parties' environmental requirements • Step 6, Ecodesign review

Fig. 1 .

 1 Fig.1.: Model for engineer/cross-disciplinary skills levels for Step 1

Fig. 2 .

 2 Fig.2.: cross disciplinary skills engaged in ecodesign

Table 1 .

 1 State-of-the-art on key competencies for sustainability

	Authors	Key competencies for sustainability
	De Haan, 2006	foresighted thinking, working in interdisciplinarity,
	[7]	cosmopolitan perception, transcultural understanding,
		learning participatory, planning and implementation
		skills, capacity of empathy, self-motivation and
		motivating others, distanced reflection
	Barth et al,	Interdisciplinary cooperation, socio-communicative
	2007 [8]	skills, team leading
	Segalas et al,	Self-learning, cooperation and transdisciplinarity, SD
	2009 [9]	problem solving, systemic thinking, critical thinking,
		social participation
	Wiek et al, 2011	Systemic thinking, anticipatory, normative
	[10]	competence, strategic competence, interpersonal
		competence
	Frisk, Larson,	System thinking and understanding of
	2011 [11]	interconnectedness, long term foresighted reasoning
		and strategizing, stakeholder engagement and group
		collaboration, action-oriented and change-agent skills

Table 2 .

 2 Cross-disciplinary competencies for sustainability[START_REF] Quelhas | Engineering education and the development of competencies for sustainability[END_REF]

Table 3 .

 3 Cross skills and engineer skills.

	Cross-disciplinary skills Enginneer's skills	Critical	thinking	Collaboration	Solve a	complex	problem	Systemic	thinking	Normative	competence	Self-	knowledge	Anticipatory	Strategic	competence
	Knowledge and understanding
	Engineering Analysis
	Engineering Design
	Investigations
	Engineering Practice
	Making Judgements
	Communication and Team-
	working
	Lifelong Learning


Pierre Marcasuzaa

Maud Save

Pierre Gérard

Laurent Billon
email: laurent.billon@univ-pau.fr

Laurent Billon

When a pH-triggered nanopatterned shape transition drives the wettability of a hierarchically self-organized film: a bio-inspired effect of "sea Anemone"

Keywords: Hierarchical structure, pH sensitive surface, wettability, micro and nano structuration, Honeycomb film

Hypothesis

Hierarchically structured surfaces including sensitive materials presents the advantage to exalt wettability variation due to the combination of micro structure effect directed by Cassie Baxter and/or Wenzel behaviour which is tuned by the surface energy variation of sensitive polymer films.

Experiments

Herein is reported the synthesis and the hierarchical structuration of a pH sensitive diblock copolymer P(S-stat-MMA)-b-P4VP with a pH-sensitive Poly 4-vinylpyridine P4VP block.

Applying the Breath Figure method casting (minute time scale process), this diblock copolymer allows to obtain a micro porous honeycomb film while a wall nano-structuration due to self-assembly of diblock copolymer is observed.

Findings

The pH-triggered wettability is studied and correlated with the morphology evolution of P4VP nano-domains investigated by AFM in a liquid cell. Indeed, a nano-dots to nanorings/donuts transition is highlighted when decreasing the pH below the pKa of the P4VP. This nano "sea Anemone" shape transition induces the macroscopic changes of the wettability of a hierarchically self-organized honeycomb film, explained by the protonation of P4VP chains inducing electrostatic repulsion and then hydrophilic surface.

Introduction

Sensitive materials able to switch properties according to different triggers as temperature, pH, light [START_REF] Gohy | Photo-responsive block copolymer micelles: design and behavior[END_REF][START_REF] Schattling | Multi-stimuli responsive polymers -the all-inone talents[END_REF] are widely developed for a large range of application as drug delivery, antifogging, self-cleaning or tissue engineering [START_REF] Mendes | Stimuli-responsive surfaces for bio-applications[END_REF][START_REF] Huang | Stimuli-Responsive Surfaces for Tunable and Reversible Control of Wettability[END_REF]. Among them, the switch from hydrophilic to hydrophobic form of polymeric surfaces is quite challenging and interesting [START_REF] Sun | Reversible Switching between Superhydrophilicity and Superhydrophobicity[END_REF]. Different polymer families were identified as polyPEG or polyNIPAM sensitive to temperature [START_REF] Ye | Fabrication of a Self-Cleaning Surface via the Thermosensitive Copolymer Brush of P(NIPAAm-PEGMA)[END_REF][START_REF] Zhao | Research progress on self-assembly of block copolymers of Nisopropyl acrylamide[END_REF], polyazobenzene to light [START_REF] Park | Reversible change of wettability in poly(??caprolactone/azobenzene) honeycomb-patterned films by UV and visible light illumination[END_REF][START_REF] Kim | Marangoni instability driven surface relief grating in an azobenzene-containing polymer film[END_REF][START_REF] Li | In situ fully light-driven switching of superhydrophobic adhesion[END_REF][START_REF] Zhang | Smart UV/Visible light responsive polymer surface switching reversibly between superhydrophobic and superhydrophilic[END_REF], polyamine to CO2 [START_REF] Yin | CO2-induced tunable and reversible surface wettability of honeycomb structured porous films for cell adhesion[END_REF][START_REF] Liu | CO2-Responsive polymer materials[END_REF][START_REF] Marcasuzaa | CO2-Driven reversible wettability in a reactive hierarchically patterned bio-inspired honeycomb film[END_REF] or polyacid and polybase to pH [START_REF] Escalé | PH sensitive hierarchically selforganized bioinspired films[END_REF][START_REF] De León | Switchable and pH responsive porous surfaces based on polypeptide-based block copolymers[END_REF][START_REF] Wang | Fabrication of highly ordered microporous thin films by PS-b-PAA self-assembly and investigation of their tunable surface properties[END_REF]. However, in case of hydrophilic polymer, the problem of polymer surface solubility in water appears, but can be overcome by using amphiphilic diblock copolymers combining hydrophobic and switchable blocks, i.e. hydrophobic/hydrophilic, respectively. Moreover, wettability effect can be enhanced using porous film by reducing the contact surface and more particularly if the change from hydrophilic towards hydrophobic forms is completed by a wettability transition from non-wetting Cassie Baxter to wetting Wenzel states [START_REF] Kim | Wetting theory for small droplets on textured solid surfaces[END_REF].

