Julie Villessèche
email: julie.villesseche@gmail.com

Sandrine Oriez
email: langues.sandrine.oriez@univ-rennes2.fr

Anne-Laure Besnard

Olivier Le Bohec
email: olivier.lebohec@univ-rennes2.fr

Christophe Quaireau
email: numériques.christophe.quaireau@univ-rennes2.fr

Yvonnick Noël
email: yvonnick.noel@univ-rennes2.fr

Fanny De

La Haye
email: haye@espe-bretagne.fr

Jérémie Nogues
email: tacit.jeremie.nogues@univ-rennes2.fr

Karine Lavandier
email: tacit.karine-lavandier@wanadoo.fr

Adaptive E-Learning as a Possible Solution to Counter Heterogeneity in Classrooms ? English Grammar Seen Through Cognitive Psychology and Psychometric Methods

et questionnaires psychologiques, ainsi que les modèles statistiques de l'apprentissage par renforcement.

en One of the main difficulties met in teaching Languages for Specialists of Other Disciplines (Lansod) is the heterogeneity of the learners' levels. Handling a classroom is thus a challenge for teachers. In English, the improvement of both expression and comprehension competences, both written and spoken, depends on the knowledge of grammar, as shown by Ellis's research (Second Language Acquisition). The project presented in this text (TACIT-Grammar) is part of the dynamic created in the universities in order to develop the use of digital technologies. The platform Tacit targets the evaluation and adaptive remediation of crossdisciplinary cognitive and linguistic competences. In this context, a module dedicated to English grammar is currently in progress to meet the specific needs of the Lansod1 .

Adaptive system, autonomous learning, English grammar, e-learning, Item Response Model (IRM), Rasch Model, tailored teaching L'apprentissage adaptatif en ligne comme une solution possible à l'hétérogénéité des classes ? La grammaire anglaise au prisme des méthodes de psychologie cognitive et de psychométrie L'une des principales difficultés rencontrées dans le domaine des langues pour les spécialistes d'autres disciplines (Lansad) est l'hétérogénéité du niveau des apprenants. La gestion des enseignements est alors un défi pour les enseignant.e.s. En anglais, la maîtrise de la grammaire est une des composantes indispensables à l'amélioration des compétences d'expression et de compréhension, à l'écrit comme à l'oral, comme le montrent notamment les travaux d 'Ellis (Second Language Acquisition). Le projet présenté dans ce texte s'inscrit dans la dynamique de développement de l'usage du numérique au sein des universités. Le portail Tacit vise à l'évaluation et à la remédiation adaptative des compétences cognitives et linguistiques transversales. Dans ce cadre, un module dédié à la grammaire anglaise est en cours de construction pour répondre aux besoins spécifiques des Lansad. apprentissage en autonomie, apprentissage en ligne, grammaire anglaise, modèles dits de la réponse à l'item (MRI), modèle de Rasch, enseignement différencié, système adaptatif 1. Introduction : Adaptive e-learning 21 st century teaching has progressively witnessed slow but steady transformations of the classroom, especially with the rapid development and diffusion of the Internet and its related technologiesstarting with desktop computers, quickly followed by laptops and smartphones. The latter two have increased the possibilities in terms of both teaching and learning with the growth of online classes and tools, complementing teaching on site. Uniting URL (Uniform Resource Locator) and IRL (In Real Life) is still the ideal a great number of universities are trying to reach.

Languages, as many other disciplines, have benefited from the arrival of these new tools, which have become common in the university curriculum of most students : online dictionaries, tests, exercises, smartphone applications, etc. They have also challenged the way universities or even classrooms work : the list could probably be longer, but to quote just a fewchanges occurred with the creation of so-called IT (Information Technology) rooms to give students the opportunity to use computers on campus, with the integration of personal laptops or smartphones in the classroom, but also with the introduction of rules restricting the use of smartphones during exams.

Besides, teachers and students are given new opportunities in terms of class organisation (MOOC -Massive Open Online Course, interactive classroom presentation, etc.) and activities (online exchanges, Skype, etc.). So there is a general movement nowadays for the improvement of the use of digital tools in universities. Thus, for languages, as for many other disciplines, the development of those IT rooms and tools has had quite a positive effect on the teacher/student relationship and as a consequence, on the interaction between teaching and learning (see part 2). The idea, as noted earlier, is neither to give the student full autonomy nor to imagine completely virtual teachers, but rather to create tools which will allow teachers to create individual curricula for students. The programs of most French universities, however, do not leave much room for individual follow-up as there is a lack of time, often also linked to a lack of funds, and all this can be coupled with difficulties inherent to Lansod2 i.e. a frequent turn-over among teachers who often hold temporary positions. This is why the development of innovative digital tools has become so crucial: efficient digital tools, such as adaptive systems, are relatively inexpensive individualized solutions providing enhanced learning.

Studies investigating individualized curricula have proved their superiority in terms of learners' performance because those systems solve the problem of the "one-size-fits-all" approach (Vandewaetere et al.). Though those systems are difficult to implementfor the reasons mentioned abovetheir effectiveness encourages researchers to keep investigating that question. Bloom (4) called it the "2 sigma problem" : "the most striking of the findings is that under the best learning conditions we can devise (tutoring), the average student is 2 sigma above the average control student taught under conventional group methods of instruction". Hence, the researchers' aim is to attain the same results which can be achieved through tutoring, while reducing the cost of this method.

So the question is : what solution can we offer teachers in order to know the level of each of their students and provide students with individual help ? In the next two parts, we will further develop the reasons prompting the use of tailored systems within a Lansod context. Then, we will describe the theoretical background which helped build the module. And in the last section, we will present the module per se.

E-learning and tailored systems

The use of e-learning was demonstrated to be effective as early as the 1980s. Indeed, Bangert et al. showed a general failure of individualized systems based on a meta-analysis of 51 studies but underlined the fact that computer-based systems produced better learning, because they guide students from one level of difficulty to another (153). Computer-based systems may also play a role in terms of motivation : programs which offer a variety of exercises of adequate difficulty with quick feedback (following the recommendations of the American Psychological Associationthe APAto enhance teaching and learning) tend to keep students interested and encourage them to continue. Thus, this study shows that individualized systems do not mean leaving students on their own, but providing educational support for each student : students are given the opportunity to follow a course in a one-toone configuration in the same way as with tutoring ; the pace of the teaching process is thus adapted to the pace of the learner. This is the principle of a tailored system.

There is one form of tailored systems which goes even further in the sense that it matches not only the pace of the learners but also their level of competence : it is called adaptive elearning. The core of an adaptive system is individual performance. Such systems have been established as effective in several studies, such as Di Giacomo et al., Truong, Yang et al. There are different levels of possible adaptations : adaptive systems can be tailored content, presentation, tools, they can take into account individual preferences or knowledge, they can give personal objectives, etc. Here, we are interested in the relation between individual performance and item difficulty. Several possibilities are available to adapt content and instruction to the learners' needs and characteristics. Vandewaetere et al. (119) identify three types of instructions : (a) macro-adaptive instruction, (b) aptitude-treatment interaction, (c) micro-adaptive instruction. The first approach corresponds to mastery learning ; the second one is a more comprehensive approach, which takes into account learners' personal characteristics such as anxiety. For example, anxious students prefer structured systems of instructions. Mödritscher et al. (2) underline the fact that it is better to give fixed tasks so as to limit the possibilities of control by the students, especially for those "with low-prior knowledge" regarding the pace of the task, for example.

