A 3D discrete model of the diaphragm and human trunk

In this paper, a 3D discrete model is presented to model the movements of the trunk during breathing. In this model, objects are represented by physical particles on their contours. A simple notion of force generated by a linear actuator allows the model to create forces on each particle by way of a geometrical attractor. Tissue elasticity and contractility are modeled by local shape memory and muscular fibers attractors. A specific dynamic MRI study was used to build a simple trunk model comprised of by three compartments: lungs, diaphragm and abdomen. This model was registered on the real geometry. Simulation results were compared qualitatively as well as quantitatively to the experimental data, in terms of volume and geometry. A good correlation was obtained between the model and the real data. Thanks to this model, pathology such as hemidiaphragm paralysis can also be simulated.Comment: published in: "Lung Modelling", France (2006

In vivo measurement of human brain elasticity using a light aspiration device

The brain deformation that occurs during neurosurgery is a serious issue impacting the patient "safety" as well as the invasiveness of the brain surgery. Model-driven compensation is a realistic and efficient solution to solve this problem. However, a vital issue is the lack of reliable and easily obtainable patient-specific mechanical characteristics of the brain which, according to clinicians' experience, can vary considerably. We designed an aspiration device that is able to meet the very rigorous sterilization and handling process imposed during surgery, and especially neurosurgery. The device, which has no electronic component, is simple, light and can be considered as an ancillary instrument. The deformation of the aspirated tissue is imaged via a mirror using an external camera. This paper describes the experimental setup as well as its use during a specific neurosurgery. The experimental data was used to calibrate a continuous model. We show that we were able to extract an in vivo constitutive law of the brain elasticity: thus for the first time, measurements are carried out per-operatively on the patient, just before the resection of the brain parenchyma. This paper discloses the results of a difficult experiment and provide for the first time in-vivo data on human brain elasticity. The results point out the softness as well as the highly non-linear behavior of the brain tissue.Comment: Medical Image Analysis (2009) accept\'

In vivo measurement of surface pressures and retraction distances applied on abdominal organs during surgery

This study undertook the in vivo measurement of surface pressures applied by the fingers of the surgeon during typical representative retraction movements of key human abdominal organs during both open and hand-assisted laparoscopic surgery. Surface pressures were measured using a flexible thin-film pressure sensor for 35 typical liver retractions to access the gall bladder, 36 bowel retractions, 9 kidney retractions, 8 stomach retractions, and 5 spleen retractions across 12 patients undergoing open and laparoscopic abdominal surgery. The maximum and root mean square surface pressures were calculated for each organ retraction. The maximum surface pressures applied to these key abdominal organs are in the range 1 to 41 kPa, and the average maximum surface pressure for all organs and procedures was 14 ± 3 kPa. Surface pressure relaxation during the retraction hold period was observed. Generally, the surface pressures are higher, and the rate of surface pressure relaxation is lower, in the more confined hand-assisted laparoscopic procedures than in open surgery. Combined video footage and pressure sensor data for retraction of the liver in open surgery enabled correlation of organ retraction distance with surface pressure application. The data provide a platform to design strategies for the prevention of retraction injuries. They also form a basis for the design of next-generation organ retraction and space creation surgical devices with embedded sensors that can further quantify intraoperative retraction forces to reduce injury or trauma to organs and surrounding tissues

A Method to Compute Respiration Parameters for Patient-based Simulators

Best Poster AwardInternational audienceWe propose a method to automatically tune a patient-based virtual environment training simulator for abdominal needle insertion. The key attributes to be customized in our framework are the elasticity of soft-tissues and the respiratory model parameters. The estimation is based on two 3D Computed Tomography (CT) scans of the same patient at two different time steps. Results are presented on four patients and show that our new method leads to better results than our previous studies with manually tuned parameters

