Analytical method to measure three-dimensional strain patterns in the left ventricle from single slice displacement data

BACKGROUND

Displacement encoded Cardiovascular MR (CMR) can provide high spatial resolution measurements of three-dimensional (3D) Lagrangian displacement. Spatial gradients of the Lagrangian displacement field are used to measure regional myocardial strain. In general, adjacent parallel slices are needed in order to calculate the spatial gradient in the through-slice direction. This necessitates the acquisition of additional data and prolongs the scan time. The goal of this study is to define an analytic solution that supports the reconstruction of the out-of-plane components of the Lagrangian strain tensor in addition to the in-plane components from a single-slice displacement CMR dataset with high spatio-temporal resolution. The technique assumes incompressibility of the myocardium as a physical constraint.

RESULTS

The feasibility of the method is demonstrated in a healthy human subject and the results are compared to those of other studies. The proposed method was validated with simulated data and strain estimates from experimentally measured DENSE data, which were compared to the strain calculation from a conventional two-slice acquisition.

CONCLUSION

This analytical method reduces the need to acquire data from adjacent slices when calculating regional Lagrangian strains and can effectively reduce the long scan time by a factor of two.

Inflammatory BCG adenitis associated with immune reconstitution following allogeneic haematopoietic stem cell transplant in infancy

We describe four cases of a localized, granulomatous reaction to BCG including ipsilateral painful, suppurative lymphadenopathy associated with donor immune reconstitution following allogeneic haematopoietic stem cell transplant performed in infancy and preceded by uneventful, routine BCG immunisation. The management of the inflammatory disease in these cases with surgery, antimycobacterial chemotherapy and steroids, is discussed.

Reversing blood flows act through klf2a to ensure normal valvulogenesis in the developing heart

The directionality of local blood flow in the zebrafish embryonic heart is essential for proper heart valve formation.

Heart valve anomalies are some of the most common congenital heart defects, yet neither the genetic nor the epigenetic forces guiding heart valve development are well understood. When functioning normally, mature heart valves prevent intracardiac retrograde blood flow; before valves develop, there is considerable regurgitation, resulting in reversing (or oscillatory) flows between the atrium and ventricle. As reversing flows are particularly strong stimuli to endothelial cells in culture, an attractive hypothesis is that heart valves form as a developmental response to retrograde blood flows through the maturing heart. Here, we exploit the relationship between oscillatory flow and heart rate to manipulate the amount of retrograde flow in the atrioventricular (AV) canal before and during valvulogenesis, and find that this leads to arrested valve growth. Using this manipulation, we determined that klf2a is normally expressed in the valve precursors in response to reversing flows, and is dramatically reduced by treatments that decrease such flows. Experimentally knocking down the expression of this shear-responsive gene with morpholine antisense oligonucleotides (MOs) results in dysfunctional valves. Thus, klf2a expression appears to be necessary for normal valve formation. This, together with its dependence on intracardiac hemodynamic forces, makes klf2a expression an early and reliable indicator of proper valve development. Together, these results demonstrate a critical role for reversing flows during valvulogenesis and show how relatively subtle perturbations of normal hemodynamic patterns can lead to both major alterations in gene expression and severe valve dysgenesis.

The growth and development of vertebrates are critically dependent on efficient cardiac output to drive blood circulation. An essential step of heart development is the formation of heart valves, whose leaflets are made through a complex set of cellular rearrangements of endothelial cells. Endothelial cells experience high flow forces as blood circulates. Moreover, heart valves and associated structures can be malformed when flow forces are abnormal, suggesting that these flow forces are in fact required for proper valve formation. Whether it is the force of the blood flow, its directionality (forward or reverse), or both that are important is not clear. We studied the interplay during valve development between key genes known to be involved in the process and epigenetic influences such as flow forces. Using zebrafish, whose optical clarity allows analyzing blood flow patterns at high resolution, we identified the presence of reversing flows specifically at the level of valve precursors. By manipulating blood flow patterns, we show that reversing flows are essential for valve morphogenesis. Specifically, we show that the expression of the gene klf2a depends on the presence of reversing flows and is required for valve development. We predict that by influencing levels of klf2a, reversing flows constitute an important stimulus controlling the appropriate biological responses of endothelial cells during valve formation.

