High-Frame-Rate Contrast Echocardiography Using Diverging Waves: Initial In Vitro and In Vivo Evaluation

Assessment of Coronary Microcirculation with High Frame-Rate Contrast-Enhanced Echocardiography

OBJECTIVE

Assessing myocardial perfusion in acute myocardial infarction is important for guiding clinicians in choosing appropriate treatment strategies. Echocardiography can be used due to its direct feedback and bedside nature, but it currently faces image quality issues and an inability to differentiate coronary macro- from micro-circulation. We previously developed an imaging scheme using high frame-rate contrast-enhanced ultrasound (HFR CEUS) with higher order singular value decomposition (HOSVD) that provides dynamic perfusion and vascular flow visualization. In this study, we aim to show the ability of this technique to image perfusion deficits and investigate the potential occurrence of false-positive contrast detection.

METHODS

We used a porcine model comprising occlusion and release of the left anterior descending coronary artery. During slow contrast agent infusion, the afore-mentioned imaging scheme was used to capture and process the data offline using HOSVD.

RESULTS

Fast and slow coronary flow was successfully differentiated, presumably representing the different compartments of the micro-circulation. Low perfusion was seen in the area that was affected, as expected by vascular occlusion. Furthermore, we also imaged coronary flow dynamics before, during and after release of the occlusion, the latter showing hyperemia as expected. A contrast agent destruction test showed that the processed images contained actual contrast signal in the cardiac phases with minimal motion. With larger tissue motion, tissue signal leaked into the contrast-enhanced images.

CONCLUSION

Our results demonstrate the feasibility of HFR CEUS with HOSVD as a viable option for assessing myocardial perfusion. Flow dynamics were resolved, which potentially helped to directly evaluate coronary flow deficits.

Requirements and Hardware Limitations of High-Frame-Rate 3-D Ultrasound Imaging Systems

The spread of high frame rate and 3-D imaging techniques has raised pressing requirements for ultrasound systems. In particular, the processing power and data transfer rate requirements may be so demanding to hinder the real-time (RT) implementation of such techniques. This paper first analyzes the general requirements involved in RT ultrasound systems. Then, it identifies the main bottlenecks in the receiving section of a specific RT scanner, the ULA-OP 256, which is one of the most powerful available open scanners and may therefore be assumed as a reference. This case study has evidenced that the “star” topology, used to digitally interconnect the system’s boards, may easily saturate the data transfer bandwidth, thus impacting the achievable frame/volume rates in RT. The architecture of the digital scanner was exploited to tackle the bottlenecks by enabling a new “ring“ communication topology. Experimental 2-D and 3-D high-frame-rate imaging tests were conducted to evaluate the frame rates achievable with both interconnection modalities. It is shown that the ring topology enables up to 4400 frames/s and 510 volumes/s, with mean increments of +230% (up to +620%) compared to the star topology.

In Vivo Comparison of Multiline Transmission and Diverging Wave Imaging for High-Frame-Rate Speckle-Tracking Echocardiography

High-frame-rate (HFR) speckle-tracking echocardiography (STE) assesses myocardial function by quantifying motion and deformation at high temporal resolution. Among the proposed HFR techniques, multiline transmission (MLT) and diverging wave (DW) imaging have been used in this context both being characterized by specific advantages and disadvantages. Therefore, in this article, we directly contrast both approaches in an in vivo setting while operating at the same frame rate (FR). First, images were recorded at baseline (resting condition) from healthy volunteers and patients. Next, additional acquisitions during stress echocardiography were performed on volunteers. Each scan was contoured and processed by a previously proposed 2-D HFR STE algorithm based on cross correlation. Then, strain curves and their end-systolic (ES) values were extracted for all myocardial segments for further statistical analysis. The baseline acquisitions did not reveal differences in estimated strain between the acquisition modes ( ); myocardial segments ( ); or an interaction between imaging mode and depth ( ). Similarly, during stress testing, no difference ( p = 0.7 ) was observed for the two scan sequences, stress levels or an interaction sequence-stress level ( p = 0.94 ). Overall, our findings show that MLT and DW compoundings give comparable HFR STE strain values and that the choice for using one method or the other may thus rather be based on other factors, for example, system requirements or computational cost.