The last type, micro-adaptive instruction, is the one which corresponds to our research here : it "diagnoses learner's specific learning needs during instruction and subsequently provides appropriate instructional prescription for these needs" (Vandewaetere et al. 119). It is a dynamic model, which can fit into a classroom environment. It extends the possibilities of a teacher-learner interaction through an online tutoring system. Consequently, the teacher can follow the individual progress of each student, creating the environment needed for blended learning (Kakosimos). The principle is that on the one hand, each student is provided with content adapted to their needs, while teachers can provide feedback individually or to groups of students and on the other hand, the system does not need any external interventionsuch as teachers selecting exercises -to provide the content environment which fits each student's abilities. Of course, the system, like the one we will present later, can sometimes leave the possibility of external intervention to take into account motivational factors for example. Thus, in this case, the clear matching of the level of competence of the learners with the difficulty level of the exercises allows the teachers to easily select, via the interface, the level of difficulty of the items. These items, for example, will be chosen from a student's area of competence or a slightly higher area (see the classic notion of a proximal zone of development, Vygotski 270), or in a lower zone to put the learner in a motivating situation of success. These possibilities of choice allow concrete individualized remedies and facilitate the management of the heterogeneity of skills within the same class.

Adaptive e-learning environment use in a Lansod

Teachers in Lansod are confronted to the high heterogeneity of students, especially if the class groups are based on the faculty where the student is registered and not on their level of competence for languages. So, generally, a same class group is made of students following different course pathways at university and who do not have the same level. For their main second language, students are supposed to have a B2 level after the baccalauréat (K-12 national examination). But this is not generally the case and this level is far from being acquired. In France, this heterogeneity is difficult to handle as the average number of language course hours in Lansod is about two hours a week, and because of a lack of staff, some universities have even turned toward alternative systems such as two intensive weeks per semester, one at the beginning and one at the end, which makes it difficult to handle heterogeneous levels, especially if, as mentioned above, groups are not based on students' competences. In this case, the rest of the semester leaves students with full autonomy, with means to support this autonomy which vary from one university to another. Thus, with this short amount of time allocated to second languages, the time to revise grammatical bases is generally very limited.

Besides, creating homogeneous groups requires time and equipment. Creating online tests means students will need computers to take the tests : the number of students makes it very difficult to organize. Some solutions are proposed : for example, when students register at home, they are required to take a test in order to choose the group where they will best fit in. However, such a system depends on the students' honesty for the test (no other material than the test itself should be used, such as dictionaries, or grammar assistance) and therefore, for the choice of a group. The choice of tests also varies from one university to another (Brudermann et al.).

But adaptive e-learning is a means to create individualized tailored systemsas seen previouslyso it is at the same time a pedagogical and research tool, in the sense that it gives teachers and researchers access to a great deal of data, which, in pedagogical terms, enables teachers to know more precisely the level of each student, and possibly, to create smaller groups within the classroom according to levels of competence. Such a system is a possible answer to the problem of heterogeneous classes, when organising students according to their levels cannot be done before the beginning of the semester. Indeed, it can either be used as a support system outside the classroom thanks to the adaptive module, or as a means of setting up smaller groups depending on the students' levels within the classroom. It also enables students to obtain individual help to strengthen the bases they may not have acquired before registering at the university and it does not add more constraints to the teachers' tasks, as the system determines the students' level without any external intervention.

Hence, an adaptive e-learning platform seems to be a potential solution to help solve the difficulties generated by the heterogeneity of groups and the lack of time induced by a limited number of hours of in-class teaching/learning.

Methodology : an English grammar module based on an explicit approach

In France, for decades, grammar teaching was the fundamental basis when learning a foreign language (Puren), contrary to English-speaking countries where teachers and researchers favoured language practice over teaching grammar rules very early on. The latter approach was rooted in the comparison between first and second language acquisition : as native speakers do not need grammar lessons to speak their mother tongue, second language learners could acquire grammar in the same wayi.e. linguistic immersion (Kraschen "Second Language Acquisition"). Therefore, for a very long time, the unconscious and implicit acquisition of the grammatical competence was favoured by the "Anglophone method", while a conscious and explicit approach of grammar was preferred by the "French method".

However, the advent of the CEFRL (Common European Framework of Reference for Languages) and the definitions of levels common to every language progressively changed the way languages were taught, even in France. The problem of the conscious and explicit approach to grammar was the way it was taught at the time (until the 1970s) : the most popular method was structural exercises or drills. And, as Newby (13) puts it, since learners are presented with pre-fabricated sentences, processing is limited to applying a rule deductively and is, as a result, very shallow. The fact that so much language is provided precludes the need for or the possibility of schematization. Such exercises are, of course, fairly boring and satisfy neither cognitive nor affective needs. Commitment is therefore likely to be low. (Newby 13) What the CEFRL has brought is a new approach based on action-oriented tasks. The principal aim is communication, and it is enhanced by the integration of the interlanguage (Selinker). This interlanguage is considered to be part of the learners' competences, which are the linguistic, the sociolinguistic and the pragmatic competences. So when it comes to evaluation, the idea is to insist on what the learner can write and say, instead of insisting on the importance of using the language properly. The system, centred on the actions and competences of the learner, necessarily puts aside grammatical questionsnot in the sense that grammar is not supposed to be taught, but mistakes are only corrected when the learner makes them, and they are not anticipated by specific grammar lessons. This was meant to avoid heavy grammar teaching, which has been proved demotivating for students (Freinet ; Falout et al.). For that reason, in this situation, one may question the relevance of reintroducing grammar through a module entirely dedicated to it.

Reintroducing grammar as an independent variable

As we pointed out earlier, the question is not really whether or not grammar should be taught but rather : what kind of grammar teaching should be implemented ? Should it be explicit or implicit ?

The action-based approach is built on the naturalistic point of view in L2 acquisition research. This idea, argued by researchers such as (Corder "The Significance" ; Krashen "Second Language Acquisition" or Ellis "Current Issues") is that learners appeared to follow a natural order and sequence of acquisition (i.e. they mastered different grammatical structures in a relatively fixed and universal order and they passed through a sequence of stages of acquisition on route to mastering each grammatical structure (Ellis "Current Issues" 85).

