The field of medical imaging benefits from the research and development of applied physics and electronics, especially in areas such as instrumentation, image acquisition and modeling. Due to its completely non-invasive nature, ultrasound occupies a special position in various imaging modalities, which provides a reliable method for visceral organ research. Ultrasonic technology has been used for medical purposes for more than half a century. However, such a necessary device is bulky and expensive, and it has only recently been exclusively manufactured using discrete devices.
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This trend is changing due to advances in semiconductor process technology. Now, semiconductor ICs can be completely used to manufacture ultrasonic transducers. Lower voltage IC technology now makes ultrasonic receiver chips with significantly high gain and low noise performance a reality. Similarly, at higher voltage terminals, there is increasing interest in the manufacture of transmitter ICs that drive ultrasonic transducers. This article outlines some of the advances in ultrasonic transmitter chip design and the many challenges that exist.
Ultrasound System Overview: Transmit and Receive Functions In a nutshell, an ultrasound system works by generating sound waves for the patient and then receiving and processing the reflected signals to form an image of the patient's body. The original sound waves sent to the body are generated by a transmitter that is typically excited by electrical pulses generated by the transmitter. Similarly, the reflected sound waves are received by the transmitter and then converted back to electrical form, and the resulting signal is processed to determine the internal structure of the relevant body part.
Figure 1 shows a typical construction of a complete medical ultrasound system. There are several different ways to implement a send path. The path may consist of a beamformer and a number of level shifters, gate drivers and high voltage switches whose outputs are sent to the ultrasonic transducer. In general, the transmitter is made of a piezoelectric material that converts the high voltage electrical signal into sound waves, the final output of the system.
Figure 1 Typical construction of a complete medical ultrasound system
In some systems, in the transmission path that drives the output stage through digital logic, the digital properties of the hold signal are obtained from start to finish. However, you can also create and send signals to the transmitter in an analog manner. It involves a digital to analog converter (DAC) that converts the beamformer output to an analog format. The analog amplification is then used to generate the signal before it is sent to the transmitter.
In terms of the receiving path of the ultrasonic system, an analog method is used. Because the amplitude of the received signal is much lower than the transmitted signal, the front end includes a low noise amplifier followed by some kind of gain control module. After filtering out the uncorrelated high frequency portion, the resulting signal is converted to digital form by an analog to digital converter (ADC), and the output of the analog to digital converter is processed by the beamformer.
Other important parts of the ultrasonic transceiver system include a multiplexer that interacts with multiple channel activities, and a receive/transmit switch that controls the flow of signals between the transmitter and the transceiver electronics. A key function of the transmit/receive switch is to protect the receiver during the transmit event because the transmit event involves an excessive transmit line voltage that is much higher than the receiver module's ability to withstand.
Ultrasound System Requirements: Transmit Path Challenge Voltage Range and Operating Frequency The ultrasonic systems described so far can produce a variety of signal images to meet the requirements of different imaging modes. In extreme ranges, you can get high voltage (60 to 100V), low duty cycle (0.5 to 2.0%) signals for B-type display and harmonic imaging applications. In the other extreme case, a low voltage (3 to 10V), 100% duty cycle signal required for continuous wave (CW) Doppler imaging mode can be obtained.
That is to say, in the fundamental frequency range of 1 to 20 MHz, the transmitter circuit of the ultrasonic system is required to generate an output voltage of ±3V to ±100V under the corresponding duty ratio.
Obviously, the ±100V output of the transmitter requires some high voltage switch. When the transmitter includes an IC, this requirement translates into a high voltage transistor and is optimized to withstand large electric fields. Again, they perform poorly at low pressures (<10V), which are typically used for CW operation. There is still a serious challenge in designing a transmitter to meet the product specifications at the extreme end of the voltage range.
The wide range of output voltages is not the only challenge in the manufacture of ultrasonic transmitter devices, and there are more challenges.
