Comprehensive model for the simulation of left ventricle mechanics

A comprehensive numerical model, based on the finite-element method, for the continuous simulation of the complete cardiac cycle is presented. The model uses real, measuredin vivo, three-dimensional geometry of the ventricle, and accounts for the anisotropy of the ventricular wall, the large deformations it undergoes during the cardiac cycle, the material nonlinearity of the myocardium and its mechanical activation. The simulation process is carried out incrementally while adjusting the mechanical activation for each increment so as to produce the same change in cavity volume as that measured experimentally. A detailed analysis of a complete cycle of the canine heart is presented in Part 2 of the paper.     

Comprehensive model for the simulation of left ventricle mechanics

The model of the left ventricle (LV) mechanics presented in Part 1 of the paper is used to simulate an entire cardiac cycle. Time-sequential canine heart data obtained by dynamic computerised tomography serve to initiate the simulation as well as to provide real data for evaluation of its results. The numerical predictions of the dynamic geometric changes are in good agreement with the tomographically determined changes that the ventricle undergoes throughout the cycle. Moreover, the simulation allows the evaluation of the time-varying stress and strain distributions in the ventricular wall and the active forces prevailing in the myocardial fibres. The simulated shape of the LV during the entire cardiac cycle reasonably compares with the experimental data. Furthermore, the twist angle of the ventricle as well as its maximal mean fibre strain are found to be in good agreement with physiological findings. Finally, a parametric study gives the relative influence of the anisotropy of the myocardium, its geometric and material nonlinearities and the mechanical activation on the mechanics of the left ventricle.     

Mechanical model for the simulation of ischaemia and infarction of the left ventricle

Several ischaemic states, including immediate infarction, occurring in a canine left ventricle at the onset of the ejection phase and affecting regions of varying sizes are simulated, employing a recently developed comprehensive finite-element model. The analysis assumes an instantaneous partial or complete loss of contractility in the damaged region, whereas the passive mechanical properties of the tissue are yet unaltered. The results indicate a progressive deterioration of the cardiac performance, as well as considerable geometrical changes in the kinematics of the whole ventricle, directly related to both ischaemia level and the ischaemic region size. Owing to the reduction in the stroke volume, the simulation predicts a degradation of up to 33 per cent in the ejection fraction for an infarct affecting 43 per cent of the ventricular wall volume. A quantitative relationship between the ejection fraction, the level of ischaemia and the size of the ischaemic zone is derived and presented.     

Dynamic stiffness and implications for assisting the operation of the left ventricle

The distribution of bending strain and stiffness in the wallof the left ventricle (LV) is relevant to the augmentation ofits function by a modified skeletal-muscle wrap in the new surgicalprocedure of cardiomyoplasty. A novel approach to ventricularmechanics is presented which blends some finite-element resultsin engineering with new data available on ventricular geometry.Two simplified axisymmetric strip-element models of the LV areused to illustrate aspects of myocardial stiffness in the bending-strain-energydistribution and the effect on wrap synchronization of a changein cross-fibre stiffness when the heart has nonuniform or ectopicbeats. The nonlinear and time-dependent nature of both dampingand wall stiffness is derived from differential equations governingthe dynamic paths from systole to diastole of finite wall elementsaround the periphery of an oblique LV slice using magnetic resonanceimaging (MRI) data. This leads to a geometric method for determiningthese parameters. Results for time-dependent stiffnesses ofelements in their trajectories are presented for a normal heart.     

A model of the mechanics of the left ventricle

The relation between cardiac muscle mechanics and left ventricular (LV) pump function is simulated by a mathematical model. In the following article special attention is paid to the relation between LV pressure and LV volume on the one hand and the transmural distribution of sarcomere length and fiber stress on the other. The LV is simulated by a thick-walled cylinder composed of 8 concentric shells. The myocardial material is assumed to be anisotropic. The orientation and sequential activation of the muscle fibers across the LV wall are considered per shell. Twisting of the base with respect to the apex around the axis of the LV is simulated by rotation of the upper cross-sectional surface of the cylinder with respect to the lower one aroud the axis of the cylinder. The model reveals that twisting of the LV is an important means to equalize transmural differences in sarcomere shortening and end-systolic fiber stress. When torsion is allowed, transmural differences in sarcomere shortening and end-systolic fiber stress are less than 18% and 16%, respectively. When torsion is prevented as in most of the models of LV-mechanics described in literature, these transmural differences increase up to 32% and 42%, respectively. Supported by the Foundation for Medical Research FUNGO, which is subsidized by the Netherlands organization for the Advancement of Pure Research.     

