Today, the majority of coronary stents are balloon-expandable and are deployed using a balloon-tipped catheter. To improve deliverability, the membrane of the angioplasty balloon is typically folded about the catheter in a pleated configuration. As such, the deployment of the angioplasty balloon is governed by the material properties of the balloon membrane, its folded configuration and its attachment to the catheter. Despite this observation, however, an optimum strategy for modelling the configuration of the angioplasty balloon in finite element studies of coronary stent deployment has not been identified, and idealised models of the angioplasty balloon are commonly employed in the literature. These idealised models often neglect complex geometrical features, such as the folded configuration of the balloon membrane and its attachment to the catheter, which may have a significant influence on the deployment of a stent. In this study, three increasingly sophisticated models of a typical semi-compliant angioplasty balloon were employed to determine the influence of angioplasty balloon configuration on the deployment of a stent. The results of this study indicate that angioplasty balloon configuration has a significant influence on both the transient behaviour of the stent and its impact on the mechanical environment of the coronary artery. Copyright © 2013 John Wiley & Sons, Ltd.

To date, an optimum strategy for modelling the configuration of the angioplasty balloon in finite element studies of balloon-expandable coronary stent deployment has not been identified. In this study, three increasingly sophisticated models of a typical semi-compliant angioplasty balloon were employed to determine the influence of angioplasty balloon configuration on the deployment of a stent. The results of this study indicate that angioplasty balloon configuration has a significant influence on both the transient behaviour of the stent and its impact on the mechanical environment of the coronary artery.

A multiphysics simulation approach is developed for predicting cardiac flows as well as for conducting virtual echocardiography (ECHO) and phonocardiography (PC) of those flows. Intraventricular blood flow in pathological heart conditions is simulated by solving the three-dimensional incompressible Navier–Stokes equations with an immersed boundary method, and using this computational hemodynamic data, echocardiographic and phonocardiographic signals are synthesized by separate simulations that model the physics of ultrasound wave scattering and flow-induced sound, respectively. For virtual ECHO, a Doppler ultrasound image is reproduced through Lagrangian particle tracking of blood cell particles and application of sound wave scattering theory. For virtual PC, the generation and propagation of blood flow-induced sounds (‘hemoacoustics’) is directly simulated by a computational acoustics model. The virtual ECHO is applied to reproduce a color M-mode Doppler image for the left ventricle as well as continuous Doppler image for the outflow tract of the left ventricle, which can be verified directly against clinically acquired data. The potential of the virtual PC approach for providing new insights between disease and heart sounds is demonstrated by applying it to modeling systolic murmurs caused by hypertrophic cardiomyopathy. Copyright © 2013 John Wiley & Sons, Ltd.

A multiphysics simulation approach is developed for predicting cardiac flows as well as for conducting virtual echocardiography and phonocardiography of those flows. Intraventricular blood flow in pathological heart conditions is simulated by solving the three-dimensional incompressible Navier–Stokes equations with an immersed boundary method, and using this computational hemodynamic data, echocardiographic and phonocardiographic signals are synthesized by separate simulations that model the physics of ultrasound wave scattering and flow-induced sound, respectively.

Tumor angiogenesis, the growth of new capillaries from preexisting ones promoted by the starvation and hypoxia of cancerous cell, creates complex biological patterns. These patterns are captured by a hybrid model that involves high-order partial differential equations coupled with mobile, agent-based components. The continuous equations of the model rely on the phase-field method to describe the intricate interfaces between the vasculature and the host tissue. The discrete equations are posed on a cellular scale and treat tip endothelial cells as mobile agents. Here, we put the model into a coherent mathematical and algorithmic framework and introduce a numerical method based on isogeometric analysis that couples the discrete and continuous descriptions of the theory. Using our algorithms, we perform numerical simulations that show the development of the vasculature around a tumor. The new method permitted us to perform a parametric study of the model. Furthermore, we investigate different initial configurations to study the growth of the new capillaries. The simulations illustrate the accuracy and efficiency of our numerical method and provide insight into the dynamics of the governing equations as well as into the underlying physical phenomenon. Copyright © 2013 John Wiley & Sons, Ltd.