Slew Rate The transmitter may have to generate a slew rate of up to 8V/ns, depending on the previously mentioned voltage swing and operating frequency range. Combined with the typical parallel loads of 100Ω and 300pF representing the transmitter, the transmitter provides nearly 3A of transient current under the most demanding conditions (see Figure 2).
Figure 2. Typical ultrasonic transmitter output at ±100V supply and corresponding instantaneous current into 100Ω and 300pF parallel loads
The ideal output of a harmonic distortion ultrasonic transmitter is a sinusoidal signal that meets the highest voltage amplitude and operating frequency requirements. Instead of creating such an analog signal that is difficult to generate, you can generate a rectangular pulse. After being limited to the low-pass filtering characteristics of the transmitter, this pulse is reduced to only the first few of its harmonics. Among the remaining even harmonics, the second harmonic is generally the culprit. Therefore, the amount of suppression of the second harmonic becomes the main quality factor of the ultrasonic transmitter.
Pulse symmetry and zeroing we can intuitively understand the symmetry requirements of the ultrasonic transmitter output. However, it is important to understand here that the output signal does not have to be a long burst. It may include a single positive and negative pulse pair with a pulse pair of 0V. Again, the quality of the signal to 0V becomes critical. Sometimes it is called the "damping" function (see Figure 3) and has a huge impact on some ultrasound modes, such as harmonic imaging of the main source of information.
Figure 3 Fast return to zero (damping) function
Therefore, the symmetry from the positive pulse to 0V and the negative pulse to 0V and the speed at which they occur become factors that determine the linear quality of the output signal.
The resistance of the output transistor in the on-resistance state is critical to the operation of the ultrasonic transmitter. First, the on-resistance together with the load determines the rise and fall time of the output signal, which sets the achievable output frequency. Second, it directly affects power consumption. According to the voltage and current ranges mentioned above, a large amount of power consumption occurs during the ultrasonic transmission event. The degree of power consumption depends on the interaction between the high and low duty cycles of the B mode display or harmonic imaging and the low and continuous operation of the CW Doppler imaging mode.
Other important performance parameters of the ultrasonic transmitter system include output signal jitter and phase noise, as well as delay matching between channels.
The emergence of semiconductors In the past few decades, semiconductor technology has been the basis for advances in the communications and computer industries. Now, they are about to bring similar breakthroughs in medical technology, especially in imaging applications. Ultrasound is no exception, and it has witnessed an ongoing movement from a custom-used discrete system to a fully integrated semiconductor chip-type solution. Due to its inherent high speed, low power consumption and small size, semiconductor ICs can help medical imaging manufacturers reduce time-to-market, portability of end devices, and improve product reliability and performance while maintaining cost controllability.
Now, the single-chip IC solution can be used to implement the receive/transmit and receive/transmit switch functions. Some of the currently available IC transmitters are capable of generating ±100V output voltages up to 8V/ns slew rate and second harmonic distortions below 40dBc. Pulse symmetry and fast zeroing can be achieved with an active damping architecture. For example, TI's TX734 is a ±90V, ±2A, 3-level, 4-channel, integrated transmitter with active damping. The integrated ultrasonic pulse generator with the AFE5851 (a 16-channel analog front-end chip) and the TX810 (an 8-channel receive/transmit switch) are examples of ultrasonic system IC solutions.
Conclusion In the past few decades, many significant advances have been made in the field of medical imaging. Ultrasonic technology plays a special role in these advancements and has proven to be a universal diagnostic tool for many applications. These applications range from obstetrics to angiography, to needle guidance in some procedures, and even to the treatment of certain benign and malignant tumors. Semiconductor IC technology is supporting this development at an increasingly faster pace. Thanks to the emergence of various ICs, all the main functions of the ultrasound system have been realized, enabling the majority of clinicians and other users to enjoy important technological advances such as portability, high image resolution and high product reliability.
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