Arash Kheradvar

Emerging trends in CV flow visualization.

Blood flow patterns are closely linked to the morphology and function of the cardiovascular system. These patterns reflect the exceptional adaptability of the cardiovascular system to maintain normal blood circulation under a wide range of workloads. Accurate retrieval and display of flow-related information remains a challenge because of the processes involved in mapping the flow velocity fields within specific chambers of the heart. We review the potentials and pitfalls of current approaches for blood flow visualization, with an emphasis on acquisition, display, and analysis of multidirectional flow. This document is divided into 3 sections. First, we provide a descriptive outline of the relevant concepts in cardiac fluid mechanics, including the emergence of rotation in flow and the variables that delineate vortical structures. Second, we elaborate on the main methods developed to image and visualize multidirectional cardiovascular flow, which are mainly based on cardiac magnetic resonance, ultrasound Doppler, and contrast particle imaging velocimetry, with recommendations for developing dedicated imaging protocols. Finally, we discuss the potential clinical applications and technical challenges with suggestions for further investigations.

Echocardiographic particle image velocimetry: a novel technique for quantification of left ventricular blood vorticity pattern.

BACKGROUND In this study, the functionality of echocardiographic particle imaging velocimetry (E-PIV) was compared with that of digital particle imaging velocimetry (D-PIV) in an in vitro model. In addition, its capability was assessed in the clinical in vivo setting to obtain the ventricular flow pattern in normal subjects, in patients with dilated cardiomyopathy, and in patients with mechanical and bioprosthetic mitral valves. METHODS A silicon sac simulating the human left ventricle in combination with prosthetic heart valves, controlled by a pulsed-flow duplicator, was used as the in vitro model. Particle-seeded flow images were acquired (1) using a high-speed camera from the mid plane of the sac, illuminated by a laser sheet for D-PIV, and (2) using a Siemens Sequoia system at a frame rate of 60 Hz for E-PIV. Data analysis was performed with PIVview software for D-PIV and Omega Flow software for E-PIV. E-PIV processing was then applied to contrast echocardiographic image sets obtained during left ventricular cavity opacification with a lipid-shelled microbubble agent to assess spatial patterns of intracavitary flow in the clinical setting. RESULTS The velocity vectors obtained using both the E-PIV and the D-PIV methods compared well for the direction of flow. The streamlines were also found to be similar in the data obtained using both methods. However, because of the superior spatial resolution of D-PIV, some smaller scale details were not revealed by E-PIV. The application of E-PIV to the human heart resulted in reproducible flow patterns in echocardiographic images taken within different time frames or by independent examiners. CONCLUSIONS The E-PIV technique appears to be capable of evaluating the major flow features in the ventricles. However, the bounded spatial resolution of ultrasound imaging limits the small-scale features of ventricular flow to be revealed.

A combined deep-learning and deformable-model approach to fully automatic segmentation of the left ventricle in cardiac MRI☆

Segmentation of the left ventricle (LV) from cardiac magnetic resonance imaging (MRI) datasets is an essential step for calculation of clinical indices such as ventricular volume and ejection fraction. In this work, we employ deep learning algorithms combined with deformable models to develop and evaluate a fully automatic LV segmentation tool from short-axis cardiac MRI datasets. The method employs deep learning algorithms to learn the segmentation task from the ground true data. Convolutional networks are employed to automatically detect the LV chamber in MRI dataset. Stacked autoencoders are used to infer the LV shape. The inferred shape is incorporated into deformable models to improve the accuracy and robustness of the segmentation. We validated our method using 45 cardiac MR datasets from the MICCAI 2009 LV segmentation challenge and showed that it outperforms the state-of-the art methods. Excellent agreement with the ground truth was achieved. Validation metrics, percentage of good contours, Dice metric, average perpendicular distance and conformity, were computed as 96.69%, 0.94, 1.81 mm and 0.86, versus those of 79.2-95.62%, 0.87-0.9, 1.76-2.97 mm and 0.67-0.78, obtained by other methods, respectively.