In this paper, we develop a numerical method for a continuum-discrete, tumor-induced angiogenesis model. The formulation of the continuous description of the model involves high-order partial differential equations, which are solved by means of isogeometric analysis. The method couples the discrete mobile cells with the continuous description of the capillary network and allows us to perform several simulations that provide insight into the governing equations and the biological phenomenon.

This work aims at identifying and quantifying uncertainties from various sources in human cardiovascular system based on stochastic simulation of a one-dimensional arterial network. A general analysis of different uncertainties and probability characterization with log-normal distribution of these uncertainties is introduced. Deriving from a deterministic one-dimensional fluid–structure interaction model, we establish the stochastic model as a coupled hyperbolic system incorporated with parametric uncertainties to describe the blood flow and pressure wave propagation in the arterial network. By applying a stochastic collocation method with sparse grid technique, we study systemically the statistics and sensitivity of the solution with respect to many different uncertainties in a relatively complete arterial network with potential physiological and pathological implications for the first time. Copyright © 2013 John Wiley & Sons, Ltd.

This work aims at identifying and quantifying uncertainties from various sources in human cardiovascular system. We establish the stochastic model as a coupled hyperbolic system incorporated with parametric uncertainties to describe the blood flow and pressure wave propagation in a relatively complete arterial network. By applying a stochastic collocation method with sparse grid technique, we carry out statistical and sensitivity analysis for the stochastic solution with potential physiological and pathological implications for the first time.

Although computational modeling of the prospective electrical activity in the cardiac tissue is well established and robust, the retrospective extrapolation of this activity has not been explored to date. Here, we establish an algorithm for the backward-in-time extrapolation of electrical activity from measurements taken in the present. Using minimal human cardiac kinetic models and a modified Newton–Raphson algorithm, we demonstrate the feasibility of past activity reconstruction in a single cell and in a linear strand. In a single cell, reconstruction of state variables' shape, the action potential morphology, and the time of stimulation was successful for up to 1300 ms poststimulation and for data with signal-to-noise ratio levels higher than 40 dB. For linear strands, the action potential morphology was reconstructed for 500 ms poststimulation, and the reconstructed conduction velocity remained unaffected for signal-to-noise ratio levels higher than 50 dB. Moreover, tissue restitution properties due to various pacing rates were successfully reconstructed by the backward-in-time algorithm. These preliminary results demonstrate that past cardiac activity may be reconstructed from measurements in the present. We envision that this methodology could be implemented in future clinical applications, for example to trace the location and timing of ectopic foci during ablation procedures. Copyright © 2013 John Wiley & Sons, Ltd.

Although computational modeling of the prospective electrical activity in the cardiac tissue is well established, the retrospective extrapolation of this activity has not been explored to date. Using minimal human cardiac kinetic models and a Newton–Raphson algorithm, we demonstrate the feasibility of past activity reconstruction in a single cell and a linear strand. We envision that this methodology could be implemented in future clinical applications, for example to trace the location and time foci during ablation procedures.

Myocardial infarction therapies involving biomaterial injections have shown benefits in inhibiting progression towards heart failure. However, the underlying mechanisms remain unclear. A finite element model of the human left ventricle was developed from magnetic resonance images. An anteroapical infarct was represented at acute (AI) and fibrotic (FI) stage. Hydrogel injections in the infarct region were modelled with layered (L) and bulk (B) distribution. In the FI, injectates reduced end-systolic myofibre stresses from 291.6% to 117.6% (FI-L) and 115.3% (FI-B) of the healthy value, whereas all AI models exhibited sub-healthy stress levels (AI: 90.9%, AI-L: 20.9%, AI-B: 30.5%). Reduction in end-diastolic infarct stress were less pronounced for both FI (FI: 294.1%, FI-L: 176.5%, FI-B: 188.2%) and AI (AI: 94.1%, AI-L: 35.3%, AI-B: 41.2%). In the border zone, injectates reduced end-systolic fibre stress by 8–10% and strain from positive (AI) and zero (FI) to negative. Layered and bulk injectates increased ejection fraction by 7.4% and 8.4% in AI and 14.1% and 13.7% in FI. The layered injectate had a greater impact on infarct stress and strain at acute stage, whereas the bulk injectate exhibited greater benefits at FI stage. These findings were confirmed by our previous in vivo results. Copyright © 2013 John Wiley & Sons, Ltd.

