Configurable beamformer broadens imaging capabilities
- Beamforming technology
- Architectures and implementations
- Market trends
Ultrasound is one of the fastest growing medical imaging modalities due to its significantly lower cost compared with other medical imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI). Ultrasound is a safe non-ionizing operation, it’s excellent at observing motion (such as blood flow), and it’s portable. A vast underserved healthcare market in third world and developing countries is adopting ultrasound to add diagnostic imaging capability to hospitals and clinics in small towns and villages. Ultrasound is also finding many new applications outside of the traditional radiology and OB/GYN departments. Regardless of their size, cost, or clinical use, all ultrasound machines employ transducers that are constructed of arrays of elements that transmit and receive acoustic energy into and from the body. Beamforming is a common signal processing technique used to create directional or spacial selectivity of signals sent to, or received from just such an array. The beamformer is one of its most critical components in the front-end electronics sub-system of any ultrasound machine.
This article will describe the technology, architectures, and a unique configurable beamforming implementation that is driving innovation and growth in the ultrasound market.
Beamforming function in medical ultrasound
Beamforming is a common signal processing technique used in various applications such as wireless communications, including cellular and WiFi (802.11n), RADAR, SONAR, and ultrasound. These applications use an antenna that consists of an array of sensors or elements, numbering from two, to thousands of elements for transmitting and receiving radio or sound waves. Beamforming is what allows for the directional or spatial selectivity of signal transmission or reception. By coherently combining the signals from the multiple elements of the array, transmit or receive gain is achieved. Beamforming can be applied to both the transmit and the receive paths of the system to achieve this gain. In medical ultrasound, transmit beamforming can be used to create focused beams of ultrasound using a phased array. By forming a pattern of beams pointing in the same direction, receive beamforming is used to focus the echo signals received as reflections from different tissue structures in a region of interest within the human or animal body. In the receive beamformer, focusing is achieved by appropriately delaying echo signals arriving at different elements to align them in a way that creates an isophase plane. These aligned echoes are then all summed coherently, which is what provides the processing gain. This basic signal processing is called delay-and-weighting function in the time domain.
High-performance medical ultrasound beamforming poses some unique challenges to designers. Unlike RADAR or SONAR applications, receive beamforming in ultrasound is unique in that the system must capture reflections from various depths both very close and further away from the transducer. This requires dynamic focusing and an apodization capability which is a major source of differentiation for manufacturers.
Architectures and implementations
Historically, the beamformer was implemented in the analog domain. Variable analog delay lines delayed the signal from each element or channel, followed by an analog adder. Then the beamformed data was sampled by an analog-digital-converter (ADC). In contrast, in digital beamformers, more commonly used today, the signal from each channel is digitized upfront using an ADC, followed by a memory device to implement the phase delay, a multiplier to weight (apodize) the signal, and finally an adder to sum the delayed and apodized data from all the channels.
Several trends in the last decade, both in semiconductor devices and in the ultrasound market, have determined the technology used for implementing the beamformer function. Large manufacturers traditionally designed and built custom application specific integrated circuits (ASICs) that implemented the digital beamformer function. However, as ASIC process technologies advanced to 65 nm and below, the nonrecurring engineering costs for ASICs became difficult to amortize across the limited volume for a single ultrasound OEM. At the same time, field programmable gate array devices (FPGAs) have become larger and more capable of implementing sophisticated digital signal processing (DSP). These high-end FPGAs thus become the preferred device in which to implement the beamformer. The advent of DSP functions in low-end, low-cost FPGAs also allowed many smaller ultrasound companies to implement their own beamformer in these devices. Still, the performance of the beamformer algorithm is directly related to the amount of logic and memory of the chosen FPGA, with more sophisticated algorithms requiring more sophisticated and expensive FPGA.
Additionally, the high power consumption of an FPGA beamforming implementation can be the limiting factor on battery life thermal envelopes especially in small-form factor ultrasound machines. Systems in this segment, such as hand-carried ultrasound (HCU), which has a laptop form factor, can operate exclusively on batteries. While these machines initially supported only rudimentary ultrasound imaging, clinicians are now demanding, and expecting, portable ultrasound machines to have the high-performance imaging modes once found only in high-end console systems.
Ultrasound OEMs wanting to bring small-form factor machines to market are now faced with a dilemma as development of a beamformer ASIC is extremely expensive, and FPGA solutions are either performance constrained (using low-end devices) or cost, power, and size constrained (using high-end devices). To address these issues, a new implementation is needed.
Samplify Systems, a Silicon Valley semiconductor and solutions start-up, has developed an innovative beamformer technology using phased-array processing called AutoFocus. The technology is a highly configurable beamformer that allows each OEM to implement a highly differentiated beamformer. The technology’s high degree of programmability also allows the OEM to differentiate within its internal machine lineup.