To combine these two effects, the present work consisted on the elaboration of a hierarchically self-organized film presenting a double structure: the first one at the macroscopic scale, as a porous honeycomb structure, and the second one at the nanoscale as nano-segregation of a diblock copolymer. Herein, a fast bottom-up process based on solvent evaporation in humid atmosphere, so-called "breath figure" process, was used to create the first level of porosity. The directed self-assembly in "breath figure" templating of block copolymers was then able to target the second one by using an amphiphilic diblock [START_REF] Escalé | PH sensitive hierarchically selforganized bioinspired films[END_REF][START_REF] Escalé | Highly structured pH-responsive honeycomb films by a combination of a breath figure process and in situ thermolysis of a polystyrene-block-poly(ethoxy ethyl acrylate) precursor[END_REF][START_REF] Bertrand | Hierarchically porous bioinspired films prepared by combining "breath figure" templating and selectively degradable block copolymer directed self-assembly[END_REF][START_REF] Escale | When block copolymer selfassembly in hierarchically ordered honeycomb films depicts the breath figure process[END_REF][START_REF] Benoot | Hierarchically organized honeycomb films through block copolymer directed self-assembly in "breath figure" templating and soft microwave-triggered annealing[END_REF][START_REF] Marcasuzaa | Reactive nano-patterns in triple structured bio-inspired honeycomb films as a clickable platform[END_REF]. A P4VP block was chosen as pH sensitive polymer for its ability to switch wettability properties according to the media. Indeed, the protonation of pyridine moiety below its pKa [START_REF] Narayanan | Anhydrous proton-conducting polymeric electrolytes for fuel cells[END_REF] introduces some intra/inter electrostatic repulsions in the polymers chains provoking simultaneously a transition from a hydrophobic to hydrophilic surface and then a decrease of the contact angle value. This macroscopic behaviour is attributed to the re-organization of the P4VP nano-dots into nano-rings domains as a nano "sea Anemone" effect. Indeed, here the mimicry between the nano to macroscopic scale is made by comparison of the macromolecular chains and the sea anemone tentacles behaviors. The P4VP chains behaviour can be compared to sea anemone tentacles which can be collapsed on its mouth, i.e. nano-dots (Figure 1a

Materials and methods

To combine hierarchical structure and pH sensitive surface, the synthesis of an amphiphilic diblock copolymer with hydrophilic P4VP and hydrophobic P(S-stat-MMA) blocks was performed.

The Styrene S allowed a good control of the MMA polymerization [START_REF] Nicolas | Living character of polymer chains prepared via nitroxidemediated controlled free-radical polymerization of methyl methacrylate in the presence of a small amount of styrene at low temperature[END_REF]. Thus, a well-defined high molar mass P(S-stat-MMA)-b-P4VP copolymer was synthesized by nitroxide-mediated polymerization (NMP) [START_REF] Nicolas | Nitroxide-Mediated Polymerization[END_REF] with a number average molecular weight Mn of 48,300 g.mol -1 and a dispersity value Ð of 1.29 with a symmetrical molar mass distribution (ESI Figure SI 1,2 & 3). The final composition of the purified copolymer is of 78% of P(S-stat-MMA) and 22% P4VP in volume as measured by 1 H NMR in CDCl3. P(S-stat-MMA)-b-P4VP copolymer structure allowed manufacturing honeycomb films by the "Breath Figure" method [START_REF] Widawski | Self-organized honeycomb morphology of starpolymer polystyrene films[END_REF]. This method consisted on a fast evaporation of solvent under humidity [START_REF] Escalé | Recent advances in honeycomb-structured porous polymer films prepared via breath figures[END_REF][START_REF] Munoz-Bonilla | Towards hierarchically ordered functional porous polymeric surfaces prepared by the breath figures approach[END_REF]. In case of amphiphilic diblock copolymer, polymer solution is casted on a glass lamella and placed under humid air flow. While the solvent was evaporated (endothermic phenomena), the temperature of the solution decreased and induced condensation of surrounding humidity at the liquid/air interface. Thus, the hydrophilic part allowed the stabilization of droplet while hydrophobic segment precipitated with water and formed an anti-coalescence layer by a so-called "bag-effect". Then, droplet self-organized in hexagonal structure due to Bénard-Marangoni convection [START_REF] Nicolas | Comprehensive modeling study of nitroxide-mediated controlled/living radical copolymerization of methyl methacrylate with a small amount of styrene[END_REF]. After total evaporation of the solvent and water, a porous honeycomb structure was obtained by water droplet templating.

Results and discussion

Hierarchical structure characterization

Micro structure: honeycomb porosity Micro structure of honeycomb films was checked by optical microscopy which revealed porous surface (ESI Figure SI 5a). Thus, the different images allowed determining the surface ratio of polymer using two methods. The first one, by manual measurement gave a pores radius of 0.5 µm and a distance between two centres of pores of 1.6 µm. Then, using a basic model considering a diamond joining four pores centre which represented an elementary mesh of network, the surface fraction was done by the ratio between surface area (diamond -pore area) and total area (diamond area). This model gave a value of surface fraction of 0.645 (Scheme 1). This value is comparable with the surface ratio of 0.65 ± 0.01 obtained with an ImageJ threshold treatment describe in ESI.

Nano structure: self-assembly of diblock copolymer

The nano structure was investigated by AFM on continuous and honeycomb films (Figure 1 c & e). Continuous films reveals a surface structuration characteristic of the nano-segregation between P(S-stat-MMA) and P4VP blocks. On continuous films, characteristic length between two nano-dots P4VP domains is 50 nm which is in accordance with molecular length of two diblock copolymers containing around 500 units of monomers. Images obtained from diblock copolymer honeycomb reveals a hierarchically structured organization. First of all, micro porosity observed by optical microscopy is confirmed by AFM (Figure 1e).

Nevertheless, the principal aim of this characterization was to visualize the nano-segregation of the block copolymer into the walls between pores (Figure 1e) which is similar with the nano-structuration obtained on continuous films (Figure 1c).

Surface properties

Surface properties of materials were investigated by contact angle measurement. This method allows comparing different states of materials wettability according to the variation of composition or the macro-structuration of polymers. Considering a polymer "A" presenting a contact angle value θA, the introduction of a second polymer "B" with a contact angle value θB, led to a resulting contact angle θ dependent of the surface ratio of each polymers according to the equation 1.

(Equation 1)

In case of rough surfaces, the contact angle can be differently affected in accordance with Cassie-Baxter or Wenzel models. The difference between these two models is due to the physical behavior of the surface. With Wenzel model, the droplet covered the entire surface and the contact angle θ * is led by the roughness r (r = Surface area/Surface area projected on a plane) and the Young contact angle θ (due to the chemical nature of polymer which can be determined from a continuous film), for a final equation equal to . When the roughness is higher enough, the energy needed to cover the surface is more important than the energy with air in pore. Therefore, the droplet rests on the top of ruggedness and the residual contact angle θ * is led by the surface ratio of polymer φs and Young contact angle θ, corresponding to the Cassie-Baxter model:

(Equation 2)
where φs is the area in contact with liquid / Total area.

