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

A Heterogeneous Ultrasound Open Scanner for the Real-Time Implementation of Computationally Demanding Imaging Methods

Ultrasound (US) open scanners have recently boosted the development and validation of novel imaging techniques. They are usually split into hardware- or software-oriented systems, depending on whether they process the echo data using embedded field programmable gate arrays (FPGAs)/digital signal processors (DSPs) or a graphics processing unit (GPU) on a host personal computer (PC). The goal of this work was to realize a high-performance heterogeneous open scanner capable of leveraging the strengths of both hardware- and software-oriented systems. The elaboration power of the 256-channel ultrasound advanced open platform (ULA-OP 256) was further enhanced by embedding a compact co-processing (CP) GPU system-on-module (SoM). By carefully avoiding latencies and overheads through low-level optimization work, an efficient peripheral component interconnect express (PCIe) communication interface was established between the GPU and the processing devices onboard the ULA-OP 256. As a proof of concept of the enhanced system, the high frame rate (HFR) color flow mapping (CFM) technique was implemented on the GPU SoM and tested. Compared to a previous DSP-based implementation, higher real-time frame rates were achieved together with unprecedented flexibility in setting crucial parameters such as the ensemble length (EL). For example, by setting EL =64 and a continuous-time high-pass filter (HPF), the flow was investigated with high temporal and spatial resolution in the femoral vein bifurcation (frame rate =1.1 kHz) and carotid artery bulb (4.3 kHz), highlighting the flow disturbances due to valve aperture and secondary velocity components, respectively. The results of this work promote the development of other computational-expensive processing algorithms in real time and may inspire the next generation of the US high-performance heterogeneous scanners.

Design and Implementation of Analog-Digital Hybrid Beamformers for Low-Complexity Ultrasound Systems: A Feasibility Study

Low-complexity ultrasound systems are increasingly desired for both wearable, point-of-care ultrasound and high-end massive-channel ultrasound for 3-D matrix imaging. However, the imaging capabilities, including spatial resolution and contrast, could suffer as low complexity systems are pursued, which remains as an unresolved tradeoff. To mitigate this limitation, this study revisits the general structures of analog and digital beamformers and introduces a hybrid approach, referred to as analog-digital hybrid beamforming, to implement efficient ultrasound systems. The suggested hybrid beamforming takes two stages sequentially, where the first analog stage partially beamforms M-channel RF signals to N sum-out data (i.e., M-to-N beamforming), and the second digital stage beamforms N partial sums to single final beamformed data (i.e., N-to-1 beamforming). Our approach was systematically designed and implemented with only four major integrated circuits, which was capable of driving full 64-channel transmission and reception. The developed system was demonstrated with a customized 64-channel 1-D phased array using a commercial tissue mimicking phantom. From the phantom imaging results, signal-to-noise ratio, contrast-to-noise ratio, and full beam width at half maximum values were quantitatively evaluated. The demonstrated results indicate that the analog-digital hybrid beamforming can be applied to any type of array for sophisticated 3-D imaging and tiny wearable ultrasound applications.

3-D Coherent Multitransducer Ultrasound Imaging With Sparse Spiral Arrays

Coherent multitransducer ultrasound (CoMTUS) creates an extended effective aperture through the coherent combination of multiple arrays, which results in images with enhanced resolution, extended field-of-view, and higher sensitivity. The subwavelength localization accuracy of the multiple transducers required to coherently beamform the data is achieved by using the echoes backscattered from targeted points. In this study, CoMTUS is implemented and demonstrated for the first time in 3-D imaging using a pair of 256-element 2-D sparse spiral arrays, which keep the channel count low and limit the amount of data to be processed. The imaging performance of the method was investigated using both simulations and phantom tests. The feasibility of free-hand operation is also experimentally demonstrated. Results show that, in comparison with a single dense array system using the same total number of active elements, the proposed CoMTUS system improves spatial resolution (up to ten times) in the direction where both arrays are aligned, contrast-to-noise ratio (CNR; up to 46%), and generalized CNR (gCNR; up to 15%). Overall, CoMTUS shows a narrower main lobe and higher CNR, which results in an increased dynamic range and better target detectability.