Layered and bulk therapeutic intra-myocardial injectates were studied in acute and fibrotic cardiac infarcts utilising a human patient-specific left ventricular geometry. Compared to the healthy case, injectates reduced the end-systolic infarct myofibre stress from 290% to approximately 115% in the fibrotic case and from 90% to between 20% and 30% in the acute case. The layered injectate was more beneficial in the acute infarct whereas the bulk injectate was superior in the fibrotic infarct agreeing with previous in vivo findings.

There have been several research studies on efficient methods for analysis and classification of electromyography (EMG) signals and adoption of wavelet functions, which is a promising approach for determining the spectral distribution of the signal. This study compares distinct time-frequency analysis methods for investigating the EMG activity of the thigh and calf muscles during gait among non-diabetic subjects and diabetic neuropathic patients. It also attempts to verify, by adaptive optimal kernel and discrete wavelet transform, whether there are EMG alterations related to diabetic neuropathy in the lower limb muscles during gait. The results show that diabetics might not keep up with the mechanical demands of walking by changing muscle fibre recruitment strategies, as seen in the control group. Moreover, principal components analysis indicates more alterations in diabetic motor strategies, and we identify that diabetic subjects need other strategies with different muscle energy production and frequencies to carry out their daily activities. Copyright © 2013 John Wiley & Sons, Ltd.

This study compares distinct time-frequency analysis methods for investigating the electromyographic activity of the thigh and calf muscles during gait among non-diabetic subjects and diabetic neuropathic patients. It also attempts to verify, by adaptive optimal kernel and discrete wavelet transform, whether there are electromyographic alterations related to diabetic neuropathy in the lower limb muscles during gait. The results show that diabetics might not keep up with the mechanical demands of walking by changing muscle fibre recruitment strategies, as seen in the control group.

We present a fully automatic procedure for the mesh generation of tubular geometries such as blood vessels or airways. The procedure is implemented in the open-source Gmsh software and relies on a centerline description of the input geometry. The presented method can generate different type of meshes: isotropic tetrahedral meshes, anisotropic tetrahedral meshes, and mixed hexahedral/tetrahedral meshes. Additionally, a multiple layered arterial wall can be generated with a variable thickness. All the generated meshes rely on a mesh size field and a mesh metric that is based on centerline descriptions, namely the distance to the centerlines and a local reference system based on the tangent and the normal directions to the centerlines. Different examples show that the proposed method is very efficient and robust and leads to high quality computational meshes. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a new automatic meshing algorithm for the generation of computational meshes from a segmented tubular geometry. The proposed methodology is based on different centerline-based descriptors and operators. Different types of computational meshes can be generated with this method: isotropic tetrahedral meshes, anisotropic tetrahedral meshes, or mixed hexahedral/tetrahedral meshes as well as boundary layer meshes for the lumen wall. The mesh size field is a function of the centerline-based descriptor.

Aiming reliable detection and localization of cerebral blood flow and emboli, embolic signals were added to simulated middle cerebral artery Doppler signals and analysed. Short-time Fourier transform (STFT) and continuous wavelet transform (CWT) were used in the evaluation. The following parameters were used in this study: the powers of the embolic signals added were 5, 6, 6.5, 7, 7.5, 8 and 9 dB; the mother wavelets for CWT analysis were Morlet, Mexican hat, Meyer, Gaussian (order 4) and Daubechies (orders 4 and 8); and the thresholds for detection (equated in terms of false positive, false negative and sensitivity) were 2 and 3.5 dB for the CWT and STFT, respectively. The results indicate that although the STFT allows accurately detecting emboli, better time localization can be achieved with the CWT. Among the CWT, the current best overall results were obtained with Mexican Hat mother wavelet, with optimal results for sensitivity (100% detection rate) for nearly all emboli power values studied.Copyright © 2013 John Wiley & Sons, Ltd.

