Ultrasound equipment is best known for letting expectant mothers follow development of their babies. But many other ultrasound applications go beyond the classical OBGYN experience.

In the veterinary world, for example, ultrasound is commonly used to diagnose ailment in expensive animals as well as monitoring food production from large-animals. The equipment also lets industrial users scan bridges and beams for cracks and defects.

With emerging applications, there is growing demand for ultrasound machines with high performance and portability. Performance driven applications, such as cardiology and 4D image processing, call for the largest number of channels, features, and options. Power is not a driving factor because this equipment is used for human diagnoses at the patient's bedside, operating room suite, or nurses' triage. Portable equipment, however, is needed where reliable electrical power is scarce or nonexistent, such as in remote village clinics, emergency medical services, and animal farming.

Designing ultrasound equipment means many component trade-offs. For example, portability must be balanced with performance. To provide the balance, Analog Devices developed the AD927x series analog front-ends (AFEs) for ultrasound equipment. The term refers to the full integration of the LNA/VGA/AAF and ADC in one device. This is the common analog signal chain that makes up any ultrasound machine before the signals are digitally processed through the beamformer. The low-power AD9273 processor (100 mW per channel) targets portable designs that satisfy a need for complex units that can fit in a pocket, while the low-noise AD9272 focuses on the performance demands of the highest dynamic range cart-based systems.

Improved resolution

Image quality is key to all ultrasound devices. Clearer images lead to quick and accurate diagnoses. As the acoustic signals penetrate through tissue, they are attenuated by about 1 dB/cm/MHz. Using an 8-MHz probe and 4-cm depth penetration — and accounting for both outgoing and return attenuation — signal returns from internal tissue differ by 64 dB (4 × 8 × 2) from near-surface reflections. This means 40+ dB of imaging resolution is needed to account for losses due to bone, cables, and other mismatches. A required dynamic range approaches 120 dB, depending on application. To put this into perspective, a 0.5-V p-p full-scale signal with a 0.86-nV/√Hz noise floor in a 10-MHz bandwidth implies a 96-dB input dynamic range. Multiple channels give an additional dynamic range of 10log(N channels). For example, 128 channels increase the dynamic range by 21 dB. This establishes a practical limit for dynamic range of 117 dB (or 96 + 21).

A higher levelintegration

Combining an ultrasound's full time grain-compression path has several advantages over discrete, multichannel devices. Multichannel, multicomponent integration generally provides a simpler design approach because it reduces requirements on PCB size and power. For example, previous processors required a separate low-noise amplifier, a variable-gain amplifier, anti-aliasing filter, and analog-to-digital converter (LNA/VGA/AAF/ADC). Each required days to tweak so it would work with the next. Most ultrasound systems have many channels, anywhere from 16 to 256 of them. Each channel has the LNA/VGA/AAF/ADC function in its path. Previously, designers of ultrasound equipment had to use discrete components for each function which makes for large system boards.

As chip designers build in more functions, advantages follow in lower cost, wider availability, smaller size, and lower power consumption. These also generate less heat and longer battery life in portable units, while keeping up with performance demands of high image quality.

The AD927x family

The AD927x processors combine eight LNA/VGA/AAF/ADC channels and a crosspoint switch in a complete time-gain compression path, the most common receiver path found in ultrasound equipment. This helps reduce the number of required components and size of the boards that hold them. The new processors offer ultrasound equipment designers the flexibility to trade power consumption for performance. For example, the low-noise, high-performance AD9272 features 0.86 nV/√Hz noise for an entire channel, while the low-power AD9273 consumes only 100 mW per channel at 40 M samples/sec. Both processors are pin compatible and use serial I/O to keep the pin count low, thus reducing per-channel area by more than 33% as compared to multi-component designs.

Advances in integrated multichannel devices are pushing system flexibility. As the ultrasound market grows, function packed devices and configuration tools that shorten system design cycles will give rise to innovative online products. These will provide a means of developing a wide range of configurable, scalable, diversified ultrasound products that meet the demands for better image quality.

FOR FURTHER READING

Brunner, Eberhard, “How ultrasound system consideration influence front end component choice” Analog Dialogue 36, Part 1 (2002).

Kisslo, Joseph A. and David B. Adams, Principles of Doppler Echocardiography and the Doppler Examination #1, London: Ciba-geigy, 1987

Kuijpers, F. A., “The role of technology in future Medical Imaging,” Medicamundi, 1995, Vol. 40, No. 3, Philips Medical Systems.

Bandes, Alan, “How Are Your Bearings Holding Up? Find Out with Ultrasound,” Sensors Magazine, July, 2006, pp. 24-27.

Meire, Hylton B. and Pat Farrant, Basic Ultrasound, Wiley, 1995, pp. 1-66.

Reeder, Rob. “Ultrasound Portable Partitioning,” Medical Design Technology, February 2008.

Reeder, Rob and Corey Petersen, “8-Channel 12-Bit, 10-50-MSPS Front End: The AD9271 — A Revolutionary Solution for Portable Ultrasound,” Analog Dialogue 41, July 2007.

Reeder, Rob. “New components offer flexibility in ultrasound system design”, Planet Analog, January 2009.

ADI AD927x Configuration Tool. For a copy please send an email to highspeed.converters@analog.com