A. F. Frangi

Personalization of a cardiac electromechanical model using reduced order unscented Kalman filtering from regional volumes

Patient-specific cardiac modeling can help in understanding pathophysiology and therapy planning. However it requires to combine functional and anatomical data in order to build accurate models and to personalize the model geometry, kinematics, electrophysiology and mechanics. Personalizing the electromechanical coupling from medical images is a challenging task. We use the Bestel-Clément-Sorine (BCS) electromechanical model of the heart, which provides reasonable accuracy with a reasonable number of parameters (14 for each ventricle) compared to the available clinical data at the organ level. We propose a personalization strategy from cine MRI data in two steps. We first estimate global parameters with an automatic calibration algorithm based on the Unscented Transform which allows to initialize the parameters while matching the volume and pressure curves. In a second step we locally personalize the contractilities of all AHA (American Heart Association) zones of the left ventricle using the reduced order unscented Kalman filtering on Regional Volumes. This personalization strategy was validated synthetically and tested successfully on eight healthy and three pathological cases.

Patient-specific computational hemodynamics of intracranial aneurysms from 3D rotational angiography and CT angiography: an in vivo reproducibility study

BACKGROUND AND PURPOSE: Patient-specific simulations of the hemodynamics in intracranial aneurysms can be constructed by using image-based vascular models and CFD techniques. This work evaluates the impact of the choice of imaging technique on these simulations. MATERIALS AND METHODS: Ten aneurysms, imaged with 3DRA and CTA, were analyzed to assess the reproducibility of geometric and hemodynamic variables across the 2 modalities. RESULTS: Compared with 3DRA models, we found that CTA models often had larger aneurysm necks (P = .05) and that most of the smallest vessels (between 0.7 and 1.0 mm in diameter) could not be reconstructed successfully with CTA. With respect to the values measured in the 3DRA models, the flow rate differed by 14.1 ± 2.8% (mean ± SE) just proximal to the aneurysm and 33.9 ± 7.6% at the aneurysm neck. The mean WSS on the aneurysm differed by 44.2 ± 6.0%. Even when normalized to the parent vessel WSS, a difference of 31.4 ± 9.9% remained, with the normalized WSS in most cases being larger in the CTA model (P = .04). Despite these substantial differences, excellent agreement (κ ≥ 0.9) was found for qualitative variables that describe the flow field, such as the structure of the flow pattern and the flow complexity. CONCLUSIONS: Although relatively large differences were found for all evaluated quantitative hemodynamic variables, the main flow characteristics were reproduced across imaging modalities.

Reproducibility of image-based computational hemodynamics in intracranial aneurysms: Comparison of CTA AND 3DRA

Hemodynamics play an important role in the pathogenesis of intracranial aneurysms and are believed to provide valuable information to predict aneurysmal rupture. Using image-based vascular models and computational fluid dynamics (CFD) techniques, the inter-aneurysmal hemodynamics can be studied in depth. In this paper, the effect of using different image-modalities is evaluated by investigating 4 middle cerebral arteries bifurcation aneurysms imaged with three-dimensional rotational angiography (3DRA) and computed tomographic angiography (CTA). The presented visualizations show that the main flow characteristics are preserved. However, there are large discrepancies in quantitative measurements.

Cardiac Modelling for Pathophysiology Research and Clinical Applications. The Need for an Automated Pipeline

A flexible pipeline for construction of computer models for electrophysiology simulation is presented. It allows the construction of 3D FEM models from clinical images with little user interaction. The processing pipeline allows to segment a patient-specific heart geometry from a scan data set, mesh it, and include the necessary functional structures to build a com- putational model. This structures include approximated fiber orientation and generic fast conduction system. The pipeline is expected to be used to construct models for study heart electro- physiology with special emphasis in cardiac resynchronization therapy in the context of the euHeart project. This framework will allow processing cases to perform clinical in a systematic and fast way.

Comparison of steady-state and transient blood flow simulations of intracranial aneurysms

Hemodynamics play an important role in the pathogenesis of intracranial aneurysms and patient-specific computational hemodynamic simulations could provide valuable information to clinicians. Transient simulations that capture the pulsatility of blood flow are commonly used for research purposes. However, steady-state simulations might provide enough information at a lower computational cost, which could help facilitate the introduction of hemodynamic simulations into clinical practice. In this study, we compared steady-state simulations to transient simulations for two aneurysms. The effect of a change in flow rate waveform was investigated and virtual treatment techniques were employed to compare post-treatment flow reduction predictions. We found that the difference in the time-averaged wall shear stress on the aneurysm was less than 5% and the distribution of wall shear stress was qualitatively assessed to be very similar.