In this configuration, instructed learners (or classroom learners) like naturalistic ones are supposed to follow the same inner syllabus. They are supposed to acquire grammatical structures when they access them through different types of natural input. Different attempts were made in this direction : some supposed the teaching of grammar structures should follow the same pattern as natural grammar acquisition (for example, Pica) ; others (Krashen "Principles") thought that input should be limited to simple structures as complex ones were too difficult to acquire in instructed situations. However, more recent research (Ellis "Language Teaching", Spada and Tomita) has shown that evidence supports explicit grammar teaching. Indeed, those opposite claims argue that implicit teaching cannot suffice when dealing with complex structures (Hulstijn and de Graaff), especially because those structures occur less frequently in natural discourse. So natural input is not enough for them to be noticed. Another argument in favour of explicit grammar teaching is the necessity for the learner to be able to access grammatical knowledge with ease in order to fulfil instantaneous and fluent communication as, according to Ellis ("Current Issues" 97), explicit knowledge can convert into implicit knowledge if the learner is ready to acquire the targeted feature and that this conversion occurs by priming a number of key acquisitional processes, in particular noticing and noticing the gap (Schmidt, 1990). That is, explicit knowledge of a grammatical structure makes it more likely that learners will attend to the structure in the input and carry out the cognitive comparison between what they observe in the input and their own output. (Ellis "Current Issues" 97) Since then, the benefits of explicit grammar teaching have been shown by a large amount of research (Ellis "Language Teaching"). They show that the learners who follow courses including explicit grammar teaching learn faster and reach a higher level than the learners who do not have access to such courses. Thus explicit teaching favours the acquisition of a language, allowing the learner to be conscious of their own mistakes and to correct them.

Besides, the automatisation of explicit knowledge helps students learn grammatical forms progressively (DeKeyser) and it also favours the implicit knowledge of grammar (Lantolf and Thorne). This question of automatisation of explicit knowledge targets a specific problem : what kind of exercises can tackle the automatisation of explicit knowledge ? 4.2. Multiple-choice tests as an effective adaptive solution ?

We established earlier that an adaptive e-learning platform was a possible solution to help teachers deal with the heterogeneity of student groups. Such a system has some constraints : the main one is to make sure that the answers provided by the students are indeed a reflection of what they have learned, i.e. are not influenced by the format of the questions in a way that would impede the learning process. To the best of our knowledge, there are two solutions to deal with that problem.

One of them is to use open fields for answers, but for some exercises, there could be several correct answers, which means that they all need to be recognized as such. Though this system might seem ideal, it is extremely costly in terms of time concerning the creation of the exercises but also concerning the calibration of the exercises (i.e. classifying from the easiest to the most difficult). Indeed, an adaptive system means that the student's level of competence can be known because the exercises have been calibrated. In order to carry out this classification, the exercises must have been submitted to a great number of students in the first place. In a fixed amount of time, the number of exercises completed by students is generally less important when they have to write a full answer and not just pick one. This leaves us with the second solution.

The second way to deal with the numerous different answers students may give is multiplechoice questions (Figure 1).

Multiple-choice questions : a 'drill and kill' method ?

The problem of multiple-choice tests is that they are associated to the 'drill and kill' method, which consists in trying to create automatisation of grammatical structures through repetition. Here, the multiple-choice tests are based on the form recommended by Khodamoradi and Khaki. The exercises must have meaning and should be based on the will to say something, not on the structures, which would only lead to check the knowledge of a grammatical form. The solution we have chosen to avoid a 'drill and kill' practice is based on two axes : first of all, grammatical structures are embedded in context ; secondly, all exercises are translated.

The main reason justifying the use of translation here is to shift the focus from the form to the meaning ; another aim is to keep learners motivated, providing them with a pool of repeated opportunities to practice and succeed (translation maximizes opportunities for success by bypassing capacity limits of working memory and helping attention guidance toward the grammatical aspects). And, as authors such as Cook or Ellis ("A Theory") showed, the first language (L1) is a resource for learners trying to acquire a second language (L2). Widdowson (152) even underlines that learning a language is a bilingual experience, and that, whatever the method, learners keep going back and forth between their L1 and L2 in their minds. Furthermore, Hall (73) argues that L1 can also be a resource for the teacher, for explanations or in order to underline differences or similarities between languages. So translation is both a resource for learners to deal with those exercises, and it is also a means to fulfil a particular goal on the platform : the problem here is to be able to evaluate learners' competence in grammar, not in comprehension ; so translating the exercises is a way of isolating the evaluation of this particular competence.

Multiple-choice questions : methodology

Multiple-choice questions have been proved effective both in the case of learning (Roediger and Marsh ; Butler et al. "The Effect") and more precisely in the case of language acquisition (Khodamoradi and Khaki ; Hemmati and Ghaderi). Multiple-choice questions have benefits in terms of retention both in the short and long terms. The main problem of that retention of tested information is that, in the case of learners who choose a wrong answer, there also is possible retention of that incorrect information (Little and Bjork). This is mitigated by the number of options submitted to the learner for each question : increased numbers of similar lures on multiple-choice tests can impair participants' performance on the multiple-choice test and general knowledge as assessed on a later cued recall test. (Butler et al. "When" 954) So, in terms of methodology to choose a format for those multiple-choice questions, there were two problems to solve : (a) how to avoid the retention of incorrect information ; (b) how many lures should be used.

In the case of the retention of incorrect information, Butler and Roediger proposed a solution : feedback. Indeed, Pashler, et al. tested this question and came to the following conclusion : the effectiveness of the MCQ is considerably enhanced if the correct answer is given (i.e. telling the learner that their answer is incorrect is not enough, a correction is needed as a complement). They also concluded that feedback does not have a significant impact if the learners gave the correct answer. However, if they chose the wrong answer, the feedback had, on the contrary, a particularly significant effect. The study of confirms those conclusions and also shows that if information has been tested a first time, the learners' results are better the second time whatever the form of the test (whether MCQ or free recall test).

The second question, which is also related to the problem of learners memorising incorrect information, is about lures. How many options should we give learners ? And what kind of options ? The number of lures is above all related to the general use of MCQ : they are generally used for tests and less as a pedagogical tool. Many studies such as Frary, Haladyna, et al., Burton ("Reliability Measures" and "Myths" or Vegada et al. "Comparison" try to find measures to prevent students from guessing and to make MCQ as efficient as possiblei.e. MCQ should make it possible to rank students. So what they generally propose is the increase of the number of lures per question, or putting in place negative marking, etc. But, as stated by Butler et al., increasing the number of options given to learners also increases the possibility of retention of incorrect information. Moreover, Haladyna and Downing have shown that it is very difficult to increase the number of lures while maintaining the credibility of the lures. Hence they advised, as Burton ("Quantifying"), that the focus should be kept on the quality of lures and that three or four luresthese numbers include the correct answerare a good limit to keep in mind.

These reflexions are the backbone of the module we have created. The next part will present the resultthe English grammar modulebut also the possibilities this module offers in terms of teacher-learner relations. Indeed, the creation of such a tool is entangled with the needs of Lansod, which must address the heterogeneity of student groups without increasing the costs in terms of staff, budget or time.

