Christian Moisan

Modelling liver tissue properties using a non-linear visco-elastic model for surgery simulation

In this work, we introduce an extension of the linear elastic tensor-mass method allowing fast computation of non-linear and visco-elastic mechanical forces and deformations for the simulation of biological soft tissue. We aim at developing a simulation tool for the planning of cryogenic surgical treatment of liver cancer. Percutaneous surgery simulation requires accurate modelling of the mechanical behaviour of soft tissue, and previous experimental characterizations have shown that linear elasticity is only a coarse approximation of the real properties of biological tissues. We first show that our model can simulate different types of non-linear and visco-elastic mechanical behaviours at speeds which are compatible with real-time applications. Then an experimental setup is presented which was used to characterize the mechanical properties of deer liver tissue under perforation by a biopsy needle. Experimental results demonstrate that a linear model is not suitable for simulating this application, while the proposed model succeeds in accurately modelling the axial load measured on the needle.

Acromiohumeral distance in a seated position in persons with impingement syndrome

To compare the acromiohumeral distance (AHD) of both shoulders in subjects with a unilateral shoulder impingement syndrome (SIS) and healthy subjects in a seated position during free shoulder movements of large amplitude.

DETECT2000: an improved Monte-Carlo simulator for the computer aided design of photon sensing devices

We introduce a new version of DETECT. DETECT is a Monte-Carlo simulator developed for the Computer Aided Design (CAD) of optical photon sensing devices. The simulator generates individual emission photons in specified locations of a photon-emitting device and tracks their passage and interactions in active and passive components of the system. Extensive options are available in the simulator to model the geometry of the photon sensing device, to account for the time and wavelength distribution of emission photons, to track their interactions with surfaces, to account for their possible absorption and re-emission by a wave-shifting components and to model their detection by pixelated photomultipliers or photodiodes. DETECT2000 is a very significant upgrade of DETECT97, which has long been established in the nuclear medicine instrumentation community for its accuracy to model the performances of high resolution energy and position sensitive gamma-ray detectors. The 2000 version of DETECT offers an accelerated version of the simulator which has been redesigned in the object-oriented C++ language. New features such as the tracking of the time and wavelength history of individual optical photons have been added.

Monte-Carlo modeling of scintillator crystal performance for stratified PET detectors with DETECT2000

In order to determine the theoretical performance of a multi-layer scintillator crystal used in small PET scanners, we have used the DETECT2000 Monte-Carlo simulation of the light transport in scintillation crystal software. The results given by this software demonstrate that some of the individual crystals in the block could not be distinguished. Also we have found out that the layer connecting the different crystals degrades the performance of the scintillation block. Simulation of thinner interconnecting layers suggest that much better crystal identification could be obtained.

Improved image contrast with mangafodipir trisodium (MnDPDP) during MR-guided percutaneous cryosurgery of the liver

The present study was undertaken to measure the gain observed in the liver-to-tumor contrast of perioperative images when using mangafodipir trisodium, a liver-specific contrast agent, during percutaneous cryosurgery of the liver performed under the guidance of magnetic resonance images. Retrospective quantitative analyses of MR images were performed on eleven patients having a total of 30 liver tumors treated by MR-guided percutaneous cryosurgery. An initial group of four patients were treated with no contrast agent, and was compared with a second group of 7 patients who received an intravenous injection of 5 microM/kg of mangafodipir for their cryosurgery. The percutaneous cryosurgery was monitored under the near-real-time-imaging mode of a 0.5T open-configuration MRI system using a T(1)-weighted Gradient-recalled echo pulse sequence. A significant improvement in the liver-to-tumor contrast-to-noise ratio was observed with mangafodipir (p < 0.05, paired t test) in 0.5T preoperative images. Along with the stability of the mangafodipir contrast enhancement during the entire cryosurgical procedure, the resulting gain in contrast allowed for better visualizing the presence of residual untreated tumor margins at the periphery of the cryosurgery iceball directly from perioperative images acquired with patients under narcosis. Consequently, it not only became easier for the interventionalist to determine the need for an additional cryoprobe to increase the size of the iceball during the procedure, but also to decide on the appropriate end point of the cryosurgery.