A dynamic double helical band as a model for cardiac pumping

We address here, by means of finite-element computational modeling, two features of heart mechanics and, most importantly, their timing relationship: one of them is the ejected volume and the other is the twist of the heart. The corner stone of our approach is to take the double helical muscle fiber band as the dominant active macrostructure behind the pumping function. We show that this double helical model easily reproduces a physiological maximal ejection fraction of up to 60% without exceeding the limit on local muscle fiber contraction of 15%. Moreover, a physiological ejection fraction can be achieved independently of the excitation pattern. The left ventricular twist is also largely independent of the type of excitation. However, the physiological relationship between the ejection fraction and twist can only be reproduced with Purkinje-type excitation schemes. Our results indicate that the proper timing coordination between twist and ejection dynamics can be reproduced only if the excitation front originates in the septum region near the apex. This shows that the timing of the excitation is directly related to the productive pumping operation of the heart and illustrates the direction for possible bioinspired pump design.

Computational models of heart pumping efficiencies based on contraction waves in spiral elastic bands

We present a framework for modeling biological pumping organs based on coupled spiral elastic band geometries and active wave-propagating excitation mechanisms. Two pumping mechanisms are considered in detail by way of example: one of a simple tube, which represents a embryonic fish heart and another more complicated structure with the potential to model the adult human heart. Through finite element modeling different elastic contractions are induced in the band. For each version the pumping efficiency is measured and the dynamics are evaluated. We show that by combining helical shapes of muscle bands with a contraction wave it is possible not only to achieve efficient pumping, but also to create desired dynamics of the structure. As a result we match the function of the model pumps and their dynamics to physiological observations.

Evidence for the existence of a functional helical myocardial band

Characterization of local and global contractile activities in the myocardium is essential for a better understanding of cardiac form and function. The spatial distribution of regions that contribute the most to cardiac function plays an important role in defining the pumping parameters of the myocardium like ejection fraction and dynamic aspects such as twisting and untwisting. In general, myocardium shortening, tangent to the wall, and ventricular wall thickening are important parameters that characterize the regional contribution within the myocardium to the global function of the heart. We have calculated these parameters using myocardium displacement fields, which were captured through the displacement-encoding with stimulated echoes (DENSE) MRI technique in three volunteers. High spatial resolution of the acquired data revealed transmural changes of thickening and tangential shortening with high fidelity in beating hearts. By filtering myocardium regions that showed a tangential shortening index of <0.23, we were able to identify the complete or a portion of a macrostructure composed of connected regions in the form of a helical bundle within the left ventricle mass. In this study, we present a representative case that shows the complete morphology of a helical myocardial band as well as two other cases that present ascending and descending portions of the helical myocardial band. Our observation of a helical functional band based on dynamics is in agreement with diffusion tensor MRI observations and gross dissection studies in the arrested heart.

Circumferential strain in the wall of the common carotid artery: comparing displacement-encoded and cine MRI in volunteers

The walls of conduit arteries undergo cyclic stretching from the periodic fluctuation of arterial pressure. Atherosclerotic lesions have been shown to localize to regions of excessive stretching of the arterial wall. We employed a displacement encoding with stimulated echoes (DENSE) sequence to image the motion of the common carotid artery wall and map the two-dimensional (2D) circumferential strain. The sequence utilizes a fully-balanced steady-state free-precession (SSFP) readout with 0.60 mm in-plane resolution. Preliminary results in volunteers at 1.5T (N = 4) and 3.0T (N = 17) are compared to measurements of the lumen circumference from cine images. The agreement between the two independent measurements at both field strengths (P < or = 0.001) supports the use of DENSE as a means to map the pulsatile strain in the carotid artery wall.

Evidence of self-correcting spiral flows in swimming boxfishes

The marine boxfishes have rigid keeled exteriors (carapaces) unlike most fishes, yet exhibit high stability, high maneuverability and relatively low drag given their large cross-sectional area. These characteristics lend themselves well to bioinspired design. Based on previous stereolithographic boxfish model experiments, it was determined that vortical flows develop around the carapace keels, producing self-correcting forces that facilitate swimming in smooth trajectories. To determine if similar self-correcting flows occur in live, actively swimming boxfishes, two species of boxfishes (Ostracion meleagris and Lactophrys triqueter) were induced to swim against currents in a water tunnel, while flows around the fishes were quantified using digital particle image velocimetry. Significant pitch events were rare and short lived in the fishes examined. When these events were observed, spiral flows around the keels qualitatively similar to those observed around models were always present, with greater vortex circulation occurring as pitch angles deviated from 0 degrees . Vortex circulation was higher in live fishes than models presumably because of pectoral fin interaction with the keel-induced flows. The ability of boxfishes to modify their underlying self-correcting system with powered fin control is important for achieving high levels of both stability and maneuverability. Although the challenges of performing stability and maneuverability research on fishes are significant, the results of this study together with future studies employing innovative new approaches promise to provide valuable inspiration for the designers of bioinspired aquatic vehicles.