Short-time Fourier transform (STFT) and different configurations of continuous wavelet transform (CWT) were tested for detection and localization embolic events. Simulated Doppler ultrasound blood flow signals of middle cerebral artery added with embolic signals and four different locations of emboli in the cardiac cycle were tested. Compared with mother wavelets for CWT analysis were Morlet, Mexican hat, Meyer, Gaussian (order 4) and Daubechies (orders 4 and 8), and the thresholds for detection (equated in terms of false positive, false negative and sensitivity) were 2 and 3.5 dB for the CWT and STFT, respectively. The results indicate that although the STFT allows accurately detecting emboli, better time localization can be achieved with the CWT. Among the CWT, the current best overall results were obtained with Mexican hat mother wavelet, with optimal results for sensitivity (100% detection rate) for nearly all emboli power values studied.

Support vector machines (SVM) are machine learning techniques that have been used for segmentation and classification of medical images, including segmentation of white matter hyper-intensities (WMH). Current approaches using SVM for WMH segmentation extract features from the brain and classify these followed by complex post-processing steps to remove false positives. The method presented in this paper combines advanced pre-processing, tissue-based feature selection and SVM classification to obtain efficient and accurate WMH segmentation. Features from 125 patients, generated from up to four MR modalities [T1-w, T2-w, proton-density and fluid attenuated inversion recovery(FLAIR)], differing neighbourhood sizes and the use of multi-scale features were compared. We found that although using all four modalities gave the best overall classification (average Dice scores of 0.54  ±  0.12, 0.72  ±  0.06 and 0.82  ±  0.06 respectively for small, moderate and severe lesion loads); this was not significantly different (p = 0.50) from using just T1-w and FLAIR sequences (Dice scores of 0.52  ±  0.13, 0.71  ±  0.08 and 0.81  ±  0.07). Furthermore, there was a negligible difference between using 5 × 5 × 5 and 3 × 3 × 3 features (p = 0.93). Finally, we show that careful consideration of features and pre-processing techniques not only saves storage space and computation time but also leads to more efficient classification, which outperforms the one based on all features with post-processing. Copyright © 2013 John Wiley & Sons, Ltd.

This paper presents a white matter hyper-intensity segmentation method that combines advanced pre-processing, tissue-based feature selection and supports vector machines classification. The proposed pipeline has been validated on a database of 125 patients with four magnetic resonance image modalities. The classification performance has been evaluated with regard to the relative input of each modality, the feature type, the impact of feature selection, and compared with other supervised algorithms.

When a patient is examined by positron emission tomography (PET), radiotracer dose amount (activity) has to be determined. However, the rules for activity correction according to patients’ weight used nowadays do not correspond with practical experience. Very high image quality is achieved for slim patients, whereas noisy images are produced for obese patients. There is opportunity to modify the correction rule with the aim to equalize image quality within the broad spectrum of patients and to diminish radiation risk to slim patients, with special importance for children.

We have built a model of a particular PET scanner and approximated human trunk, which is our region of interest, by a cylindrical model with segments of liver, outer adipose tissue, and the rest. We have performed Monte Carlo simulations of PET imaging using the GATE simulation package. Under reasonably simplifying assumptions and for special parameters, we have developed curves that recommend amount of injected activity based on body parameters to give PET images of constant quality, the quality being expressed in terms of noise equivalent counts. The dependence qualitatively differs from the rules used in clinical practice nowadays, and the results indicate potential for improvement.Copyright © 2012 John Wiley & Sons, Ltd.

To achieve constant quality of positron emission tomography images for different patients, we have derived curves recommending the amount of injected activity based on body parameters. These curves show rather convex tendency on the contrary to today's linear or sublinear standard recommendation. This indicates there is opportunity to optimize the current recommendation with the aim to standardize the diagnostic process.

The current study proposes an automatic method for the segmentation and tracking of red blood cells flowing through a 100- μm glass capillary. The original images were obtained by means of a confocal system and then processed in MATLAB using the Image Processing Toolbox. The measurements obtained with the proposed automatic method were compared with the results determined by a manual tracking method. The comparison was performed by using both linear regressions and Bland–Altman analysis. The results have shown a good agreement between the two methods. Therefore, the proposed automatic method is a powerful way to provide rapid and accurate measurements for in vitro blood experiments in microchannels. Copyright © 2012 John Wiley & Sons, Ltd.