Non-linear Soft Tissue Deformations for the Simulation of Percutaneous Surgeries

We introduce a non-linear extension of the tensor-mass method for real-time computation of biological soft tissue deformations. We aim at developing a simulation tool for the planning of cryogenic surgical treatment of liver cancer. This therapy requires careful planning, therefore accurate modeling of the mechanical behavior of organs is required. Our method presently allows real-time computation of non-linear elastic tissue deformations, and further extension towards viscoelasticity modeling is planned.

An analytic method to predict the thermal map of cryosurgery iceballs in MR images

This paper presents a newly developed method to estimate, in magnetic resonance (MR) images, the temperatures reached within the volume of an iceball produced by a cryogenic probe. Building on the direct measurements of the MR signal intensity and its correlation with independent temperature variations at the phase transition from liquid to solid, the thermal information embedded in the images was accessed. The volume and diameter of the growing iceball were estimated from a time series of MR images. Using regressions over the volume in the time and thermal domains, this method predicted the cryogenic temperatures beyond the range of sensitivity of the MR signal itself. We present a validation of this method in samples of gelatin and ex vivo pig liver. Temperature predictions are shown to agree with independent thermosensor readings over a range extending from 20/spl deg/C down to -65/spl deg/C, with an average error of less than 6/spl deg/C.

Integrating geometric and biomechanical models of a liver tumour for cryosurgery simulation

In this paper, we present a 3D reconstruction approach of a liver tumour model from a sequence of 2D MR parallel cross-sections, and the integration of this reconstructed 3D model with a mechanical tissue model. The reconstruction algorithm uses shape-based interpolation and extrapolation. While interpolation generates intermediate slices between every pair of adjacent input slices, extrapolation performs a smooth closing of the external surface of the model. Interpolation uses morphological morphing, while extrapolation is based on smoothness surface constraints. Local surface irregularities are further smoothed with Taubins surface fairing algorithm [5]. Since tumour models are to be used in a planning and simulation system of image-guided cryosurgery, a mechanical model based on a non-linear tensor-mass algorithm was integrated with the tumour geometry. Integration allows the computation of fast deformations and force feedback in the process of cryoprobe insertion.

Three-dimensional reconstruction of the bony structures involved in the articular complex of the human shoulder using shape-based interpolation and contour-based extrapolation

Here, we propose a 3D reconstruction approach using shape-based interpolation and contour-based extrapolation. This approach aims to generate a 3D geometric model of a human shoulder from a sequence of MR parallel 2D cross-sections. While interpolation generates intermediate slices between every pair of adjacent input slices, extrapolation performs a smooth closing of the external surface of the model. We propose a new interpolation method based on conditional morphological dilation. Our extrapolation approach is based on surface smoothness constraints and gradually shrinks every extreme 2D cross-section of a bony structure towards a point, respectively. Surface rendering is accomplished through the generation of a triangular mesh using a parametric representation of 2D slice contours. After surface rendering, local surface irregularities are smoothed with Taubins surface fairing algorithm [G. Taubin, (1995)].

SKALPEL-ICT: SIMULATION KERNEL APPLIED TO THE PLANNING AND EVALUATION OF IMAGE-GUIDED CRYOTHERAPY

The SKALPEL-ICT project (Simulation Kernel Applied to the Planning and Evaluation of Image-guided Cryotherapy) aims at the developing of a virtual environment for the simulation of liver cancer cryotherapy. This simulator will mainly be used to plan the configuration and the positioning of the cryo-probes for an image-guided cryo-surgical intervention. Moreover, the prototype of the simulator will constitute an excellent training tool for future surgeons. The design of a simulator of this type involves several challenges related to the multidisciplinary nature of the project, which imposes the use of a modular approach. Presently, we are developing simultaneously three parallel models in the SKALPEL-ICT project: the thermal model associated with the prediction of the temperature of an ice-ball, the geometric model associated with the 3D reconstruction of hepatic tumours and the mechanical model which reproduces the real deformations of the liver and force feed-back during the cryo-probe insertion process. The final phase of the project is to deal with the integration of these three models into a virtual environment using an interaction-centric modeling approach.