Ultrasound imaging analog front-end design for various systems

Ultrasound imaging is a widely used medical imaging method. Traditional ultrasound imaging systems use frequencies of 2 to 15 MHz with millimeter accuracy levels and have been widely used to monitor fetuses and diagnose internal diseases. For more than 20 years, traditional desktop ultrasound systems have dominated medical ultrasound applications due to the large number of channels and large signal processing required by ultrasound systems. The aging population, rising health care costs, and the needs of emerging economies have all led to a growing demand for innovative medical solutions.

Ultrasonic product manufacturers need small size, low power consumption and high performance of ultrasonic analog front end (AFE) and DSP. More importantly, ultrasonic product manufacturers want a design that can be used in various systems to minimize their development cycle and accelerate the product launch process.

Ultrasonic system structure

Ultrasound systems vary in their functionality and performance. For example, some high-end systems often have 3D, 4D, and harmonic imaging modes, while some low-end systems may only have 2D B-mode imaging and spectral Doppler imaging modes. Functional differentiation mainly depends on the digital backend. High-end ultrasound systems require more and faster high-end DSP computing resources to achieve near real-time signal processing. Obviously, it is difficult to share signal processing units between high-end and portable systems. However, ignoring the different performance requirements, ultrasound systems generally have similar receive channel architectures.

As shown in Figure 1, the receive analog front end of an ultrasound system consists of common blocks such as a low noise amplifier (LNA), a time gain control (TGA) amplifier, a voltage controlled amplifier (VCA), a programmable gain amplifier (PGA), a low pass filter, and an analog-to-digital converter (ADC). In any case, the performance of the AFE greatly affects the performance of the entire system. Therefore, as long as AFE products that meet different performance requirements exist in pin-to-pin compatible packages, the AFE design can be standardized and reused in various systems. This standardization can be easily implemented in mid- and low-end systems that do not require special analog signal conditioning.

Figure 1 Ultrasonic system structure diagram

However, most current AFE products cannot meet the needs of ultrasound product manufacturers. Therefore, some separate chips must be selected to meet the various performance requirements of pocket and desktop systems. For example, a desktop system may allow higher power consumption but must achieve lower noise, or vice versa, so a redesign is necessary.

Some new AFE devices, such as TI AFE5805, maintain the same external pinout. They are targeted at various ultrasound systems, both portable and desktop. Pin-to-pin compatibility will allow ultrasound product manufacturers to standardize AFE designs and design innovative products with significant cost savings and fast time to market.

Relationship between analog front-end characteristics and system performance

Always remember that designing an ultrasound system is a complex matter and that every characteristic of the AFE affects the performance of the entire system. The ability to balance the various parameters for each system category is undoubtedly an art.

Power consumption is a key consideration for portable ultrasound systems. Low power consumption means longer operating time with less battery charge. However, it affects other parameters such as input signal range, input equivalent noise, harmonic distortion, etc., although these performance degradations are usually acceptable for portable (low-end) systems.

In addition to power consumption, AFE noise is the second factor that ultrasonic system designers need to consider. The received signal level of an ultrasonic transmitter may vary between 10μVPP and 1VPP. The smaller the signal that can be detected, the higher the sensitivity of the system. Both the input equivalent current and input equivalent voltage noise affect the system sensitivity. Generally speaking, the noise parameters of 0.7 to 1.5nV/rt(Hz) (RTI) are used for systems from high-end to low-end.

一些具体系统应用证实这些噪声参数足以产生高质量的图像。虽然可以使用一款更低噪声的放大器，但是其对最终超声波图像质量并无显著提高，因为需要考虑输入等效电流噪声和发射/接收(T/R)开关的噪声。除输入等效电压噪声以外，闪烁噪声（即1/f噪声）也是成像应用中的一个重要因素。在存在混频的连续波(CW)模式下，低频噪声频谱移至载波频率，从而降低了相关频率的信噪比(SNR)。具有白噪声性能的放大器优于其宽泛的工作频率。

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In some ultrasound applications, the gain control range plays an important role in achieving the image dynamic range. When the VCA has a higher gain control range, the final image has a wider dynamic range, resulting in higher image quality. Combined with the ADC's SNR, the system's dynamic range can be calculated using the following equation:

Dynamic Range = SNR + Gain Control Range (Equation 1)