Result : the TACIT-Grammar module

The TACIT (Testing adaptatif des compétences individuelles transversales) platform which will support the English grammar module is already online and accessible from any browser, as a first module for enhancing the comprehension of implicit information in French texts was launched in 2012 and another in 2014 with a focus on French vocabulary. The platform is divided between a part aimed at being used by speech therapists, and another one intended for the classroom. So far, it has been used by primary and secondary schools.

In 2013, a project was born, from a collaboration between two research teams, TACITmainly composed of researchers from the LP3C (Laboratoire de psychologie : cognition, comportement, communication) and ACE (Anglophonie : communautés et écritures), both in the university Rennes 2. The collaboration was then extended to the Lansod of that university. The module offers two modalities : a synchronous one (for use in the classroom) and an asynchronous one (for autonomous use). This module can thus be used to create blendedlearning-type courses. It enables teachers to have precise and genuine progress indicators for each of their students.

Earlier, we gave the reason that drove us to choose multiple-choice questions over other types of exercises (2.2.2)including their effectiveness and compatibility with IRM (here, Rasch Model). The IRM are statistical models which allow the adaptive system to represent both the learners' level of competence and the level of difficulty of the exercises on the same continuum. Though they are not used very much in applied psychometrics in France, they enable researchers to create adaptive pedagogical systems, where the exercises are selected according to the learner's level of competence. The model which was used here is the Rasch model :

The examiner can use the target person's performance on such a variable segment of self-chosen items to estimate his ability […], the process is self-tailoring. As the target person takes the test he finds for himself the items in the test booklet of difficulty best for him. (Wright and Douglas 2) We have also sided with conclusions which favour the quality over the quantity of options given for each question. That is why the exercises are built in two stages : one to determine the incorrect options given for each question and the other one to evaluate the level of difficulty of each exercise.

In the first stage, exercises are submitted to students, not as MCQ, but as open-field exercises in order to collect incorrect answers produced by the learners themselves. This enables us to cover all the strategies which prevent learners from connecting the meaning of a sentence with the correct grammatical form. Then, the incorrect answers are chosen according to three criteria : if they are the most frequently used, if they fit in terms of quality (spelling, vocabulary, etc.) and if they correspond to erroneous strategies (for example, if the incorrect answer given is a calque). In this stage, the learners are given one possible good answer after they validate their own answer. The exercises thus submitted are in their final form, in the sense that the translation associated is already there. See Figures 2 and3. In the second stage, the exercises are now in an MCQ form. The purpose is to evaluate the level of difficulty of the exercises in order to be able to match this level of difficulty with a level of competence, which can then be attributed to each learner. The ranking of the exercises follows the IRM. In this case, the experience takes the form of a test : the learners do not know if the answer they have given is correct or incorrect. This measure is taken to avoid a possible learning effect during the test. This stage is completed once we have enough material to prepare five evaluations of 20 exercisesthose exercises are the ones which reach the gold standard according to the IRM. The rest of the exercises are used for practice. To be finalised, the exercises have to be submitted to a great number of learners : each exercise has to be completed at least 120 times.

The base is composed of almost ten thousand exercises, which have been divided into 52 grammatical categories. The categories can be regrouped into three types : noun phrase (1824 exercises, 19.21 %), verb phrase (4183 exercises, 44.06 %) and syntax of the sentence (3486 exercises, 36.72 %). The purpose was to cover as many grammatical forms as possible while also creating categories based on difficulties often met by students : for example, translating the future or managing "the syntax of the sentence" (Figure 4). So far, more than 3,500 exercises have already gone through the first stage, and are in MCQ form. We are currently completing the first stage, collecting the most interesting incorrect answers for each unfinalized exercise (as shown in Figures 2 and3 above). We hope that the module will be ready in 2019.

Once completed, TACIT-grammar will offer two possibilities :  In-class use will be based on the same parameters which already exist on the platform for two other modules : Implicit comprehension and Vocabulary (in French). In this case, learners start with an evaluation of 20 exercises. This evaluation places the learner on a competence scale. Then, the exercises presented to the learner match the learner's competence in terms of difficulty. Learners have to take another evaluation in order to re-evaluate their level and see their progress. In this case, the platform provides teachers with visual summaries of the learners' history for the module and progress. The teacher may give homework to the learners -TACIT-grammar exercises automatically distributed according to the learners' level. In this situation, the teacher is provided with all the information : they neither have to select the exercises nor define their learners' competence level.

 Use in autonomy: This time, the system is entirely adaptive, which means that after a first exercise is proposed to the learners, the system then adapts the exercises given to the learners according to their achievements or failures (Quaireau et al. 4).

This means that the module is built to circumvent the heterogeneity of the classroom, in the sense that teachers do not have to find ways to monitor their learners' competence level, and learners are not given random exercises which would not specifically match their competence level.

Discussion

We have seen to what extent grammar is essential to the development of English competences, both written and spoken. A lot of learners arrive at university with difficulties they have accumulated over time : they did not manage to develop their grammar competence during their secondary school curriculum and they need further help, which often cannot be provided for lack of time, staff and budget. One element of solution can come from the proven efficiency of blended learning, with face-to-face courses coupled with (adaptive) elearning tools. Indeed, a systematic search of the research literature from 1996 through July 2008, prepared for the U.S. Department of Education, identified more than a thousand empirical studies on online learning. One of the most interesting results is that "Instruction combining online and face-to-face elements had a larger advantage relative to purely face-toface instruction than did purely online instruction" (U.S. Department of Education,17).

Functional since 2012, with now about 85,000 primary or secondary school students working on L1 French, the TACIT adaptive e-learning platform may also benefit university students learning L2, both in face-to-face situations and in distant e-learning.

In face-to-face interaction, it is much easier for teachers to help each non-self-regulated learner (Zimmerman) to begin their learning with the forethought phase (i.e. careful consideration of what will be necessary to succeed in learning ; task analysis, strategic planning, goal setting). They can also help learners to become more autonomous, to procrastinate less often, to develop different strategies that can make learning "easier, faster, more enjoyable, more self-directed, more effective, and more transferable to new situations" (Oxford 8). Nevertheless, simply telling students that they need to develop different learning strategies to progress in L2 and to earn good grades will not usually lead to develop autonomy and motivation. Still, it is important to teach students that some tasks, even tasks that are necessary to master, might be uninteresting at first yet require consistent, sometimes tedious, engagement for learning. Once learned, new skills may become their own reward. (APA 22) Another important point needs to be noted : to pursue L2 learning, at university, is less often "adding to existing student knowledge, known as conceptual growth", and more often "transforming or revising student knowledge, known as conceptual change" (APA 7).