The current study proposes an automatic method for segmentation and tracking of red blood cells flowing through a 100- μm glass capillary. The original images were obtained by means of a confocal system and then processed in MATLAB using the Image Processing Toolbox. The measurements obtained with the proposed automatic method were compared with the results determined by a manual tracking method. The comparison was performed by using linear regressions and Bland-Altman analysis. The results showed a good agreement between the two methods.

Ultrasonic-measurement-integrated (UMI) simulation of blood flow is used to analyze the velocity and pressure fields by applying feedback signals of artificial body forces based on differences of Doppler velocities between ultrasonic measurement and numerical simulation. Previous studies have revealed that UMI simulation accurately reproduces the velocity field of a target blood flow, but that the reproducibility of the pressure field is not necessarily satisfactory. In the present study, the reproduction of the pressure field by UMI simulation was investigated. The effect of feedback on the pressure field was first examined by theoretical analysis, and a pressure compensation method was devised. When the divergence of the feedback force vector was not zero, it influenced the pressure field in the UMI simulation while improving the computational accuracy of the velocity field. Hence, the correct pressure was estimated by adding pressure compensation to remove the deteriorating effect of the feedback. A numerical experiment was conducted dealing with the reproduction of a synthetic three-dimensional steady flow in a thoracic aneurysm to validate results of the theoretical analysis and the proposed pressure compensation method. The ability of the UMI simulation to reproduce the pressure field deteriorated with a large feedback gain. However, by properly compensating the effects of the feedback signals on the pressure, the error in the pressure field was reduced, exhibiting improvement of the computational accuracy. It is thus concluded that the UMI simulation with pressure compensation allows for the reproduction of both velocity and pressure fields of blood flow. Copyright © 2012 John Wiley & Sons, Ltd.

Existing ultrasonic-measurement-integrated simulation reproduces blood flow field by the effect of feedback signals proportional to differences between measured and computed Doppler velocities, but reproducibility of pressure field is not necessarily satisfactory. In this study, the effect of feedback on the pressure field was examined theoretically, and a pressure compensation method was devised. Validity of theoretical results was confirmed through a numerical experiment for a synthetic steady flow in a thoracic aneurysm, exhibiting improvement in reproducibility of the pressure field.

We propose a new 3D biphasic constitutive model designed to incorporate structural data on the sample/patient-specific collagen fiber network . The finite strain model focuses on the load-bearing morphology, that is, an incompressible, poroelastic solid matrix, reinforced by an inhomogeneous, dispersed fiber fabric, saturated with an incompressible fluid at constant electrolytic conditions residing in strain-dependent pores of the collagen–proteoglycan solid matrix. In addition, the fiber network of the solid influences the fluid permeability and an intrafibrillar portion that cannot be ‘squeezed out’ from the tissue. We implement the model into a finite element code. To demonstrate the utility of our proposed modeling approach, we test two hypotheses by simulating an indentation experiment for a human tissue sample. The simulations use ultra-high field diffusion tensor magnetic resonance imaging that was performed on the tissue sample. We test the following hypotheses: (i) the through-thickness structural arrangement of the collagen fiber network adjusts fluid permeation to maintain fluid pressure (Biomech. Model. Mechanobiol. 7:367–378, 2008); and (ii) the inhomogeneity of mechanical properties through the cartilage thickness acts to maintain fluid pressure at the articular surface (J. Biomech. Eng. 125:569–577, 2003). For the tissue sample investigated, both through-thickness inhomogeneities of the collagen fiber distribution and of the material properties serve to influence the interstitial fluid pressure distribution and maintain fluid pressure underneath the indenter at the cartilage surface. Tissue inhomogeneity appears to have a larger effect on fluid pressure retention in this tissue sample and on the advantageous pressure distribution. Copyright © 2012 John Wiley & Sons, Ltd.