For example, a system including a 12-bit, 70dB SNR and 40dB gain control range VCA can achieve a dynamic range of 110dB. In other words, considering the attenuation coefficient of 0.7dB/cm.MHz of the human body, the imaging depth of 10cm and the 7.5MHz transmitter, a dynamic range of 105dB can be calculated by 10×2×0.7×7.5. In some current ultrasound systems, 10-15MHz probes are usually used to image some small areas. Therefore, a dynamic range of more than 100dB is usually required, which leads to the conclusion that an AFE with a large gain control range is preferred. In addition, an AFE with higher overall gain is an aid in detecting small signals and compensating for the insertion loss caused by other circuits (for example, the insertion loss of passive high-order filters).

Amplifier saturation and overload recovery are also important system parameters. It is more valuable to evaluate and calculate these two parameters together rather than discussing them separately. Basically, the ideal input signal range of an amplifier is limited by its linear output voltage (related to the supply voltage) and gain.
(Equation 2)
Therefore, lower gain and higher supply voltage are beneficial to this parameter. However, low gain will reduce the input equivalent voltage noise, while high supply voltage will increase the overall power consumption, so a compromise must be used. For some portable and mid-range systems, parameters of 200 to 400mVPP are usually selected. Ultrasound amplifier saturation is usually caused by high-voltage pulse leakage or large signals reflected from near-surface objects with very different acoustic impedances. Such examples include epidermal tissue or bone, where only a small amount of clinical information is available. In most cases, the loss of information in these areas may not affect the clinical diagnosis. However, if the amplifier cannot recover in time, a large amount of information will be lost. The fast overload recovery time of the AFE ensures that the ultrasound system can obtain as much useful information as possible. The overload recovery time of the AFE can be expressed in the number of ADC clock cycles. An overload recovery time of one clock cycle is ideal.

Another effect of ultrasound amplifier saturation is increased harmonic distortion. Due to the use of popular contrast agents, more and more systems (even portable systems) require low second harmonic distortion of the entire system to ensure smooth harmonic imaging. Generally speaking, the harmonic signal received by the transmitter can be as high as 40dB (below the fundamental signal), depending on the combination of contrast agent acoustic properties, transmitter voltage and tissue characteristics. Therefore, the amplifier's HD2 should be less than 40dBc, which enables the system to obtain ideal harmonic images. In addition, high HD2 may cause artificial Doppler shifts. In some clinical situations, this artifact may affect the accuracy of diagnosis. In the final Doppler image, the artificial Doppler shift will cause the directional isolation of the Doppler system. Some literature shows that for some CW and PW Doppler systems, 45-50dB of directional isolation may be sufficient. Due to the above factors, the linear input range of the AFE should be specified when the HD2 is less than 40dBc.

Crosstalk, which affects image accuracy, is another parameter that needs to be considered in ultrasound systems. The main crosstalk in ultrasound systems is caused by some transducer arrays arranged at -30 to -35dBc, depending on the spacing, frequency, design, materials, etc. of the transducer elements. Generally speaking, the crosstalk of ICs or PCBs is much lower than -35dBc. Therefore, circuit crosstalk will not degrade system performance.

Ultrasonic Analog Front End

To meet the above criteria, an ultrasound AFE, such as TI's AFE5805, is an ideal choice. Best-in-class BiCMOS and CMOS technologies are used to optimize power consumption and noise performance. The BiCMOS process is the best choice for the VCA portion of the AFE5805 because of its low power consumption, small chip size, and low flicker noise. The CMOS process is well suited for analog-to-digital converters. These innovative combinations can reduce size by 50%, reduce power consumption by 20%, and reduce noise by 40% compared to similar solutions. The constant noise performance shown in Figure 2 covers the entire operating frequency range. This allows the design of portable ultrasound systems to achieve low power consumption and higher image quality.

Summarize

In the next few years, the demand for portable, low-cost ultrasound equipment is expected to grow rapidly in all regions of the world. For ultrasound equipment manufacturers, opportunities and challenges coexist. The advanced technology of the new ultrasound analog front end allows ultrasound equipment manufacturers to adjust the performance to suit various system sizes. Based on a single design, manufacturers can release multiple products, greatly saving the development cost and time of portable devices and high-channel density mid-range ultrasound systems.