Teachers can be instrumental in achieving conceptual change in students, for example in L2 grammar acquisition, by using tools that precipitate cognitive conflict or dissonance in the minds of students by helping make them aware of the discrepancy between their own thinking and correct curricular material or concepts. (APA 13)

The English grammar module was built to help both learners and teachers in this task. Indeed, in addition to the own-pace training mode, the TACIT platform integrates a tutored training mode. This second mode allows the teacher to conduct training sessions by introducing phases of interaction and explanation of the exercises encountered. Tutored training makes it possible to take into account the heterogeneity of the levels of knowledge3 and also the misconceptions that "tend to be entrenched in reasoning and resistant to change" (APA 13). A very large set of exercises is needed for this conceptual change to take place, which is why the Tacit-grammar module requires about 9,500 various exercises.

What has been presented here is the background of a new adaptive-system module created for students and teachers within the university Rennes 2. Both MCQ and adaptive systems are recognised as valuable and effective pedagogical tools, but they are also both objects of researchwhich was the purpose of this article, and means of researchwhich will help to better understand the place of the grammatical competence within learners' overall competence in English.

Indeed, the main objective of the research we are currently conducting using TACIT is to help students make progress, especially those in their first year at university, who have not yet reached a B2 level. The use of the TACIT platform will also make it possible to collect data from those learners and to compare it with their results to other tests (CLES, TOEFL, etc.). In this way, we hope to reach an equilibrium between researchers', teachers' and learners' expectations. The platform should be made available to all universities wishing to sign an agreement with the University of Rennes 2.

Figure 1 .

 1 Figure 1. Screenshot of TACIT showing an example of a finalized MCQ exercise in the category "Translate the conditional"

Figure 2 .

 2 Figure 2. Screenshot of TACIT showing an example of an unfinalized MCQ exercise on the passive

Figure 4 .

 4 Figure 4. Screenshot of TACIT showing the breakdown of the different categories of exercises for one of the three major types of exercises

This research was presented at the

RANACLES conference (23-25 November, University of Corsica, Corte). See Quaireau et al. and Villessèche et al. for presentations of the TACIT platform and the Ortho and Implicit modules.

Languages for Specialists of Other Disciplines (Van der Yeught), translation of the French acronym Lansad originally proposed by Michel Perrin.

Acquiring long-term knowledge and skill is largely dependent on practice but this practice needs to be welladapted (neither too easy nor too difficult) to be efficient and motivating. "Providing information that is not too elementary to be easily understood and not too complex to be out of range of understanding even with assistance represents the perfect level of entry for new material" (APA 9). "As students develop increasing competence, the knowledge and skills that have been developed provide a foundation to support the more complex tasks, which become less effortful and more enjoyable. When students have reached this point, learning often becomes its own intrinsic reward" (APA 16).


Christophe Reynier

Jocelyne Troccaz

Philippe Fourneret

André Dusserre

Cécile Gay-Jeune

Jean-Luc Descotes

Michel Bolla

Jean-Yves Giraud

MRI/TRUS data fusion for prostate brachytherapy. Preliminary results

teaching and research institutions in France or abroad, or from public or private research centers.

INTRODUCTION

Prostate cancer is the most common cause of cancer in men in many countries including the European Community and NorthAmerica. 40209 cases were reported out of a 60 million population in France in 2002 [START_REF] Abenhaim | Rapport de la Commission d'Orientation sur le Cancer[END_REF] matching a standardized incidence of 75.3 (rate per 100000) and 10004 individuals subsequently died. Many options -careful watch, conventional or laparoscopic surgery, brachytherapy, conventional or conformal external radiotherapy including IMRT, focused ultrasound, cryosurgery, etc. -are available and based on a multidisciplinary approach with informed patient consents. Individual screening from blood analysis using PSA allows for increased detection of early and localized stages. Recent progresses tend towards improving local control whilst minimizing side effects.

Using permanently placed radioactive seeds (a technique known as brachytherapy) back in the 1970s produced initial results that were as good as those obtained with other radiotherapy techniques and presented low impotence risks or other side effects. However seeds were placed freehand during laparotomy at that time [START_REF] Sylvester | Modern Prostate Brachy[END_REF] and resulted in non-homogeneous irradiation where cold regions may have contributed to disappointing clinical results.

Treatment planning and seed implantation is now based on the intensive use of transrectal ultrasound imaging. A stepper is used to acquire parallel transverse images with constant inter-slice distance. Moreover, needle insertion through the perinea is made easier via a grid, also called template, rigidly connected to the TRUS probe. This improved clinical outcome but such an approach is still limited by two factors. Firstly, the prostate apex and base are sometimes difficult to visualize in the transverse TRUS images and, secondly, intra-and interobservers variations are very frequent in ultrasound image interpretation. These problems could possibly be solved with image processing automation. Results regarding prostate automatic segmentation [START_REF] Shen | Segmentation of prostate boundaries from ultrasound images using statistical shape model[END_REF] whilst encouraging are generally obtained for images of the prostate acquired rather far from its extremities and therefore easier to process.

Whilst prostate imaging is still controversial, Magnetic Resonance Imaging (MRI) has a great potential. Its sensitivity and specificity have been demonstrated. Radioactive seeds are currently implanted inside an open interventional MR (iMRI) machine in a small number of centres [START_REF] D'amico | Real-time magnetic resonance image guided interstitial brachytherapy in the treatment of select patients with clinically localized prostate cancer[END_REF]. Robots may also be used in conjunction with CT or MRI imaging for improved accuracy and increased possible trajectories [START_REF] Fichtinger | System for robotically assisted prostate biopsy and therapy using intraoperative CT guidance[END_REF]. Yet, iMRI is prohibitive for most centres and makes surgical procedures much more complex. Moreover, even when using iMRI for prostate procedures (biopsy or brachytherapy), pre-operative MRI is also acquired because it provides enhanced image quality and is correlated with iMRI [START_REF] Hata | MRI-guided prostate biopsy using surgical navigation software: device validation and feasability[END_REF].

The alternative solution suggested is "bringing the MR virtually" in the operating room using MRI/TRUS data fusion. This approach allows refining prostate localization in the TRUS images with MRI data acquired pre-operatively. A similar approach was developed for biopsy guidance by Kaplan et al [START_REF] Kaplan | Real-Time MRI-Ultrasound Image Guided Stereotactic Prostate Biopsy[END_REF]. Those authors suggested registering MRI to TRUS by matching six fiducial prostate points defined in both modalities. Whereas, for sake of accuracy and robustness improvement in data fusion, we elected to record large sets of 3D points. In the presented approach, contours segmented in the intra-operative TRUS data with a satisfactory level of confidence are used for registration. MRI data are then superimposed to TRUS images to verify and improve TRUS segmentation at the apex and base of the prostate in particular. The objective is to improve data acquisition, homogeneity of dosimetry with a better definition of the D90 isodose and the reliability of urethra and rectum Dose Volume Histograms (DVH). This might help control cancer while reducing bladder, rectum and urethra side effects.

II.

MATERIAL AND METHODS Two types of protocols are used for prostate brachytherapy. The first is based on a preplanning stage whilst the treatment planning stage is taking place intra-operatively in the second one. The second protocol is that applied in our institution.

A.