3D reconstruction of intervertebral discs from T1-weighted magnetic resonance images

Low back pain is a current and increasing problem closely related to intervertebral disc degeneration, which is responsible for over 90% of spinal surgical procedures. In clinical routine, clinicians base their diagnosis of disc degeneration on 2D analysis of Magnetic Resonance (MR) images. In this work, an automatic 3D segmentation method, based on active shape models, is presented for both degenerated and normal intervertebral discs. A database of 25 intervertebral discs was used to semi-automatically build a shape statistical model and intensity models. Then, a 3D reconstruction was achieved by using those models to deform an initial shape. The method was evaluated using the 25 intervertebral discs and a leave-one-out cross validation, resulting in a mean shape accuracy of 1.6mm and a mean dice similarity index of 83.6%. This automatic and accurate 3D reconstruction method opens the way for an improved diagnosis of disc degeneration.

Cerebral Aneurysms: A Patient-Specific and Image-Based Management Pipeline

This work presents an image- and biomechanics-based data processing pipeline able to build patient-specific models of cerebral aneurysms. The pipeline also contemplates the virtual modeling and release of endovascular devices such as stents and coils. As a result of the morphological, morphodynamic, hemodynamic and structural analyses, a set of complex descriptors relevant for aneurysm’s diagnosis and prognosis is derived. On the one hand these will bring an insight into the processes behind aneurysm genesis, growth and rupture. On the other one, the inclusion of virtual devices enables the in silicopersonalized evaluation of alternative treatment scenarios before intervention and constitutes a valuable tool for the industrial design of more effective devices. Several of its components have been evaluated in terms of robustness and accuracy. The next step should comprehensively assess the complete pipeline, also proving its clinical value. This pipeline illustrates some of the ideas behind the Virtual Physiological Human (VPH) and the integration of complex data for a better understanding of human physiology in health, disease and its treatment.

Effect of coil surface area on the hemodynamics of a patient-specific intracranial aneurysm: A computational study

The purpose of this work is to evaluate the influence of coil diameters on the hemodynamics of intracranial aneurysms by using computational fluid dynamic (CFD) simulations. Three virtual treatments were performed varying packing density and coil surface area, by changing coil diameter. Hemodynamic parameters such as wall shear stress (WSS), average velocity and dye concentration at the aneurysm site were qualitative and quantitatively analyzed. Simulations with same coil area (958 mm2) but different packing density (21.0% and 41.6%) show similar hemodynamic alterations. Besides, the treated model with the highest coil area (1596 mm2) and intermediate packing density (33.2%) exhibited the highest reduction in the studied hemodynamic variables. Our findings show that coil area plays an important role in the resistance (pressure and frictional drags) of blood flow through the coils.

Comparison of two techniques of endovascular coil modeling in cerebral aneurysms using CFD

Coiling is the most common endovascular therapy for cerebral aneurysms. In this work, the influence of coil embolization on intra-aneurysmal hemodynamics was studied using two techniques for modeling coils. The first technique represented each coil explicitly and the second one approximated the coil structure with a porous medium. CFD simulations of pre- and post-treatment conditions were compared for one anatomically realistic cerebral aneurysm model. We observed a larger decrease in time- and space-averaged velocity in the aneurysm with the explicit model (92.3%) than with the porous medium model (71.4%). The difference between the two techniques was also demonstrated using virtual contrast injection. Whereas with the explicit model there was a large decrease in the amount of contrast entering the aneurysm and an increase in washout time, these phenomena were not observed with the porous medium model.

Statistical deformable models for cardiac Segmentation and Functional Analysis In Gated-Spect Studies

This chapter describes the use of statistical deformable models for cardiac segmentation and functional analysis in Gated Single Positron Emission Computer Tomography (SPECT) perfusion studies. By means of a statistical deformable model, automatic delineations of the endoand epicardial boundaries of the left ventricle (LV) are obtained, in all temporal phases and image slices of the dynamic study. A priori spatio-temporal shape knowledge is captured from a training set of high-resolution manual delineations made on cine Magnetic Resonance (MR) studies. From the fitted shape, a truly 3D representation of the left ventricle, a series of functional parameters can be assessed, including LV volume–time curves, ejection fraction, and surface maps of myocardial perfusion, wall motion, thickness, and thickening. We present encouraging results of its application on a patient database that includes rest/rest studies with common cardiac pathologies, suggesting that statistical deformable models may serve as a robust and accurate technique for routine use.