ARTICLE FOCUS:

  • FPGA and AFE make for dynamic duo
  • How ultrasound imaging prototype was achieved
  • Scalable and customizable system is end result

Reducing time-to-market while increasing the rate at which new technologies are integrated into today's medical imaging systems are presenting design challenges. A difference of even a few months in the release of a product can significantly impact the return on investment (ROI) of the project, both from missed revenue and a missed market window. Meanwhile, medical imaging system developers also are being required to integrate the latest technology to build systems with excellent analog performance, complex processing and visualization, and high data throughput resulting from higher speed analog-to-digital converters (ADCs) and increasing channel counts.

Several tools are now available to help engineers quickly prototype new designs and deliver the best performance in their systems. These tools help developers use reconfigurable field-programmable gate array (FPGA) technologies and integrated analog front-ends (AFEs) coupled with flexible integration platforms to develop prototype imaging systems faster. Developers are now able to combine modular FPGA hardware, integrated AFEs, higher-level design tools, and industry-standard platforms to create highly flexible, scalable, and customizable imaging systems.

Prototype ultrasound imaging system in just three months
A UK-based company, Diagnostic Sonar (diagnosticsonar.com) demonstrated this concept in a novel phased-array ultrasound imaging system. Designing around off-the-shelf FPGA hardware, application-specific integrated AFEs, and using higher-level design tools, it went from specifying the architecture to creating a working prototype system showing real-time ultrasound imaging in less than three months. The team was able to achieve such a short time to their first working prototype system by building the system around modular FPGA and AFE hardware. This provided significant flexibility and the ability to be customized, allowing them to focus on their domain expertise in ultrasound processing algorithms and I/O interfacing.

FPGAs can enable a lot of design flexibility to try out new ideas and reduce risk earlier in the development of a system. Since FPGAs can be reconfigured through software, a designer can save development time, demonstrating hardware-based processing while being able to reprogram the FPGA to accommodate modifications that are unknown during initial specification.

One challenge of using FPGAs for prototyping is that programming a system using a traditional hardware description language like VHDL can be very time consuming, lengthening project timelines. However, recent advancements in development tools have made FPGA programming more efficient by allowing higher level graphical tools to be used for overall system design. This leverages existing VHDL IP (Xilinx CORE Generator, in-house developed, third-party, etc,) where appropriate. When used properly, these tools can enable very fast development of a prototype system so that algorithms and hardware performance can be evaluated and refined.

The team from Diagnostic Sonar built their system around National Instruments tools such as NI FlexRIO modular FPGA hardware programmed with the LabVIEW FPGA Module, a graphical design language that can be used to design FPGA circuitry without needing to know VHDL coding. NI FlexRIO combines interchangeable, customizable I/O adapter modules with a user-programmable FPGA module in a PXI or PXI Express chassis. The Virtex series of Xilinx FPGAs are used on the board to achieve the I/O and signal processing performance required for applications like medical imaging. Diagnostic Sonar developed boards with FPGAs in the past, but NI FlexRIO was appealing to them because they could to build around known good hardware, which already included infrastructure components for I/O connectivity, PCI Express bus interfacing and DRAM communication. Developing these components in-house can consume a lot of time and can distract developers from new innovations where they add the most value.

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Once Diagnostic Sonar made the decision to build around a modular FPGA architecture using NI FlexRIO, the next step was to define the I/O for the system. The NI FlexRIO platform has a variety of analog and digital adapter modules that can meet many application needs, but it also allows a system developer to design their own customized I/O that interfaces to the FPGA using the adapter module developer kit (MDK). Diagnostic Sonar already had experience designing ultrasonic front-ends. However, they realized that in order to achieve the channel density requirements for the best system performance, they needed to use a fully integrated AFE designed for ultrasound applications.

Higher performance systems developed with integrated AFEs
The performance of an ultrasound system can be influenced significantly by its analog circuitry. Therefore, every feature in the AFE is critical for any ultrasound system design.