Proposed method The PROCUR system presented in this paper is technically approached as follows. An MRI exam is performed with a transrectal coil and acquisition proceeds in the 3 orthogonal phases on the day preceding seed implantation. The patient is lying supine. Data are recorded in the DICOM format and transmitted to the computer for image processing. The contours of the prostate are manually segmented in the three planes. Data are segmented from the sagittal and coronal planes and superimposed onto those contours to facilitate apex segmentation in the MRI horizontal images. This step helps construct a three dimensional (3-D) cloud of n points, S MRI , representing the prostate in the MRI reference frame R MRI .

Intra-operatively, the patient is placed in the gynaecologic position. The TRUS probe is inserted and transverse images are collected regularly with a stepper as in the conventional procedure. The prostate is swept as a whole and approximatively 8 to 10 parallel images are collected. The urologist delineates the prostate on every image whenever possible. The data are then fed in the computer and the urologist segmentation is recorded. This results in a second set of m 3-D points, S TRUS , representing the prostate in the ultrasound reference system R TRUS .

The next stage involves registering both cloud points automatically. The system computes the T TRUS/MRI transform between R TRUS and R MRI allowing for optimal S TRUS to S MRI superimposition. This is in fact minimizing the following energy as a function of the T TRUS/MRI unknown:

2 / 1 2 / )) ( , ( 1 )
( TRUS i MRI TRUS MRI m i i MRI TRUS M T S dist T E ∑ = = σ
where M i TRUS is the i th point belonging to S TRUS , σ i is the estimated data-related error and S MRI represents the MRI data set. dist(S,P) is the Haussdorf distance between a point P and a surface S.

A pre-registration consisting in superimposing the S TRUS and S MRI centres of gravity initializes T TRUS/MRI before minimization. Two types of registration have been tested. In the first one called rigid registration, T TRUS/MRI is a homogeneous transform combining rotations and translations (6 parameters). The second one is an elastic registration where T TRUS/MRI is much more complex allowing for rotation and translation between data sets as well as local deformations or distortions. Those methods are derived from the octree-spline elastic registration published by Szeliski and Lavallée [START_REF] Szeliski | Matching 3-D anatomical surfaces with non-rigid deformations using octree-splines[END_REF]. The optimization procedure is the Levenberg-Marquardt algorithm (see [START_REF] Press | Numerical recipes in C: the art of scientific computing[END_REF]). Elastic registration makes use of an adaptive, hierarchical and regularized free-form deformation of one volume to the other coordinate system. Since S MRI density is much higher than S TRUS (n>>m), the octree-spline construction is based on S MRI . The result is a 3-D function f TRUS/MRI transforming any point given in R TRUS to R MRI .

Once the data have been registered (see figure 1) the software may compute the corresponding slice in the MRI transverse volume for every acquired TRUS image using f TRUS/MRI and the interpolation tools. An image presenting both the TRUS and MRI data is computed to help the urologist visualize results and verify or improve prostate segmentation. The resulting image is split into four quadrants with a virtual cross: the upper right and lower left quadrants display TRUS data whilst the upper left and bottom right ones present the corresponding re-sliced MRI data (see figure 2). The users are allowed to move the cross in this bi-modality image. This representation helps them explore prostate limits in both modalities. Contours extracted from the MRI data may also be superimposed to the TRUS images. MRI data help confirm or modify the original segmentation close to the apex and base of the prostate in particular. A 3-D representation is also available to the clinician. Three intersecting orthogonal MRI slices are computed in the three MRI volumes for any given needle i.e. a point defined in a transverse TRUS image at a normal direction to the image plane.

This improved segmentation can then be used for treatment planning. The 3-D view may facilitate appreciating the needle position with respect to the whole gland and surrounding structures.

B.

Materials

The MRI system is a Philips Gyroscan ACS II 1.5 Tesla operated with a transrectal coil. T2 Turbo Spin Echo sequences are used for transverse, sagital and coronal acquisitions. The repetition time is 1800ms and echo time 120ms. The region explored is 15cmx15cm; image resolution is 256x256. The slice thickness is 3mm. The echographic system is a BK-Medical one. The stepper is designed to acquire TRUS images every 5mm. The probe is a 8551 model (5-10Mhz intrarectal probe). The TRUS image size is 576x768 pixels. A tissue-equivalent ultrasound prostate phantom 1 from CIRS (Computerized Imaging Reference Systems, Inc.) was used for technical evaluation and accuracy quantification. The PROCUR system is written in Visual C++. The computer is a PC Intel Pentium4, 1.6GHz with 256MB of RAM and equipped with a Matrox MeteorII video frame grabber. The PC is operated with Windows2000 Professional.

C.

Validation approach It must first be stated that treatment was planned and administered as usual to the patients participating in those evaluations. Imaging data (pre-operative MRI and intra-operative TRUS) were recorded. The PROCUR system was evaluated post-operatively from acquired data. Some evaluations were also performed with the phantom.

Technical evaluation

Registration is a key step in the PROCUR system. The registration methodology as presented in section II.1 involves minimizing the distance between two sets of 3-D points. It can be evaluated in several ways. Firstly, one can visually confirm that the two sets of points have a good fit. This evaluation is purely qualitative but should registration be poor, it may be useful to determine where mismatches were made. A second and more interesting qualitative computed transform evaluation is based on user interaction with the bi-modality image produced after data fusion. The continuity of prostate contours from one modality to the other is a very good indicator of registration quality.

Quantitatively, information regarding the distance remaining between both sets of points is an indicator of registration quality. Mean, maximum, minimum values as well as standard deviations are reported in section III.1. However, small residual distances are not true equivalent of optimal registration and optimization algorithms may be trapped into local minima of the energy function. Pre-registration reduces this risk but it is not completely eliminated. A second registration accuracy assessment methodology consequently needs to be defined without any gold standard -the exact computed transform value cannot be determined -and it should be as minimally invasive as possible for the patients -i.e. avoiding marker placement as in [START_REF] Balter | Measurement of prostate movement over the course of routine radiotherapy using implanted markers[END_REF] for instance. It was decided to verify the registration accuracy of the urethra. This is a significant structure in treatment planning; it is internal to the prostate and is not included in registration data. As far as the phantom is concerned, the urethra is easily visible in both modalities. It has to be visible for patients and this is why the procedure described below was adopted. A probe is inserted into the patient's urethra and retained for the MRI examination as well as the implantation procedure the next day. This allows for urethra visibility in both MRI data and TRUS images. The PROCUR system is then operated as described in section II.A and produces a f TRUS/MRI value. The bi-modality image is computed for each echographic plane. The urethra position (lumen centre) is recorded in the TRUS and MRI components for every bi-modality image (see figure 3). The distance between 1 Model 053 (see http:\\www.cirs.com) those two points is computed. Mean, minimum, maximum values and standard deviations valid for phantom experiments and series of 4 patients are presented in section III.1

Potential impact on treatment

Other evaluations of a more clinically advanced level have also been performed. The effects resulting from the use of three orthogonal MRI acquisitions have been quantified for series of 8 patients. The objective was to compare MRI transverse image segmentation with and without the assistance of the two other volumes. The effects of data fusion and MRI superimposition on prostate delineation were determined in two different manners. First, the number of contoured echographic images with and without data fusion have been compared close to the apex and to base of the gland for 11 patients. Secondly the prostate volume was computed without and with data fusion for the same patients. These data have a serious potential impact for the treatment planning stage. Results are presented in section III.2.

