A review is presented of measurement techniques to characterise dispersed multiphase flows, which are not accessible by means of conventional optical techniques. The main issues that limit the accuracy and effectiveness of optical techniques are briefly discussed: cross-talk, a reduced signal-to-noise ratio, and (biased) data drop-out. Extensions to the standard optical techniques include the use of fluorescent tracers, refractive index matching, ballistic imaging, structured illumination, and optical coherence tomography. As the first non-optical technique, a brief discussion of electrical capacitance tomography is given. While truly non-invasive, it suffers from a low resolving power. Ultrasound-based techniques have rapidly evolved from Doppler-based profiling to recent 2D approaches using feature tracking. The latter is also suitable for time-resolved flow studies. Magnetic resonance velocimetry can provide time-averaged velocity fields in 3D for the continuous phase. Finally, X-ray imaging is demonstrated to be an important tool to quantify local gas fractions. While potentially very powerful, the impact of the techniques will depend on the development of acquisition and measurement protocols for fluid mechanics, rather than for clinical imaging. This requires systematic development, aided by careful validation experiments. As theoretical predictions for multiphase flows are sparse, it is important to formulate standardised ‘benchmark’ flows to enable this validation.

/pdf/measurement-in-opaque-flows-a-review-of-measurement-m7w5bbmdss.pdf

Measurement in opaque flows: a review of measurement techniques for dispersed multiphase flows

Left ventricular (LV) blood flow is an inherently complex time-varying 3-D phenomenon, where 2-D quantification often ignores the effect of out-of-plane motion. In this study, we describe high frame rate 4-D echocardiographic particle image velocimetry (echo-PIV) using a prototype matrix transesophageal transducer and a dynamic LV phantom for testing the accuracy of echo-PIV in the presence of complex flow patterns. Optical time-resolved tomographic PIV (tomo-PIV) was used as a reference standard for comparison. Echo-PIV and tomo-PIV agreed on the general profile of the LV flow patterns, but echo-PIV smoothed out the smaller flow structures. Echo-PIV also underestimated the flow rates at greater imaging depths, where the PIV kernel size and transducer point spread function were large relative to the velocity gradients. We demonstrate that 4-D echo-PIV could be performed in just four heart cycles, which would require only a short breath-hold, providing promising results. However, methods for resolving high velocity gradients in regions of poor spatial resolution are required before clinical translation.

https://biblio.ugent.be/publication/8648300/file/8648727.pdf

4-D Echo-Particle Image Velocimetry in a Left Ventricular Phantom

Time-resolved particle image velocimetry

Ultrasound vector Doppler techniques for three-dimensional (3-D) blood velocity measurements are currently limited by low temporal resolution and high computational cost. In this paper, an efficient 3-D high-frame-rate vector Doppler method, which estimates the displacements in the frequency domain, is proposed. The novel method extends to 3-D an approach so far proposed for two-dimensional (2-D) velocity measurements by approximating the (x, y, z) displacement of a small volume through the displacements estimated for the 2-D regions parallel to the y and x directions, respectively. The new method was tested by simulation and experiments for a 3.7 MHz, 256-element, 2-D piezoelectric sparse spiral array. Simulations were also performed for an equivalent 7 MHz Capacitive Micromachined Ultrasonic Transducer spiral array. The results indicate performance (bias ± standard deviation: 6.5 ± 8.0) comparable to the performance obtained by using a linear array for 2-D velocity measurements. These results are particularly encouraging when considering that sparse arrays were used, which involve a lower signal-to-noise ratio and worse beam characteristics with respect to full 2-D arrays.

