Geometrical models of left ventricular contraction from MRI of the normal and spontaneously hypertensive rat heart

Increased cardiac work provides a link between systemic hypertension and heart failure

The spontaneously hypertensive rat (SHR) is an established model of human hypertensive heart disease transitioning into heart failure. The study of the progression to heart failure in these animals has been limited by the lack of longitudinal data. We used MRI to quantify left ventricular mass, volume, and cardiac work in SHRs at age 3 to 21 month and compared these indices to data from Wistar-Kyoto (WKY) controls. SHR had lower ejection fraction compared with WKY at all ages, but there was no difference in cardiac output at any age. At 21 month the SHR had significantly elevated stroke work (51 ± 3 mL.mmHg SHR vs. 24 ± 2 mL.mmHg WKY; n = 8, 4; P < 0.001) and cardiac minute work (14.2 ± 1.2 L.mmHg/min SHR vs. 6.2 ± 0.8 L.mmHg/min WKY; n = 8, 4; P < 0.001) compared to control, in addition to significantly larger left ventricular mass to body mass ratio (3.61 ± 0.15 mg/g SHR vs. 2.11 ± 0.008 mg/g WKY; n = 8, 6; P < 0.001). SHRs showed impaired systolic function, but developed hypertrophy to compensate and successfully maintained cardiac output. However, this was associated with an increase in cardiac work at age 21 month, which has previously demonstrated fibrosis and cell death. The interplay between these factors may be the mechanism for progression to failure in this animal model.

Sensitivity of Noninvasive Cardiac Electrophysiological Imaging to Variations in Personalized Anatomical Modeling

OBJECTIVE

Noninvasive cardiac electrophysiological (EP) imaging techniques rely on anatomically-detailed heart-torso models derived from high-quality tomographic images of individual subjects. However, anatomical modeling involves variations that lead to unresolved uncertainties in the outcome of EP imaging, bringing questions to the robustness of these methods in clinical practice. In this study, we design a systematic statistical approach to assess the sensitivity of EP imaging methods to the variations in personalized anatomical modeling.

METHODS

We first quantify the variations in personalized anatomical models by a novel application of statistical shape modeling. Given the statistical distribution of the variation in personalized anatomical models, we then employ unscented transform to determine the sensitivity of EP imaging outputs to the variation in input personalized anatomical modeling.

RESULTS

We test the feasibility of our proposed approach using two of the existing EP imaging methods: epicardial-based electrocardiographic imaging and transmural electrophysiological imaging. Both phantom and real-data experiments show that variations in personalized anatomical models have negligible impact on the outcome of EP imaging.

CONCLUSION

This study verifies the robustness of EP imaging methods to the errors in personalized anatomical modeling and suggests the possibility to simplify the process of anatomical modeling in future clinical practice.

SIGNIFICANCE

This study proposes a systematic statistical approach to quantify anatomical modeling variations and assess their impact on EP imaging, which can be extended to find a balance between the quality of personalized anatomical models and the accuracy of EP imaging that may improve the clinical feasibility of EP imaging.

Positron emission tomography in the assessment of left ventricular function in healthy rats: a comparison of four imaging methods

OBJECTIVE

To measure left ventricular (LV) function parameters in heart of healthy rats by three different positron emission tomography (PET) imaging techniques and by magnetic resonance imaging (MRI).

METHODS

ECG-gated microPET examinations were obtained in seven healthy rats with 2-deoxy-2-[(18)F]fluoro-D-glucose (FDG) for calculation of LV-function from the blood-pool phase of the dynamic recording (FDGBP), and also from the later myocardial uptake (FDGMyo). On subsequent days, we re-measured LV-function using the novel blood-pool tracer (68)Ga-albumin (AlbBP) and again by FDG (FDGMyo2) in one setting. Cine-MRI examination provided the reference standard measurement.

RESULTS

The mean LV ejection fractions (LVEF) were 56 ± 3 (FDGBP), 55 ± 3 (FDGMyo), 56 ± 3 (FDGMyo2), 57 ± 3 (AlbBP), and 57 ± 2 (MRI). There were good to excellent correlations found between the LVEF-values as compared to MRI reference standard for FDGBP (r = 0.71), FDGMyo (r = 0.86) and AlbBP (r = 0.88). Both of the blood-pool methods significantly overestimated the magnitudes of end-diastolic-volume and end-systolic-volume, whereas FDGMyo matched closely to the MRI reference standard. There was no significant bias for both blood-pool methods and a minor negative bias for FDGMyo regarding the LV ejection fraction (LVEF) when compared to cine-MRI results. There was no significant difference between the means of FDGMyo and FDGMyo2 (P = .50).

