A Comparison of Acoustic Radiation Force-Derived Indices of Cardiac Function in the Langendorff Perfused Rabbit Heart

Application of mean maximum Young's modulus value as a new parameter for differential diagnosis of prostate diseases

Transrectal shear wave elastography (T-SWE) can be used non-invasively to diagnose prostate cancer (PCa) and benign prostatic hyperplasia (BPH). The prostate tissue can be viewed as an ellipsoidal sphere with viscoelastic characterization. Linear elastic model has been used to characterize soft tissues, and the simplification of partial characterization provides incomplete information. This retrospective study included 144 suspected prostate disease patients who had completed T-SWE from our hospital. The mean maximum Young's modulus value (m-Emax) of the maximum transverse section of prostate was obtained by calculating the mean of 12 measured maximum Young's modulus value (Emax) in the four quadrants. M-Emax was statistically correlated with and good discriminability for PCa and BPH. There was a nonlinear dose-response relationship between m-Emax and PCa risk, as well as between m-Emax and BPH risk. The relationship between m-Emax and prostate disease was consistent with the nonlinear effect showed by tissues as an elastic model in normal physiological activity areas. When stiffness increasing, the stress-strain relationship was approximates linear. M-Emax can be used as an innovative parameter of Young's modulus value, which represents the stiffness of prostate tissue in T-SWE, and has a good effect in the differential diagnosis of prostate diseases.

Novel Imaging Technologies for Accurate Assessment of Cardiac Allograft Performance

Purpose of the Review

The current lack of objective and quantitative assessment techniques to determine cardiac graft relative viability results in risk-averse decision-making, which negatively impact the utilization of cardiac grafts. The purpose of this review is to highlight the current deficiencies in cardiac allograft assessment before focusing on novel cardiac assessment techniques that exploit conventional and emerging imaging modalities, including ultrasound, magnetic resonance, and spectroscopy.

Recent Findings

Extensive work is ongoing by the scientific community to identify improved objective metrics and tools for cardiac graft assessment, with the goal to safely increasing the number and proportion of hearts accepted for transplantation.

Summary

This review briefly discusses the in situ and ex vivo tools currently available for clinical organ assessment, before focusing on the individual capabilities of ultrasound, magnetic resonance, and spectroscopy to provide insightful, non-invasive information regarding cardiac graft functional and metabolic status that may be used to predict outcome after transplantation.

Assessing cardiac stiffness using ultrasound shear wave elastography

Shear wave elastography offers a new dimension to echocardiography: it measures myocardial stiffness. Therefore, it could provide additional insights into the pathophysiology of cardiac diseases affecting myocardial stiffness and potentially improve diagnosis or guide patient treatment. The technique detects fast mechanical waves on the heart wall with high frame rate echography, and converts their propagation velocity into a stiffness value. A proper interpretation of shear wave data is required as the shear wave interacts with the intrinsic, yet dynamically changing geometrical and material characteristics of the heart under pressure. This dramatically alters the wave physics of the propagating wave, demanding adapted processing methods compared to other shear wave elastography applications as breast tumor and liver stiffness staging. Furthermore, several advanced analysis methods have been proposed to extract supplementary material features such as viscosity and anisotropy, potentially offering additional diagnostic value. This review explains the general mechanical concepts underlying cardiac shear wave elastography and provides an overview of the preclinical and clinical studies within the field. We also identify the mechanical and technical challenges ahead to make shear wave elastography a valuable tool for clinical practice.

Measuring Intraventricular Pressure Using Ultrasound Elastography

OBJECTIVES

Intraventricular pressure (IVP) is one of the most important measurements for evaluating cardiac function, but this measurement is not currently easily assessable in the clinic. The primary reason for this is the absence of a noninvasive technique for measuring IVP. In this study, we investigate the relationship between IVP and dynamic myocardial stiffness measured by shear wave elasticity imaging (SWEI) and assess the feasibility of measuring IVP using SWEI.

METHODS

In 8 isolated working rabbit hearts, IVP was recorded in the left ventricle using a pressure catheter. Simultaneously, myocardial stiffness was recorded by SWEI. Using the peak values for IVP and SWEI measured stiffness, SWEI measurements were calibrated and converted to IVP.

RESULTS

A linear relationship with zero intercept was observed between IVP and SWEI, with the average slope of 0.318 kPa/mm Hg, R²  = 0.89. Using one point on the IVP/SWEI curve, SWEI measurements were converted to IVP. Estimated pressure using SWEI and IVP were linearly correlated with the slope of 0.95, R²  = 0.88 (mean end diastolic pressure by pressure catheter = 12.716 mm Hg and by SWEI=14.726 mm Hg), indicating the near equivalence of the 2 measurements.

CONCLUSION

We have shown that SWEI measurements are linearly related to IVP; therefore, pressure-based indices could potentially be derived from SWEI ultrasound elastography. The feasibility of using SWEI to estimate IVP with a single point calibration was also shown in this study.