Continuous film wettability

By contact angle investigation, an experimental φs (P4VP) = 0.19 was determined using equation 3 with a contact angle of the diblock copolymer equal to 78° while P(S-stat-MMA) and P4VP respectively presented contact angle values of 85° and 48°.

(Equation 3)

This value was close to the value of the volume fraction (0.22) that testified that block copolymer was uniformly distributed between bulk and surface. The pH effect on film surface can be tuned by the protonation/deprotonation of the pyridine moiety of P4VP. Diblock copolymer presents an increase of contact angle between 45° to 78° when increasing pH from 1 to 10 (Figure 2). This variation follows a typical titration curve with two plateau on both sides of a transition state around pH 5. This last one is attributed to the P4VP pKa value of 5.2. Indeed, above this value, deprotonated amino group are pre-dominant while protonated forms appear below the pKa. This protonation leads to the formation of a pyridinium salt which possessed more affinities with water, i.e. much hydrophilic form, than deprotonated one, i.e. much hydrophobic. Such behavior can explain the pH-triggered modulation of wetting on the continuous film.

Honeycomb film wettability

Whatever the pH is, honeycomb films present contact angle upper than continuous films which are lower than 90° (Figure 2). For contact angle values below than 90°, Wenzel model predicts lower contact angle for porous films than continuous film (Young angle). So, this model can't be applied for the present system. Considering Cassie-Baxter approach and assuming to keep the same chemical nature of the surface from continuous film and in between pores for honeycomb film, a theoretical value of θ* can be determined assuming the experimental θ value of a continuous film, φs of the porous film (0.65 ± 0.01) and the equation 2.

Theoretical and experimental values are in accordance for pH above 6 where a nice fit is observed with the Cassie-Baxter model (Figure 2). However, below this value, the model diverges from the experimental values which are much lower than the predicted ones. This divergence occurs for the pH values reaching the pKa of the P4VP and thus enhancing its hydrophilic polyelectrolyte character. So, for the protonated copolymer, the Cassie-Baxter model cannot be applied. Indeed, considering Cassie-Baxter model, the contact angle experimental values lead to a φs of 0.83 while SEM images of a honeycomb film after treatment at pH1 reveals a φs,pH1 of 0.63 ± 0.01 close to initial value φs,initial of 0.65 ± 0.01. The non-variation of the surface fraction can be explained by the glassy state of the PMMA block with a glass transition temperature Tg close to 100 °C. Therefore, the increase of the surface fraction cannot be assigned to the modification of micro-structure but to the water penetration into the pores without total wetting due to the air confinement, avoiding then to be in Wenzel model. This phenomenon was already observed in case of pillared surfaces in which one water droplet entering to structure without contact with bottom part due to air confinement [START_REF] Papadopoulos | How superhydrophobicity breaks down[END_REF][START_REF] Elbourne | Imaging the air-water interface: Characterising biomimetic and natural hydrophobic surfaces using in situ atomic force microscopy[END_REF].

For fitting the experimental curve, to consider another model taking into account this parameter and determining the depth of penetration of water into pores is necessary. A model is proposed and used considering the hypothesis that pores were in contact as describe in Scheme 1. So, the pore radius (R) is determined as the half of the distance between to center of pores (d). Thus, it is possible to calculate the pore depth (2R-h) where h is the height of virtual dome covering the pore and dicted by where r corresponds to the radius of virtual dome. The surface fraction is then calculated by equation 4.

(Equation 4)

where SSA is the polymer surface area (top of the film), SWSIP the wetting surface into pore and SAC the air contact area.

In this equation, the polymer surface area is constant, but wetting surface into pore and air contact area depends of the penetration depth (p). Expression of wetting surface into pore and air contact area are different if the penetration is higher (p < R-h) or lower (p > R-h) than the pore diameter. These two behaviours are given by equations 5; 6 and 7; 8 (Scheme 1), respectively. Using equation 4, 5, 6, 7 and 8, the φs versus the penetration depth can be drawn (ESI Figure SI 4). Experimental values of contact angle give an experimental φs = 0.83, corresponding to a penetration depth of 1.04 µm. Consequently, the height of air in bottom of pore is estimated to be around 400 nm.

The pH-triggered effect on the surface wettability of the microscopic structure is well understood, but we also wanted to focus on the nanostructure impact by AFM with liquid cell at different pH.

pH effect on P4VP nano domains

In case of continuous films, the contact angle values are close to the non-treated films for high pH, as explained by equation 2. Indeed, a media at pH 10 does not modify the structure of the diblock copolymer self-assembly. Image of the immersed surface reveals a hexagonal-closepacked hcp pattern with a characteristic distance of 50 nm between P4VP nano-domains (Figure 1c). However, at pH 1 the domains of P4VP are protonated, showing up a shape transition to nano-rings or nano-donuts, as a nano "sea anemone" effect (Figure 1d). Even if the surface structure seems to be totally different from pH10 to pH1, a similar hexagonal nano-pattern is observed in both cases with a characteristic distance between nano-donuts center of 50 nm (Figure 1i). This shape transition of the P4VP nano-domains is attributed to the protonation of P4VP inducing a quaternization and the appearance of positive charges along macromolecular chain [START_REF] Wang | Gold nanoring arrays from responsive block copolymer templates[END_REF]. Thus, inter and intra macromolecular chain repulsions are created and provoke the nano-holes at the center of P4VP domains (Figure 1g & h). The chain stretching in the 3D directions induces a decrease of the external distance between P4VP nano-donuts which seem to be almost in contact (Figure 1f & i). A concomitant effect is the strong increase of the height of the P4VP nano-domains to deep holes inside the nano-donuts, from 3 nm at pH=10 to 8 nm at pH=1, respectively (Figure 1i). The P4VP chains stretching by a 2.5 times can be associated to a transition from a collapsed polymer brush in a nonsolvent to an extended polymer brush in a good solvent [START_REF] Bendejacq | Water-dispersed lamellar phases of symmetric poly(styrene)-block-poly(acrylic acid) diblock copolymers: Model systems for flat dense polyelectrolyte brushes[END_REF]. This pH-triggered shape transition lead to an increase of P4VP surface ratio which can be determined by using the Correlation between the macroscopic and nanoscopic observations allow to well-understand the water penetration into the pores covered by P4VP at low pH. Indeed, growing of P4VP surface ratio lead to a total modification of surface properties and explained the large decrease of the contact angle values for pH below pKa leading to the gap with Cassie Baxter state towards a Wenzel state 33 .

