Realistic CT simulation using the 4D XCAT phantom

The authors develop a unique CT simulation tool based on the 4D extended cardiac-torso (XCAT) phantom, a whole-body computer model of the human anatomy and physiology based on NURBS surfaces. Unlike current phantoms in CT based on simple mathematical primitives, the 4D XCAT provides an accurate representation of the complex human anatomy and has the advantage, due to its design, that its organ shapes can be changed to realistically model anatomical variations and patient motion. A disadvantage to the NURBS basis of the XCAT, however, is that the mathematical complexity of the surfaces makes the calculation of line integrals through the phantom difficult. They have to be calculated using iterative procedures; therefore, the calculation of CT projections is much slower than for simpler mathematical phantoms. To overcome this limitation, the authors used efficient ray tracing techniques from computer graphics, to develop a fast analytic projection algorithm to accurately calculate CT projections directly from the surface definition of the XCAT phantom given parameters defining the CT scanner and geometry. Using this tool, realistic high-resolution 3D and 4D projection images can be simulated and reconstructed from the XCAT within a reasonable amount of time. In comparison with other simulators with geometrically defined organs, the XCAT-based algorithm was found to be only three times slower in generating a projection data set of the same anatomical structures using a single 3.2 GHz processor. To overcome this decrease in speed would, therefore, only require running the projection algorithm in parallel over three processors. With the ever decreasing cost of computers and the rise of faster processors and multi-processor systems and clusters, this slowdown is basically inconsequential, especially given the vast improvement the XCAT offers in terms of realism and the ability to generate 3D and 4D data from anatomically diverse patients. As such, the authors conclude that the efficient XCAT-based CT simulator developed in this work will have applications in a broad range of CT imaging research.     

Large-Scale 3-D Geometric Reconstruction of the Porcine Coronary Arterial Vasculature Based on Detailed Anatomical Data

The temporal and spatial distribution of coronary blood flow, pressure, and volume are determined by the branching pattern and three-dimensional (3-D) geometry of the coronary vasculature, and by the mechanics of heart wall and vascular tone. Consequently, a realistic simulation of coronary blood flow requires, as a first step, an accurate representation of the coronary vasculature in a 3-D model of the beating heart. In the present study, a large-scale stochastic reconstruction of the asymmetric coronary arterial trees (right coronary artery, RCA; left anterior descending, LAD; and left circumflex, LCx) of the porcine heart has been carried out to set the stage for future hemodynamic analysis. The model spans the entire coronary arterial tree down to the capillary vessels. The 3-D tree structure was reconstructed initially in rectangular slab geometry by means of global geometrical optimization using parallel simulated annealing (SA) algorithm. The SA optimization was subject to constraints prescribed by previously measured morphometric features of the coronary arterial trees. Subsequently, the reconstructed trees were mapped onto a prolate spheroid geometry of the heart. The transformed geometry was determined through least squares minimization of the related changes in both segments lengths and their angular characteristics. Vessel diameters were assigned based on a novel representation of diameter asymmetry along bifurcations. The reconstructed RCA, LAD and LCx arterial trees show qualitative resemblance to native coronary networks, and their morphological statistics are consistent with the measured data. The present model constitutes the first most extensive reconstruction of the entire coronary arterial system which will serve as a geometric foundation for future studies of flow in an anatomically accurate 3-D coronary vascular model.     

On the design of the coronary arterial tree: a generalization of Murray's law

Murray's law has been generalized to provide morphometric relationships among various subtrees as well as between a feeding segment and the subtree it perfuses. The equivalent resistance of each subtree is empirically determined to be proportional to the cube of a subtree's cumulative arterial length (L) and inversely proportional to a subtree's arterial volume (V) raised to a power of approximately 2.6. This relationship, along with a minimization of a cost function, and a linearity assumption between flow and cumulative arterial length, provides a power law relationship between V and L. These results, in conjunction with conservation of energy, yield relationships between the diameter of a segment and the length of its distal subtree. The relationships were tested based on a complete set of anatomical data of the coronary arterial trees using two models. The first model, called the truncated tree model, is an actual reconstruction of the coronary arterial tree down to 500 microm in diameter. The second model, called the symmetric tree model, satisfies all mean anatomical data down to the capillary vessels. Our results show very good agreement between the theoretical formulation and the measured anatomical data, which may provide insight into the design of the coronary arterial tree. Furthermore, the established relationships between the various morphometric parameters of the truncated tree model may provide a basis for assessing the extent of diffuse coronary artery disease.     