A dose volume histogram (DVH) was computed with the MRI-enhanced echographic data to predict the potential impact of volume modification on treatment outcome. It was compared to that applicable for the conventional procedure. A comparison is presented for one patient (No 5).

III. RESULTS

III.1 Technical evaluation

Phantom 11 images of the phantom were obtained (see figures 4 and 5) with the TRUS probe (5.2MHz). Registration results are presented in table I. One can see a satisfactory match for prostate contours in registered modalities (Fig. 5). The urethra match measurements on bi-modality images after registration are presented in the following table. These values correspond to the 11 bi-modality computed images of the phantom. Please note that the phantom urethra diameter is about 10mm. In Figure 6, the urethra distance is presented as a function of the slice number. Each curve corresponds to one patient. It clearly appears that errors are systematically higher for apical images and decrease fairly regularly. This will be discussed in section IV. Figure 7 shows the average urethra distance obtained for each slice and all 4 patients.

III.2 Pre-clinical evaluation MRI segmentation

Sagital and coronal MRI acquisitions helped increase the number of segmented slices in the MRI horizontal plane (see table V). On average, 1.12 slices representing 9.78% could be segmented additionally for the prostate base. 1.62 slice representing 14.13% were added for the apex.

Volume measurement

Prostate volume was measured for the original TRUS segmentation and modified segmentation resulting from MRI enhancement in each patient. The following formula was applied to compute this volume:

d S S S S V n i i i i i * 3 * 1 1 1 1 ∑ - = + + + + =
Where n is the number of slices, S i the surface of the i th slice and d the inter-slice distance.

As regards the phantom, the volume corresponding to the original segmentation is 65.92cc; it is 67.97cc after elastic registration and MRI-enhancement. This corresponds to a difference of 2.05cc and is equivalent to 3.11% of the original volume. It was not possible to compare these computed volumes with the real one because the dimensions given by the phantom provider are averages only. The difference between those two values mainly comes from the significant increase in surface (5.94cm 2 ) of the first slice (apex) for which contour detection on the original TRUS image proved very difficult. In absolute (or signed) values, the surface difference (cf. figure 9) is: max = 5.94cm 2 , min = 0.05cm 2 , mean = 1.01cm 2 , std.dev. = 1.69cm 2 (max = 5.94cm 2 , min = -0.66cm 2 , mean = 0.63cm 2 , std.dev. = 1.88cm 2 respectively).

As can be seen on figure 10, the volume computed from enhanced images after elastic registration is higher than the original volume for all patients. In terms of percentage variation relatively to the initial volume, the minimum, maximum, mean values and the standard deviations are: min = 1.03% (patient No 6), max = 48.24% (patient No 8), mean =15.86%, std. dev. = 13.57%. The largest three volume differences (patients No 7, 8 and 10) correspond to cases where one additional echographic image was segmented after registration (see table VI). Concerning the other two patients (No 2 and 11) for whom the number of echographic images changed, the volume difference is lower. As regards patient No 2, the removed slice is compensated in terms of volume by an increase in computed surfaces on the other slices. As regards patient No 11, the added slice is partly compensated by the fact that the largest surface (slice No 5) is smaller in modified data compared to the original ones.

Dose volume histograms

Planned treatment in clinical practice is such that 90% of prostate volume should receive between 160 and 180Gy. The D90 planned on the original segmented contours of patient No 5 corresponds to 165Gy i.e. 90% of the prostate is supposed to receive at least 165Gy. When the D90 is recomputed from modified contours after elastic registration, the corresponding value is 105Gy (i.e. 90% of the prostate is supposed to receive at least 105Gy). 68.45% only of the prostate would receive at least 165Gy for these modified contours. The difference between those two values is very significant (-36%). However, this is an individual test from which no conclusion can be drawn. Still, as pointed out in [START_REF] Perrotti | Prospective evaluation of endorectal magnetic resonance imaging to detect tumor foci in men with prior negative prostatic biopsy: a pilot study[END_REF], image processing may significantly contribute to inaccuracy of dose distribution and measurement. Our results are very preliminary but they show that a significant difference in TRUS prostate definition coming from MRI enhancement may have a very large impact on potential clinical outcomes.

IV.

DISCUSSION AND CONCLUSION Concerning surface registration, qualitative results based on visual evaluation are generally quite positively appreciated from a clinician's standpoint. Quantitative results are close to 1mm in average and this is very satisfactory as the different sources of errors (image resolution, manual segmentation2 ) are taken into consideration. Rigid and elastic registrations give rather similar results in terms of residual distance measurements for both sets of points. It is likely that the rather similar positions of the patient during MRI and TRUS acquisition, which both involve placing an intra-rectal sensor, result in rather similar deformations of the prostate, if any. Prostate motion depending on bladder and rectum filling are ideally taken into account with rigid registration. Elastic registration produces lower mean and minimum values as regards the residual distance between data, but the standard deviation and maximum residual distance are somewhat higher. This can be explained by the fact that elastic registration imposes regularization to retain surface smoothness. This means that reducing the distance of one TRUS point closer to the MRI surface may increase the distance of an adjacent TRUS point to retain the TRUS surface continuity. Elastic registration will be the preferred methodology because it may render potential deformations. Both processes are realtime ones.

Our method differs from [START_REF] Kaplan | Real-Time MRI-Ultrasound Image Guided Stereotactic Prostate Biopsy[END_REF] on several accounts: the density of registered data (6 points versus large data sets) and the type of registration (point-to-point versus set-to-set and rigid versus rigid or elastic). The accuracy of small data sets point-to-point matching is highly dependent on the ability to precisely define the corresponding points in both modalities. However, the approach presented in [START_REF] Kaplan | Real-Time MRI-Ultrasound Image Guided Stereotactic Prostate Biopsy[END_REF] helps simplify the protocol. In the PROCUR system, the fact that contours are used without any need for explicit point-to-point equivalence between MRI and TRUS data has a related cost i.e. MRI prostate segmentation (TRUS segmentation is a standard stage in brachytherapy protocols). No quantitative comparison can be made with our work since no quantitative results are provided in reference [START_REF] Kaplan | Real-Time MRI-Ultrasound Image Guided Stereotactic Prostate Biopsy[END_REF].