Conclusions

In the large area of sensitive surface material already reported in literature [START_REF] Ye | Fabrication of a Self-Cleaning Surface via the Thermosensitive Copolymer Brush of P(NIPAAm-PEGMA)[END_REF][START_REF] Zhao | Research progress on self-assembly of block copolymers of Nisopropyl acrylamide[END_REF][START_REF] Park | Reversible change of wettability in poly(??caprolactone/azobenzene) honeycomb-patterned films by UV and visible light illumination[END_REF][START_REF] Kim | Marangoni instability driven surface relief grating in an azobenzene-containing polymer film[END_REF][START_REF] Li | In situ fully light-driven switching of superhydrophobic adhesion[END_REF][START_REF] Zhang | Smart UV/Visible light responsive polymer surface switching reversibly between superhydrophobic and superhydrophilic[END_REF][START_REF] Yin | CO2-induced tunable and reversible surface wettability of honeycomb structured porous films for cell adhesion[END_REF][START_REF] Liu | CO2-Responsive polymer materials[END_REF][START_REF] Marcasuzaa | CO2-Driven reversible wettability in a reactive hierarchically patterned bio-inspired honeycomb film[END_REF][START_REF] Escalé | PH sensitive hierarchically selforganized bioinspired films[END_REF][START_REF] De León | Switchable and pH responsive porous surfaces based on polypeptide-based block copolymers[END_REF][START_REF] Wang | Fabrication of highly ordered microporous thin films by PS-b-PAA self-assembly and investigation of their tunable surface properties[END_REF], this work describes the synergy of pH-triggered properties and hierarchal structure obtained using the directed self-assembly in "breath figure" templating of a P(S-stat-MMA)-b-P4VP block copolymers. This copolymer, with a high χ value close to 6.5 between the P(S-stat-MMA)

and the pH-sensitive P4VP blocks, drives the formation of the self-assembled P4VP nano-dots into the walls between the micropores and leads to a nano-patterned honeycomb film, as a hierarchically structured surface. Then, a pH sweep from 10 to 1 allows to trigger the wettability of the honeycomb film and reveals a transition from a Cassie Baxter model towards a model considering a partial water penetration into the pores below the P4VP pKa.

The variation of the wettability properties of P4VP material based is enhanced by the roughness effect induced by the hierarchical organization. This switch of model is explained by AFM observations in liquid cell which revealed an increase of surface ratio of P4VP at low pH. Indeed, due to the intra and inter electrostatic repulsion between P4VP chains, a shape transition from nano-dots to nano-donuts is observed as a nano "sea anemone" effect without affecting the surface ratio but provokes the increase of the hydrophilicity of the surface and a better wettability. This pH effect on porous film was already observed for other pH sensitive polymer like PAA [START_REF] Wang | Fabrication of highly ordered microporous thin films by PS-b-PAA self-assembly and investigation of their tunable surface properties[END_REF] and a polypeptide [START_REF] De León | Switchable and pH responsive porous surfaces based on polypeptide-based block copolymers[END_REF], but going beyond the synthesis of a new diblock copolymer presenting a pH-triggered hierarchical structure, this work established the correlation of the variation of polymer nano domains from nano dots to nano "sea anemone" shape leading to the macroscopic variation of the contact angle. This pH sensitive porous material opens a large range of application as adsorption/desorption of species according to pH, use P4VP as a precursor of gold salt reduction or for example use it as grafting function [START_REF] Poggi | Preparation of Janus nanoparticles from block copolymer thin films using triazolinedione chemistry[END_REF] to fix some specific species. For the two last applications, according to pH applied, the morphology of the grafted species can be dots or nano rings.

Figures and Scheme captions

  & c), or stretched around the mouth, i.e. nano-rings (Figure 1b & d) just by a modification of their close environment.

experimental contact angle values in equation 3 .

 3 Figure SI 5 b, c & d, respectively pH 8, pH 6 & pH 4)).

1 2Figure 1 -

 11 Figure 1 -Sea anemone pictures with collapsed (a) and stretched (b) tentacles. AFM height profile microscopy images (scale bar = 300nm) of continuous film at pH 10 (c) and pH 1 (d) and of honeycomb film at pH 10 (e) pH 1 (f). Schematic representation of neutral and protonated P4VP (g and h). Height & distance profile of P(MMA-stat-S)-b-P4VP nano selfassembly at pH 10 (full line) and pH 1 (dotted line) (i).

Figure 2 -

 2 Figure 2 -Contact angle evolution with pH for continuous films (full diamonds) and honeycomb films (empty diamonds). Theoretical value of θ* according to Cassie-Baxter model (dotted line).

Funding

ARKEMA is thanked for funding this work and the postdoc fellowship of PM in the framework of the GENESIS project.

Conflicts of interest

The authors declare no conflict/competing interest


J M Hamard

H Teissier

J P Pasquier

D Beregi

Dabli

Joël Greffier
email: joel.greffier@chu-nimes.fr

J Frandon

A Hamard

J M Teissier

H Pasquier

J P Beregi

D Dabli

Impact of iterative reconstructions on image quality and detectability of focal liver lesions in low-energy monochromatic images

Keywords: Multidetector Computed tomography, Dual-energy, Iterative reconstruction, Task-based image quality assessment ASIR: Adaptive Statistical Iterative Reconstruction CT: Computed Tomography CTDIvol: Volume CT dose index d': Detectability index DECT: Dual-energy CT ESF: Edge spread function FBP: Filtered Back Projection HCC: Hepatocellular carcinoma IR: Iterative reconstruction KVSCT: Fast kV switching CT Noise power spectrum SECT: Single-energy CT