Figure 1 shows a high-level block diagram of the AutoFocus beamformer. This device has a high-performance data path supporting parallel processing of four beams simultaneously for improved imaging and frame-rates. An on-board calculation engine updates the delay coefficients and apodization weights on every sample clock for continuous dynamic focus and apodization. The inputs to this calculator come from multiple user programmable registers and tables providing multiple layers of configurability.
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The ultrasound market is highly fragmented, with about 50% of market share owned by smaller manufacturers rather than the “big 4” (GE, Philips, Siemens, and Toshiba). Because of this, several noticeable trends have emerged. Large tier-one original equipment manufacturers (OEMs) have switched from a mostly vertically integrated model to becoming system integrators. These OEMs are starting to outsource their hardware development, especially for the low-end systems in their line-ups. With new clinical applications for ultrasound being introduced, a new type of OEM has emerged. These new specialty OEMs focus on building ultrasound systems targeting a specific application such as breast imaging, abdominal imaging, or imaging for surgeons in the operating room. A common attribute for both of these cases – the tier-one and specialty OEMs – is that they see their core competency as well as their differentiating advantage in either the transducer technology or back-end image processing and clinical workflow. The sub-system bridging the transducer and the back-end computer is the front-end electronics, with the beamformer at its center. For these OEMs, the front-end electronics sub-system has to be of high-enough performance to allow their own subsystems, be it the transducer, back-end software, or both to shine. But apart from performance and features, a beamforming subsystem has to be highly configurable and easy to integrate into the OEM’s final product. The final product includes, of course, lots of imaging and control software used to configure the many functions and features in the front-end, and grab the acquired data in multiple formats.
Kit aids design efforts
As ultrasound market fragmentation increases competition, time-to-market for new machines is being reduced. Even so, developing a new ultrasound machine can take more than two years because the hardware design has to be stable enough before software integration can begin.
To simplify OEM designs using the AutoFocus beamformer and in response to the need to reduce time-to-market, Samplify has developed a front-end electronics subsystem using its AutoFocus beamformer called the AutoFocus Beamforming Platform (Figure 2). This system is a complete transmit/receive 64-channel beamforming front-end that includes both the transmit and receive analog circuitry and digital beamformers. The system interfaces at one end to a transducer and on the other side to a back-end host via USB and PCIe (Figure 3). The platform is provided as a production-ready sub-system that can be used in an OEM’s final product. Many of the specialty OEMs that have no expertise or desire to reinvent the wheel and spend precious R&D dollars can now focus on developing their specialty transducer or image processing algorithms and get into clinical trial and certification months faster thanks to a new ultrasound development kit. The kit includes a high-level application programming interface (API), drivers, and a license to use the entire design as a reference design. This AutoFocus Beamforming Development kit allows OEMs and original design manufacturers (ODMs) of all tiers and types to bring machines to market within a year (times vary by certification processes required) using their own designed boards and software.
The development kit’s high-level ultrasound API interfaces with the OEM’s software and with the platform’s hardware. This API obfuscates the low-level details of the hardware in the system while exposing all the functionality of the beamformer to the user. The API is a C++ middleware software layer that provides a convenient and easy-to use interface for the configuration and setup of platform for various ultrasound scan types. The user can control all the parameters and functions of the hardware including the transmit beamformer, receive beamformer, and various mid-processor functions, via high-level classes of scans, frames, and beams. The API supports the AutoFocus beamformer’s arbitrary probe geometry configurations, which allow the user to define any probe by specifying its element location in 3D space, center frequency, etc. The API can be used to integrate with user-specific ultrasound post processing and user interface and display applications. The API provides an abstraction layer between the user imaging software and the platform’s hardware and its low-level drivers (Figure 4). It provides software investment protection as the underlying hardware and physical interfaces can change with no effect on the high-level software. The hardware can change when better (higher performance/lower power/more cost effective) components become available, or when a new physical interface (e.g. USB 3.0) is added.
The platform is a golden known-good-hardware reference on which user software can be developed right away. Hardware design is done in parallel and since the same beamformers (and other key components) are used in the customized hardware along with the same API, all software developed using the platform is compatible with the custom design.
The novel implementation of ultrasound beamformer technology described in this article is driving change in the ultrasound industry by enabling traditional ultrasound OEMs to outsource the front-end hardware and quickly and easily develop new software where they can differentiate their products.
Further, it enables new specialty OEM entrants to create new clinical applications for ultrasound allowing everyone to push the limits of image quality and clinical applications.
Those are clearly sound results.