High-Frame-Rate Echo-Particle Image Velocimetry Can Measure the High-Velocity Diastolic Flow Patterns

Left ventricular flow patterns have been studied as potential early-stage markers of cardiac dysfunction.¹ A relatively new method of measuring left ventricular flow patterns, named echo-particle image velocimetry (echoPIV), tracks the motion of ultrasound contrast agent microbubbles in the blood using echocardiography. However, the low frame rates (50–70 Hz) permitted by the current generation of clinical ultrasound scanners cause velocity magnitudes to be severely underestimated during filling and ejection (<40 cm/s at 50 Hz).² High-frame-rate (HFR) echocardiography, using diverging wave transmission schemes, has allowed for frame rates of ≤100× faster than conventional line-scanning echocardiography. The image quality improvements when using HFR contrast-enhanced ultrasound over conventional contrast-enhanced ultrasound have recently been described.³ Still, measurement of the high-energy and high-velocity transmitral jet has yet to be demonstrated in humans. We have shown previously, in an in vitro left ventricular phantom study, that HFR echoPIV can accurately measure the high-energy diastolic flow patterns.⁴ In this work, we demonstrate that this holds true in a patient with heart failure.

A patient (19, woman, 1.65 m, 66 kg) with dilated cardiomyopathy and dual chamber pacing implantable cardioverter-defibrillators was admitted for decompensatio cordis. Apical 3-chamber views were obtained using both a clinical scanner (EPIQ 7 with X5-1 probe; Philips Healthcare, Best, the Netherlands) and a research scanner (Vantage 256; Verasonics, Kirkland, WA) with a P4-1 probe (Philips Healthcare). Pulsed-wave Doppler measurements were obtained, using the clinical scanner, in the region of the mitral valve tips. Ultrasound contrast agent (SonoVue; Bracco Imaging SpA, Milan, Italy) was then continuously infused at 0.6 mL/min (VueJect BR-INF 100; Bracco Imaging SpA), and its arrival in the left ventricle was verified with the clinical scanner. The research scanner was then used to obtain HFR contrast-enhanced ultrasound acquisitions using a 2-angle (−7° and 7°) diverging wave sequence with 2-pulse contrast scheme (pulse inversion; mechanical index ≈0.06 to 0.01) at a pulse repetition frequency of 4900 Hz, resulting in an imaging frame rate of 1225 Hz. EchoPIV analysis was performed in the polar domain, using custom particle image velocimetry software that used correlation compounding on ensembles of 5 frames for each angle.⁴ The final vector-grid resolution was 1.25° by 1.25 mm. HFR echoPIV magnitudes were validated by comparing the mean temporal velocity profile to the pulsed-wave Doppler spectrum captured in the same location. This study was approved by Erasmus Medical Center Medical Ethics Committee (NL63755.078.18).

The velocities measured with HFR echoPIV agreed well with the pulsed-wave Doppler spectrum (Figure [A]), with peak velocities ≤80 cm/s measured in this patient. This is the first demonstration of echoPIV measuring the high velocities present in the transmitral jet in adults. The high temporal resolution also permits study of the flow patterns in greater detail (Movie I in the Data Supplement). For example, the large, central clockwise vortex was observed pinching off the transmitral jet before migrating apically (Figure [B through D], *). Smaller, more transient vortices were also observed, such as the counterclockwise vortex between the jet and the free wall (Figure [B], †).

We have demonstrated in a patient with heart failure that HFR echoPIV can measure the, previously unobtainable, high-velocity flow patterns in 2-dimensions. This development has potential to become a useful tool in the study of intraventricular blood flow and its relation with ventricular function.