On Mitral Valve Dynamics and its Connection to Early Diastolic Flow

In the field of cardiology, the current ability to accurately detect diastolic dysfunction is unsatisfactory due to the lack of an effective diagnostic index. Isolated indexes obtained from echocardiography are all restricted to a certain aspect of ventricular diastolic function only, and individually cannot be regarded as a global representative for the left heart diastolic function. Due to complexity of cardiac motion, a reliable measure for diastolic performance should be a parameter that independently correlates with several aspects of cardiac function. The presence of trans-mitral vortex ring and its influence on dynamics of the mitral valve is a topic that has been recently received more attention in cardiovascular research. One obvious reason for this attention is to find better solutions to overcome our inability in interpretation of Doppler mitral inflow patterns for distinguishing a normal trans-mitral flow from a pseudonormal pattern. In the present study, we investigated the relationship among the ventricular early pressure drop, trans-mitral thrust as a force generated during diastolic filling and mitral annulus recoil through the index of vortex formation time. As a result, we found that vortex formation time is independently correlated to trans-mitral thrust, minimal ventricular pressure and pressure drop time-constant of isovolumic relaxation phase. Results also showed that trans-mitral thrust is maximized when the non-dimensional vortex formation time is in the range of 4 and 5.5 regardless of the shape of the waveform or the value of the pressure drop time-constant. In conclusion, this study confirms that vortex formation time, a non-dimensional measure for duration of E-wave, can be used as an index to assess diastolic ventricular function.

An anatomic study of the lingual nerve in the third molar region.

PURPOSE The aim of this study was to determine the location of the lingual nerve in the lower third molar region. PATIENTS AND METHODS In this study, 669 nerves from 430 fresh cadavers were examined. Measurements on each cadaver were made using a micrometer caliper to determine the horizontal and vertical position of the lingual nerve in the lower third molar region. RESULTS In 94 cases (14.05%), the nerve was above the lingual crest, and in 1 case (0.15%), the nerve was in the retromolar pad region. In the remaining 574 cases (85.80%), the mean horizontal and vertical distances of the nerve to the lingual plate and the lingual crest 2.06 +/- 1.10 mm (range, 0.00 to 3.20 mm) and 3.01 +/- 0.42 mm (range, 1.70 to 4.00 mm), respectively. In 149 cases (22.27%), the nerve was in direct contact with the lingual plate of the alveolar process. CONCLUSIONS This study confirms the relatively unsafe position of the lingual nerve in relation to some oral and maxillofacial surgery procedures.

Correlation between vortex ring formation and mitral annulus dynamics during ventricular rapid filling.

One of the most important fluid phenomena observed in the left ventricle during diastole is the presence of vortex rings that develop with a strong jet entering through the mitral valve. The present study is focused on the rapid filling phase of diastole, during which the left ventricle expands and receives blood through the fully open mitral valve. The atrioventricular system during the rapid filling phase was emulated experimentally with a simplified mechanical model in which the relevant pressure decay and the dimension of mitral annulus approximate the physiologic and pathologic values. Digital particle image velocimetry measurements were correlated with the force measurements on the mitral annulus plane to analyze the relation between flow and the mitral annulus motion. The recoil force on the displaced annulus plane was computed on the basis of plane acceleration and plane velocity and correlated with the inflow jet. Measurements of the recoil force for different values of the mitral annulus diameter showed that the recoil force was generated during fluid propulsion and that it is maximal for an annulus diameter close to the normal adult value in a healthy left ventricle. We also tested annulus diameters smaller and larger than the normal one. The smaller annulus corresponds to the stenotic valves and the larger annulus exists in dilated cardiomyopathy cases. In both conditions, the recoil force was found to be smaller than in the normal case. These observations are consistent with the previously reported results for dilated cardiomyopathy and mitral stenosis clinical conditions.

Contrast echocardiography for assessing left ventricular vortex strength in heart failure: a prospective cohort study