Adaptation and applications of a realistic digital phantom based on patient lung tumor trajectories

Digital phantoms continue to play a significant role in modeling and characterizing medical imaging. The currently available XCAT phantom incorporates both the flexibility of mathematical phantoms and the realistic nature of voxelized phantoms. This phantom generates images based on a regular breathing pattern and can include arbitrary lung tumor trajectories. In this work, we present an algorithm that modifies the current XCAT phantom to generate 4D imaging data based on irregular breathing. First, a parameter is added to the existing XCAT phantom to include any arbitrary tumor motion. This modification introduces the desired tumor motion but, comes at the cost of decoupled diaphragm, chest wall and lung motion. To remedy this problem diaphragm and chest wall motion is first modified based on initial tumor location and then input to the XCAT phantom. This generates a phantom with synchronized respiratory motion. Mapping of tumor motion trajectories to diaphragm and chest wall motion is done by adaptively calculating a scale factor based on tumor to lung contour distance. The distance is calculated by projecting the initial tumor location to lung edge contours characterized by quadratic polynomials. Data from ten patients were used to evaluate the accuracy between actual independent tumor location and the location obtained from the modified XCAT phantom. The RMSE and standard deviations for ten patients in x, y, and z directions are: (0.29 ± 0.04, 0.54 ± 0.17, and0.39 ± 0.06) mm. To demonstrate the utility of the phantom, we use the new phantom to simulate a 4DCT acquisition as well as a recently published method for phase sorting. The modified XCAT phantom can be used to generate more realistic imaging data for enhanced testing of algorithms for CT reconstruction, tumor tracking, and dose reconstruction.     

Computational high-resolution heart phantoms for medical imaging and dosimetry simulations

Cardiovascular disease in general and coronary artery disease (CAD) in particular, are the leading cause of death worldwide. They are principally diagnosed using either invasive percutaneous transluminal coronary angiograms or non-invasive computed tomography angiograms (CTA). Minimally invasive therapies for CAD such as angioplasty and stenting are rendered under fluoroscopic guidance. Both invasive and non-invasive imaging modalities employ ionizing radiation and there is concern for deterministic and stochastic effects of radiation. Accurate simulation to optimize image quality with minimal radiation dose requires detailed, gender-specific anthropomorphic phantoms with anatomically correct heart and associated vasculature. Such phantoms are currently unavailable. This paper describes an open source heart phantom development platform based on a graphical user interface. Using this platform, we have developed seven high-resolution cardiac/coronary artery phantoms for imaging and dosimetry from seven high-quality CTA datasets. To extract a phantom from a coronary CTA, the relationship between the intensity distribution of the myocardium, the ventricles and the coronary arteries is identified via histogram analysis of the CTA images. By further refining the segmentation using anatomy-specific criteria such as vesselness, connectivity criteria required by the coronary tree and image operations such as active contours, we are able to capture excellent detail within our phantoms. For example, in one of the female heart phantoms, as many as 100 coronary artery branches could be identified. Triangular meshes are fitted to segmented high-resolution CTA data. We have also developed a visualization tool for adding stenotic lesions to the coronaries. The male and female heart phantoms generated so far have been cross-registered and entered in the mesh-based Virtual Family of phantoms with matched age/gender information. Any phantom in this family, along with user-defined stenoses, can be used to obtain clinically realistic projection images with the Monte Carlo code penMesh for optimizing imaging and dosimetry.     

