Detailed explanation of the principles and system division of portable ultrasound equipment

In the early 1990s, portable phones were all the rage. As laptop computers shrank in size, they became known as “backpack phones.” Today, the electronics industry has come a long way, and today’s cell phones can send emails and text messages, take photos, check stock prices, schedule meetings, and of course, talk to anyone anywhere in the world. Similarly in the medical field, so-called portable ultrasound systems were previously mounted on carts and could be towed, but in reality, they were difficult to tow. Fortunately, ultrasound systems continue to improve and are being called “the new stethoscope” by doctors.
This article will review the classic ultrasound signal chain, discuss different system partitioning strategies and their advantages and disadvantages, and show the significance of these system partitioning strategies in portable ultrasound applications.

Ultrasonic signal link

Figure 1 shows a simplified schematic of an ultrasound system. The transducers of the system are located at the end of relatively long cables, which are approximately two meters long. These cables contain at least eight to 256 micro coaxial cables and are one of the most expensive parts of the system. In almost every system, the cable is driven directly by the transducer unit. The capacitance of the cable acts as a load on the transducer element, causing significant signal loss, which places sensitivity requirements on the receiver in order to maintain dynamic range and achieve optimal system performance.

Figure 1. Typical ultrasound signal chain.

On the transmit side (Tx path), a beamformer determines the delay pattern and pulse train that is tailored for the desired focus. The output of the beamformer is then amplified by a high-voltage transmit amplifier that drives the sensor. These amplifiers, which can be controlled by a digital-to-analog converter (DAC) or an array of high-voltage FET switches, shape the transmit pulses for optimal energy delivery to the sensor element. On the receive side, a transmit/receive (T/R) switch (usually a diode bridge) blocks the Tx high-voltage pulses. High-voltage (HV) multiplexers/demultiplexers are used in some arrays to reduce the complexity of the transmit and receive hardware, but this sacrifices flexibility.

The time gain control (TGC) path consists of a low noise amplifier (LNA), a variable gain amplifier (VGA), and an analog-to-digital converter (ADC). Under operator control, the TGC path is used to maintain image uniformity during scanning. Good noise performance depends on the LNA, which can reduce the noise contribution of the following VGA. For applications that benefit from input impedance matching, active impedance control can optimize noise performance.

The wide dynamic range input signal is compressed by the VGA to meet the input range requirements of the ADC. The input-referred noise of the LNA limits the minimum resolvable input signal, while the output-referred noise is mainly determined by the VGA, which limits the maximum instantaneous dynamic range at a specific gain control voltage. This limit is set based on the quantization noise floor, which is determined by the resolution of the ADC.

The anti-aliasing filter (AAF) limits the signal bandwidth and also limits other noise in the TGC path before the ADC.

Beamforming for medical ultrasound is defined as the phase alignment and summing of signals generated by a common source but received by a multi-element ultrasound transducer at different points in time. In the CWD path, the receiver channels are phase-shifted and summed to extract consistent information. Beamforming has two functions: one is to indicate the direction to the transducer, i.e., to increase its gain, and the other is to define a focal point in the human body from which the echo is located.

For beamforming, two distinct approaches can be taken: analog beamforming (ABF) and digital beamforming (DBF). The main difference between ABF and DBF systems is the way the beamforming is accomplished; both approaches require good channel-to-channel matching. In ABF, analog delay lines and summing are used. Only one (very high resolution) high-speed ADC is required. In DBF systems, on the other hand, multiple high-speed, high-resolution ADCs are required. Logarithmic amplifiers are sometimes used before the ADC in ABF systems to compress the dynamic range. In DBF systems, the signal is acquired as close to the sensor unit as possible, then delayed and digitally summed. Simplified schematics of these two types of beamforming architectures are shown in Figures 2 and 3.

Figure 2. Simplified schematic of the ABF system.

Figure 3. Simplified schematic of a DBF system.

Since DBF is more flexible, it is the method commonly used by most modern image acquisition ultrasound systems, but it should be noted that the advantages and disadvantages between ABF and DBF are relative.

Advantages of DBF over ABF:

Analog delay lines tend to have poor matching between channels The number of delay taps in an analog delay line is limited and must use fine-tuning circuits After the data is acquired, the digital storage and summing is "perfect", so the matching between channels is also perfect Multiple beams can be easily formed by summing data at different locations in the FIFO As memory becomes cheaper, larger FIFOs can be used to provide finer delays Systems can be made to function differently using software alone The performance of digital ICs continues to improve at a very high rate

Disadvantages of DBF compared to ABF:

Multiple high-speed, high-resolution ADCs are required (Pulse Width Doppler requires approximately 60 dB of dynamic range, which requires at least a 10-bit ADC). High power consumption due to the use of multiple ADCs and digital beamforming ASICs. The sampling rate of the ADC directly affects the resolution and accuracy of the phase delay adjustment between channels; the higher the sampling rate, the finer the phase delay.

System partitioning strategy

Although today's systems have a lot of advanced technology, ultrasound system design is still complex. As with other complex systems, there are many ways to partition the system. In this section, we will discuss a variety of ultrasound system partitioning strategies, all of which focus on solving the problem of system portability.