Performance evaluation of a pinhole SPECT system for myocardial perfusion imaging of mice

The increasing use of transgenic mice as models of human physiology and disease has motivated the development of dedicated in vivo imaging systems for anatomic and functional characterization of mice as an adjunct to or a replacement for established ex vivo techniques. We have developed a pinhole single photon emission computed tomography (SPECT) system for high resolution imaging of mice with cardiovascular imaging as the primary application. In this work, we characterize the system performance through phantom studies. The spatial resolution and sensitivity were measured from images of a line source and point source, respectively, and were reported for a range of object-to-pinhole distances and pinhole diameters. Tomographic images of a uniform cylindrical phantom, Defrise phantom, and grid phantom were used to characterize the image uniformity and spatial linearity. The uniform phantom image did not contain any ring or reconstruction artifacts, but blurring in the axial direction was evident in the Defrise phantom images. The grid phantom images demonstrated excellent spatial linearity. A novel phantom modeling perfusion of the left ventricle of a mouse was designed and built with perfusion defects of varying sizes to evaluate the system performance for myocardial perfusion imaging of mice. The defect volumes were measured from the pinhole SPECT images and correlated to the actual defect volumes calculated according to geometric formulas. Linear regression analysis produced a correlation coefficient of r = 0.995 (p < 0.001), demonstrating the feasibility for measurement of perfusion defect size in mice using pinhole SPECT. We have performed phantom studies to characterize the spatial resolution, sensitivity, image uniformity, and spatial linearity of the pinhole SPECT system. Measurement of the perfusion defect size is a valuable phenotypic assessment and will be useful for hypothesis testing in murine models of cardiovascular disease.     

Quantitative gated SPECT: the effect of reconstruction filter on calculated left ventricular ejection fractions and volumes

Gated SPECT (GSPECT) offers the possibility of obtaining additional functional information from perfusion studies, including calculation of left ventricular ejection fraction (LVEF). The calculation of LVEF relies upon the identification of the endocardial surface, which will be affected by the spatial resolution and statistical noise in the reconstructed images. The aim of this study was to compare LVEFs and ventricular volumes calculated from GSPECT using six reconstruction filters. GSPECT and radionuclide ventriculography (RNVG) were performed on 40 patients; filtered back projection was used to reconstruct the datasets with each filter. LVEFs and volumes were calculated using the Cedars-Sinai QGS package. The correlation coefficient between RNVG and GSPECT ranged from 0.81 to 0.86 with higher correlations for smoother filters. The narrowest prediction interval was 111 +/- 2%. There was a trend towards higher LVEF values with smoother filters, the ramp filter yielding LVEFs 2.55 +/- 3.10% (p < 0.001) lower than the Hann filter. There was an overall fall in ventricular volumes with smoother filters with a mean difference of 13.98 +/- 10.15 ml (p < 0.001) in EDV between the Butterworth-0.5 and Butterworth-0.3 filters. In conclusion, smoother reconstruction filters lead to lower volumes and higher ejection fractions with the QGS algorithm, with the Butterworth-0.4 filter giving the highest correlation with LVEFs from RNVG. Even if the optimal filter is chosen the uncertainty in the measured ejection fractions is still too great to be clinically acceptable.     

A new hybrid electro-numerical model of the left ventricle

The paper presents a new project of a hybrid numerical-physical model of the left ventricle. A physical part of the model can be based on electrical or hydraulic structures. Four variants of the model with numerical and physical heart valves have been designed to investigate an effect of a heart assistance connected in series and in parallel to the natural heart. The LabVIEW(TM) real time environment has been used in the model to increase its accuracy and reliability. A prototype of the hybrid electro-numerical model of the left ventricle has been tested in an open loop and closed loop configuration.     

Quantitative analysis of gated SPECT images using an efficient physical deformation model

In this paper, we present quantitative analysis of cardiac images using an efficient physical deformation model to evaluate ventricular function. By using this model we can accurately and efficiently compute ventricular volume, myocardial mass, endo- and epi-cardial wall motions and wall thickness over a full cardiac cycle. Patients with cardiac diseases were studied in our modeling and measurement framework using gated single-photon emission computed tomographic images. The results show that quantitative analysis using the model is very useful for the assessment of the extent and severity of myocardial ischemia or infarction. And it could be helpful to improve the decision-making process in the treatment of patients with cardiac diseases.     

