Portable ultrasound becomes a reality
12 June 2008
New ICs help to enhance portability and also reduce power consumption in a typical ultrasound system with a beamformer
On the transmit side of the ultrasonic transducer, a beamformer determines the delay pattern for the desired focal point. The beamformer outputs are amplified by high-voltage transmit amplifiers to drive the transducers. These amplifiers are controlled by digital-toanalogue converters (DACs), which shape the transmit pulses for better energy delivery. On the receive side, a transmit/receive switch blocks the highvoltage pulses. A high-voltage multiplexer is sometimes used to reduce the hardware complexity at the expense of flexibility.
The time-gain control (TGC) path consists of a low-noise amplifier (LNA), variable-gain amplifier (VGA), and analogue-to-digital converter (ADC). Under operator control, the TGC path maintains image uniformity during the scan. Good noise performance relies on the LNA, which minimises the noise contribution in the following VGA. Active impedance control optimises noise performance for applications that benefit from input impedance matching.
The VGA compresses a wide-dynamicrange input signal to fit within the input span of the ADC. The input-referred noise of the LNA limits the minimum resolvable input signal, whereas the output-referred noise – which depends primarily on the VGA – limits the maximum instantaneous dynamic range. This limit is set in accordance with the quantisation noise floor, as determined by the ADC resolution.
The anti-aliasing filter (AAF) limits the signal bandwidth and rejects unwanted noise in the TGC path prior to the ADC.
Beamforming techniques
Beamforming, as applied to medical ultrasound, is the phase alignment and summation of signals that are generated from a common source, but received at different times by a multi-element ultrasound transducer. In the continuous-wave Doppler path, receiver channels are phase-shifted and summed together to extract coherent information. Beamforming has two functions; it imparts directivity to the transducer (enhancing its gain) and it defines a focal point within the body, from which the location of the returning echo is derived.
There are two distinct approaches to beamforming; analogue beamforming (ABF) and digital beamforming (DBF). The main difference between an ABF and DBF system is the way the beamforming is done; both require perfect channel-to-channel matching. In ABF, an analogue delay line and summation are used. Only one precision high-speed ADC is needed. In a DBF system, many high-speed, high-resolution ADCs are needed. Sometimes a logarithmic amplifier is used in the ABF systems to compress the dynamic range before the ADC. In DBF, commonly used in modern image-acquisition ultrasound systems, the signal is sampled as close to the transducer elements as possible; the signals are then delayed and summed digitally.
System partitioning
Before multi-channel VGAs, ADCs, and DACs were available, manufacturers implemented ultrasound systems with custom ASICs, allowing them to incorporate flexible functions, reduce costs and minimise external component count. The disadvantage was that once designed, ASICs could not be updated. Their digital technology is not optimised for analogue circuitry, so analogue functions could not be integrated efficiently. Although portable systems could be designed, battery life was limited by the ASIC’s high power consumption.
With the advent of quad and octal TGCs, ADCs, and DACs, both size and power were further reduced, enabling new types of portable systems. Multichannel components allow sensitive circuits to be divided between multiple boards, providing system scaling and re-use of the electronic circuits. System scaling depends on the system split, and multichannel components make wiring difficult and pose thermal challenges for mechanical designers.
Further integration of the TGC path using multi-channel, multi-component integration makes the design easier by reducing PCB size and power requirements. As higher level integration become more predominant, advantages follow in cost, size, and power reduction, enabling cooler systems and longer battery life. The AD9271 is a 14mm x 14mm x 1.2mm package that consumes only 150mW per entire TGC channel at 40 Msample/sec. It employs serial I/O to keep the pin count low, reducing total area per channel by more than a third, and power dissipation by more than a quarter.
Multichannel integration of commodity devices, including quad and octal ADCs, puts an end to high cost components and endless tweaking of individual TGC paths, and brings the truly portable ultrasound scanner one step closer.
COREY PETERSEN is senior design engineer, high-speed converter group, Analog Devices
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