Fetal scanning is one of the most common applications of ultrasound imaging and serves as a source of vital information about maternal and fetal health. Visualization of clinically relevant structures, however, can be severely compromised in difficult-to-image patients due to poor resolution and the presence of high levels of acoustical noise or clutter. We have developed novel coherence-based beamforming methods called Short-Lag Spatial Coherence (SLSC) imaging and Harmonic Spatial Coherence imaging (HSCI) and applied them to suppress the effects of clutter in fetal imaging. This method is used to create images of the spatial coherence of the backscattered ultrasound as opposed to images of echo magnitude. We present the results of a patient study to assess the benefits of coherence-based beamforming in the context of first trimester fetal exams. Matched fundamental B-mode, SLSC, harmonic B-mode and HSCI images were generated using raw RF data collected on 11 volunteers in the first trimester of pregnancy. The images were compared for qualitative differences in image texture and target conspicuity as well as using quantitative imaging metrics such as SNR, CNR and contrast. SLSC and HSCI showed statistically significant improvements across all imaging metrics compared to B-mode and harmonic B-mode respectively. These improvements were greatest for poor quality B-mode images where contrast of anechoic targets was improved from 15 dB in fundamental B-mode to 27 dB in SLSC and 17 dB in harmonic B-mode to 30 dB in HSCI. CNR improved from 1.4 to 2.5 in the fundamental images and 1.4 to 3.1 in the harmonic case. These results exhibit the potential of coherence-based beamforming to improve image quality and target detectability, especially in high noise environments.
Myocardial stiffness exhibits cyclic variations over the course of the cardiac cycle. These trends are closely tied to the electro-mechanical and hemodynamic changes in the heart. Characterization of dynamic myocardial stiffness can provide insights into the functional state of the myocardium as well as allow for differentiation between the underlying physiological mechanisms that lead to congestive heart failure. Previous work has demonstrated the potential of acoustic radiation force impulse (ARFI) imaging to capture temporal trends of myocardial stiffness in experimental preparations such as the Langendorff heart as well as on animals in open-chest and intracardiac settings. This study aims to investigate the potential of ARFI to measure dynamic myocardial stiffness on human subjects, in a noninvasive manner through transthoracic imaging windows. ARFI imaging was performed on twelve healthy volunteers to track stiffness changes within the interventricular septum (IVS) in the parasternal long-axis (PLAX) and short-axis (PSAX) views. Myocardial stiffness dynamics over the cardiac cycle were quantified using five indices: stiffness ratio, rates of relaxation and contraction, and time constants of relaxation and contraction. Yield of ARFI acquisitions was evaluated based on metrics of signal strength and tracking fidelity such as displacement signal-to-noise, signal-to-clutter level, temporal coherence of speckle, and spatial similarity within the region-of-excitation (ROE). These were quantified using the mean ARFinduced displacements over the cardiac cycle, the contrast between the myocardium and the cardiac chambers, the minimum correlation coefficients of RF signals, and the correlation between displacement traces across simultaneously-acquired azimuthal beams, respectively.
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