Introduction

Dual-Energy CT (DECT) is increasingly used in clinical practice [START_REF] Cecco | White Paper of the Society of Computed Body Tomography and Magnetic Resonance on Dual-Energy CT, Part 4: Abdominal and Pelvic Applications[END_REF][START_REF] Agrawal | Oncologic applications of dual-energy CT in the abdomen[END_REF][START_REF] Goo | Dual-Energy CT: New Horizon in Medical Imaging[END_REF][START_REF] Marin | State of the art: dual-energy CT of the abdomen[END_REF][START_REF] Mccollough | Dual-and Multi-Energy CT: Principles, Technical Approaches, and Clinical Applications[END_REF][START_REF] Wang | Quantitative analysis of the dual-energy CT virtual spectral curve for focal liver lesions characterization[END_REF][START_REF] Zhang | Objective characterization of GE discovery CT750 HD scanner: gemstone spectral imaging mode[END_REF][START_REF] Matsumoto | Virtual monochromatic spectral imaging with fast kilovoltage switching: improved image quality as compared with that obtained with conventional 120-kVp CT[END_REF][START_REF] Yu | Virtual monochromatic imaging in dual-source dual-energy CT: radiation dose and image quality[END_REF]. It is based on the attenuation difference of various materials as a function of energy [START_REF] Mccollough | Dual-and Multi-Energy CT: Principles, Technical Approaches, and Clinical Applications[END_REF][START_REF] Alvarez | Energy-selective reconstructions in X-ray computerized tomography[END_REF]. The principle of DECT imaging is to use both low-and high-energy photon spectra to calculate the photoelectric and Compton scattering effects separately in order to obtain the attenuation difference of various materials as a function of energy [START_REF] Mccollough | Dual-and Multi-Energy CT: Principles, Technical Approaches, and Clinical Applications[END_REF][START_REF] Alvarez | Energy-selective reconstructions in X-ray computerized tomography[END_REF]. DECT is particularly useful in abdominal indications by generating virtual monochromatic images. Many studies have shown the advantage of using 50-60 keV energy levels for the detection and characterization of hepatic, pancreatic or renal lesions in abdominal imaging [1-4, 6, 11]. Compared to 70 keV simulating 120 kVp Single-Energy CT (SECT) [START_REF] Jacobsen | Intermanufacturer Comparison of Dual-Energy CT Iodine Quantification and Monochromatic Attenuation: A Phantom Study[END_REF], images obtained with lower monochromatic levels enhanced the iodine contrast in these lesions.

Several DECT platforms exist with different acquisition techniques to obtain both low-and highenergy photon spectra. Among those, Gemstone Spectral Imaging (GSI -GE Healthcare) is a fast kVp switching CT (KVSCT) technique that uses one X-ray tube / detector pair and switches from 80 to 140 kVp in less than 0.5 milliseconds [START_REF] Goo | Dual-Energy CT: New Horizon in Medical Imaging[END_REF][START_REF] Marin | State of the art: dual-energy CT of the abdomen[END_REF][START_REF] Siegel | White Paper of the Society of Computed Body Tomography and Magnetic Resonance on Dual-Energy CT, Part 1: Technology and Terminology[END_REF]. The tube current (mA) is fixed and the exposure time is adjusted to obtain an uniform distribution of the dose between both kVp.

Several studies on phantoms and patients have evaluated the spectral performance of GSI on Filtered Back Projection (FBP) images using classical metrics such as the accuracy of HU values and the iodine bias [START_REF] Jacobsen | Intermanufacturer Comparison of Dual-Energy CT Iodine Quantification and Monochromatic Attenuation: A Phantom Study[END_REF][START_REF] Jacobsen | Dual-Energy CT: Lower Limits of Iodine Detection and Quantification[END_REF][START_REF] Sellerer | Dual-energy CT: a phantom comparison of different platforms for abdominal imaging[END_REF][START_REF] Washio | Accuracy of Quantification of Iodine and Hounsfield Unit Values on Virtual Monochromatic Imaging Using Dual-Energy Computed Tomography: Comparison of Dual-Layer Computed Tomography With Fast Kilovolt-Switching Computed Tomography[END_REF]. However, Iterative Reconstruction (IR) algorithms such as Adaptive Statistical Iterative Reconstruction (ASIR) are now available and used routinely on DECT platforms, especially for low-energy monochromatic images to decrease the noise magnitude. ASIR reconstructs the CT projections by including the system statistics in the process and using an FBP image as a building block for each individual image reconstruction [START_REF] Geyer | State of the Art: Iterative CT Reconstruction Techniques[END_REF]. ASIR incorporates matrix algebra to convert the measured value of each pixel to an updated estimate of the pixel value. This pixel value is then evaluated and compared to the ideal value that is predicted with noise statistics. Those steps are repeated iteratively until the difference between the final estimation and the ideal pixel values is inferior to a stopping threshold. The ASIR image is then reconstructed and blended with traditional FBP in 10% increments according to user preference.

Many studies performed on SECT have shown that specifically-adapted metrics must be used when assessing the image quality of IR [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Greffier | Comparison of noise-magnitude and noise-texture across two generations of iterative reconstruction algorithms from three manufacturers[END_REF][START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF][START_REF] Verdun | Image quality in CT: From physical measurements to model observers[END_REF]. Hence, a task-based image quality assessment is now commonly applied in routine when considering IR. It employs metrics such as the Noise Power Spectrum (NPS) to assess both the magnitude and texture of the noise; and the Task-based Transfer function (TTF) to evaluate the spatial resolution. A detectability index (d') can then be computed to estimate the radiologist's ability to perform a clinical task. The detectability index value reflects the noise and resolution properties (NPS and TTF outcomes) as they relate to the ability of the system to perform a task of interest.

The purpose of this study was to assess the impact of ASIR on GSI low-energy monochromatic images by measuring the noise characteristics, spatial resolution and detectability of small focal liver lesions.

Materials and methods

Phantoms used

A 20-cm-diameter ACR QA phantom (Gammex 464, Middleton, WI) placed inside a body ring (diameter of 33 cm and length of 24 cm) (Figure 1.a) was scanned to measure the NPS (Figure 1.b), the TTF (Figure 1.c) and detectability indexes.

Acquisition and reconstruction parameters

Acquisitions were performed on a fast KVSCT system (Revolution HD, GE Healthcare).

Three GSI presets adapted to abdominal imaging were used, resulting in CTDIvol of 12.72, 10.76 and 8.79 mGy, respectively. These three dose levels correspond to the 75th, 50th and 25th percentiles of the data distribution used to define our national Diagnostic Reference Level for abdomen-pelvic CT examination, respectively. Detailed acquisition parameters of each GSI preset are listed in Table 1.

Raw data were reconstructed using FBP and ASIR at 30, 50, 70 and 100% levels for five monochromatic energy levels (40, 50, 60, 70 and 80 keV). Images were reconstructed with the standard reconstruction kernel, a slice thickness of 1.25 mm (1.25 mm increment) and a field of view of 250 mm.

