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Ultrasound systems have become more portable, with some evolving into ultra compact palm-sized devices. In the not so distant future, an ultrasound system could become a specialised personal digital assistant that is as common as the doctor’s stethoscope.

From the patient’s perspective, the move to portable ultrasound scanning means faster attention and more convenience. In the hospital ward, the ultrasound system can easily be transported from the patient’s bedside to other clinical departments, as necessary, much like an intravenous pump. For outpatient treatment, a doctor’s clinic may be able to perform an ultrasound scan without needing to send the patient to the local hospital or a specialist clinic. Emergency medical teams in an ambulance can scan the patient before reaching the hospital, thereby saving valuable time.

In addition, as ultrasound scanning becomes easier and increasingly available, more scans are likely to be taken, which will increase the rate at which patients can be diagnosed during the early stage of an illness. With higher integration and more input channels, the image quality is improving without a large increase in cost. This is helping to make ultrasound more affordable to a wider range of medical environments.

Until recently, most ultrasound systems were developed from discrete components and multiple integrated circuits (ICs). The receive signal chain consists primarily of a low-noise amplifier (LNA), a variable-gain amplifier (VGA), an anti-aliasing filter (AAF) and an analogue-to-digital converter (ADC). Each of these components is replicated many times in common digital beam forming (DBF) architectures, and between them they acquire the signal from the ultrasound probe, shape it and digitise it to create the image data.

In a DBF system, multiple channels are spatially summed with a beamformer that combines the multiple individual channels to create one image. Increasing the number of channels improves the dynamic range and hence the image quality, providing the channel noise is uncorrelated. Additional dynamic range is achieved by using multiple channels, 10×log (N channels); for example, 128 channels increase the dynamic range by 21 dB. Sixty-four to 256 channels are most common for higher-end systems and 16 to 64 channels are common for portable, mid- to low-end ultrasound systems.

To meet the demands of portable equipment by increasing the channel counts, the vendors of ICs are designing integrated components that combine the necessary parts of the receive signal chain. By combining these different blocks, the power consumption and board space required can be substantially reduced.

In ultrasound equipment, the heat generated by the system components is a problem that requires careful design attention; it is a particular problem for small form factor portable scanners. By choosing components in the receive signal chain that dissipate as little power as possible (a low noise amplifier, time gain or voltage gain amplifier, and analogue to digital converter), issues of heating and cooling are minimised.

Improved image quality is an important goal for ultrasound design; better images can help doctors diagnose medical conditions faster and more accurately. The image quality is limited by the dynamic range and noise performance of the receive signal chain; the stronger the signal in relation to the noise, the better the resulting image.

As the signals penetrate through the body they are attenuated by approximately 1 dB/cm/MHz. For example, with an 8-MHz probe and 4-cm depth penetration, the signal amplitude variation from reflections near the surface will be 64 dB (that is, 4 multiplied by 2 multiplied by 8). If 50 dB of imaging resolution is required, and accounting for loss from bone, cables and other mismatches, the desired dynamic range approaches 119 dB.

The achievable dynamic range is determined by the front-end components. Because the entire dynamic range is not needed instantaneously, an ADC with less dynamic range can be used by sweeping the gain of the VGA to match the attenuation of the received reflection over time. This is called time–gain compensation.

To meet these requirements, two new ultrasound front-ends are now available. The AD9272 is optimised for the best noise performance and primarily aimed at cart-based ultrasound systems, and the AD9273 is a low power device intended for portable equipment. Each device builds on the AD9271 platform and combines eight channels; both comprise a LNA, VGA, AAF and a 12-bit ADC with low voltage differential signalling output interface, which means the AD927x channels can be directly connected to the signal processors being used, without the need for any additional driver, level shifter or conditioner.

The devices in the AD927x family provide medical equipment designers with the flexibility to scale power as well as performance for different imaging modes by adjusting bias currents in the analogue input stages. This may be used in a portable system, where a doctor could start the scanning process at a lower image quality to maintain battery life and then adjust the quality as required to focus on a particular point of interest.

The medical technology market is growing year on year and design cycles are shortening. In ultrasound systems, there will be continuing demands for better image quality, lower power dissipation and reduced overall cost. Currently, the latest generation of devices handles eight channels each; in the future this number is expected to increase as customers request more integration.

Information supplied by Jan-Hein Broeders Healthcare Business Development and Application Engineer Europe, Analog Devices Inc., e-mail: jan.broeders@analog.com, www.analog.com