Controlled partial embedding of carbon nanotubes within flexible transparent layers

Applications of carbon nanotubes (CNTs) like field emission displays, super-capacitors, and cell growth scaffolds can benefit from controllable embedding of the CNTs in a material such that the CNTs are anchored and protrude a desired length. We demonstrate a simple method for anchoring densely packed, vertically aligned arrays of CNTs into silicone layers using spin-coating, CNT insertion, curing, and growth substrate removal. CNT arrays of 51 and 120 µm in height are anchored into silicone layers of thickness 26 and 36 µm, respectively. Scanning electron microscopy (SEM) and optical microscopy are used to characterize the sample morphology, a 5.5 m s(-1) impinging water jet is used to apply shear stress, and a tensile test shows that the silicone layer detaches from the substrate before the CNTs are ripped from the layer. The CNTs are thus well anchored in the silicone layers. The spin-coating process gives control over layer thickness, and the method should have general applicability to various nanostructures and anchoring materials.

Three-dimensional real-time imaging of cardiac cell motions in living embryos

While quantitative analysis of dynamic biological cell motions in vivo is of great biomedical interest, acquiring 3-D (plus time) information is difficult due to the lack of imaging tools with sufficient spatial and temporal resolution. A novel 3-D high-speed microscopic imaging system is developed to enable 3-D time series data acquisition, based on a defocusing technique (DDPIV). Depth coordinate Z is resolved by the triangular image patterns generated by a mask with three apertures forming an equilateral triangle. Application of this technique to microscale imaging is validated by calibration of targets spread over the image field. 1-microm fluorescent tracer particles are injected into the blood stream of 32 h post-fertilization developing zebrafish embryos to help describe cardiac cell motions. 3-D and velocity fields of cardiovascular blood flow and trajectories of heart-wall motions are obtained.

Estimation of elastic and viscous properties of the left ventricle based on annulus plane harmonic behavior

Assessment of left ventricular (LV) function with an emphasis on contractility has been a challenge in cardiac mechanics during the recent decades. The LV function is usually described by the LV pressure-volume (P-V) diagram. The standard P-V diagrams are easy to interpret but difficult to obtain and require invasive instrumentation for measuring the corresponding volume and pressure data. In the present study, we introduce a technique that can estimate the viscoelastic properties of the LV based on harmonic behavior of the ventricular chamber and it can be applied non-invasively as well. The estimation technique is based on modeling the actual long axis displacement of the mitral annulus plane toward the cardiac base as a linear damped oscillator with time-varying coefficients. The time-varying parameters of the model were estimated by a standard recursive linear least squares (RLLS) technique. LV stiffness at end-systole and end diastole was in the range of 61.86-136.00 dyne/g.cm and 1.25-21.02 dyne/g.cm, respectively. The only input used in this model was the long axis displacement of the annulus plane, which can also be obtained non-invasively using tissue Doppler or MR imaging.

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.

Electrocardiographic characterization of embryonic zebrafish

The zebrafish (Danio rerio) has emerged as one of the primary experimental models of developmental cardiovascular research. Recent progress in flow visualization techniques along with the existing genetic map of the species has made zebrafish amenable to a variety of experiments relating cardiac developmental structure and function. One essential tool in establishing the proper functioning of the heart is the electrocardiogram (ECG). This study presents a methodology whereby the ECGs of embryonic zebrafish could be used in assessing the electrophysiological consequences of genetically-, mechanically-, or pharmacokinetically-induced cardiac perturbations. Five day post-fertilization (dpf) embryos produced repeating bimodal ECGs with clearly distinguished atrial (P) and ventricular (R) depolarization waves. P-R intervals along with P-P intervals are cited.

Role of Vortices in Growth of Microbubbles at Mitral Mechanical Heart Valve Closure

This study is aimed at refining our understanding of the role of vortex formation at mitral mechanical heart valve (MHV) closure and its association with the high intensity transient signals (HITS) seen in echocardiographic studies with MHV recipients. Previously reported numerical results described a twofold process leading to formation of gas-filled microbubbles in-vitro: (1) nucleation and (2) growth of micron size bubbles. The growth itself consists of two processes: (a) diffusion and (b) sudden pressure drop due to valve closure. The role of diffusion has already been shown to govern the initial growth of nuclei. Pressure drop at mitral MHV closure may be attributed to other phenomena such as squeezed flow, water hammer and primarily, vortex cavitation. Mathematical analysis of vortex formation at mitral MHV closure revealed that a closing velocity of approximately 12 m/s can induce a strong regurgitant vortex which in return can instigate a local pressure drop of about 0.9 atm. A 2D experimental model of regurgitant flows was used to substantiate the impact of vortices. At simulated flow and pressure conditions, a regurgitant vortex was observed to drastically enlarge micron size hydrogen bubbles at its core.