Noise power spectrum

Noise characteristics were measured by NPS, using module 3 of the ACR phantom. For each reconstruction, four 128×128 pixels regions of interest (ROIs) within 20 consecutive axial slices of the module were used to compute the 2D-NPS. Each ROI was detrended by subtracting a fitted 2D second-order paraboloid to remove the low-frequency non-uniformity caused by x-ray scatter and beam hardening [START_REF] Zhou | Techniques to improve the accuracy of noise power spectrum measurements in digital x-ray imaging based on background trends removal[END_REF]. The 2D-NPS was then computed as the area-normalized Fourier transform of the ROI [START_REF] Measurements | ICoRUa. Medical imaging -the assessment of image quality[END_REF]:

NPS f , f = ∆ ∆ N N ∑ |F ROI x, y) -FIT x, y) |² ! N "#$ , (1)
where ∆ % and & % denote respectively the pixel size and the number of pixels of a ROI along axis ' respectively; ( … corresponds to the 2D Fourier transform; *+, -., /) is the i th ROI in the spatial domain; (,0 -., /) is the 2D second-order paraboloid fit of the i th ROI and & 123 is the total number of ROIs. 2D-NPS measurements were radially averaged and binned into 64 frequency bins to provide 1D-NPS curves. To quantify the changes of magnitude and texture of noise between reconstructions, the square root of the Area Under the Curve and average spatial frequency of the NPS curve (fav) were measured respectively.

Task-based transfer function

Spatial resolution was assessed using a TTF in module 1 of the ACR phantom from 15 consecutive axial slices according to the methodology previously reported [START_REF] Richard | Towards task-based assessment of CT performance: system and object MTF across different reconstruction algorithms[END_REF] and [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF].

TTF was computed only on the acrylic insert to be close to the contrast of the two simulated lesions for the detectability index computation. A ROI was placed around this, and a circular-edge technique was employed to measure the edge spread function (ESF) by plotting the NCT of each pixel as a function of the distance to the center of the insert. The line spread function (LSF) was then obtained by derivation of the ESF. The TTF was computed from the normalized Fourier transformation of the LSF. To quantify the loss/benefit of spatial resolution between reconstructions, the spatial frequency at which the TTF reduced to 50% (f50) was measured [START_REF] Burgess | Visual signal detectability with two noise components: anomalous masking effects[END_REF].

Detectability

A detectability index (4′) was computed to assess the detection of liver metastasis and hepatocellular carcinoma as function of energy level, iterative level and dose level. Detectability combines resolution and noise characteristics of an imaging system with a predefined function, noted Wtask, representative of a clinical imaging task to estimate how well a human observer would perform the considered task [START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF].

This index was based on a Non PreWhitening matched filter with Eye filter and Internal noise (NPWEi) model observer as [START_REF] Richard | Towards task-based assessment of CT performance: system and object MTF across different reconstruction algorithms[END_REF][START_REF] Christianson | An Improved Index of Image Quality for Task-based Performance of CT Iterative Reconstruction across Three Commercial Implementations[END_REF]:

4′² 6789- = ∬ ;0( <=>% ? @, A)B <=>% ? @, A)C ? @, A)4@4A ² ∬ &DE @, A);0( <=>% ? @, A)B <=>% ? @, A)C F @, A) + ;0( <=>% ? @, A)B <=>% ? @, A)& -@, A)4@4A ,

where @ and A are the spatial frequencies, C @, A) is the eye filter and Ni denotes the internal noise.

The eye filter employed in this study was identical to the one used by Burgess et al. [START_REF] Burgess | Visual signal detectability with two noise components: anomalous masking effects[END_REF], which was modeled on the contrast sensitivity function curve measured by Barten et al. for the human visual system [START_REF] Barten | Contrast sensitivity of the human eye and its effects on image quality[END_REF]:

C I) = I J exp -MI ? ), (3)
where I designates the radial spatial frequency such as I² = @² + A². Setting N as 1.3 has been shown

to yield good agreement with measurements and the value of M is determined such that C I) peaks at four cycles/degree for a viewing distance set to 40 cm [START_REF] Webster | Contrast adaptation and the spatial structure of natural images[END_REF][START_REF] Eckstein | Visual signal detection in structured backgrounds. IV. Figures of merit for model performance in multiple-alternative forced-choice detection tasks with correlated responses[END_REF]; here M ≈ 3. Assuming that the NPS is constant, the internal noise was considered uncorrelated and defined as a fraction (0.02 at a viewing distance of 100 cm) [START_REF] Burgess | Visual signal detectability with two noise components: anomalous masking effects[END_REF] of the NPS scaled to the observer's viewing distance Q:

& -@, A) = 0.02 U Q 100 W ? &DE. (4)
Wtask is a mathematical function representative of a given imaging task in the frequency domain. It is given by the Fourier transform of the difference between the signal present ℎ ! ., /) and signal absent ℎ ? ., /) hypotheses [START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF][START_REF] Measurements | ICoRUa. Medical imaging -the assessment of image quality[END_REF]:

B <=>% @, A) = ( ℎ ! ., /) -ℎ ? ., /) . (5)
Two task functions were defined to model the detection task of a liver metastasis (LM) and hepatocellular carcinoma (HCC). These task functions were assumed to represent a circular signal with a diameter of 10 mm. To account for lesion enhancement variation as a function of keV, the contrast between each clinical task and the liver parenchyma was defined according to the curves of HU variations measured on patients and published by Wang et al. for LM, HCC and the liver parenchyma [START_REF] Wang | Quantitative analysis of the dual-energy CT virtual spectral curve for focal liver lesions characterization[END_REF] (Table 2).

For each detection task, d' values obtained at 40, 50 and 60 keV were compared with those obtained for a SECT acquisition at 120 kVp with the same acquisition and reconstruction parameters.

Results

Noise power spectrum

Noise magnitude

Overall, for all three GSI presets, noise magnitude decreased as the dose increased (Figure 2).

Across the monochromatic energy range considered, for the three GSI presets and all reconstructions, the noise magnitude decreased from 40 keV to a minimum at 70 keV and then increased at 80 keV.

From 40 to 70 keV, noise magnitude was reduced on average by -68%±1% with FBP; -61%±3% with ASIR50 and -52%±6% with ASIR100.

At all energy levels, the noise magnitude was inferior with iterative reconstruction and further decreased when increasing the percentage of ASIR. The difference in noise reduction using ASIR compared to FBP was greatest at 40 keV and decreased as the monochromatic energy increased. The noise reductions with the different ASIR levels compared to FBP were similar at all dose levels. On average, noise magnitude was reduced by -39%±2% with ASIR50 and -66%±2% with ASIR100 at 40 keV and -26%±1% and -49%±2% at 70 keV, respectively.

Noise texture

The noise texture was impacted by the monochromatic energy level of the reconstructed images. Across the energy range considered, for all doses and reconstruction algorithms, the fav of the NPS was the greatest and therefore most favorable at 70 keV (Figure 3). The fav of the NPS shifted towards higher frequencies from 40 to 70 keV and shifted towards lower frequencies from 70 to 80 keV.