CONCLUSIONS

Relative to reference standard MRI measurements of LVEF, there was excellent agreement between PET-based measurements, notably for the novel blood-pool tracer (68)Ga-albumin.

The effects of asymmetric ventricular filling on left-right ventricular interaction in the normal rat heart

Heart failure is characterised by ventricular dysfunction and with the potential for changes to ventricular volumes constraining the mechanical performance of the heart. The contribution of this interaction from geometric changes rather than fibrosis or metabolic changes is unclear. Using the constant pressure Langendorff-perfused rat heart, the volume interaction between left ventricle (LV) and right ventricle (RV) was investigated. RV diastolic stiffness (P < 0.001) and developed pressure (P < 0.001) were significantly lower than LV. When the RV was fixed at the end-diastolic volume (EDV) or EDV + 50 %, both LV systolic and diastolic performance were unaffected with increasing LV balloon volume. However, at fixed LV volume, RV systolic performance was significantly decreased when LV volume increased to EDV + 50 % when RV volume was increased incrementally between 50 and 300 μl (P < 0.001). Systolic interaction in RV was noted as declining RV peak systolic load with increasing LV systolic pressure (P < 0.05) and diastolic interaction was noted for RV when LV volume was increased from EDV to EDV + 50 % (P < 0.05). RV diastolic wall stress was increased with increasing LV balloon volume (P < 0.05), but LV wall stress was unaltered at fixed RV balloon volume. Taken together, increasing LV volume above EDV decreased systolic performance and triggered ventricular constraint in the RV but the RV itself had no effect on the performance of the LV. These results are consistent with overload of the LV impairing pulmonary perfusion by direct ventricular interaction with potential alteration to ventilation-perfusion characteristics within the lung.

Weighted least-squares finite element method for cardiac blood flow simulation with echocardiographic data

As both fluid flow measurement techniques and computer simulation methods continue to improve, there is a growing need for numerical simulation approaches that can assimilate experimental data into the simulation in a flexible and mathematically consistent manner. The problem of interest here is the simulation of blood flow in the left ventricle with the assimilation of experimental data provided by ultrasound imaging of microbubbles in the blood. The weighted least-squares finite element method is used because it allows data to be assimilated in a very flexible manner so that accurate measurements are more closely matched with the numerical solution than less accurate data. This approach is applied to two different test problems: a flexible flap that is displaced by a jet of fluid and blood flow in the porcine left ventricle. By adjusting how closely the simulation matches the experimental data, one can observe potential inaccuracies in the model because the simulation without experimental data differs significantly from the simulation with the data. Additionally, the assimilation of experimental data can help the simulation capture certain small effects that are present in the experiment, but not modeled directly in the simulation.

Simultaneous measurement of blood and myocardial velocity in the rat heart by phase contrast MRI using sparseq-space sampling

PURPOSE

To measure cardiac blood flow patterns and ventricular wall velocities through the cardiac cycle in anesthetized Wistar Kyoto (WKY) rats.

MATERIALS AND METHODS

A gradient-echo cine pulse sequence incorporating pulsed field gradients (PFGs) provided phase contrast (PC) motion encoding. We achieved a range of velocity sensitivity that was sufficient to measure simultaneously the large flow velocities within the cardiac chambers and aortic outflow tract (up to 70 cm s(-1) during systole), and the comparatively small velocities of the cardiac wall (0-3 cm s(-1)). A scheme of sparsely sampling q-space combined with a probability-based method of velocity calculation permitted such measurements along three orthogonal axes, and yielded velocity vector maps in all four chambers of the heart and the aorta, in both longitudinal and transverse sections, for up to 12 time-points in the cardiac cycle.

RESULTS

Left ventricular systole was associated with a symmetrical laminar flow pattern along the cardiac axis, with no appearance of turbulence. In contrast, blood showed a swirling motion within the right ventricle (RV) in the region of the pulmonary outflow tract. During left ventricular diastole a plume of blood entered the left ventricle (LV) from the left atrium. The ventricular flow patterns could also be correlated with measurements of left ventricular wall motion. The greatest velocities of the ventricular walls occurred in the transverse cardiac plane and were maximal during diastolic refilling. The cardiac wall motion in the longitudinal axis demonstrated a caudal-apical movement that may also contribute to diastolic refilling.

CONCLUSION

The successful measurements of blood and myocardial velocity during normal myocardial function may be extended to quantify pathological cardiac changes in animal models of human cardiac disease.

Finite element modelling and simulations in cardiovascular mechanics and cardiology: A bibliography 1993–2004

The paper gives a bibliographical review of the finite element modelling and simulations in cardiovascular mechanics and cardiology from the theoretical as well as practical points of views. The bibliography lists references to papers, conference proceedings and theses/dissertations that were published between 1993 and 2004. At the end of this paper, more than 890 references are given dealing with subjects as: Cardiovascular soft tissue modelling; material properties; mechanisms of cardiovascular components; blood flow; artificial components; cardiac diseases examination; surgery; and other topics.

