Marek Bucki

Finite element speaker-specific face model generation for the study of speech production

In situations where automatic mesh generation is unsuitable, the finite element (FE) mesh registration technique known as mesh-match-and-repair (MMRep) is an interesting option for quickly creating a subject-specific FE model by fitting a predefined template mesh onto the target organ. The irregular or poor quality elements produced by the elastic deformation are corrected by a ‘mesh reparation’ procedure ensuring that the desired regularity and quality standards are met. Here, we further extend the MMRep capabilities and demonstrate the possibility of taking into account additional relevant anatomical features. We illustrate this approach with an example of biomechanical model generation of a speakers face comprising face muscle insertions. While taking advantage of the a priori knowledge about tissues conveyed by the template model, this novel, fast and automatic mesh registration technique makes it possible to achieve greater modelling realism by accurately representing the organ surface as well as inner anatomical or functional structures of interest.

Biomechanical modeling to prevent ischial pressure ulcers

With 300,000 paraplegic persons only in France, ischial pressure ulcers represent a major public health issue. They result from the buttocks׳ soft tissues compression by the bony prominences. Unfortunately, the current clinical techniques, with - in the best case - embedded pressure sensor mats, are insufficient to prevent them because most are due to high internal strains which can occur even with low pressures at the skin surface. Therefore, improving prevention requires using a biomechanical model to estimate internal strains from skin surface pressures. However, the buttocks׳ soft tissues׳ stiffness is still unknown. This paper provides a stiffness sensitivity analysis using a finite element model. Different layers with distinct Neo Hookean materials simulate the skin, fat and muscles. With Young moduli in the range [100-500 kPa], [25-35 kPa], and [80-140 kPa] for the skin, fat, and muscles, respectively, maximum internal strains reach realistic 50 to 60% values. The fat and muscle stiffnesses have an important influence on the strain variations, while skin stiffness is less influent. Simulating different sitting postures and changing the muscle thickness also result in a variation in the internal strains.

TexiCare: an innovative embedded device for pressure ulcer prevention. Preliminary results with a paraplegic volunteer.

This paper introduces the recently developed TexiCare device that aims at preventing pressure ulcers for people with spinal cord injury. This embedded device is aimed to be mounted on the user wheelchair. Its sensor is 100% textile and allows the measurement of pressures at the interface between the cushion and the buttocks. It is comfortable, washable and low cost. It is connected to a cigarette-box sized unit that (i) measures the pressures in real time, (ii) estimates the risk for internal over-strains, and (iii) alerts the wheelchair user whenever necessary. The alert method has been defined as a result of a utility/usability/acceptability study conducted with representative end users. It is based on a tactile-visual feedback (via a watch or a smartphone for example): the tactile modality is used to discreetly alarm the person while the visual modality conveys an informative message. In order to evaluate the usability of the TexiCare device, a paraplegic volunteer equipped his wheelchair at home during a six months period. Interestingly, the first results revealed bad habits such as an inadequate posture when watching TV, rare relief maneuvers, and the occurrence of abnormal high pressures.

Foot ulcer prevention using biomechanical modelling

Foot ulcers are a common complication of diabetes and are the consequence of trauma to the feet and a reduced ability to perceive pain in persons with diabetes. Ulcers appear internally when pressures applied on the foot create high-internal strains below bony structures. It is therefore important to monitor tissue strains in persons with diabetes. We propose to use a biomechanical model of the foot coupled with a pressure sensor to estimate the strains within the foot and to determine whether they can cause ulcer formation. Our biomechanical foot model is composed of a finite element mesh representing the soft tissues, separated into four Neo-Hookean materials with different elasticity: plantar skin, non-plantar skin, fat and muscles. Rigid body models of the bones are integrated within the mesh to rigidify the foot. Thirty-three joints connect those bones around cylindrical or spherical pivots. Cables are included to represent the main ligaments in order to stabilise the foot. This model simulates a realistic behaviour when the sole is subjected to pressures measured with a sensor during bipedal standing. Surface strains around 5% are measured below the heel and metatarsal heads, while internal strains are close to 70%. This strain estimation, when coupled to a pressure sensor, could consequently be used in a patient alert system to prevent ulcer formation.

Influence of the calcaneus shape on the risk of posterior heel ulcer using 3D patient-specific biomechanical modeling.

AbstractMost posterior heel ulcers are the consequence of inactivity and prolonged time lying down on the back. They appear when pressures applied on the heel create high internal strains and the soft tissues are compressed by the calcaneus. It is therefore important to monitor those strains to prevent heel pressure ulcers. Using a biomechanical lower leg model, we propose to estimate the influence of the patient-specific calcaneus shape on the strains within the foot and to determine if the risk of pressure ulceration is related to the variability of this shape. The biomechanical model is discretized using a 3D Finite Element mesh representing the soft tissues, separated into four domains implementing Neo Hookean materials with different elasticities: skin, fat, Achilles’ tendon, and muscles. Bones are modelled as rigid bodies attached to the tissues. Simulations show that the shape of the calcaneus has an influence on the formation of pressure ulcers with a mean variation of the maximum strain over 6.0 percentage points over 18 distinct morphologies. Furthermore, the models confirm the influence of the cushion on which the leg is resting: a softer cushion leading to lower strains, it has less chances of creating a pressure ulcer. The methodology used for patient-specific strain estimation could be used for the prevention of heel ulcer when coupled with a pressure sensor.