Towards the mechanical characterization of abdominal wall by inverse analysis

The aim of this study is to characterize the passive mechanical behaviour of abdominal wall in vivo in an animal model using only external cameras and numerical analysis. The main objective lies in defining a methodology that provides in vivo information of a specific patient without altering mechanical properties. It is demonstrated in the mechanical study of abdomen for hernia purposes. Mechanical tests consisted on pneumoperitoneum tests performed on New Zealand rabbits, where inner pressure was varied from 0 mmHg to 12 mmHg. Changes in the external abdominal surface were recorded and several points were tracked. Based on their coordinates we reconstructed a 3D finite element model of the abdominal wall, considering an incompressible hyperelastic material model defined by two parameters. The spatial distributions of these parameters (shear modulus and non linear parameter) were calculated by inverse analysis, using two different types of regularization: Total Variation Diminishing (TVD) and Tikhonov (H1). After solving the inverse problem, the distribution of the material parameters were obtained along the abdominal surface. Accuracy of the results was evaluated for the last level of pressure. Results revealed a higher value of the shear modulus in a wide stripe along the craneo-caudal direction, associated with the presence of linea alba in conjunction with fascias and rectus abdominis. Non linear parameter distribution was smoother and the location of higher values varied with the regularization type. Both regularizations proved to yield in an accurate predicted displacement field, but H1 obtained a smoother material parameter distribution while TVD included some discontinuities. The methodology here presented was able to characterize in vivo the passive non linear mechanical response of the abdominal wall

