Department of Biomedical Engineering, Duke University, Durham, North Carolina, USA.
Department of Cardiology, Duke University Hospital, Durham, North Carolina, USA.
Ultrasound Med Biol. 2019 May;45(5):1112-1130. doi: 10.1016/j.ultrasmedbio.2018.12.011. Epub 2019 Mar 16.
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.
心肌僵硬度在心脏周期中表现出周期性变化。这些趋势与心脏的机电和血液动力学变化密切相关。动态心肌僵硬度的特征可以深入了解心肌的功能状态，并允许区分导致充血性心力衰竭的潜在生理机制。先前的工作表明，声辐射力脉冲（ARFI）成像具有捕获实验制剂（如 Langendorff 心脏）和开胸及心内环境中心肌僵硬度时间趋势的潜力。本研究旨在通过经胸成像窗以非侵入性方式研究 ARFI 测量人类受试者动态心肌僵硬度的潜力。对 12 名健康志愿者进行 ARFI 成像，以跟踪胸骨旁长轴和短轴视图中室间隔内的僵硬度变化。使用 5 个指数来量化心动周期内心肌僵硬度的动态变化：僵硬度比、松弛和收缩率以及松弛和收缩的时间常数。基于信号强度和跟踪保真度的指标（如位移信噪比、信号与杂波比、散斑的时间相干性和激励区域内的空间相似性）来评估 ARFI 采集的有效性。这些指标通过整个心动周期的平均 ARF 诱导位移、心肌与心脏腔室之间的对比度、射频信号的最小相关系数以及同时获取的方位波束之间的位移轨迹的相关性来量化。使用 1.5μm 的平均 ARF 诱导位移阈值，将 41%的 ARFI 采集确定为“成功”。与“不成功”采集相比，“成功”采集具有更高的 (i) 信号与杂波比、(ii) 时间相干性和 (iii) 空间相似性。两组之间这三个指标的中位数分别为 13.42dB 与 5.42dB、0.988 与 0.976 和 0.984 与 0.849。信号与杂波比、时间相干性和空间相似性也相互关联。在健康志愿者队列中，测量的僵硬度比为 2.74 ± 0.86；松弛率为 7.82 ± 4.69/s；收缩率为-7.31±3.79/s。松弛时间常数为 35.90 ± 20.04ms，收缩时间常数为 37.24 ± 19.85ms。ARFI 衍生的心肌僵硬度指数在两个视图中相似。这些结果表明，使用 ARFI 以非侵入性方式测量动态心肌僵硬度趋势是可行的，并突出了在经胸成像环境中实施该方法的技术挑战。