As regards urethra experiments, figures 6 and 7 clearly show that measurements are dependent on the TRUS slice position for the patients. The distance measured between both modalities is always higher close to the apex. This may be partly explained by the fact that the TRUS apex slices often are the smallest surfaces and the most difficult to read. The uncertainty regarding urethra position is certainly a factor contributing to this error. Moreover, defining the lumen centre manually may be somewhat difficult and perhaps inaccurate even for a 'satisfactory' TRUS image. Errors are much smaller with the phantom because the urethra boundary is much more visible on TRUS images. A semi-automated procedure would probably reduce the centre localization inaccuracy. Those results do not seem to be correlated to the quality of surface registration but this would have to be confirmed with statistical analysis for a larger number of cases. Reference [START_REF] Lindsay | Modelling uncertainties in the delivery and evaluation of prostate brachytherapy implants[END_REF] describes CT/MRI registration for dose evaluation using urethral catheters. The authors underline that the least precise results were obtained near the prostate base and this is explained by potential balloon buoyancy. Concerning our experiments, the potential variability of urethral probe traction between MRI acquisition and the intra-operative situation could only be a low contributor to those results. One can also observe that our results globally improved from patient to patient and that may have been a learning curve effect for the placement of the urethral probe in a stable position. This should be confirmed with a significant number of cases.

We demonstrated with series of 8 patients that using three MRI volumes acquisition facilitates prostate segmentation for the apical region and the base in particular. We also demonstrated that MRI enhancement allows modifying the number of segmented TRUS images especially at the apex and base of the prostate. Experiments made regarding volume measurements have shown that MRI enhancement always results in computed volume increases. The formula used for volume computation may be discussed and different expressions could be used (a simple average for two successive slices or a formula taking into account the fact that a prostate section may be unreported before the first slice and after the last one because of TRUS step or MRI inter-slice distance -both were experimented). However, general conclusions can be drawn because the formulae used for TRUS and MRI-enhanced images are identical. We have seen that underestimating the prostate volume in the TRUS data could have a very significant potential impact on dose distribution. This needs to be confirmed with a clinical evaluation intended to correlate clinical factors (post-brachytherapy PSA measurements for instance) with those simulations.

The next project stage is the clinical evaluation. Other long-term extensions could also be envisioned. Introducing MRI affords extensive opportunities. Several centres are attempting to evaluate its potential use in cancer localization (see reference [START_REF] Amdut | Prostate seed implant quality assessment using MR and CT imag fusion[END_REF] for instance) and this would allow for more precise and selective seeds positioning. In the same vein, seed placement could be based on MRI navigation with pre-operative data as brain and spine surgeons have been doing for many years now [START_REF] Watanabe | Three dimensional digitizer (neuro-navigator): a new equipment for CT guided stereotaxy[END_REF][START_REF] Reinhardt | Computer aided surgery with special focus on neuronavigation[END_REF].

List of figures:

Figure 1 :Figure 2 :

 12 Figure 1: Data superimposition after registration

Figure 3 :

 3 Figure 3: Urethra distance computation

Figure 4 :

 4 Figure 4: Prostate TRUS phantom: (left) photograph (right) inside.

Figure 5 :Figure 6 :

 56 Figure 5: Phantom experiment: (a) MRI image -(b) TRUS image -(c) composite image after data fusion

Figure 7 :

 7 Figure 7: Mean residual distance of the urethra

Figure 8 :Figure 9 :

 89 Figure 8: Segmentation of the apical region: (a) original non segmented TRUS image -(b) corresponding MRI recomputed slice -(c) contours from the MRI volumes (blue for frontal, yellow for sagital and red for transverse)

Figure 10 :

 10 Figure 10: Volume analysis (patients): elastic registration case

Figures

  Figures

Figure 1 :

 1 Figure 1: Data superimposition after registration

Figure 2 :

 2 Figure 2: Data fusion: the computed image presents MRI (top-left/bottom-right) data and TRUS data (top-right/bottom-left) for a given TRUS acquisition plane. Two examples.

Figure 3 :

 3 Figure 3: Urethra distance computation

Figure 4 :Figure 5 :Figure 8 :Figure 9 :

 4589 Figure 4: Prostate TRUS phantom: (left) photograph (right) inside.

Figure 10 :

 10 Figure 10: Volume analysis (patients): elastic registration case

  Rigid and elastic registrations are very similar because the phantom is non-deformable.

	Table II	Distance between the urethra detected
			on TRUS and on MRI (mm)
	Registration method	Mean [min; max]	Std. Dev.
	Rigid	1.30 [0.58; 2.63]	0.58
	Elastic	1.57 [0.82; 2.87]	0.62
	Patients
	Table III reports results obtained for 11 patients.

  patients i.e. 45.45% of the cases. The apex segmentation was modified for 4 of the 11 patients i.e. 36.36% of the cases. Figure8shows a case where the prostate apex was not segmented on TRUS; MRI data provided additional information and enabled contour definition.

	Table VI		Contribution of MRI data to the segmentation of apex and base
			of the prostate on TRUS images
	Patient number	Before	After registration (number of images added or removed)
	registration		Apex	Base
			Rigid	Elastic	Rigid	Elastic
	1	11	0	0	0	0
	2	12	-1	0	-2	-1
	3	8	0	0	0	0
	4	8	0	0	0	0
	5	7	0	0	0	0
	6	8	0	0	0	0
	7	8	0	0	1	1
	8	5	1	1	1	1
	9	10	0	0	0	0
	10	7	0	1	1	1
	11	9	2	1	2	1
	Modified	-	27.27% of the	27.27%	45.45%	45.45%
	segmentation		cases
			36.36%	45.45%
	Table V		Contribution of the sagittal and coronal MRI data to the
			segmentation of the prostate on the transverse MRI volume
	Patient number	Nb of segmented	Nb of additional	Nb of additional
			transverse slices	slices (base)	slices (apex)
	1		10	2	1
	2		10	1	0
	3		14	1	2
	4		11	1	2
	5		14	0	3
	6		13	1	2
	7		10	1	2
	8		10	2	1
	Mean		11.5	1.125	1.625
	%		-	9.78%	14.13%

TRUS segmentation

Using MRI data resulted in a modification of the number of segmented TRUS transverse images (see table VI): slices were added (or removed) for 4 (respectively 1) of the 11 patients i.e. 36.16% (or 9.1% respectively) of the cases. The base segmentation was modified for 5 of the 11

Table I :

 I Residual distance error between S TRUS and S MRI (mm). Phantom experiments.

Table II :

 II Distance between the urethra detected on TRUS and MRI (mm).Phantom experiments.

Table III :

 III Residual distance of the S TRUS and S MRI surface points. Patient experiments.

Table IV :

 IV Distance between the urethra detected on TRUS and MRI (mm). Patient experiments.

Table V :

 V Contribution of sagittal and coronal MRI data to segmentation of the prostate on the transverse MRI volume

Table VI :

 VI Contribution of MRI data to the segmentation of apex and base of the prostate on TRUS images.

In MRI images where pixel size is 0.58mm x 0.58mm for instance, a variation of 2 pixels in manual segmentation is equivalent to a 1.16mm distance on the contour ..

ACKNOWLEDGMENTS

This project is supported by the French Ministry of Health (Projet Hospitalier de Recherche Clinique PHRC "Prostate-écho").