Validation of Image-Based Method for Extraction of Coronary Morphometry

An accurate analysis of the spatial distribution of blood flow in any organ must be based on detailed morphometry (diameters, lengths, vessel numbers, and branching pattern) of the organ vasculature. Despite the significance of detailed morphometric data, there is relative scarcity of data on 3D vascular anatomy. One of the major reasons is that the process of morphometric data collection is labor intensive. The objective of this study is to validate a novel segmentation algorithm for semi-automation of morphometric data extraction. The utility of the method is demonstrated in porcine coronary arteries imaged by computerized tomography (CT). The coronary arteries of five porcine hearts were injected with a contrast-enhancing polymer. The coronary arterial tree proximal to 1 mm was extracted from the 3D CT images. By determining the centerlines of the extracted vessels, the vessel radii and lengths were identified for various vessel segments. The extraction algorithm described in this paper is based on a topological analysis of a vector field generated by normal vectors of the extracted vessel wall. With this approach, special focus is placed on achieving the highest accuracy of the measured values. To validate the algorithm, the results were compared to optical measurements of the main trunk of the coronary arteries with microscopy. The agreement was found to be excellent with a root mean square deviation between computed vessel diameters and optical measurements of 0.16 mm (<10% of the mean value) and an average deviation of 0.08 mm. The utility and future applications of the proposed method to speed up morphometric measurements of vascular trees are discussed.     

Rest period duration of the coronary arteries: implications for magnetic resonance coronary angiography

Magnetic resonance (MR) and computed tomography coronary imaging is susceptible to artifacts caused by motion of the heart. The presence of rest periods during the cardiac and respiratory cycles suggests that images free of motion artifacts could be acquired. In this paper, we studied the rest period (RP) duration of the coronary arteries during a cardiac contraction and a tidal respiratory cycle. We also studied whether three MR motion correction methods could be used to increase the respiratory RP duration. Free breathing x-ray coronary angiograms were acquired in ten patients. The three-dimensional (3D) structure of the coronary arteries was reconstructed from a biplane acquisition using stereo reconstruction methods. The 3D motion of the arterial model was then recovered using an automatic motion tracking algorithm. The motion field was then decomposed into separate cardiac and respiratory components using a cardiac respiratory parametric model. For the proximal-to-middle segments of the right coronary artery (RCA), a cardiac RP (<1 mm 3D displacement) of 76+/-34 ms was measured at end systole (ES), and 65+/-42 ms in mid-diastole (MD). The cardiac RP was 80+/-25 ms at ES and 112+/-42 ms at MD for the proximal 5 cm of the left coronary tree. At end expiration, the respiratory RP (in percent of the respiratory period) was 26+/-8% for the RCA and 27+/-17% for the left coronary tree. Left coronary respiratory RP (<0.5 mm 3D displacement) increased with translation (32% of the respiratory period), rigid body (51%), and affine (79%) motion correction. The RCA respiratory RP using translational (27%) and rigid body (33%) motion correction were not statistically different from each other. Measurements of the cardiac and respiratory rest periods will improve our understanding of the temporal and spatial resolution constraints for coronary imaging.     

The Coronary Vasculature and its Reconstruction

Recently, we have developed several new innovations in the morphometry of vascular trees. These innovations have been used to study the anatomy of the coronary circulation in the pig and have yielded a complete set of morphometric data on the entire coronary vasculature. Since, the innovations are applicable to any organ that has a tree-like vasculature, their utility in describing the quantitative anatomy of intraorgan vasculature is evident. These advancements in morphometry along with the automation of vascular tree reconstruction, data analysis, and hemodynamic applications should make the data base on intraorgan vasculature more abundant and more useful. The morphometric data will contribute to the Circulatory Network Physiome that will serve as an anatomical basis for the Physiome Project. This article covers several topics: (1) intraorgan vascular trees in terms of their anatomy, mechanical properties, physiological behavior, and adaptation, (2) reconstruction of the coronary vasculature, and (3) some of the shortcomings of the present morphometric data base and some proposed remedies. Finally, a discussion of the constitution of the circulatory network physiome in terms of the available morphometric data on intraorgan vasculature will be presented. © 2000 Biomedical Engineering Society. PAC00: 8719Uv, 8719Rr     