AIMS This study investigated the incremental role of echocardiographic-contrast particle image velocimetry (Echo-PIV) in patients with heart failure (HF) for measuring changes in left ventricular (LV) vortex strength (VS) during phases of a cardiac cycle. METHODS AND RESULTS Echo-PIV was performed in 42 patients, including 23 HF patients and 19 controls. VS was measured as a fluid-dynamic parameter that integrates blood flow rotation over a given area and correlated with non-invasively derived indices of LV mechanical performance. In comparison with early and late diastole, the VS was higher during isovolumic contraction (IC) for control and HF patients with the preserved ejection fraction (P = 0.002 and P = 0.01, respectively), but not for HF patients with the reduced ejection fraction (P = 0.41). On multivariable regression analysis, the VS during IC (VSIC) was independently related to late-diastolic VS and LV longitudinal strain (R(2)= 0.63, P < 0.001 and P = 0.003, respectively). Patients in whom diastolic VS was augmented during IC showed a higher LV stroke volume (P = 0.01), stroke work (P = 0.02), and mechanical efficiency (P = 0.008). Over a median follow-up period of 2.9 years, eight (34%) HF patients were hospitalized for decompensated HF. In comparison with the rest, these eight patients showed markedly reduced longitudinal strain (P = 0.002), and lower change in VS (P = 0.004). CONCLUSION Our preliminary data suggest that the persistence of vortex from late diastole into IC is a haemodynamic measure of coupling between diastole and systole. The change in VS is correlated with LV mechanical performance and shows association with adverse clinical outcomes seen in HF patients.

Influence of Ventricular Pressure Drop on Mitral Annulus Dynamics Through the Process of Vortex Ring Formation

Several studies have suggested that the mitral annulus displacement and velocity in early diastole can be used as indicators of diastolic performance. The peak velocity of the mitral annulus away from the LV apex during early diastole, which indicates the rate of longitudinal expansion of the LV, is reduced in patients with impaired diastolic relaxation. With the intention of relating the trans-mitral flow to mitral annulus plane dynamics, we measured mitral annulus recoil force for different valve sizes, while applying an exponential pressure drop in a simplified model of the ventricle. The temporal changes in diameter of the valve during rapid filling phase were also considered. The process of ventricular vortex formation was studied together with the measurement of mitral annulus recoil force within different pressure drop conditions. Matching the vorticity contour plots with the recoil force measurements resulted in the fact that the magnitude of recoil is maximal once the vortex ring is about to pinch off, regardless of the valve size or the characteristics of ventricular pressure drop. This study showed that the mitral annulus recoil is maximal once occurs at the vortex formation time ranging from 3.5 to 4.5. It was also shown that the presence of leaflets would dissipate the annulus recoil force.

The Effects of Transcatheter Valve Crimping on Pericardial Leaflets

BACKGROUND Transcatheter aortic valve replacement has emerged as a promising therapy for treatment of severe aortic stenosis. Although it has been shown that these valves can be safely delivered and implanted, studies of valve longevity are lacking because of the infancy of the technology. Particularly, the effects of stent crimping on the valves leaflets have not yet been sufficiently investigated. In this study, we have characterized the effects of crimping on pericardial leaflets in time and through the depth of the tissue. METHODS To test the structural changes at the surface and deep layers of bovine pericardial leaflets, scanning electron microscopy and second-harmonic generation microscopy were used. An uncrimped tissue sample was imaged, followed by imaging a segment of tissue after crimping in a stented transcatheter valve, immediately after, at 20 minutes, and 60 minutes after crimping. The crimping experiment was performed for multiple crimping sizes (ie, 14F, 16F, and 18F). We defined a damage index that quantifies the level of leaflet structural changes as a result of crimping. RESULTS Based on the calculated damage indices and analyses of the raw images, it was determined that crimping does measurable damage to the leaflet tissue that persists with time. CONCLUSIONS Significant tissue damage was observed at the surface layers of the leaflets. In the deeper tissue layers, damage was substantial for 14F crimping; however, it became less significant but still visible for larger collapse profiles. Crimping may induce substantial structural damage to pericardial leaflets that does not improve with time.

Emerging Trends in Heart Valve Engineering: Part I. Solutions for Future

As the first section of a multi-part review series, this section provides an overview of the ongoing research and development aimed at fabricating novel heart valve replacements beyond what is currently available for patients. Here we discuss heart valve replacement options that involve a biological component or process for creation, either in vitro or in vivo (tissue-engineered heart valves), and heart valves that are fabricated from polymeric material that are considered permanent inert materials that may suffice for adults where growth is not required. Polymeric materials provide opportunities for cost-effective heart valves that can be more easily manufactured and can be easily integrated with artificial heart and ventricular assist device technologies. Tissue engineered heart valves show promise as a regenerative patient specific model that could be the future of all valve replacement. Because tissue-engineered heart valves depend on cells for their creation, understanding how cells sense and respond to chemical and physical stimuli in their microenvironment is critical and therefore, is also reviewed.