We propose a new 3D biphasic, finite strain constitutive model and show representative finite element results for an indentation experiment with an impermeable, plane-ended cylinder of diameter 1 mm compressing a cartilage sample to 1% global strain in 150 s-column 1: normal Green-Lagrange strain in the axial (indentation) direction; column 2: interstitial fluid pressure; column 3: von Mises stress; row 1: constitutive model including patient-specific collagen fiber network and inhomogeneous material properties; row 2: model without collagen fiber network and homogeneous material properties. For the tissue sample investigated, through-thickness inhomogeneity of both the collagen fiber network and the material properties maintains interstitial fluid pressure underneath the indenter at the cartilage surface and reduces tissue stress.

Finite element analysis is nowadays a well-assessed technique to investigate the impact of stenting on vessel wall and, given the rapid progression of both medical imaging techniques and computational methods, the challenge of using the simulation of carotid artery stenting as procedure planning tool to support the clinical practice can be approached. Within this context, the present study investigates the impact of carotid stent apposition on carotid artery anatomy by means of patient-specific finite element analysis. In particular, we focus on the influence of the vessel constitutive model on the prediction of carotid artery wall tensional state of lumen gain and of vessel straightening. For this purpose, we consider, for a given stent design and CA anatomy, two constitutive models for the CA wall, that is, a hyperelastic isotropic versus a fiber-reinforced hyperelastic anisotropic model. Despite both models producing similar patterns with respect to stress distribution, the anisotropic model predicts a higher vessel straightening and a more evident discontinuity of the lumen area near the stent ends as observed in the clinical practice. Although still affected by several simplifications, the present study can be considered as further step toward a realistic simulation of carotid artery stenting.Copyright © 2012 John Wiley & Sons, Ltd.

The study investigates the impact of carotid stent apposition on vascular anatomy by patient-specific finite element analysis. In particular, we consider, for a given stent design and artery model, two constitutive models for the vessel wall, that is, a hyperelastic isotropic versus a fiber-reinforced hyperelastic anisotropic model. Despite both models predicting a similar stress distribution, the anisotropic model predicts a higher vessel straightening and a more evident discontinuity of the lumen near the stent ends, as observed in the clinical practice.

Craniosynostosis is a skull malformation because of premature fusing of one or more cranial sutures. The most common types of craniosynostosis are scaphocephaly (with the sagittal suture fused) and trigonocephaly (with the metopic suture fused). In this paper we describe and discuss how finite element analysis and three-dimensional modeling can be used for preoperative planning of the correction of craniosynostosis and for the postoperative evaluation of the treatment results. We used the engineering software MIMICS MATERIALISE to obtain three-dimensional geometry from computed tomography scans, and applied finite element method for the sake of biomechanical analysis. These simulations help to improve the surgical treatment, making it more accurate, safer, and faster. Copyright © 2012 John Wiley & Sons, Ltd.

This paper presents the engineering preoperative planning of skull shape correction. The MIMICS software was used to calculate the three-dimensional geometrical model and to plan the incisions, and the ANSYS software was used to perform the finite element analysis and to find the optimal variant of correction. This procedure of planning can be applied in both classic and endoscopic skull surgeries in diagnosed craniosynostosis (trigonocephaly, scaphocephaly, plagiocephaly, or brachycephaly).

Accurate and efficient segmentation of the whole brain in magnetic resonance (MR) images is a key task in many neuroscience and medical studies either because the whole brain is the final anatomical structure of interest or because the automatic extraction facilitates further analysis. The problem of segmenting brain MRI images has been extensively addressed by many researchers. Despite the relevant achievements obtained, automated segmentation of brain MRI imagery is still a challenging problem whose solution has to cope with critical aspects such as anatomical variability and pathological deformation. In the present paper, we describe and experimentally evaluate a method for segmenting brain from MRI images basing on two-dimensional graph searching principles for border detection. The segmentation of the whole brain over the entire volume is accomplished slice by slice, automatically detecting frames including eyes. The method is fully automatic and easily reproducible by computing the internal main parameters directly from the image data. The segmentation procedure is conceived as a tool of general applicability, although design requirements are especially commensurate with the accuracy required in clinical tasks such as surgical planning and post-surgical assessment. Several experiments were performed to assess the performance of the algorithm on a varied set of MRI images obtaining good results in terms of accuracy and stability. Copyright © 2012 John Wiley & Sons, Ltd.