Contrast-Enhanced High-Frame-Rate Ultrasound Imaging of Flow Patterns in Cardiac Chambers and Deep Vessels

Cardiac function and vascular function are closely related to the flow of blood within. The flow velocities in these larger cavities easily reach 1 m/s, and generally complex spatiotemporal flow patterns are involved, especially in a non-physiologic state. Visualization of such flow patterns using ultrasound can be greatly enhanced by administration of contrast agents. Tracking the high-velocity complex flows is challenging with current clinical echographic tools, mostly because of limitations in signal-to-noise ratio; estimation of lateral velocities; and/or frame rate of the contrast-enhanced imaging mode. This review addresses the state of the art in 2-D high-frame-rate contrast-enhanced echography of ventricular and deep-vessel flow, from both technological and clinical perspectives. It concludes that current advanced ultrasound equipment is technologically ready for use in human contrast-enhanced studies, thus potentially leading to identification of the most clinically relevant flow parameters for quantifying cardiac and vascular function.

Microbubble Radiation Force-Induced Translation in Plane-Wave Versus Focused Transmission Modes

Due to the primary radiation force, microbubble displacement has been observed previously in the focal region of single-element and array ultrasound probes. This effect has been harnessed to increase the contact between the microbubbles and targeted endothelium for drug delivery and ultrasound molecular imaging. In this study, microbubble displacements associated with plane-wave (PW) transmission are thoroughly investigated and compared to those obtained in focused-wave (FW) transmission over a range of pulse repetition frequencies, burst lengths (BLs), peak negative pressures, and transmission frequencies. In PW mode, the displacements, depending upon the experimental conditions, are in some cases consistently higher (e.g., by 28%, when the longest BL was used at PRF = 4 kHz), and the axial displacements are spatially more uniform compared to FW mode. Statistical analysis on the measured displacements reveals a slightly different frequency dependence of statistical quantities compared to transient peak microbubble displacements, which may suggest the need to consider the size range within the tested microbubble population.

High-Frame-Rate Contrast-Enhanced Echocardiography Using Diverging Waves: 2-D Motion Estimation and Compensation

Combining diverging ultrasound waves and microbubbles could improve contrast-enhanced echocardiography (CEE), by providing enhanced temporal resolution for cardiac function assessment over a large imaging field of view. However, current image formation techniques using coherent summation of echoes from multiple steered diverging waves (DWs) are susceptible to tissue and microbubble motion artifacts, resulting in poor image quality. In this study, we used correlation-based 2-D motion estimation to perform motion compensation for CEE using DWs. The accuracy of this motion estimation method was evaluated with Field II simulations. The root-mean-square velocity errors were 5.9% ± 0.2% and 19.5% ± 0.4% in the axial and lateral directions, when normalized to the maximum value of 62.8 cm/s which is comparable to the highest speed of blood flow in the left ventricle (LV). The effects of this method on image contrast ratio (CR) and contrast-to-noise ratio (CNR) were tested in vitro using a tissue mimicking rotating disk with a diameter of 10 cm. Compared against the control without motion compensation, a mean increase of 12 dB in CR and 7 dB in CNR were demonstrated when using this motion compensation method. The motion correction algorithm was tested in vivo on a CEE data set acquired with the Ultrasound Array Research Platform II performing coherent DW imaging. Improvement of the B-mode and contrast-mode image quality with cardiac motion and blood flow-induced microbubble motion was achieved. The results of motion estimation were further processed to interpret blood flow in the LV. This allowed for a triplex cardiac imaging technique, consisting of B mode, contrast mode, and 2-D vector flow imaging with a high frame rate of 250 Hz.