Single-FPGA, scalable, low-power, and high-quality 3D ultrasound beamformer

Abstract

Abstract-We present an efficient FPGA architecture suitable for a medical 3D ultrasound beamformer. We tackle the delay calculation bottleneck, which is the heart and the most critical part of the beamformer, by proposing a computationally efficient design that is able to perform volumetric real-time beamforming on a single-chip FPGA. The design has been demonstrated for a 32×32-channel receive probe, and we extrapolated the requirements of the architecture for 80×80 channels.I. MOTIVATION Medical ultrasound (US) imaging is well established, being used in a wide range of applications including detecting static structures, such as tumors, and studying dynamic phenomena like blood flow and valve functionality. US imaging is comprises three main processes: insonification, beamforming (BF), and visualization. Insonification is the process of emitting Radio Frequency (RF) acoustic waves from a piezoelectric transducer, called probe, through a body region. The waves are reflected from inhomogeneous tissues interfaces that act as scatterers due to acoustic impedance mismatches. The returned echoes are digitized and processed through an algorithm called Beamforming (BF). Finally, a post-processing step should be performed, including mapping the beamformed signals into screen image pixels.Recently, 3D US imaging has become available. A key advantages is that, since whole volumes are acquired at once, it is possible to remove the traditional dependence on having a trained sonographer operating the probe, in order to locate minute anatomical structures by fine adjustments of the position and orientation of the transducer. This enables telesonography, where even an unskilled operator can upload scans to a hospital where trained radiologists will issue a diagnosis. Unfortunately, present-day 3D imagers are bulky and expensive, suitable only for clinics and hospitals. A portable US platform with cheap, battery-operated electronics would be a breakthrough, enabling telesonography in rescue environments, in rural areas, and in developing countries, with major societal benefits. To this end, we undertake to implement 3D beamforming on a single FPGA. II. PROBLEM DEFINITION AND PREVIOUS WORKBeamforming is the core of any US imaging machine. It is the process of mapping the echoes to their origins by summing them along a certain delay profile, that represents the two-way time-offlight of the acoustic wave from the origin to each scatterer, and back to the all the piezoelectric elements. BF also includes apodization, the weighting of the delayed echoes by a factor that compensates for antenna directivity effects.In volumetric US imaging, a software-based implementation of the beamformer is not optimal if we target a battery-powered platform, whereas a hardware design offers major potential energy savings. One of the critical challenges of 3D US imaging is the number of receiving channels of high-end transducers, up to 100×100 elements, and the correspondingly massive computations required for image reconstruction. Different state-of...

Tetris

“…In 2D imaging, delays can be trivially stored in lookup tables [12], but the scale of 3D imaging has typically precluded such precomputation due to the vast number of delays required. Prior works [2,3,5,6,16,18] have instead used specialized hardware to approximate time delays on the fly; but, all of these systems fall orders of magnitude short in achieving the 1,000+ volumes per second target for cardiac, vector flow imaging, and shear wave elastography applications.…”