Morphometric Studies of Human Coronary Artery Trees in Healthy and Disease

Objective According to the report from American Heart Association(AHA),cardiovascular diseases(CVDs)are the leading causes of death globally,and coronary artery disease(CAD),known as coronary atherosclerotic plaques,accounts for over 30%of cardiovascular diseases.Therefore,it is of great clinical significance to study the relationship between coronary bifurcations morphometrical feature change and coronary artery disease.Although coronary atherosclerosis has been extensively investigated,there is a lack of in-deep study on the differences in morphometric features between optimal and realistic geometry of coronary arterial trees.The purpose of the present paper is to determine the morphological changes in patients with CAD lesion compared with non-coronary artery disease(non-CAD)subjects.Methods Due to the difficulty of studying the coronary bifurcations in vivo,image-based in vitro anatomical 3D models have been widely used as a noninvasive method for morphometric measurement and clinical diagnosis of the coronary bifurcations.With the development of coronary computed tomography angiography(CTA)hardware and software technologies,the CTA imaging technique has been shown a promising application in the characterization,visualization,and identification of coronary artery disease in recent decades.The CTA images used to reconstruct three-dimensional(3D)coronary arterial trees are from Asia populations(Southern Chinese populations),including five cadavers without CAD lesion and 102 patients with CAD lesion.The best fit artery diameter was calculated as twice the average radius between the points in the centerlines and the points on the coronary arterial inner wall.The bifurcation angles between larger daughter artery and smaller daughter artery were determined by the intersection angle of their centerlines.Murray's law was introduced to assess the deviation of the realistic vascular networks from its optimal state.Results Based on the morphometric analysis of coronary artery bifurcations in non-CAD subjects and patients with CAD lesion subjects,the most important finding is that morphological feature parameters of non-CAD subjects are closer to the optimal values than those of patients with CAD lesion.Moreover,by comparing the morphometric data between the left and right coronary arteries,the right coronary artery exhibits a structure closer to the optimal one in morphological feature than the left coronary artery.In addition,coronary arterial trees with CAD lesion have higher asymmetry and larger area expansion ratio(AER)than those of the coronary arterial trees without CAD lesion.Conclusions We morphologically found that the coronary arterial trees with CAD lesion and left are more likely to deviate from the optimal structure predicted by Murray's law than those without CAD lesion and right.The degree to which coronary arterial system deviating from their optimal state may directly affect the incidence of coronary artery disease.This computer morpho-logical analysis strategy is illustrated to be effective in the distinguishing of the geometric differences between the healthy and diseased coronary arteries,and the analysis method may have a large potential in cardiovascular disease earlier diagnosis.     

In vivo validation of the design rules of the coronary arteries and their application in the assessment of diffuse disease

The conventional rationale that uses per cent diameter reduction to assess diffuse coronary artery disease is not appropriate because no normal reference segments exist. In a recent publication, we have proposed a theoretical model based on physical principles that relate the various morphological and haemodynamic parameters (cross-sectional area, length, volume and flow) of the normal coronary arterial tree. The model was validated using haemodynamic simulations based on detailed morphological data of the pig coronary arterial tree. This paper extends the model validation to in vivo swine studies. Coronary arteriography was performed in five swine (15-18 kg body weight) after power injection of contrast material into the coronary artery. Coronary arterial length was obtained using a 3D reconstruction technique. The arterial volume, cross-sectional area and blood flow were measured using videodensitometry. The proposed relationships between these quantities were validated. Furthermore, a sensitivity analysis was demonstrated based on a simulation of diffuse coronary artery disease (approximately 40% reduction in cross-sectional area). The results of a sensitivity analysis based on a simulation of diffuse coronary artery disease suggest that the relationships between arterial volume, cross-sectional area, blood flow and the distal arterial length can be utilized to quantify moderate levels of diffuse coronary artery disease.     