Evaluation of isovolumic relaxation phase in the process of ventricular remodeling following myocardial infarction

The course of cardiac remodeling after an acute cardiac MI, might affect the orientation of the cardiac muscle fibers as well as their contraction behavior. This may result in alteration of the untwisting during isovolumic relaxation phase, which might have effects on rapid early filling phase. In the present article, the variation of the time constant of isovolumic pressure drop (tau) has been studied during the course of cardiac remodeling after different types of induced myocardial infarction (MI) in sheep. The results for each group show different patterns of change in tau. The normalized tau curve in all three groups of anteroapical, anterobasal and posterobasal MI group show a rise 30 minutes after infarction. Two weeks later, the pressure drop constants decline to a lower level than baseline and by eight weeks after infarction, the time constant reached around the baseline level.

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.

Rapid three-dimensional imaging and analysis of the beating embryonic heart reveals functional changes during development

We report an accurate method for studying the functional dynamics of the beating embryonic zebrafish heart. The fast cardiac contraction rate and the high velocity of blood cells have made it difficult to study cellular and subcellular events relating to heart function in vivo. We have devised a dynamic three-dimensional acquisition, reconstruction, and analysis procedure by combining (1) a newly developed confocal slit-scanning microscope, (2) novel strategies for collecting and synchronizing cyclic image sequences to build volumes with high temporal and spatial resolution over the entire depth of the beating heart, and (3) data analysis and reduction protocols for the systematic extraction of quantitative information to describe phenotype and function. We have used this approach to characterize blood flow and heart efficiency by imaging fluorescent protein-expressing blood and endocardial cells as the heart develops from a tube to a multichambered organ. The methods are sufficiently robust to image tissues within the heart at cellular resolution over a wide range of ages, even when motion patterns are only quasiperiodic. These tools are generalizable to imaging and analyzing other cyclically moving structures at microscopic scales.

Assessment of left ventricular viscoelastic components based on ventricular harmonic behavior

BACKGROUND

Assessment of left ventricular (LV) function with an emphasis on contractility has been a challenge in cardiac mechanics during the recent decades. The LV function is usually described by the LV pressure-volume (P-V) relationship. Based on this relationship, the ratio of instantaneous pressure to instantaneous volume is an index for LV chamber stiffness. The standard P-V diagrams are easy to interpret but difficult to obtain and require invasive instrumentation for measuring the corresponding volume and pressure data. In the present study, we introduce a technique that can estimate viscoelastic properties, not only the elastic component but also the viscous properties of the LV based on oscillatory behavior of the ventricular chamber and it can be applied non-invasively as well.

MATERIALS AND METHODS

The estimation technique is based on modeling the actual long axis displacement of the mitral annulus plane toward the cardiac base as a linear damped oscillator with time-varying coefficients. Elastic deformations resulting from the changes in the ventricular mechanical properties of myocardium are represented as a time-varying spring while the viscous components of the model include a time-varying viscous damper, representing relaxation and the frictional energy loss. To measure the left ventricular axial displacement ten healthy sheep underwent left thoracotomy and sonomicrometry transducers were implanted at the apex and base of the LV. The time-varying parameters of the model were estimated by a standard Recursive Linear Least Squares (RLLS) technique.

RESULTS

LV stiffness at end-systole and end-diastole was in the range of 61.86-136 dyne/gxcm and 1.25-21.02 dyne/gxcm, respectively. Univariate linear regression was performed to verify the agreement between the estimated parameters, and the measured values of stiffness. The averaged magnitude of the stiffness and damping coefficients during a complete cardiac cycle were estimated as 58.63+/-12.8 dyne/gxcm and 0 dynexs/gxcm, respectively.

CONCLUSION

The results for the estimated elastic coefficients are consistent with the ones obtained from force-displacement diagram. The trend of change in the estimated parameters is also in harmony with the previous studies done using P-V diagram. The only input used in this model is the long axis displacement of the annulus plane, which can also be obtained non-invasively using tissue Doppler or MR imaging.