In the present paper, we describe and experimentally evaluate a method for segmenting brain from MRI images basing on two-dimensional graph searching principles for border detection. The method is fully automatic and easily reproducible by computing the internal main parameters directly from the image data. The segmentation procedure is conceived as a tool of general applicability, although design requirements are especially commensurate with the accuracy required in clinical tasks such as surgical planning and post-surgical assessment.

Non-Cartesian acquisition strategies are widely used in MRI to dramatically reduce the acquisition time while at the same time preserving the image quality. Among non-Cartesian reconstruction methods, the least squares non-uniform fast Fourier transform (LS_NUFFT) is a gridding method based on a local data interpolation kernel that minimizes the worst-case approximation error. The interpolator is chosen using a pseudoinverse matrix. As the size of the interpolation kernel increases, the inversion problem may become ill-conditioned. Regularization methods can be adopted to solve this issue. In this study, we compared three regularization methods applied to LS_NUFFT. We used truncated singular value decomposition (TSVD), Tikhonov regularization and L₁-regularization. Reconstruction performance was evaluated using the direct summation method as reference on both simulated and experimental data. We also evaluated the processing time required to calculate the interpolator. First, we defined the value of the interpolator size after which regularization is needed. Above this value, TSVD obtained the best reconstruction. However, for large interpolator size, the processing time becomes an important constraint, so an appropriate compromise between processing time and reconstruction quality should be adopted. Copyright © 2013 John Wiley & Sons, Ltd.

Least squares non-uniform fast Fourier transform (LS_NUFFT) is a gridding method used to reconstruct data from non-Cartesian sampling in the Fourier domain. In this work, we compared three regularization methods applied to LS_NUFFT.We found that truncated singular value decomposition obtains the smallest reconstruction error, whereas Tikhonov regularization is calculated with the smallest amount of processing time.

This paper is motivated by a recent experimental research by Levitan and colleagues (Wong MY, Zhou C, Shakiryanova D, Lloyd TE, Deitcher DL, Levitan ES. Neuropeptide delivery to synapses by long-range vesicle circulation and sporadic capture. Cell 2012; 148(5): 1029–1038), which discovered and explained the circulation of dense core vesicles (DCVs) between the neuron soma and nerve terminals. We developed a compartmental mechanistic model to simulate this circulation. The model includes five compartments, the axonal compartment and four en passant boutons, which are small axonal varicosities that are present in many axon terminals. By postulating expressions for DCV fluxes between the compartments and for the rates of DCV capture in the boutons and utilizing conservation of DCVs, ODEs modeling concentrations in each of the compartments were developed. The equations were then solved numerically. The obtained results provide insight into how DCV circulation develops, what the circulation is at steady state, and how it may be affected by defects in retrograde transport. Copyright © 2013 John Wiley & Sons, Ltd.

A compartmental mechanistic model that simulates the circulation of neuropeptides transported inside dense core vesicles between the neuron soma and nerve terminals is developed. The model simulates how concentrations in five compartments, the axonal compartment and four en passant boutons, evolve with time. The obtained results provide insight into how the circulation of dense core vesicles develops, what the circulation is at steady state, and how it may be affected by defects in retrograde transport.

We introduce here a novel approach for the numerical simulation of nonlinear, hyperelastic soft tissues at kilohertz feedback rates necessary for haptic rendering. This approach is based upon the use of proper generalized decomposition techniques, a generalization of PODs. Proper generalized decomposition techniques can be considered as a means of a priori model order reduction and provides a physics-based meta-model without the need for prior computer experiments. The suggested strategy is thus composed of an offline phase, in which a general meta-model is computed, and an online evaluation phase in which the results are obtained at real time. Results are provided that show the potential of the proposed technique, together with some benchmark test that shows the accuracy of the method. Copyright © 2013 John Wiley & Sons, Ltd.

We introduce here a novel approach for the numerical simulation of nonlinear, hyperelastic soft tissues at kilohertz feedback rates necessary for haptic rendering.