Multi-branched model of the human arterial system

A model of the human arterial system was constructed based on the anatomical branching structure of the arterial tree. Arteries were divided into segments represented by uniform thin-walled elastic tubes with realistic arterial dimensions and wall properties. The configuration contains 128 segments accounting for all the central vessels and major peripheral arteries supplying the extremities including vessels of the order of 2·0 mm diameter. Vascular impedance and pressure and flow waveforms were determined at various locations in the system and good agreement was found with experimental measurements. Use of the model is illustrated in investigating wave propagation in the arterial system and in simulation of arterial dynamics in such pathological conditions as arteriosclerosis and presence of a stenosis in the femoral artery.     

Analysis of pig’s coronary arterial blood flow with detailed anatomical data

Blood flow to perfuse the muscle cells of the heart is distributed by the capillary blood vessels via the coronary arterial tree. Because the branching pattern and vascular geometry of the coronary vessels in the ventricles and atria are nonuniform, the flow in all of the coronary capillary blood vessels is not the same. This nonuniformity of perfusion has obvious physiological meaning, and must depend on the anatomy and branching pattern of the arterial tree. In this study, the statistical distribution of blood pressure, blood flow, and blood volume in all branches of the coronary arterial tree is determined based on the anatomical branching pattern of the coronary arterial tree and the statistical data on the lengths and diameters of the blood vessels. Spatial nonuniformity of the flow field is represented by dispersions of various quantities (SD/mean) that are determined as functions of the order numbers of the blood vessels. In the determination, we used a new, complete set of statistical data on the branching pattern and vascular geometry of the coronary arterial trees. We wrote hemodynamic equations for flow in every vessel and every node of a circuit, and solved them numerically. The results of two circuits are compared: oneasymmetric model satisfies all anatomical data (including the meanconnectivity matrix) and the other, asymmetric model, satisfies all mean anatomical data except the connectivity matrix. It was found that the mean longitudinal pressure drop profile as functions of the vessel order numbers are similar in both models, but the asymmetric model yields interesting dispersion profiles of blood pressure and blood flow. Mathematical modeling of the anatomy and hemodynamics is illustrated with discussions on its accuracy.     

Investigation of dynamic SPECT measurements of the arterial input function in human subjects using simulation, phantom and human studies

Computer simulations, a phantom study and a human study were performed to determine whether a slowly rotating single-photon computed emission tomography (SPECT) system could provide accurate arterial input functions for quantification of myocardial perfusion imaging using kinetic models. The errors induced by data inconsistency associated with imaging with slow camera rotation during tracer injection were evaluated with an approach called SPECT/P (dynamic SPECT from positron emission tomography (PET)) and SPECT/D (dynamic SPECT from database of SPECT phantom projections). SPECT/P simulated SPECT-like dynamic projections using reprojections of reconstructed dynamic (94)Tc-methoxyisobutylisonitrile ((94)Tc-MIBI) PET images acquired in three human subjects (1 min infusion). This approach was used to evaluate the accuracy of estimating myocardial wash-in rate parameters K(1) for rotation speeds providing 180° of projection data every 27 or 54 s. Blood input and myocardium tissue time-activity curves (TACs) were estimated using spatiotemporal splines. These were fit to a one-compartment perfusion model to obtain wash-in rate parameters K(1). For the second method (SPECT/D), an anthropomorphic cardiac torso phantom was used to create real SPECT dynamic projection data of a tracer distribution derived from (94)Tc-MIBI PET scans in the blood pool, myocardium, liver and background. This method introduced attenuation, collimation and scatter into the modeling of dynamic SPECT projections. Both approaches were used to evaluate the accuracy of estimating myocardial wash-in parameters for rotation speeds providing 180° of projection data every 27 and 54 s. Dynamic cardiac SPECT was also performed in a human subject at rest using a hybrid SPECT/CT scanner. Dynamic measurements of (99m)Tc-tetrofosmin in the myocardium were obtained using an infusion time of 2 min. Blood input, myocardium tissue and liver TACs were estimated using the same spatiotemporal splines. The spatiotemporal maximum-likelihood expectation-maximization (4D ML-EM) reconstructions gave more accurate reconstructions than did standard frame-by-frame static 3D ML-EM reconstructions. The SPECT/P results showed that 4D ML-EM reconstruction gave higher and more accurate estimates of K(1) than did 3D ML-EM, yielding anywhere from a 44% underestimation to 24% overestimation for the three patients. The SPECT/D results showed that 4D ML-EM reconstruction gave an overestimation of 28% and 3D ML-EM gave an underestimation of 1% for K(1). For the patient study the 4D ML-EM reconstruction provided continuous images as a function of time of the concentration in both ventricular cavities and myocardium during the 2 min infusion. It is demonstrated that a 2 min infusion with a two-headed SPECT system rotating 180° every 54 s can produce measurements of blood pool and myocardial TACs, though the SPECT simulation studies showed that one must sample at least every 30 s to capture a 1 min infusion input function.     

