There are major trade-offs to be considered when designing ultrasound front-end ICs. Performance parameters in the ICs affect diagnostic performance and, conversely, system configuration and objectives affect the choice of components. It is essential for designers to understand these trade-offs.
Medical ultrasound machines are among the most sophisticated signal-processing machines in widespread use today. Some system-level understanding is necessary to fully appreciate the desired front-end IC functions and performance levels, especially for the low-noise amplifier (LNA), time gain compensation amplifier (TGC; a variable-gain amplifier) and analog-to-digital converters. The front-end component characteristics define the limits on system performance; once noise and distortion have been introduced, it is virtually impossible to remove them.
A high-voltage multiplexer/demultiplexer is used in some arrays to reduce the complexity of transmit and receive hardware, but at the expense of flexibility. The most flexible systems are phased-array digital beam former systems; but they also tend to be the most costly, because of the need for full electronic control of all channels. Nonetheless, front-end ICs are pushing down cost and power per channel.
On the transmit (Tx) side, the Tx beam former determines the delay pattern that sets the desired transmit focal point. Its outputs are then amplified by high-voltage transmit amps that drive the transducers. On the receive (Rx) side is a transmit/receive switch, generally a diode bridge, which blocks the high-voltage Tx pulses. These are followed by an LNA and one or more VGAs.
After amplification comes analog (ABF) or digital (DBF) beam forming. In most modern systems the process is digital; an exception is continuous-wave (CW) Doppler processing, whose dynamic range (DR) is too large to be processed through the imaging channel. Finally, the Rx beams are processed to show a gray-scale image, a color flow overlay on the 2-D image or a Doppler output.
Both DBF and ABF systems require VGAs for channel-to-channel matching. An ABF system needs only one high-resolution, relatively slow A/D, since the signal is downconverted after summation; but a DBF system requires many high-speed, high-resolution A/Ds because it samples the RF bandpass signal.
The noise floor of the LNA determines how weak a signal can be received. But at the same time especially during CW Doppler signal processing the LNA must also be able to handle very large signals. So it is crucial to maximize the dynamic range of the LNA (in general, it is impossible to implement any filtering before the LNA, because of noise and signal distortion constraints).
CW Doppler has the largest DR of all signals in an ultrasound system: During CW, a sine wave is transmitted continuously with half of the transducer array, while the other half receives the signal. There is a strong tendency for the Tx signal to leak into the Rx side. Since Doppler signals are very weak and filtering of the large leakage signal before demodulation is not easily done, any component processing the CW signal needs to have very large DR.
At the current state of the art, CW Doppler signals cannot be processed through the main B- and F-mode path in a DBF system; for this reason, an ABF is needed for CW Doppler processing. Naturally, the holy grail in DBF ultrasound is for all modes to be processed through the DBF chain (at realistic cost and power).
Since ultrasound systems require many channels, low power consumption is critical for all components. There will always be a push to increase front-end dynamic range so as to arrive at eventual integration of all ultrasound modes into one beam former.
Brunner, Eberhard, "Ultrasound System Considerations and their Impact on Front-End Components," www.analog.com/library/analogdialogue/ archives/36-03/ultrasound/index.html.
Eberhard Brunner (firstname.lastname@example.org) is senior design engineer at Analog Devices Inc. (Norwood, Mass.).
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