In the realm of medical systems, the conventional table-type ultrasound systems have long dominated the field, owing to their extensive channel coverage and demanding signal processing requirements. Operating within a frequency range of 2 to 15 MHz, these systems offer sub-millimeter precision, making them indispensable for diagnosing various organ-related conditions such as cardiac, hepatic, splenic, pulmonary, and renal diseases.
Nevertheless, as semiconductor technology continues to advance, the pursuit of high performance and cost-effectiveness alone is no longer sufficient. The demand for convenience has emerged as a crucial factor, leading to the inevitable rise of portable ultrasound system designs. These compact devices deliver comparable performance to their traditional counterparts while offering enhanced portability. In response to this trend, TechSparks is delighted to present a comprehensive guide to designing portable ultrasound imaging systems, equipping you with valuable insights.
Structural Improvement Scheme of Ultrasonic System
Ultrasound systems offer various functions and performance levels to cater to different needs. High-end systems feature modes like 3D, 4D, and harmonic imaging, while low-end systems commonly use 2D B-mode imaging and spectral Doppler. The key differentiating factor is the back-end digital information processing capability. High-end systems require robust real-time signal processing, necessitating high-performance digital signal processors (DSPs) in their internal architecture. Implementing a shared signal processing unit poses a significant challenge for designing high-end portable systems. Nonetheless, both high-end and low-end systems employ similar receive channel architectures, regardless of performance considerations.
Figure 1 illustrates the receiving analog front end (AFE) of an ultrasound system, comprising essential modules such as the Low Noise Amplifier (LNA), Time Gain Control (TGC) Amplifier, Voltage Control Amplifier (VCA), Programmable Gain Amplifier (PGA), low-pass filter, and analog-to-digital converter (ADC).
Regardless of the system chosen, the performance of the entire system ultimately depends on the AFE. With this perspective in mind, designers can achieve AFE design by developing pin-to-pin compatible packages that meet various performance requirements. Standardizing designs and applying them across different systems simplifies the process. Such standardized designs are more feasible in low-end ultrasound system design, as they do not require specialized analog signal adjustments.
However, current AFE solutions do not fully meet the requirements of portable ultrasound manufacturers. Consequently, separate chips are necessary to address the distinct performance demands of handheld and console systems. For instance, console systems can tolerate higher power consumption but necessitate lower noise levels, while handheld systems prioritize power efficiency. This requires redesigning the AFE accordingly.
Fortunately, new AFE devices are emerging in the market, such as TI’s AFE5805, which enables ultrasound manufacturers to adopt standardized AFE designs. These devices with consistent pinouts are extensively utilized in diverse ultrasound systems, ranging from portable to console-style devices. Pin-to-pin compatibility empowers ultrasound equipment manufacturers to design innovative products while significantly reducing costs and expediting time-to-market.
Improving Ultrasound System Design for Higher Performance
Designing a portable ultrasound system entails addressing various challenges, where AFE-related characteristics significantly impact overall performance. Designers must carefully balance different parameters based on system categories.
Power consumption stands as a primary concern in portable imaging systems. The compact size imposes limitations on battery capacity, and frequent charging can impact battery life. Designers need to consider size constraints and market requirements, potentially sacrificing performance, which is acceptable for low-end systems.
Noise is another critical issue to address. Ultrasonic sensors often receive extremely small signal amplitudes, typically ranging from 10uVPP to 1VPP. The system must exhibit high sensitivity to detect these minute signals. During system operation, input equivalent current and voltage can generate noise, which affects sensitivity. After practical examination, using a noise parameter of 0.7 nV/rt (Hz)~1.5 nV/rt (Hz) enhances the image quality produced by the ultrasound imaging system. It is worth noting that opting for a lower noise amplifier has shown negligible improvement in image quality. Additionally, flicker noise significantly affects imaging quality. In continuous wave mode, when the circuit is mixed, the low-frequency noise spectrum shifts to the carrier frequency, thereby reducing the signal-to-noise ratio at relevant frequencies. Therefore, amplifiers with superior white noise performance are preferred.
In certain ultrasound scenarios, optimizing image quality can be achieved by adjusting the gain control range to obtain a wider dynamic range. This range can be calculated by combining the SNR of the ADC and the gain control range. The calculation formula is expressed as follows:
Dynamic Range = SNR + Gain Control Range (Equation 1)
For instance, consider a system equipped with a VCA having a resolution of 12 bits, an SNR of 70dB, and a gain control range of 40dB. This system would achieve a dynamic range of 110dB.
Dynamic range requirements: In ultrasound systems, a dynamic range of over 100dB is typically necessary when the imaging depth is 10cm, the sensor frequency is 7.5MHz, and the attenuation coefficient of human tissue is 0.7dB/cmMHz.
AFE selection: From a system design perspective, an AFE with a larger gain control range is preferred. This choice not only addresses the imaging quality concern but also compensates for insertion loss caused by other circuits and fulfills the detection requirements for small signals.
Amplifier saturation and overload recovery are crucial parameters to consider in ultrasound systems. Evaluating and measuring these parameters together provides more valuable insights than assessing them individually. The amplifier’s input signal range is determined by its linear output voltage (i.e., supply voltage) and gain, as expressed by the following equation:
Ideal Input Signal Range = (Linear Output Voltage) / Gain (Equation 2)
While lower gains and higher supply voltages can enhance amplifier performance, there is a trade-off between input equivalent voltage noise and overall power dissipation. For portable and mid-range systems, it is recommended to select parameters within the range of 200-400mVPP.
Amplifier saturation typically occurs when large signals, caused by high voltage pulse leakage or reflections from nearby objects, exceed the amplifier’s input range. Saturation results in the loss of information, making fast overload recovery crucial to capture as much valuable information as possible. The overload recovery time of the amplifier can be determined by the number of clock cycles of the Analog-to-Digital Converter (ADC).
Furthermore, amplifier saturation contributes to the generation of harmonic distortion, which can impact harmonic imaging. To ensure successful harmonic imaging, it is necessary to maintain low second harmonic distortion throughout the system. Typically, the amplifier’s harmonic distortion (HD2) should be lower than 40dBc to achieve satisfactory harmonic image quality.
Analog Front End
To meet the aforementioned requirements, we have selected TI’s AFE5805 as our ultrasound AFE of choice. This cutting-edge device leverages state-of-the-art BiCMOS and CMOS technologies to optimize power consumption and noise performance. The utilization of BiCMOS technology in the VCA section of the AFE5805 provides several advantages, including reduced power consumption, compact size, and lower flicker noise. Simultaneously, the CMOS process is exceptionally well-suited for analog-to-digital converters.
By leveraging these innovative advancements, we have achieved remarkable results in our design. Our portable ultrasound systems have experienced a 50% reduction in size, a 20% decrease in power consumption, and a 40% reduction in noise levels. Notably, our design ensures consistent noise performance across the entire operating frequency range. Consequently, our portable ultrasound systems deliver outstanding image quality while minimizing power consumption.