Mixed-Element Mesh for an Intra-Operative Modeling of the Brain Tumor Extraction

This paper presents a modified-octree technique that generates a mixed-element mesh. The final output mesh consider cubes, prisms, pyramids and tetrahedra. This technique is optimized for brain tumor extraction simulation in a real-time application. The proposed technique is based on the octree algorithm with a specific constraint: elements will be split only if they intersects a certain region of interest. With this approach we pursued a refined mesh only in the path from the skull opening point to the tumor. Fast computation by the Finite Element Method (FEM) is achieve thanks to the local refinement. Examples are given and comparison with other approaches are presented.

Jacobian-based repair method for finite element meshes after registration

Registration methods are used in the meshing field to “adapt” a given mesh to a target domain. Finite element method (FEM) is applied to the resulting mesh to compute an approximate solution to the system of partial differential equations (PDE) representing the physical phenomena under study. Prior to FE analysis the Jacobian matrix determinant must be checked for all mesh elements. The value of this Jacobian depends on the configuration of the element nodes. If it is negative for a given node, the element is invalid and therefore the FE analysis cannot be carried out. Similarly, some elements, although valid, can present poor quality regarding Jacobian-based indicator values, such as the Jacobian ratio. Mesh registration procedures are likely to produce invalid and/or poor quality elements if the Jacobian parameter is ignored. To repair invalid and poor quality elements after mesh registration, we propose a relaxation procedure driven by specific validity and quality energy formulations derived from the Jacobian value. The algorithm first recovers mesh validity and further improves elements quality, focusing primarily on nodes that make the elements invalid or of poor quality. Our novel approach has been developed in the context of non-rigid mesh registration and validated on a data set of 60 clinical cases in the context of orthopaedic and orthognathic hard and soft tissues modelling studies. The proposed repair method achieves a valid state of the mesh and also raises the quality of the elements to a level suitable for commercial FE solvers.

Real-time SPECT and 2D ultrasound image registration

In this paper we present a technique for fully automatic, real-time 3D SPECT (Single Photon Emitting Computed Tomography) and 2D ultrasound image registration. We use this technique in the context of kidney lesion diagnosis. Our registration algorithm allows a physician to perform an ultrasound exam after a SPECT image has been acquired and see in real time the registration of both modalities. An automatic segmentation algorithm has been implemented in order to display in 3D the positions of the acquired US images with respect to the organs.

Doppler Ultrasound Driven Biomechanical Model of the Brain for Intraoperative Brain-Shift Compensation: A Proof of Concept in Clinical Conditions

Accurate localization of the target is essential to reduce morbidity during brain tumor removal interventions. Yet, image-guided neurosurgery faces an important issue for large skull openings where brain soft-tissues can exhibit large deformations in the course of surgery. As a consequence of this “brain-shift” the pre-operatively acquired images no longer correspond to reality and subsequent neuronavigation is therefore strongly compromised. In this chapter we present a neuronavigator which addresses this issue and offers passive help to the surgeon by displaying the position of the guided tools with respect to the corrected location of the anatomical features. This low-cost system relies on localized 2D Doppler ultrasound imaging of the brain which makes it possible to track the vascular tree deformation throughout the intervention. An elastic registration procedure is used to match the shifted tree with its pre-operative structure identified within Magnetic Resonance Angiography images. A patient specific Finite Element biomechanical model of the brain further extends the resulting sparse deformation field to the overall organ volume. Finally, the estimated global deformation is applied to all pre-operatively available volumetric images or data, such as tumor contours, and the corrected planning is displayed to the surgeon. The system, tested on a patient presenting a large meningioma, was able to compensate within seconds for the intraoperatively observed brain-shift, reducing the mean error on tumor margin localization from 3.5 mm (max = 7.6 mm, RMS = 3.7 mm) to 0.9 mm (max = 1.7 mm, RMS = 1.0 mm).

Patient-specific finite element model of the buttocks for pressure ulcer prevention – linear versus non-linear modelling

Currently available techniques and/or protocols designed to prevent pressure sore formation in persons with spinal cord injury and wheelchair users are mainly based on the improvement of the skin/support interface and on postural and behavioural education. These techniques, however, seem to lack efficiency as the prevalence and incidence of pressure sores still remains very high. This study outlines a methodology aiming at the definition of an individual and personalised pressure ulcer risk assessment scale based on patient-specific Finite Element modelling of the buttocks.