In modern medicine, medical imaging has made great strides. Transmitting acoustic energy in the body and receiving and processing the reflections of the signal, ultrasound technology can generate images of our internal organs, map the tissues, and other medical-scientific information. Ultrasound is represented by a high-frequency sonar system, which measures the tiny echoes of sound waves that pass through the body. The advances in the field of microelectronics have directed the instrumentation towards portable and high-efficiency solutions, with performances that are getting closer and closer to the more complex and decidedly larger systems.

Synchronization is a must!

Ultrasound has a wide range of medical applications, including imaging. The main design elements are the transmission and reception of the signal for a correct interpretation of the images. In a pulsed wave (PW) ultrasound system, a high voltage pulse stimulates the piezoelectric transducer (a crystal), causing a corresponding mechanical compression (an inverse piezoelectric effect), and thus creating the ultrasound wave that passes through the body (Figure 1).

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Figure 1: Block diagram of an ultrasound system. (Source: Maxim Integrated)

The transmitter sends ultrasound signals, while the receiver collects the echoes that are reflected from the body structure, generally through the same piezoelectric transducer with the support of digital processing and amplification circuits.

The wave generated by the transducer through relaxation traverses the body and is focused by phased array techniques. The reflection signal will have different intensity characteristics depending on the substances it encounters, such as soft tissues, bones, fat, and blood.

The transmission and reception circuits must be synchronized to achieve the best results. The management of the synchronization clock is fundamental for the information to be consistent and the image to have minimal disturbances.

The phase noise (frequency-domain) or jitter (time-domain) is a measure of the phase of a clock signal that deviates from its ideal position and is an important parameter that can determine the impact on the ultrasound image. Higher phase noise can cause artifacts, which represents errors in the calculation and visualization of blood flow.

The power supplies are synchronized with a master clock in the range of 100 kHz - 1 MHz to avoid switching effects. Higher frequencies require additional measures in order to avoid coupling with the ultrasound signal. Synchronization at lower frequencies avoids design complications helping to reduce electromagnetic interference (EMI) too.

Amplification

The transmitted sound pulse propagates through the body as a wave in the frequency range between 1 MHz and 15 MHz. Sound waves are attenuated during the journey to the human body.

STHV1600 is a monolithic, high-voltage, high-speed pulse generator with 16 independent channels and a beamformer. The IC provides a switch structure that ensures effective isolation during the transmission phase. Both PW output stages (TX0 and TX1) can provide a peak output up to ± 2 A (Figure 2).

As the transmitted signal travels, part of the wave energy is reflected back to the transducer with very high-intensity levels. The reflections that occur long after the start of the transmission are mostly weak signals. The reflected signal is processed and displayed as an image on a screen. With appropriate amplification electronics, extremely sensitive microelectronic solutions have been developed with a wide range of dynamics. The echoes received from the focal points close to the surface require minimal or no amplification; while the others, those received from the deep focal points in the body, must be amplified by a factor of 100 or more.

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Figure 2: Block diagram of a complete ultrasound imaging layout with the external implementation of STHV1600 pulser. (Source: ST Microelectronics)

Most receivers must be able to adapt in both conditions, without saturating or distorting the signal, thus introducing a minimum noise. For ultrasound systems, the amplifier consists of a low-noise, highly linear low-noise amplifier (LNA) with digitally programmable gain and impedance; and a variable gain amplifier (VGA) before being digitized by the analog-to-digital (ADC) to provide a better signal-to-noise radio (SNR) and phase noise for the path (Figure 3).

MAX40077 is a wide-band, low-noise, operational amplifier with 2.7V to 5.5V single power supply. The family of these devices absorbs about 2.2 mA of quiescent power supply with very low distortion (0, 0002% THD + N), as well as a low density of noise in input voltage (4.2 nV/√Hz).

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Figure 3: Block diagram of the reception system for the ultrasound system. The transmit/receive (T/R) switch isolates the receiver during high voltage bursts in transmission. (Source: Maxim Integrated)

Since ultrasonic signals are attenuated as they travel through the body, the reflected (received) signal is much smaller than the transmitted signal. Its detectable peak at the entry of the LNA is limited by the linear input range of the LNA, typically 500 mVPP with an LNA gain of 18 dB.

During the transmission interval, overload conditions can occur, limiting the processing of the reflection signals and making it difficult to see small signals in the presence of larger ones or completely losing the reflected signal. Therefore, it is essential to choose a reception front-end circuit with excellent overload recovery features.

The LNA and VGA stages, as well as the ADC, should be designed in such a way that they all recover quickly from an overload condition since the receiver could be blind until the end of recovery so as not to preserve the image quality or information on the Doppler phase shift.

The MAX2082 is a fully integrated ultrasonic transceiver in the industry, optimized for high-performance, high-channel portable systems. The transmission can generate high voltage pulses up to ± 105 V. The device is composed of T/R switch, LNA, variable gain amplifier (VGA), antialiasing filter (AAF), analog to digital converter (ADC) and digital high-pass filter (HPF) with a power dissipation of 131 mW per very-low channel at 50 Mbps (Figure 4).

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Figure 4: Block diagram of the MAX2082 and an application circuit. (Source: Maxim Integrated)

The choice of an ultrasound transceiver with excellent operating characteristics can significantly simplify the HW/SW design and debugging challenges, speeding up time-to-market and improving image quality. Ultrasound imaging offers a diagnostic procedure contributing to the general diagnosis of the patient's status.