Structure and vascularization of the ventricular myocardium in Holocephali: their evolutionary significance

It was generally assumed that the ventricle of the primitive vertebrate heart was composed of trabeculated, or spongy, myocardium, supplied by oxygen‐poor luminal blood. In addition, it was presumed that the mixed ventricular myocardium, consisting of a compacta and a spongiosa, and its supply through coronary arteries appeared several times throughout fish evolution. Recent work has suggested, however, that a fully vascularized, mixed myocardium may be the primitive condition in gnathostomes. The present study of the heart ventricles of four holocephalan species aimed to clarify this controversy. Our observations showed that the ventricular myocardium of Chimaera monstrosa and Harriotta raleighana consists of a very thin compacta overlying a widespread spongiosa. The ventricle of Hydrolagus affinis is composed exclusively of trabeculated myocardium. In these three species there is a well‐developed coronary artery system. The main coronary artery trunks run along the outflow tract, giving off subepicardial ventricular arteries. The trabeculae of the spongiosa are irrigated by branches of the subepicardial arteries and by penetrating arterial vessels arising directly from the main coronary trunks at the level of the conoventricular junction. The ventricle of Rhinochimaera atlantica has only spongy myocardium supplied by luminal blood. Small coronary arterial vessels are present in the subepicardium, but they do not enter the myocardial trabeculae. The present findings show for the first time that in a wild living vertebrate species, specifically H. affinis, an extensive coronary artery system supplying the whole cardiac ventricle exists in the absence of a well‐developed compact ventricular myocardium. This is consistent with the notion derived from experimental work that myocardial cell proliferation and coronary vascular growth rely on distinct developmental programs. Our observations, together with data in the literature on elasmobranchs, support the view that the mixed ventricular myocardium is primitive for chondrichthyans. The reduction or even lack of compacta in holocephali has to be regarded as a derived anatomical trait. Our findings also fit in with the view that the mixed myocardium was the primitive condition in gnathostomes, and that the absence of compact ventricular myocardium in different actinopterygian groups is the result of a repeated loss of such type of cardiac muscle during fish evolution.     

A Novel Method for Visualization of Entire Coronary Arterial Tree

The complexity of the coronary circulation especially in the deep layers largely evades experimental investigations. Hence, virtual/computational models depicting structure-function relation of the entire coronary vasculature including the deep layer are imperative. In order to interpret such anatomically based models, fast and efficient visualization algorithms are essential. The complexity of such models, which include vessels from the large proximal coronary arteries and veins down to the capillary level (3 orders of magnitude difference in diameter), is a challenging visualization problem since the resulting geometrical representation consists of millions of vessel segments. In this study, a novel method for rendering the entire porcine coronary arterial tree down to the first segments of capillaries interactively is described which employs geometry reduction and occlusion culling techniques. Due to the tree-shaped nature of the vasculature, these techniques exploit the geometrical topology of the object to achieve a faster rendering speed while still handling the full complexity of the data. We found a significant increase in performance combined with a more accurate, gap-less representation of the vessel segments resulting in a more interactive visualization and analysis tool for the entire coronary arterial tree. The proposed techniques can also be applied to similar data structures, such as neuronal trees, airway structures, bile ducts, and other tree-like structures. The utility and future applications of the proposed algorithms are explored.     