Comparing the different reconstructions, the fav of the NPS shifted towards lower frequencies when the percentage of ASIR increased. The impact of the proportion of ASIR on noise texture was increased at lower energy but equivalent at all dose levels. Compared to FBP, fav of the NPS was reduced on average by 28%±2% with ASIR50 and 54%±2% with ASIR100 at 40 keV and 13%±0% and 28%±1% at 70 keV, respectively.

Task-based transfer function

The values of TTF at 50% (f50) decreased as the energy level increased and as the percentage of ASIR increased (Figure 4). From 40 to 80 keV, f50 decreased on average from 0.61 mm -1 to 0.24 mm -1 for FBP, from 0.57 mm -1 to 0.22 mm -1 for ASIR50 and from 0.39 mm -1 to 0.22 mm -1 for ASIR100. Spatial resolution wasn't impacted by the dose.

Detectability index

Figure 5 depicts the detectability index (d') obtained for the two simulated lesions as function of the energy level for each dose level and percentage of ASIR. Overall, the detectability of the two considered lesions increased with increasing dose level and percentage of ASIR.

For the LM, d' increased from 40 keV to 70 keV and decreased afterwards for FBP, ASIR30 and ASIR50. Using ASIR70 for GSI 40 and GSI 22, the detectability also peaked at 70 keV. However, for GSI 1, d' tended to increase with decreasing energy, peaking at 50 keV. Similarly, for ASIR100, highest d' values were found at 50 keV and detectability between 50 and 70 keV increased with the dose level (3% for GSI 40 and 23% for GSI 1).

For the HCC, when using FBP or ASIR30, d' peaked at 60 keV. For higher iterative percentage, from ASIR50 to ASIR100, d' increased when decreasing the monochromatic energy, peaking at 50 keV. The improvement of d' between 50 and 70 keV decreased when the dose level and the percentage of ASIR decreased (for GSI 40: 33% with ASIR100 and 5% with ASIR50; for GSI 1:

67% with ASIR100 and 22% with ASIR50).

d' value variations for both detection tasks from 40 to 60 keV were compared to that a SECT acquisition at 120 kVp (Table 3). For the LM, the d' values were higher than those obtained at 120 kVp at all dose levels when using ASIR70 and ASIR100 at 60 and 50 keV For GSI 1, resulting in the highest dose level, the detectability of the LM was also increased for ASIR30 and ASIR50 at 60 keV and for ASIR50 at 50 keV.. For the HCC, the d' values obtained at 60 keV were higher than for a SECT acquisition at 120 kVp for reconstruction types and dose levels. At 50 keV, the d' values were higher only for ASIR50 for GSI 22 and GSI 1, for ASIR70 and ASIR100 at all dose levels.

Discussion

This study confirmed on GSI DECT images the same behavior of ASIR previously found on SECT images [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF]. For a given energy level, the use of ASIR reduced the noise magnitude but decreased the spatial resolution and changed the image texture by increasing its smoothness. In addition, ASIR improved the detectability of the two simulated lesions and favored the use of low keV monochromatic images compared to the conventional SECT polychromatic 120 kVp images.

In the present study, a task-based image quality assessment was used to evaluate the image quality of spectral images using IR algorithms. Indeed, the use of these IR algorithms requires to assess the image quality more completely than with conventional metrics such as noise, SNR or CNR.

These metrics are used to simply evaluate the image quality but do not take into account changes in noise texture or the dependence of spatial resolution on dose and contrast on IR images. For this, new metrics such as the NPS, the TTF or the detectability index are used. Many studies have shown the interest and the superiority of these new metrics in the analysis of IR images and in the optimization process compared to classical metrics [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF][START_REF] Verdun | Image quality in CT: From physical measurements to model observers[END_REF][START_REF] Greffier | CT dose optimization for the detection of pulmonary arteriovenous malformation (PAVM): A phantom study[END_REF][START_REF] Greffier | Optimization of radiation dose for CT detection of lytic and sclerotic bone lesions: a phantom study[END_REF][START_REF] Ott | Update on the non-prewhitening model observer in computed tomography for the assessment of the adaptive statistical and modelbased iterative reconstruction algorithms[END_REF][START_REF] Rotzinger | Task-Based Model Observer Assessment of A Partial Model-Based Iterative Reconstruction Algorithm in Thoracic Oncologic Multidetector CT[END_REF][START_REF] Samei | Performance evaluation of computed tomography systems: Summary of AAPM Task Group 233[END_REF].

Our NPS results showed that noise magnitude increased from 70 to 40 keV and the average frequency of the fav of the NPS shifted towards lower frequencies. Similar results have been found by Zang et al. with the KVSCT technique but with a CTDIvol close to 26 mGy [START_REF] Zhang | Objective characterization of GE discovery CT750 HD scanner: gemstone spectral imaging mode[END_REF]. These results are linked to the decrease in the average beam energy when the keV decreases, which reduces the penetration power of the X-ray beam and therefore increases the image noise. For a given energy level, the variations in NPS outcomes as a function of the dose and the percentage of ASIR are similar to those published on SECTs [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Greffier | Comparison of noise-magnitude and noise-texture across two generations of iterative reconstruction algorithms from three manufacturers[END_REF][START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF]. Noise magnitude decreased as the dose and percentage of ASIR increased. Conversely, fav of the NPS decreased (i.e. noise texture got smoother) with increasing percentage of ASIR, but was only weakly influenced by the dose.

Our TTF outcomes show that the highest values of f50 were found at 40 keV and then spatial resolution decreased to lower values as energy increased. This variation of the TTF as a function of the keV is due to the enhancement on the border of the acrylic insert, particularly at lowest energy levels.

This enhancement introduces an overshoot on the ESF curves, which improves the TTF and therefore the spatial resolution. As for the NPS, for a given energy level, the TTF variations as a function of the dose and the percentage of ASIR are similar to those published on SECTs [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF][START_REF] Ott | Update on the non-prewhitening model observer in computed tomography for the assessment of the adaptive statistical and modelbased iterative reconstruction algorithms[END_REF]. Values of f50

were not changed by the dose level, but decreased as the percentage of ASIR increased.