Non-invasive Measurement of Dynamic Myocardial Stiffness Using Acoustic Radiation Force Impulse Imaging

Myocardial stiffness exhibits cyclic variations over the course of the cardiac cycle. These trends are closely tied to the electromechanical and hemodynamic changes in the heart. Characterization of dynamic myocardialstiffness can provide insights into the functional state of the myocardium, as well as allow for differentiation between the underlying physiologic mechanisms that lead to congestive heart failure. Previous work has revealed the potential of acoustic radiation force impulse (ARFI) imaging to capture temporal trends in myocardial stiffness in experimental preparations such as the Langendorff heart, as well as on animals in open-chest and intracardiac settings. This study was aimed at investigating the potential of ARFI to measure dynamic myocardial stiffness in human subjects, in a non-invasive manner through transthoracic imaging windows. ARFI imaging was performed on 12 healthy volunteers to track stiffness changes within the interventricular septum in parasternal long-axis and short-axis views. Myocardial stiffness dynamics over the cardiac cycle was quantified using five indices: stiffness ratio, rates of relaxation and contraction and time constants of relaxation and contraction. The yield of ARFI acquisitions was evaluated based on metrics of signal strength and tracking fidelity such as displacement signal-to-noise ratio, signal-to-clutter level, temporal coherence of speckle and spatial similarity within the region of excitation. These were quantified using the mean ARF-induced displacements over the cardiac cycle, the contrast between the myocardium and the cardiac chambers, the minimum correlation coefficients of radiofrequency signals and the correlation between displacement traces across simultaneously acquired azimuthal beams, respectively. Forty-one percent of ARFI acquisitions were determined to be "successful" using a mean ARF-induced displacement threshold of 1.5 μm. "Successful" acquisitions were found to have higher (i) signal-to-clutter levels, (ii) temporal coherence and (iii) spatial similarity compared with "unsuccessful" acquisitions. Median values of these three metrics, between the two groups, were measured to be 13.42dB versus 5.42dB, 0.988 versus 0.976 and 0.984 versus 0.849, respectively. Signal-to-clutter level, temporal coherence and spatial similarity were also found to correlate with each other. Across the cohort of healthy volunteers, the stiffness ratio measured was 2.74 ± 0.86; the rate of relaxation, 7.82 ± 4.69/s; and the rate of contraction, -7.31±3.79 /s. The time constant of relaxation was 35.90 ± 20.04ms, and that of contraction was 37.24 ± 19.85ms. ARFI-derived indices of myocardial stiffness were found to be similar in both views. These results indicate the feasibility of using ARFI to measure dynamic myocardial stiffness trends in a non-invasive manner and also highlightthe technical challenges of implementing this method in the transthoracic imaging environment.

Assessment of Diastolic Function Using Ultrasound Elastography

Shear wave elasticity imaging (SWEI) is a novel ultrasound elastography technique for assessing tissue stiffness. In this study, we investigate the potential of SWEI for providing diastolic functional assessment. In 11 isolated rabbit hearts, pressure-volume (PV) measurements were recorded simultaneously with SWEI recordings from the left ventricle free wall before and after induction of global ischemia. PV-based end diastolic stiffness increased by 100% after ischemia (p <0.05), and SWEI stiffness showed an increase of 103% (p <0.05). The relaxation time constant (τ) before and after ischemia derived from pressure and SWEI curves showed increases of 79% and 76%, respectively (p <0.05). A linear regression between pressure-derived and SWEI-based (τ) showed a slope of 1.164 with R² = 0.80, indicating the near equivalence of the two assessments. SWEI can be used to derive (τ) values and myocardial end diastolic stiffness. In global conditions, these measurements are consistent with PV measurements of diastolic function.

Recent technological advancements in cardiac ultrasound imaging

About 92.1 million Americans suffer from at least one type of cardiovascular disease. Worldwide, cardiovascular diseases are the number one cause of death (about 31% of all global deaths). Recent technological advancements in cardiac ultrasound imaging are expected to aid in the clinical diagnosis of many cardiovascular diseases. This article provides an overview of such recent technological advancements, specifically focusing on tissue Doppler imaging, strain imaging, contrast echocardiography, 3D echocardiography, point-of-care echocardiography, 3D volumetric flow assessments, and elastography. With these advancements ultrasound imaging is rapidly changing the domain of cardiac imaging. The advantages offered by ultrasound imaging include real-time imaging, imaging at patient bed-side, cost-effectiveness and ionizing-radiation-free imaging. Along with these advantages, the steps taken towards standardization of ultrasound based quantitative markers, reviewed here, will play a major role in addressing the healthcare burden associated with cardiovascular diseases.

MRI use for atrial tissue characterization in arrhythmias and for EP procedure guidance

We review the utilization of magnetic resonance imaging methods for classifying atrial tissue properties that act as a substrate for common cardiac arrhythmias, such as atrial fibrillation. We then review state-of-the-art methods for mapping this substrate as a predicate for treatment, as well as methods used to ablate the electrical pathways that cause arrhythmia and restore patients to sinus rhythm.