The AFE for an ultrasound system consists of a low-noise amplifier (LNA), voltage-controlled attenuator (VCA), programmable gain amplifier (PGA), anti-aliasing filter (AAF) and analog-to-digital converter (ADC). The LNA provides low-noise amplification for good sensitivity. The VCA and PGA are part of the time gain control (TGC) blocks and improve the system’s dynamic range. They also allow the gain to be increased with time to compensate for the increased attenuation of the signal as it passes through the body. The amplified signal is then filtered to improve its signal-to-noise ratio (SNR). The resulting signal is converted into digital format through an ADC and is processed by the receive beamformer. The performance of the AFE significantly drives the ultrasound system's characteristics in terms of size, weight, battery life, and image quality.

Before any IC design can be initiated, process selection is a key consideration for semiconductor manufacturers. The selection process must balance performance, power consumption, cost, and upgrade feasibility.

Regardless of whether the design is for a high-end cart or a hand-held portable system, channel integration in the AFE is important. Portable system developers must maximize their board space savings while a high-end system must optimize for a high-channel count. AFEs have heavily evolved within the past five years. In 2004, over 40 components were needed to design a 16-channel AFE discretely. Now it only takes two. Semiconductor process technology provides the opportunity to decrease size, reduce power, and improve the overall performance. Today's AFEs, like the AFE5808 from Texas Instruments (TI), offer twice the performance, 97% less space board, and 67% less power. A higher integration of channels in the AFE devices provides significant size reduction, cost savings due to fewer components, and simpler layouts – all leading to more cost-effective and quicker time-to-market systems.

Integrating ultrasound system components
Many designers have an application that demands the highest performance AFE they can get, which Diagnostic Sonar considered using the NI FlexRIO MDK to build their own design around one of the newest AFEs. However, they realized their application could be achieved with the performance of TI's AFE5801, which has eight channels, providing digitally controlled swept gain from –5dB to +31dB. They were able to use an off-the-shelf adapter module, the NI 5752, which integrates four of these AFEs into a 32-channel module with 50 MS/s sample rate and 12-bit resolution.

Using an existing module saved development time on the receive side of the system, and they were able to focus their hardware design efforts on a 32-channel, high-voltage pulser module for NI FlexRIO that pairs with the NI 5752. Prototyping using modular FPGA hardware allowed them to quickly get a working system and to determine required hardware changes, since the I/O was decoupled from the FPGA back-end. While their prototype system had 32 channels, by building their design around modular FPGA boards, their architecture can easily scale to 64, 128, 256, or even higher multiples of 32 channels for both transmit and receive, integrate multiplexing, as well as accommodate a variety of ultrasonic arrays. Additionally, by using the FPGA for hardware-based processing, their processing can scale as they add additional channels to the system, rather than having the CPU limit the system's imaging rate.

On the software side, Diagnostic Sonar initially developed algorithms using LabVIEW on the host – including beamforming, filtering, and rectification – along with a graphical user interface (GUI) to visualize data. After demonstrating the prototype system, they then had the ability to use LabVI EW FPGA to move algorithms to the FPGA on the NI FlexRIO board to further accelerate the processing performance. In the end, Diagnostic Sonar was able to leverage modular FPGA hardware and graphical software to create a high-performance, multichannel ultrasound acquisition and processing system that is scalable and can be customized for a range of applications. These technologies allow them to offer their customers a variety of options to either provide a more turnkey standard system configuration, or even sell the individual components like the 32-channel pulser, custom array connectivity, and beamforming IP to groups with the capability to integrate those components into their own systems.

End result: scalable and customizable systems
Diagnostic Sonar is a small company, but it leveraged the processing capabilities and reconfigurability of FPGAs and the analog performance built into TI’s ultrasound AFEs to build an ultrasound system that can be easily scaled and customized.

Moreover, it demonstrated the initial system in just three months by using a combination of off-the-shelf and custom hardware.