The outcomes of detectability index showed that d' values increased when the dose and the percentage of ASIR increased, regardless of the energy level used. Similar results were found with ASIR on SECTs for one energy level [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Samei | Assessment of the dose reduction potential of a model-based iterative reconstruction algorithm using a task-based performance metrology[END_REF]. The values of d' varied in different ways with keV as function of the percentage of ASIR and the simulated lesion. For the LM, the detectability was highest at 70 keV and did not increase with monochromatic images of lower energy reconstructed with FBP and ASIR up to 50%. Conversely, for ASIR100, the detectability decreased when lowering the energy and peaked at 50 keV. For HCC, the use of ASIR improved detectability when reducing the energy to 60 keV for FBP and ASIR30 and to 50 keV for higher ASIR percentages. These differences between lesions were directly related to the NPS and TTF outcomes and the contrast variations of the simulated lesions. With a lower contrast, the detectability of the HCC was lower than that of LM and more dependent on noise and spatial resolution variations. The reduction of the noise magnitude associated with mid-to-high percentages of ASIR benefited from the increase of lesion contrast and spatial resolution (TTF) at low keV to improve detection. However, because the contrast between the metastasis and the surrounding liver parenchyma is already sufficient at 120 kVp, the detectability of such lesions can be further improved at low-energy only when using the highest percentage of ASIR.

The results found in this study showed that it was possible and beneficial, to use ASIR to reduce image noise and promote the use of low keV for the detection of focal liver lesions in portal venous phase. Indeed, many clinical studies have shown the advantage of using energy levels between 50 and 60 keV for the detection and characterization of hepatic, pancreatic or renal lesions in abdominal imaging [1-4, 6, 11]. Using these energy levels, the photoelectric effect was then predominant, enhancing the lesion with iodine contrast. However, the results of this present study showed that image texture became smoother when lowering the energy of virtual monochromatic images.

This study has several limitations. Firstly, despite increasing the phantom attenuation to more closely resemble that of a patient, it was made of relatively uniform and geometric structures, which do not perfectly represent human anatomy. Furthermore, the use of this phantom does not take into account the movements of organs or tissues of the upper abdomen due to breathing during CT acquisitions. These movements could blur images and hence affect the detectability of small liver lesions. Further patients study should be conducted to confirm the results found on this phantom study Moreover, acquisitions were performed for a single standard reconstruction kernel, for specific GSI presets and on a single CT scan. Only two types of lesions were simulated with a fixed size. The evaluation of other acquisition/reconstruction parameters combinations and lesion characteristics representative of all vascular phases would be necessary to fully assess the performance of ASIR in low-energy monochromatic images. Finally, a new GSI platform (GSI Xtream) has been developed on more recent CT systems from the same CT vendor. It differs in the acquisition and detection process and enables the newest generation of iterative reconstruction (ASIR-V) which has demonstrated better performance in SECT [START_REF] Greffier | CT iterative reconstruction algorithms: a task-based image quality assessment[END_REF][START_REF] Marco | New adaptive statistical iterative reconstruction ASiR-V: Assessment of noise performance in comparison to ASiR[END_REF][START_REF] Lim | Initial phantom study comparing image quality in computed tomography using adaptive statistical iterative reconstruction and new adaptive statistical iterative reconstruction v[END_REF]. A future study should be conducted to assess the changes in the metrics studied between the two platforms.

Conclusion

Our phantom-based study demonstrated that using percentage of ASIR above 50% at lowenergy monochromatic images could limit the increase of the noise magnitude and benefit from spatial resolution improvement. Hence, it could enhance detectability of subtle low contrast focal liver lesions, such as HCC. Footnote: The lesions being hypodense, their HU values were lower than the surrounding liver parenchyma and hence the resulting contrast is negative.

Figure 1 .a

 1 Figure 1. a Phantom used in the study. b Regions of interest (ROIs) used for the noise power

Figure 2 .

 2 Figure 2. Values of noise magnitude corresponding to the square root of the Area Under the Curve of

Figure 3 .

 3 Figure 3. Values of average noise power spectrum spatial frequency (fav) obtained on low-energy

Figure 4 .Figure 5 .

 45 Figure 4. Values of task-based transfer function at 50% (f50) obtained on low-energy monochromatic

Table 1 .

 1 Acquisition parameters used for each GSI preset

	Preset GSI	GSI 40	GSI 22	GSI 1
	kVp	80/140	80/140	80/140
	mA	360	375	630
	Rotation time (s)	0.6	0.7	0.5
	Pitch factor	1.375	1.375	1.375
	Beam collimation (mm) 64 x 0.625 64 x 0.625 64 x 0.625
	CTDIvol (mGy)	8.79	10.76	12.72

Table 2 .

 2 Contrast between the hypodense liver metastasis or the hypodense hepatocellular carcinoma and surrounding the liver parenchyma as function of keV used to compute the detectability index. These values were extrapolated to the curves of HU variations measured on patients and published by Wang et al.[START_REF] Wang | Quantitative analysis of the dual-energy CT virtual spectral curve for focal liver lesions characterization[END_REF].

	Energy	Liver	Hepatocellular
	(keV)	metastasis	carcinoma
	40	-112	-39
	50	-78	-30
	60	-61	-23
	70	-50	-17
	80	-43	-16

Table 3 .

 3 Percentage variation of the detectability index (d') obtained with three lowest energy levels compared to those obtained with a conventional single energy CT acquisition at 120 kVp for all dose levels, reconstruction types and for both detection tasks.

	Lesions	GSI Energy (keV) FBP ASIR30 ASIR50 ASIR 70 ASIR100
			40	-39% -37%	-31%	-26%	-20%
		40	50	-22% -14%	-5%	5%	18%
			60	-3%	-2%	2%	4%	6%
			40	-39% -35%	-29%	-22%	-6%
	Liver metastasis	22	50 60	-21% -13% -3% -3%	-4% 0%	6% 1%	33% 14%
			40	-40% -28%	-21%	-15%	-2%
		1	50	-22% -4%	5%	15%	35%
			60	-5%	5%	7%	9%	13%
			40	-40% -36%	-29%	-23%	-11%
		40	50	-14% -4%	9%	22%	48%
			60	3%	5%	9%	12%	20%
	Hepatocellular		40	-39% -33%	-26%	-16%	3%
	carcinoma	22	50	-13% -1%	11%	28%	65%
			60	3%	5%	9%	14%	26%
			40	-40% -28%	-18%	-8%	13%
		1	50	-13%	6%	21%	39%	77%
			60	1%	10%	15%	20%	30%

Acknowledgements

We thank S. KABANI for her help in editing the manuscript.

The authors state that this work has not received any funding. One author declares relationship with the following company: GE Healthcare. H. PASQUIER is a GE Healthcare employee. However, he neither had access nor control on the phantom data acquisition and analysis. All other authors of this manuscript declare no relationships with any companies, whose products or services may be 13 related to the subject matter of the article.