Reproducibility of left ventricular volume and ejection fraction measurements in rat using pinhole gated SPECT

PURPOSE

The aim of this study was to investigate the intra-individual reproducibility of left ventricular volume and ejection fraction measurements in living rat using pinhole gated single-photon emission computed tomography (SPECT).

METHODS

Eight normal male Wistar rats underwent four pinhole gated SPECT acquisitions over a 1-month period. Two pinhole gated myocardial perfusion SPECT studies were acquired at a 1-week interval after injecting the animals with 439+/-52 MBq of (99m)Tc-sestamibi. Subsequently, 1 week after the perfusion studies, two pinhole gated blood pool SPECT studies were acquired at a 1-week interval after in vivo labelling of the red blood cells using 520+/-49 MBq of (99m)Tc-pertechnetate. Pinhole gated SPECT acquisitions were done on a single-head gamma camera equipped with a pinhole collimator with a 3-mm opening and 165-mm focal length. Parameters of acquisition were as follows: 44 mm radius of rotation, 360 degrees rotation using a circular orbit, 64 projections, 64x64 matrix, gating using 16 time frames and 22-min acquisition time. The projection data were reconstructed with a modified version of OSEM taking into account the pinhole geometry and incorporating a prior assumption about the temporal properties of gated SPECT studies to reduce noise. Left ventricular volumes and ejection fraction were measured using automatic quantification algorithms. Inter-study, inter-observer and intra-observer reproducibility was investigated.

RESULTS

Pinhole gated myocardial perfusion and pinhole gated blood pool images were of high quality in all animals. No significant differences were observed between the repeated measurements. The pinhole gated myocardial perfusion SPECT studies indicated that differences between repeated measurements larger than 41 microl for end-diastolic volume, 17 microl for end-systolic volume and 3% for ejection fraction were significant. The pinhole gated blood pool SPECT studies indicated that differences between repeated measurements larger than 42 microl for end-diastolic volume, 38 mul for end-systolic volume and 5% for ejection fraction were significant. In addition to the reproducibility measures, the accuracy of volume measurements in pinhole gated blood pool SPECT was confirmed by a phantom study. Excellent correlations were observed between the measured volumes and the actual phantom volumes.

CONCLUSION

Pinhole gated SPECT is an accurate and reproducible technique for cardiac studies of small animals. Because this technique is non-invasive, the same animal can be imaged repetitively, allowing follow-up studies.

Compact and efficient 3D shape description through radial function approximation

A fast and simple method for three-dimensional shape description is described. The method views a 3D object as a radial distance function on the unit sphere, and thus reduces the dimensionality of the description problem by one. The radial distance function is approximated by Fourier methods in the basis of the spherical harmonic polynomials. The necessary integration is carried out on the object boundary, rather than on the unit sphere. Consequently, there is no need of a parameterisation of the object surface. The description makes it possible to compare shapes in a computationally very simple way. Solutions on how to cope with translated and rotated objects are discussed. The method is developed for star-shaped objects, but is stable even if the input image is non-star-shaped. The method is tested in a data set from magnetic resonance imaging (MRI) of the brain. Potential medical applications are discussed.

MRI analysis of right ventricular function in normal and spontaneously hypertensive rats

Right ventricular structure and function were characterized in spontaneously hypertensive rats (SHR) using non-invasive magnetic resonance imaging (MRI) techniques. These studies therefore complement previous reports preoccupied with left ventricular changes associated with this condition. Eight SHR and eight control normotensive Wistar-Kyoto (WKY) rats were each subdivided into equal age-matched groups of 8 and 12 weeks. The right ventricle was imaged through a series of twelve contiguous 1.37-1.75 mm transverse sections at twelve equally spaced time-points that covered both systole and most of diastole thereby completely reconstructing right ventricular anatomy. This gave measurements of right ventricular myocardial mass that were consistent through all twelve time-points in all four experimental groups throughout their cardiac cycles. However, spontaneous hypertension increased this right ventricular myocardial mass, as well as the end-diastolic (EDV) and end-systolic volumes (ESV). Although stroke volume (SV) was conserved, decreases in ejection fraction (EF), a positive shift in the relationship between SV and EDV, and reduced indices of systolic ejection rates in SHR rats compared with the age-matched normal WKY controls indicated significant systolic dysfunction. Additionally, reductions in the rates of diastolic relaxation suggested the onset of diastolic dysfunction. Thus, the non-invasive nature of MRI has made it possible for the first time to demonstrate alterations in structure of the right ventricle and in quantitative indicators of its systolic and diastolic function in the SHR model of hypertension.