In this work, a computational procedure is proposed to vascularize anatomical regions supplied by many inflow sites. The proposed methodology creates a partition of the territory to be vascularized into nonoverlapping subdomains that are independently supplied by the so-called perforator arteries (inflow sites). Then, in each subdomain, the constrained constructive optimization method is used to generate a network of vessels. The identification of subdomains in a certain vascular territory perfused by many perforator arteries turns out to be a fundamental problem towards understanding the morphological conformation of peripheral beds in the cardiovascular system. The methodology is assessed through two academic examples showing the main structural features of the so-defined vascular territory partition and the corresponding arterial networks. In addition, the vascularization of a three-dimensional sheet-like tissue is presented with potential application in flap planning and design. Copyright © 2013 John Wiley & Sons, Ltd.

Vascular territories of the upper limb. A specific territory is chosen to perform concurrent arterial vascularization. Five perforator arteries supply blood to such territory. Vascular territory partitioning has been performed and vascular networks have been generated in each subdomain using the proposed methodology.

Finite element (FE) modeling based on a patient's hip dual energy X-ray absorptiometry (DXA) image is a promising tool for more accurately assessing hip fracture risk, as it is able to comprehensively consider effects from all the mechanical parameters affecting hip fracture. However, a number of factors influence the precision (also known as repeatability or reproducibility) of a DXA-based FE procedure, for example, subject positioning in DXA scanning. As a procedure is required to have adequately high precision in clinical application, we investigated the effects of the involved factors on the precision of a DXA-based patient-specific FE procedure developed by the authors, to provide insight into how the precision of the procedure can be improved so that it can meet the clinical standards. Fracture risk indices corresponding to initial and repeat DXA scans acquired in 30 typical clinical subjects were computed and compared to assess short term repeatability of the procedure. It was found that inconsistent positioning followed by manual segmentation of the projected femur contour induced significant variability in the predicted fracture risk indices. This research suggests that, to apply the DXA-based FE procedure in clinical assessment, it will be necessary to pay more strict attention to subject positioning in DXA scanning. Copyright © 2013 John Wiley & Sons, Ltd.

Key Findings:

• Precision of DXA-based finite element modeling procedure for assessing hip fracture risk was studied for the first time.
• Effects of factors affecting the precision were successfully isolated, which provides insight into the question why a DXA-based finite element procedure sounds more reasonable for assessing hip fracture risk than areal bone mineral density alone but has not established its role in clinical applications.
• The research outcome provides a clear way for improving the precision of a DXA-based finite element procedure so that it will meet clinical requirements.

The present theoretical study examines the ability to estimate cardiac stroke volume (CSV) in patients with implanted cardiac pacemaker using parametric electrical impedance tomography (pEIT) in a 2D computerized model of the thorax. CSV is a direct indicator of the cardiac pumping efficiency. The commonly used methods for measuring CSV require the invasive procedure of right heart catheterization or use expensive imaging techniques (i.e., MRI). Hence, experience with these techniques for diagnosis and monitoring has been limited to hospitalized patients. In the present study, pEIT scheme was applied in a computerized 2D model of the human thorax with implanted cardiac device to determine the left ventricular (LV) volume at different cardiac cycle phases. The LV was simulated as a prolate ellipse with its axes’ lengths as the reconstruction parameters while all other geometries and conductivity values remained constant. An optimization was carried out in order to ensure that the ellipse is the appropriate model for the LV at each cardiac cycle phase. LV volumes calculated by both the pEIT algorithm and the ellipsoid model are consistent. A high correlation (ρ = 0.99) between the true and reconstructed volumes was found. The SV calculation error was ∼1%. The results suggest that the LV volume can be estimated using the pEIT method in a 2D computerized model, and that the method has the potential to be used for monitoring patients with implanted cardiac pacemaker. Copyright © 2013 John Wiley & Sons, Ltd.

The ability to estimate cardiac stroke volume in patients with implanted cardiac pacemaker was examined using parametric electrical impedance tomography in a 2D computerized model of the thorax. The left ventricle was simulated as a prolate ellipse with its axes' lengths as the reconstruction parameters. The results suggest that the method has the potential to be used for monitoring patients with implanted cardiac pacemaker.