https://www.mdpi.com/2076-3417/10/15/5365/pdf

High-frame-rate 3-D vector flow imaging in the frequency domain

Abnormal blood flow and wall shear stress (WSS) can cause and be caused by cardiovascular disease. To date, however, no standard method has been established for mapping WSS in vivo. Here we demonstrate wide-field assessment of WSS in the rabbit abdominal aorta using contrast-enhanced ultrasound image velocimetry (UIV). Flow and WSS measurements were made independent of beam angle, curvature or branching. Measurements were validated in an in silico model of the rabbit thoracic aorta with moving walls and pulsatile flow. Mean errors over a cardiac cycle for velocity and WSS were 0.34 and 1.69%, respectively. In vivo time average WSS in a straight segment of the suprarenal aorta correlated highly with simulations (PC = 0.99) with a mean deviation of 0.29 Pa or 5.16%. To assess fundamental plausibility of the measurement, UIV WSS was compared to an analytic approximation derived from the Poiseuille equation; the discrepancy was 17%. Mapping of WSS was also demonstrated in regions of arterial branching. High time average WSS (TAWSSxz = 3.4 Pa) and oscillatory flow (OSIxz = 0.3) were observed near the origin of conduit arteries. In conclusion, we have demonstrated that contrast-enhanced UIV is capable of measuring spatiotemporal variation in flow velocity, arterial wall location and hence WSS in vivo with high accuracy over a large field of view.

/pdf/determining-haemodynamic-wall-shear-stress-in-the-rabbit-1cfmcaywnu.pdf

Determining Haemodynamic Wall Shear Stress in the Rabbit Aorta In Vivo Using Contrast-Enhanced Ultrasound Image Velocimetry

We describe a 3-D multiline parallel beamforming scheme for real-time volumetric ultrasound imaging using a prototype matrix transesophageal echocardiography probe with diagonally diced elements and separated transmit and receive arrays. The elements in the smaller rectangular transmit array are directly wired to the ultrasound system. The elements of the larger square receive aperture are grouped in 4 × 4-element sub-arrays by micro-beamforming in an application-specific integrated circuit. We propose a beamforming sequence with 85 transmit–receive events that exhibits good performance for a volume sector of 60° × 60°. The beamforming is validated using Field II simulations, phantom measurements and in vivo imaging. The proposed parallel beamforming achieves volume rates up to 59Hz and produces good-quality images by angle-weighted combination of overlapping sub-volumes. Point spread function, contrast ratio and contrast-to-noise ratio in the phantom experiment closely match those of the simulation. In vivo 3-D imaging at 22-Hz volume rate in a healthy adult pig clearly visualized the cardiac structures, including valve motion.

/pdf/fast-volumetric-imaging-using-a-matrix-transesophageal-14ineq1qg4.pdf

Fast Volumetric Imaging Using a Matrix Transesophageal Echocardiography Probe with Partitioned Transmit–Receive Array

This paper presents a power- and area-efficient approach to digitizing the echo signals received by piezoelectric transducer elements, commonly used for ultrasound imaging. This technique utilizes such elements not only as sensors but also as the loop filter of an element-level $\Delta \Sigma $ analog to digital converter (ADC). The receiver chain is thus greatly simplified, yielding savings in area and power. Every ADC becomes small enough to fit underneath a $150\,\,\mu \mathrm {m} \times 150\,\,\mu \mathrm {m}$ transducer element, enabling simultaneous acquisition and digitization from all the elements in a 2-D array. This is especially valuable for miniature 3-D probes. Experimental results are reported for a prototype receiver chip with an array of $5 \times 4$ element-matched ADCs and a transducer array fabricated on top of the chip. Each ADC consumes 800 $\mu \mathrm {W}$ from a 1.8 V supply and achieves a SNR of 47 dB in a 75% bandwidth around a center frequency of 5 MHz.

/pdf/an-element-matched-electromechanical-delta-sigma-adc-for-4cklyg0x4b.pdf

Boris Lippe

Papers

4-D Echo-Particle Image Velocimetry in a Left Ventricular Phantom

Fast Volumetric Imaging Using a Matrix Transesophageal Echocardiography Probe with Partitioned Transmit–Receive Array

An Element-Matched Electromechanical $\Delta\Sigma$ ADC for Ultrasound Imaging