Physically based deformable object modeling and soft tissue deformation

Bilgisayar destekli sanal ameliyat benzetimleri ve farklı alanlardaki uygulamaları, birçok parametrenin olası en doğru biçimde belirlendiği ve uygulandığı matematiksel modellerle mümkündür. Bu parametreler benzetimi yapılacak biyolojik yapıların geometrik sınırlarının, organ özelliklerinin, farklı kuvvetler altındaki davranışlarının ve en önemlisi dokuya ait malzeme parametrelerinin olası en doğru biçimde belirlenmesi ile elde edilir. Bu çalışma yumuşak dokuların doğrusal olmayan, viskoelastik davranışlarının matematiksel olarak modellenmesi ve önerilen bu modelin sonlu elemanlar yöntemi kullanılarak sayısal olarak çözülmesine odaklanmıştır. Örnek organ olarak seçilen insan karaciğerine ait malzeme katsayıları, hastane ortamında canlı dokular üzerinde yapılan deney sonuçları kullanılarak doğrusal olmayan eniyileme yöntemi ile elde edilmiştir. Uygulanan genel yaklaşımlardan farklı olarak benzetim sonuçlarının çözümden önce yani modelleme aşamasında yapılan ön basitleştirmelerden etkilenmemesi ilkesi benimsenmiştir. Bu nedenle, modelleme aşamasında uygulanan ve gerçeklikten uzaklaşılması ile sonuçlanan geleneksel basitleştirme adımları uygulanmamış, bu yaklaşım yerine tam çözümü veren benzetim sonuçları üzerinde model indirgeme teknikleri uygulanmıştır. İlk aşama olarak yumuşak dokuların önemli özelliklerini içeren bir model önerilmiş, sonrasında oluşturulan bu model sonlu elemanlar yöntemi kullanılarak analiz edilmiştir. Analiz sonrası elde edilen sonuçlar bir sonraki aşamada kullanılmak üzere saklanmıştır. İkinci adımda saklanan sonuçlar üzerinde Karhunen Loeve model indirgeme yöntemi kullanılarak, ilk adımda elde edilen sonuçlara yakın değerleri gerçek zamanlı sunabilen bir çözüm elde edilmiştir. Son adımda ise basitleştirilmemiş model ile indirgenmiş model sundukları hızlar ve sonuçların doğrulukları açısından karşılaştırılmıştır. Anahtar Kelimeler: Yumuşak dokular, sonlu elemanlar yöntemi, model indirgeme, viskoelastisite.As indicated by The Institute of Medicine Report, 44,000 to 98,000 people die annually as a result of procedure mistakes in surgery. The main reason cited was the young surgeons' insufficient experience of new techniques before facing real-world surgical operations. With the use of virtual surgery, surgeons have the opportunity to test different critical surgical procedures in a low- cost, ethically sound environment. The virtual surgery simulators equipped with physically based modeling engines are outstanding candidates for the simulation of deformable human organs. The real time soft tissue simulation has gained a great interest recently as a result of advancements in areas such as surgery planning and the surgical simulations. Linear deformation models may not provide the required accuracy in such areas whilst nonlinear models do not serve the real time needs. Therefore, there is a common need for a computationally simplified yet accurate, nonlinear, large deformable viscoelastic model of soft tissues to be used in these real time applications. Computer aided surgery and surgery simulation applications require accurate form and feature description as well as proper material and behavior descriptions of the biological tissues in a mathematically formulated model. This study focuses on mathematical formulation and numerical implementation of a nonlinear viscoelastic model of soft tissues using Finite Element Method (FEM). As an object, a human liver is selected in our study. The necessary material parameters are extracted from results of in vivo material tests on human liver using a nonlinear optimization method. Due to the technology limitations, today's physics based surgery simulators are forced to use different simplification methods to obtain satisfactory visual effects and response time in delivering an acceptable visual and haptic user experience. Thus, previous approaches to this problem involve techniques in the simplification of the mathematical models prior to obtain the necessary numerical solutions. The model simplification is necessary to achieve the required computational speeds however; these simplifications will inevitably result in reduced accuracy. Unlike the above mentioned approaches, our research is based on the opposite view that accuracy should not unduly influenced due to premature model simplifications prior to the analysis. Therefore, simplification step that is applied in the modeling phase, resulting in inaccurate outcome, is displaced to be applied after the solution phase. This aims to limit the degradation of already calculated values. As the first step, a complete soft tissue model is created. Subsequently this model is analyzed using Finite Element Method (FEM) and the results are stored and are used as the input to the next stage. Then, the Karhunen-Loeve decomposition technique is used to simplify the previously obtained data resulting in the final simulated model. Simplification technique is necessary to achieve acceptable real-time update rates due to the computing power available. Finally, a complete (unsimplified) model is compared with the simplified model in terms of accuracy and speed. Since soft tissues undergo large deformations under applied load and required volume preservation behavior, the use of linear strain tensor is not suitable for the accurate modeling of the resulting large deformations involved. Limitation of using a linear strain tensor is overcome by the use of nonlinear Green strain tensor in our customized FEM code. Standard material tests on human liver reveal the material nonlinearity relationship between applied forces and the resulting displacements. This nonlinear behavior is necessitated the use of hyperelastic strain energy density function in our FEM implementation. Viscoelastic behavior is also a predominant feature of soft tissues that must be included in any soft tissue deformation simulation. Therefore, the quasi-linear viscoelastic behavior of a human liver is added to our implementation to provide for this need. To implement time dependent viscoelastic behavior in improved the reduced order model, the surface nodes and its neighboring nodes are determined inside the Radius of Influence (ROI). Then, a constant unit force is applied to each of surface nodes for a period of 20 seconds. It is assumed that the creep response of the soft tissue lasts for 20 seconds due to the limitations of data storage and higher computational costs. Our proposed model, when employed, results in a final constitutive equation that successfully produces accurate results while catering for the whole previously mentioned phenomenon namely; geometric and material nonlinearity, volume preservation, viscoelasticity. Keywords: Real-time simulation; deformable models; model order reduction; nonlinear simulations; soft tissue; viscoelasticity

Modeling and parameter estimation of rheological objects for simultaneous reproduction of force and deformation

