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Ultrasound today

Image: Texas Instruments

Over the past 50 years, ultrasound technology has advanced dramatically and progressed from simple “grey-scale” imaging to the three- and four-dimensional imaging that is available today. In addition, innovative analogue and digital signal processor (DSP) solutions that provide low power consumption, extended battery life and greater on-chip integration allow console-type ultrasound systems to be replaced with cost-effective, portable hand-held devices. The arrival of more powerful, multicore DSPs with greater efficiency and low-power dissipation is driving the development of smaller devices with more precise imaging capabilities. These portable ultrasound devices can be used away from traditional hospital-based settings and play an increasingly important role in diagnosis. With this in mind, integrated analogue front end (AFE) products are being designed for these devices.

Standardising AFE design

Ultrasound systems have similar receive-channel architectures, regardless of performance requirements. The receive AFEs contain common blocks including low-noise amplifiers,time gain controlled amplifiers, voltage-controlled amplifiers (VCA), programmable gain amplifiers, low-pass filters and analogue-to-digital converters (ADC) (Figure 1). As a result, there is no reason why AFE design cannot be standardised, particularly between mid- to low-end systems. Indeed, innovative ultrasound AFE products are available that can accommodate different system performances for various system sizes and provide pin-to-pin compatibility between portable and console-type ultrasound systems. The ability to utilise a standardised AFE design has obvious cost benefits and will speed up development times.

The latest ultrasound AFE products maximise power and noise performance by using state-of-the-art bipolar complementary metal-oxide-semiconductor (BiCMOS) and complementary metal-oxide-semiconductor (CMOS) technologies. By using BiCMOSs for the VCA and CMOSs for the ADC, it is possible to reduce noise by 40%, power by 20% and size by 50%. These innovations allow the development of portable systems that provide superior image quality while minimising power consumption.

Optimising AFE features

Figure 1: Ultrasound system block diagram. (click image to enlarge)

When designing an AFE, several features should be considered including power consumption, noise, gain control range, amplifier saturation/overload recovery, harmonic distortion and crosstalk. It is important to balance these AFE features against overall system performance.

Power consumption. Low power consumption is essential for portable ultrasound systems to extend battery life and operation time. Although this requirement can affect other system parameters such as the input signal range, any performance degradations can usually be kept within acceptable levels.

AFE noise. Both input-referred-current and input-referred-voltage noise contribute to system sensitivity. Noise parameters of 0.7 nV/rt(Hz) to 1.5 nV/rt(Hz) (refer to input) should be chosen for high- to low-end systems because this is sufficient for producing good quality ultrasound images; lower noise values do not improve image quality. Flicker noise (1/f noise) is also important. In continuous wave mode, where there is mixing, the low frequency noise spectrum shifts to the carrier frequency. This reduces the signal-to-noise ratio (SNR) at a given frequency. An amplifier with flat noise performance over a wide operation frequency is preferable.

Gain control range. Gain control is an important issue because it affects the dynamic range of ultrasound systems. The higher the VCA gain control range, the wider the dynamic range, which leads to superior image quality. Dynamic range can be calculated as shown in Equation 1.

Equation 1: Dynamic range = ADC’s SNR + VCA’s Gain control range

Therefore, a system with a 12-bit 70 dB SNR and a VCA with a gain control range of 40 dB can obtain a dynamic range of 110 dB. Assuming an attenuation coefficient of 0.7 dB/cmMHz in the human body, an imaging depth of 10 cm and a 7.5 MHz transducer, a dynamic range of 105 dB can be calculated as 10 x 2 x 0.7 x 7.5. An AFE with a larger gain control range is needed because current ultrasound systems use 10 to 15 MHz probes that often require a dynamic range of 100 dB. Furthermore, higher gain is beneficial for detecting small signals and compensating for insertion loss from other circuits.

Amplifier saturation and overload recovery. These are also important considerations. An amplifier’s input signal range is determined by its linear output voltage and gain, as illustrated by the following equation in which Vin = input voltage, Voutput = output voltage, pp = peak to peak value:

Too low a gain, however, degrades input-referred-voltage noise, and too high an output voltage (that is, high power supply voltage) increases total power consumption, thus a compromise must be found. Consequently, Vin of 200 to 400 mVpp is commonly selected for portable to midrange systems. Ultrasound amplifier saturation is commonly caused by high voltage pulse leakage or large signals reflected from close-to-surface objects, for example, superficial tissues or bones. Significant imaging information can be lost if amplifiers cannot recover quickly and consistently. This can be avoided if the AFE has a fast and consistent overload recovery time, that is, a consistent overload recovery time of a one clock cycle.

Harmonic distortion. With the increasing use of contrast agents, low, second harmonic distortion is required to ensure successful harmonic imaging. The harmonic signals received by transducers are as many as 40 dB lower than fundamental signals. Therefore, the second harmonic distortion (HD2) from an amplifier should be lower than 40 dBc to achieve acceptable harmonic images. In addition, in Doppler images, artifact Doppler shift frequencies as a result of high HD2 and bad directional isolation may affect accurate image interpretation. Directional isolation of 45 to 50 dB may be sufficient for continuous wave and pulsed wave Doppler systems.^1,2 In general, the AFE’s linear input range should be specified when HD2 is less than 40 dBc.

Cross talk. In ultrasound systems, crosstalk reduces image accuracy. Array transducers (–30 to –35 dBc) are the main source of crosstalk. Usually, crosstalk from integrated circuits or printed circuit boards is much lower than –35 dBc and does not degrade system performance.

Efficient development

New, state-of-the-art ultrasound AFEs provide a single design solution that enables portable and high-channel density mid-range ultrasound systems to be developed with considerable cost and time savings. Future AFEs with higher channel density, lower power consumption, more features and better performance will continue to help system designers achieve better image quality for end users/radiologists.

Xiaochen Xu is a Systems and Applications Engineer in the Medical and High Reliability Business Unit, Harish Venkataraman is a Design Engineer in the Medical and High Reliability Business Unit, and Anand Udupa is a Design Engineer in the High Speed Data Converter Group at Texas Instruments, Haggertystrasse 1, D-85350 Freising, Germany, European toll free tel. 00800 275 83927, e-mail: epic@ti.com, www.ti.com