A hybrid (numerical-physical) model of the left ventricle

Hydraulic models of the circulation are used to test mechanical devices and for training and research purposes; when compared to numerical models, however, they are not flexible enough and rather expensive. The solution proposed here is to merge the characteristics and the flexibility of numerical models with the functions of physical models. The result is a hybrid model with numerical and physical sections connected by an electro-hydraulic interface - which is to some extent the main problem since the numerical model can be easily changed or modified. The concept of hybrid model is applied to the representation of ventricular function by a variable elastance numerical model. This prototype is an open loop circuit and the physical section is built out of a reservoir (atrium) and a modified windkessel (arterial tree). The corresponding equations are solved numerically using the variables (atrial and arterial pressures) coming from the physical circuit. Ventricular output flow is the computed variable and is sent to a servo amplifier connected to a DC motor-gear pump system. The gear pump, behaving roughly as a flow source, is the interface to the physical circuit. Results obtained under different hemodynamic conditions demonstrate the behaviour of the ventricular model on the pressure-volume plane and the time course of output flow and arterial pressure.     

Perfusion of myocardial segments of the right ventricle: role of the left coronary artery in infarction of the right ventricle

In a series of 88 human hearts, from individuals aged between 24 h and 94 yr (x= 61.09 +/- 21.96), the coronary arterial distribution of the right ventricle was studied using a modified Selvester's system of segmentation. Postmortem angiographies and microdissection techniques were used. The analysis of the six segments of the right ventricle shows that the three anterior segments, basal, mesial, and--less frequently--apical, present a type of irrigation that is practically constant and is dual. The postero-basal and postero-mesial segments are irrigated almost exclusively by the right coronary artery. In the remaining segments the vascularization was of mixed type, although a considerable degree of exclusive arterial perfusion was observed in the antero-apical segment. The segmental analysis allows us to conclude that although arterial vascularization of the right ventricle depends fundamentally on the right coronary artery, the anterior interventricular artery irrigates more than 20% of the right ventricular myocardium. Results from segmental analysis are compared with data from clinical and necropsic studies. Copyright Wiley-Liss, Inc.     

犬急性心肌梗死模型的多层螺旋CT心肌灌注和冠状动脉造影

目的 :研究急性心肌缺血和梗死的多层螺旋CT(MSCT)心肌灌注特点 ,探索MSCT冠状动脉造影对冠状动脉阻断的显示情况。方法 :犬左冠状动脉前降支结扎前、后不同时期分别进行MSCT心肌灌注扫描 ,分析其特点 ,并与病理检查相对照 ;同时做MSCT冠状动脉造影观察冠状动脉阻断的情况。结果 :犬心肌正常灌注量为 ( 69.3±1 3 .9)ml·1 0 0g- 1·min- 1、达峰值时间 ( 1 2 .8± 2 .1 )sec。左冠状动脉前降支结扎 3 0min后MSCT心肌灌注表现为灌注量减低 ,时间密度曲线低平 ,延迟 1 0min扫描局部心肌密度无显著改变。结扎 4h后 ,局部心肌呈明显延迟增强。MSCT冠状动脉造影显示冠状动脉前降支中断。结论 :MSCT心肌灌注结合冠状动脉造影可以判断心肌缺血和梗死 ,同时显示冠状动脉的阻塞状况     

Dynamic computer-generated displays of data from biplane roentgen angiography for study of the left ventricle

Computer-generated three-dimensional stereo images of the epicardial and endocardial surfaces of the left ventricle were displayed from data obtained with a biplane orthogonal roentgen video system. The silhouette of the opacified left ventricle was digitized from a video angiograph. The data were used to calculate the dynamic geometry of the ventricle and parameters such as curvature and stress. Continuous tone hidden surface displays of the ventricle were inscribed with isostress contours. The system is capable of obtaining biplane silhouettes of the ventricle at a rate of 60 pictures/sec. Dynamic displays of the beating ventricle can be obtained by recording sequential frames on a video disc recorder via a scan converter. The resulting sequence of frames can be played back in forward or reverse slow-motion or stop-action for detailed study. This investigation was supported in part by Research Grants HL 4664, HL 3532, and FR-7 from the National Institutes of Health, U. S. Public Health Service, and AHA CI 10.     