4D cardiac MRI in the mouse

With the introduction of mouse models for the study of cardiac morphogenesis, there arises a need for new imaging protocols that can capture both morphological and functional information. High-resolution 2D cardiac cine MRI has often been used to quantify left and right ventricular function. In this study we propose a 3D isotropic cardiac cine MRI protocol with a voxel size of 200 microm(3) as a means of studying cardiac multi-chamber morphology and function. A black blood sequence was used to enhance blood myocardium contrast. Manual segmentation of the ventricles was used to measure ventricular volumes at end diastole and end systole. This method is demonstrated on an Irx4-deficient mouse model. We have been able to identify the volumes of both ventricles dynamically and to show differences in ejection fraction in the mutant. We have also identified an abnormality of the papillary muscle in the mutant that had been missed in previous phenotyping with ultrasound and histology.     

Three-dimensional knowledge driven reconstruction of coronary trees

A knowledge-driven approach to the three-dimensional reconstruction of coronary artery trees by means of two X-ray projections is proposed. The spatial reconstruction of the tree skeleton is discussed. A binary tree model of the arterial structure and its projections is employed. Consequently, the reconstruction of the three-dimensional tree skeleton is achieved by (a) matching the skeletons of corresponding pairs of vascular segments in the two views and (b) back-projecting the coupled skeleton projections. From a geometrical point of view, the matching problem is, in general, ill-conditioned. For this reason, additional information sources were used. Thus, the matching phase is accomplished by using both the imaging geometry information, as well as anatomical and topological knowledge, about the coronary arteries coded in a rule base. As far as the back-projection phase is concerned, an algorithm was developed based on: (1) the imaging geometry, (2) the bounding of the back-projection error and (3) a contiguity criterion.     

Simulation of cardiac motion on non-Newtonian, pulsating flow development in the human left anterior descending coronary artery

This study aimed at investigating the effect of myocardial motion on pulsating blood flow distribution of the left anterior descending coronary artery in the presence of atheromatous stenosis. The moving 3D arterial tree geometry has been obtained from conventional x-ray angiograms obtained during the heart cycle and includes a number of major branches. The geometry reconstruction model has been validated against projection data from a virtual phantom arterial tree as well as with CT-based reconstruction data for the same patient investigated. Reconstructions have been obtained for a number of temporal points while linear interpolation has been used for all intermediate instances. Blood has been considered as a non-Newtonian fluid. Results have been obtained using the same pulse for the inlet blood flow rate but with fixed arterial tree geometry as well as under steady-state conditions corresponding to the mean flow rate. Predictions indicate that myocardial motion has only a minor effect on flow distribution within the arterial tree relative to the effect of the blood pressure pulse.     

Anatomy and clinical significance of ventricular Thebesian veins

An injection study was carried out in sheep hearts to compare the anatomy and distribution of Thebesian veins (venae cordis minimae) in the ventricles. The left azygos, middle cardiac, small cardiac, and anterior cardiac veins were ligated in 36 hearts, and India ink was injected into the right coronary artery or the left coronary artery or the coronary sinus. Examination revealed foramina Thebesii in both of these cardiac chambers. Myocardial tissue samples were taken, and 12 were subsequently studied histologically to confirm the presence of Thebesian veins. A greater number of Thebesian veins were observed in the right ventricle than in the left (P < 0.05). To identify any larger communications between the coronary arteries and cardiac chambers (arterioluminal), and between the coronary veins and cardiac chambers (venoluminal), gelatine was injected in 16 hearts. Arterioluminal vessels were identified only in the right ventricle, whereas venoluminal vessels were present in both ventricles. Venoluminal vessels are most likely responsible for the non-nutritive shunting of cardioplegic solutions delivered via the coronary sinus during surgery. Thebesian veins play a role in the drainage of blood, contributing towards right to left shunting of deoxygenated blood. It has also been suggested, although not proven, that they are able to supply blood to the myocardium in coronary arterial occlusion, thus acting as a natural form of nutrient channel. Thebesian veins may be confused with artificial nutrient channels constructed by transmyocardial laser revascularization, a possibility that should be considered during histological evaluation of this technique. Copyright Wiley-Liss, Inc.