For many years, manufacturers have implemented complex systems by designing custom ASICs. This solution usually includes two ASICs that cover the main parts of the TGC path and the Rx/Tx path, as shown in Figure 4. This approach was common before the widespread use of multi-channel VGAs, ADCs, and DACs. The custom circuits allow designers to add some inexpensive and flexible functions. This solution is considered cost-effective because it integrates most of the signal chain and reduces the number of external components used in the system. However, its disadvantage is that over time, the development of lithography technology has made these ASICs outdated and unable to meet the needs of further reducing size and power consumption. ASICs have a large number of gate circuits, and their digital technology is not optimized for integrating analog functions. In addition, only a limited number of suppliers can customize ASIC devices, which will cause designers to face a bottleneck.

Figure 4. ASIC approach

In the previous example, the portability of the ultrasound system was limited but feasible. Even so, it was an important first step in solving the system partitioning problem. Portability is not only in terms of size, but also in terms of battery life, because these circuits are very power-hungry. With the advent of four- and eight-channel TGCs, ADCs, and DACs, size and power have been further reduced, and with it, new system approaches to solving the portability problem have emerged. These multi-channel devices allow designers to place sensitive circuits on two or more boards when building a system. This can reduce system size and facilitate the reuse of the circuit on multiple development platforms. However, this approach also has disadvantages. System size reduction also depends on system partitioning. Multi-channel devices can make PCB routing extremely cumbersome, forcing designers to use devices with fewer channels, such as going from an eight-channel ADC to a four-channel ADC, and if the system size is small, it will also cause heat dissipation problems.

With further integration of the complete TGC path, as shown in Figure 5, multi-channel, multi-component integration makes design easier because their requirements for PCB size and power consumption are further reduced. With the widespread use of more advanced integration schemes, costs, the number of suppliers, system volume and power consumption can be further reduced, system heat dissipation is reduced, and battery life in portable units is extended. The AD9271 ultrasound subsystem of Analog Devices is designed to meet compactness requirements. It uses a tiny 14 mm × 14 mm × 1.2 mm package and consumes only 150 mW per complete TGC channel at 40 MSPS. The AD9271 uses a serial I/O interface to reduce the number of pins, thereby reducing the total area of ​​each channel by at least 1/3 and reducing power consumption by at least 25%.

Figure 5. TGC integration

However, the AD9271 cannot meet the requirements of every ultrasound system designer. The ideal solution is to integrate more functional units into the probe or as close to the probe element as possible. It should be noted that the cable connecting the probe unit will have some adverse effects on dynamic range and is expensive. If the front-end electronics are closer to the probe, the probe losses that affect signal sensitivity can be reduced, allowing the designer to reduce the system's LNA requirements. One approach is proposed in Figure 6, which is to integrate the LNA into the probe unit. Another approach is to place the VGA control between the probe and the components on the board. As the size of the device continues to shrink, the system can also be packaged in ultra-small packages. However, the disadvantage of this approach is that the designer needs to make a full custom design of the probe. In other words, the custom design of the probe/electronics will return the designer to the bottleneck that exists in the ASIC instance, and the suppliers are limited.

Figure 6. Probe integration

In summary, it should be commended that most ultrasound system companies today have applied the majority of their intellectual property (IP) to probe and beamforming technologies. Using multi-channel integrated common devices, including quad- and octal-channel ADCs to complete the system eliminates the need for high-cost components and simplifies the tuning and optimization of independent TGC paths. It should also be noted that further integration of other parts of the ultrasound system can also be considered. Integration of these other signal chain parts will be beneficial if production capabilities allow and market-oriented goals are appropriate.

The trend of ultrasound system portability

Many applications recognize the benefits of ultrasound and therefore place high demands on the portability of ultrasound systems. The increased portability allows these devices to be used even in remote applications where reliable power is not available. These applications include clinical care in remote rural areas, emergency medical services, large animal husbandry, and inspection of bridges, large machinery, and oil pipelines.

Ultrasound systems can be divided into three categories: high-end, mid-range, and low-end. High-end ultrasound systems use the latest technology, meet the latest market requirements, and provide the best image quality. Mid-range ultrasound systems usually have some of the features of high-end ultrasound systems without sacrificing image quality. Low-end ultrasound systems are generally smaller in size and are generally used in specific applications such as clinical medicine. Obviously, high-end ultrasound systems are very expensive and are divided differently depending on the application and market needs. However, the trend towards portability has downgraded many "high-end" features to typical low-end or portable features. Generally speaking, this trend has developed with technological advances in the industrial and electronic industries. As these advances have pushed the size, power consumption and performance indicators of devices to the limit, there is a growing demand to move portable devices from low-end systems to high-end systems. Although ultrasound systems have gradually become known as clinical medicine and preventive maintenance tools, the initial usage rate is still low. This is because the cost of portable ultrasound systems includes not only the purchase cost, but also the cost of training new users. However, as the long-term benefits completely outweigh the cost, portable ultrasound systems will become increasingly popular.

in conclusion

Understanding the nuances of complex systems such as ultrasonic systems requires many years of research and development. We should thank those initial developers for pioneering new fields and setting the research direction that has made cutting-edge electronic technology a benefit to mankind. With early use of pulse echo technology for inspecting large underwater hulls and submarines, and for crack detection in structural manufacturing, it was only a matter of time before ultrasonic technology became widely used.

There is a growing demand for portable ultrasound systems for medical and industrial applications. All of these systems have similar requirements for compactness and portability. In the near future, you may be able to send a scan of your fetus using your mobile phone.