Abstract-Many deformable objects in our living life demonstrate rheological behaviors, such as human organs and tissues, clays and various food products. Rheological objects include both elastic and plastic properties. Due to the presence of residual (permanent) deformation, it is difficult to model rheological objects, especially to reproduce both force and residual deformation simultaneously. In this paper, a series of physical models was investigated for simulating rheological behaviors. Generalized formulations of the constitutive laws were derived for serial and parallel physical models, respectively. We found that the serial models are appropriate for formulating strain, whereas the parallel models allow a convenient calculation of stress. Analytical expressions of force and residual deformation were then derived for generalized parallel models. Theoretical discussions suggested the difficulty to reproduce both force and deformation simultaneously using linear physical models. A 2D FE (finite element) model was then formulated and an efficient method for estimating physical parameters were proposed by taking the advantages of analytical force expressions. Experimental results with commercial clay and Japanese sweets material were presented to validate our modeling and parameter estimation methods. A dual-moduli viscous element was also introduced to improve our FE model for reproducing rheological force and deformation simultaneously

A new approach for the in-vivo characterization of the biomechanical behavior of the breast and the cornea

The characterization of the mechanical behavior of soft living tissues is a big challenge in Biomechanics. The difficulty arises from both the access to the tissues and the manipulation in order to know their physical properties. Currently, the biomechanical characterization of the organs is mainly performed by testing ex-vivo samples or by means of indentation tests. In the first case, the obtained behavior does not represent the real behavior of the organ. In the second case, it is only a representation of the mechanical response of the indented areas. The purpose of the research reported in this thesis is the development of a methodology to in-vivo characterize the biomechanical behavior of two different organs: the breast and the cornea. The proposed methodology avoids invasive measurements to obtain the mechanical response of the organs and is able to completely characterize of the biomechanical behavior of them. The research reported in this thesis describes a methodology to in-vivo characterize the biomechanical behavior of the breast and the cornea. The estimation of the elastic constants of the constitutive equations that define the mechanical behavior of these organs is performed using an iterative search algorithm which optimizes these parameters. The search is based on the iterative variation of the elastic constants of the model in order to increase the similarity between a simulated deformation of the organ and the real one. The similarity is measured by means of a volumetric similarity function which combines overlap-based coefficients and distance-based coefficients. Due to the number of parameters to be characterized as well as the non-convergences that the solution may present in some regions, genetic heuristics were chosen to drive the search algorithm. In the case of the breast, the elastic constants of an anisotropic hyperelastic neo-Hookean model proposed to simulate the compression of the breast during an MRI-guided biopsy were estimated. Results from this analysis showed that the proposed algorithm accurately found the elastic constants of the proposed model, providing an average relative error below 10%. The methodology was validated using breast software phantoms. Nevertheless, this methodology can be easily transferred into its use with real breasts. In the case of the cornea, the elastic constants of a hyperelastic second-order Ogden model were estimated for 24 corneas corresponding to 12 patients. The finite element method was applied in order to simulate the deformation of the human corneas due to non-contact tonometry. The iterative search was applied in order to estimate the elastic constants of the model which approximates the most the simulated deformation to the real one. Results showed that these constants can be estimated with an error of about 5%. After the results obtained for both organs, it can be concluded that the iterative search methodology presented in this thesis allows the \textit{in-vivo} estimation the patient-specific elastic constants of the constitutive biomechanical models that govern the biomechanical behavior of these two organs.Lago Ángel, MÁ. (2014). A new approach for the in-vivo characterization of the biomechanical behavior of the breast and the cornea [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/44116TESI

Comparación de volúmenes: aplicación al análisis del comportamiento de modelos biomecánicos de órganos

En este trabajo se proponen distintas métricas que permiten validar el comportamiento del modelo biomecánico. Estas métricas son los coeficientes clásicos de validación de segmentaciones basadas en el solape y las distancias entre volúmenes. Esto permite dar un grado de ajuste entre el modelo biomecánico y el comportamiento real.Lago Ángel, MÁ. (2011). Comparación de volúmenes: aplicación al análisis del comportamiento de modelos biomecánicos de órganos. http://hdl.handle.net/10251/11309Archivo delegad

Traditional indentation test for measurement of soft tissue elasticity

2011-2012 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe