Preliminary study on estimation of flow velocity vectors using focused transmit beams

High-frame-rate ultrasound imaging with plane wave transmissions is a predominant method for blood flow imaging, and methods for estimation of blood flow velocity vectors have been developed based on high-frame-rate imaging. On the other hand, in imaging of soft tissues, such as arterial walls and atherosclerotic plaques, high-frame-rate imaging sometimes suffers from high-level clutters. Even in observation of the arterial wall with a focused transmit beam, it would be highly beneficial if blood flow velocity vectors could be estimated simultaneously. We conducted a preliminary study on estimation of blood flow velocity vectors based on a multi-angle Doppler method with focused transmit beam and parallel receive beamforming. It was shown that the lowest estimation error was achieved at a steering angle of 25 degrees by simulation. Also, velocity vectors with typical velocity magnitudes and directions could be obtained by the proposed method in in vivo measurement of a carotid artery.

Enhanced Real-Time Intermediate Flow Estimation for Video Frame Interpolation

Recently, the demand for high-quality video content has rapidly been increasing, led by the development of network technology and the growth in video streaming platforms. In particular, displays with a high refresh rate, such as 120 Hz, have become popular. However, the visual quality is only enhanced if the video stream is produced at the same high frame rate. For the high quality, conventional videos with a low frame rate should be converted into a high frame rate in real time. This paper introduces a bidirectional intermediate flow estimation method for real-time video frame interpolation. A bidirectional intermediate optical flow is directly estimated to predict an accurate intermediate frame. For real-time processing, multiple frames are interpolated with a single intermediate optical flow and parts of the network are implemented in 16-bit floating-point precision. Perceptual loss is also applied to improve the cognitive performance of the interpolated frames. The experimental results showed a high prediction accuracy of 35.54 dB on the Vimeo90K triplet benchmark dataset. The interpolation speed of 84 fps was achieved for 480p resolution.

Myocardial stiffness assessed by natural shear wave elastography is related to pressure-volume loop derived parameters

Background Several cardiovascular disorders are accompanied by a stiffening of the myocardium and may result in diastolic heart failure. The non-invasive assessment of myocardial stiffness could therefore improve the understanding of the pathophysiology and guide treatment. Shear wave elastography (SWE) is a recent technique with tremendous potential for evaluating myocardial stiffness in a non-invasive way. Using high frame rate echocardiography, the propagation speed of shear waves is evaluated, which is directly related to the stiffness of the myocardium. These waves are induced by for instance mitral valve closure (MVC) and propagate throughout the cardiac muscle. However, validation of SWE against an invasive gold standard method is lacking. Purpose The aim of this study was to compare echocardiographic shear wave elastography against invasive pressure-volume loops, a gold standard reference method for assessing chamber stiffness. Methods In 15 pigs (31.2±4.1 kg) stiffness of the myocardium was acutely changed by inducing ischemia/reperfusion (I/R) injury. For this, the proximal LAD was balloon occluded for 90 minutes with subsequent reperfusion for 40 minutes. Conventional and high frame rate echocardiographic images were acquired simultaneously with pressure-volume loops during baseline conditions and after the induction of the I/R injury. Preload was reduced in order to acquire a set of pressure-volume loops to derive the end-diastolic pressure volume relation (EDPVR). From the EDPVR, the stiffness coefficient β and the operating chamber stiffness dP/dV were obtained. High frame rate echocardiographic datasets of the parasternal long axis view were acquired with an experimental ultrasound scanner (HD-PULSE) at an average frame rate of 1304±115 Hz. Tissue acceleration maps were obtained by drawing an M-mode line along the interventricular septum in order to visualize shear waves after MVC (at end-diastole). The propagation speed was assessed by semi-automatically measuring the slope (Figure A). Results I/R injury led to an elevated chamber stiffness constant β (0.09±0.03 1/ml vs. 0.05±0.01 1/ml; p<0.001) and operating chamber stiffness dP/dV (1.09±0.38 mmHg/ml vs. 0.50±0.18 mmHg/ml; p<0.01). Likewise, shear wave speed after MVC increased after the induction of the I/R injury in comparison to baseline (6.1±1.2 m/s vs. 3.2±0.8 m/s; p<0.001). Shear wave speed had a moderate positive correlation with β (r=0.63; p<0.001) (Figure B) and a strong positive correlation with dP/dV (r=0.81; p<0.001) (Figure C). Conclusion End-diastolic shear wave speed is strongly related to chamber stiffness, assessed invasively by pressure-volume loops. These results indicate that shear wave propagation speed could be used as a novel non-invasive measurement of the mechanical properties of the ventricle. FUNDunding Acknowledgement Type of funding sources: Public grant(s) – National budget only. Main funding source(s): FWO - Research Foundation Flanders

High frame rate speckle tracking echocardiography to assess diastolic function

Background Left ventricular (LV) strain rate (SR) during isovolumic relaxation (SRIVR) and early diastolic filling (SRe) has previously been shown to correlate with the invasive gold standard for LV diastolic function (i.e. the time constant of LV pressure decay tau). However, the translation of these biomarkers to the clinic has been hampered by technical limitations. Indeed, conventional speckle tracking (STE) is limited by its temporal resolution, whereas tissue Doppler imaging (TDI) is angle-dependent, labor-intensive and thus rarely used clinically nowadays. Purpose The aim of this study was to show that these limitations could be overcome by using a recently proposed STE algorithm operating on high frame rate (HFR) imaging data. Methods 37 subjects (age: 64±12, 81% male) were included in the study; 16 had cardiac amyloidosis, 12 were undergoing clinically indicated left and/or right heart cardiac catheterization and 9 were healthy volunteers. Since the sequence of left ventricular activation and thus the repolarization process (i.e. relaxation) starts at mid septum, we measured SRIVR and SRe in the mid septal segment in an apical 4 chamber view using a commercially available clinical system with: (1) TDI (frame rate (FR) ∼142 Hz); (2) STE (FR ∼65 Hz). Moreover, subjects were scanned with HD-PULSE, an experimental high frame ultrasound scanner (FR ∼915 Hz) and then a manually placed contour was tracked during the cardiac cycle by a custom-made 2D HFR STE algorithm, to compute and extract SRIVR and SRe from the mid septum. Since TDI is considered the reference method to assess SR, conventional as well as HFR STE values were correlated against the TDI SR values. Results In 3 subjects, SRIVR could not be reliably assessed with the clinical STE approach, which we attributed to the relatively low temporal resolution of the images; all other measurements could be made in all subjects. For both biomarkers, HFR STE values correlated better with the TDI reference measurements than the clinical STE estimates (Fig.1). The latter estimates showed a systematic underestimation (bias −0.19 1/s (p<0.01) and −0.46 1/s (p<0.01) for SRIVR and SRe respectively) while no significant bias was observed for the HFR STE values. Similarly, the limits of agreement of the HFR STE values were narrower (−0.45 to +0.54 1/s and −0.94 to +0.86 1/s) than those of the clinical STE measurements (−0.85 to +0.48 1/s and −1.32 to +0.41 1/s). Conclusions These results show that HFR STE offers a reliable way to assess novel biomarkers of diastolic function in a user-friendly manner and can therefore facilitate their incorporation to the clinical practice. FUNDunding Acknowledgement Type of funding sources: None.