Quantitative reconstruction for myocardial perfusion SPECT: an efficient approach by depth-dependent deconvolution and matrix rotation

An efficient reconstruction method for myocardial perfusion single-photon emission computed tomography (SPECT) has been developed which compensates simultaneously for attenuation, scatter, and resolution variation. The scattered photons in the primary-energy-window measurements are approximately removed by subtracting the weighted scatter-energy-window samples. The resolution variation is corrected by deconvolving the subtracted data with the detector-response kernel in frequency space using the depth-dependent frequency relation. The attenuated photons are compensated by recursively tracing the attenuation factors through the object-specific attenuation map. An experimental chest phantom with defects inside myocardium was used to test the method. The attenuation map of the phantom was reconstructed from transmission scans using a flat external source and a high-resolution parallel-hole collimator of a single-detector system. The detector-response kernel was approximated from measurements of a point source in air at several depths from the collimator surface. The emission data were acquired by the same detector setting. A computer simulation using similar protocols as in the experiment was performed. Both the simulation and experiment showed significant improvement in quantification with the proposed method, as compared to the conventional filtered-backprojection technique. The quantitative gain by the additional deconvolution was demonstrated. The computation time was less than 20 min on a HP/730 desktop computer for reconstruction of a 1282 x 64 array from 128 projections of 128 x 64 samples.     

An engineering analysis of myocardial fiber orientation in pig's left ventricle in systole

The concept of discrete layers and bundles of muscle as a basic structural arrangement in left ventricular myocardium was tested by measuring the helix angles at 1 mm intervals from endocardium to epicardium, using pig's heart in the contracted state. A fixed coordinate system was established which permitted measurement of corresponding sites in hearts of different dimensions. The helix angle was found to change from somewhat less than 90° endocardially to about ?90° epicardially in an almost linear clockwise sequence, like a Japanese fan opened up. Approximately the same pattern was observed in the interventricular septum and the anterior, left and posterior walls. Generally, there was no abrupt change between the helix angle of papillary fibers and that of adjacent wall myocardium. Where occasionally abrupt changes in fiber orientation were demonstrated, no intervening septum could be discerned. The deviant fibers seemed to co-exist as part of the same gross structure. The concept of a continuum more appropriately describes the basic structure of left ventricular myocardium. Lev and Simkins ('56) and Grant ('65) showed that there was no evidence of identifiable layers as defined by the presence of connective tissue septa. This study shows no evidence of identifiable layers as defined by (1) an abrupt change in fiber direction demarking the boundary of a layer and (2) a parallel fiber direction between such boundaries.     

Spatial energy balance within a structural model of the left ventricle

A model describing the local instantaneous energetic needs within the left ventricle (LV) myocardium is presented. The model, which combines the myocardial oxygen consumption (MVO₂) with the mechanical activity of the cardiac muscle, is based on the theory of cross bridge kinetics between the actin and myosin fibers within the sarcomere. The microscale relationship between the stress, stress development, strain rate and basal metabolism demand is incorporated into the LV model which describes the mechanical activities of different layers within the myocardium. The model shows a significant increase in the oxygen consumption in the endocardial layers as compared with the epicardial layers. Integrating the spatial and temporal oxygen consumption distribution within the myocardium yields the total myocardial oxygen consumption. The quantitative relationships between the heat rate, stress, contractility and external work and the MVO₂ are in agreement with known data. The model thus offers a tool to assess the local instantaneous as well as the time averaged overall energy consumption, over a wide range of loading conditions of the LV.     

Fluid-dynamics modelling of the human left ventricle with dynamic mesh for normal and myocardial infarction: preliminary study

Pulsating blood flow patterns in the left ventricular (LV) were computed for three normal subjects and three patients after myocardial infarction (MI). Cardiac magnetic resonance (MR) images were obtained, segmented and transformed into 25 frames of LV for a computational fluid dynamics (CFD) study. Multi-block structure meshes were generated for 25 frames and 75 intermediate grids. The complete LV cycle was modelled by using ANSYS-CFX 12. The flow patterns and pressure drops in the LV chamber of this study provided some useful information on intra-LV flow patterns with heart diseases.