TI Technical Article | Signal chain design considerations for ultrasound systems

High-performance ultrasound imaging systems are widely used in various medical scenarios. In the past decade, discrete circuits in ultrasound systems have been replaced by highly integrated chips (ICs). Advanced semiconductor technology continues to promote system performance optimization and miniaturization. These changes are due to various chip technologies, such as dedicated low-noise amplifiers, multi-channel low-power ADCs, integrated high-voltage transmission, optimized silicon processes, and multi-chip module packaging. As chip power consumption and size are reduced to 20% of the original. In addition, thanks to the development of low-power, high-performance silicon processes, some beamforming pre-processing modules have been integrated into general-purpose analog or mixed-signal chips instead of dedicated digital processors. At the same time, advanced high-speed serial or wireless interfaces greatly reduce the complexity of system layout and can transfer as much RF data as possible to system integrated chips (SOCs), CPUs or GPUs. The current application of ultrasound technology has also expanded from specific radiological diagnosis to various portable applications, real-time bedside monitoring, and on-site medical inspections.

Ultrasound is a sound wave with a frequency higher than 20KHz. Medical ultrasound imaging systems often use frequencies between 1 MHz and 20 MHz, which can achieve sub-millimeter resolution. The first commercial ultrasound imaging system was born in the 1970s, providing real-time 2D brightness or grayscale images. Today, ultrasound imaging has become an important medical imaging technology due to its safety, cost-effectiveness and real-time advantages. Medical ultrasound systems can effectively monitor infant development and can also be used to diagnose diseases of internal organs such as the heart, liver, gallbladder, spleen, pancreas, kidneys, and bladder.

A typical ultrasound system consists of a piezoelectric transducer, electronic circuitry, an image display unit, and DICOM (Digital Imaging and Communications in Medicine) compliant software. A simplified block diagram of a typical ultrasound system is shown below.

Figure 1: Simplified block diagram of a typical ultrasound system

Ultrasonic transducers are a key component of ultrasonic systems and consist of piezoelectric elements, connectors, and support structures. The piezoelectric effect refers to the phenomenon that the physical dimensions of a material change in response to an applied electric field, and vice versa. As shown below, most transducers in ultrasonic applications are dual-mode. The transducer converts electrical energy into mechanical energy during the transmit phase (mode). The generated mechanical waves propagate toward the medium and are reflected if the medium is not uniform. In the receive mode, the reflected mechanical waveform is received and converted into an electrical signal by the transducer.

Figure 2. Transducer vibration, sound wave propagation and reflection

In the early days of ultrasound imaging, multichannel electronics for ultrasound systems were expensive and immature. Single-element transducers driven by motors and mechanically scanned were widely used to obtain two-dimensional (2D) images. Early systems could not achieve high frame rates or high-precision imaging due to the speed and precision limitations of the mechanical structure. Today, mature array transducers and multichannel electronics support transducers with 64 to 512 elements. Images up to >100 frames/second can be obtained based on electronic scanning. To achieve electronic scanning, beamforming techniques are applied to focus the transducer's acoustic beam. The details of beamforming are discussed in the next section. Similar to optical imaging systems, ultrasound systems achieve the best spatial resolution at the focused focal point. Depending on the application, one-dimensional (1D) array transducers include linear arrays, curved linear arrays, and phased arrays. The main differences between these transducers are the beam shaping structure, imaging range, and image resolution. In addition, the latest 2D array transducers consisting of more than 2000 elements can support real-time three-dimensional (3D) imaging. The figure below shows a single-element transducer, a 1D array transducer, and a 2D array transducer.

Figure 3. Typical transducer:

(a) Single element transducer; (b) 1D array transducer; (c) 2D array transducer

As with any imaging system, image quality is an important criterion in medical ultrasound imaging. Common parameters such as spatial resolution and image penetration are mainly determined by transducer specifications and sound wave propagation theory. The longitudinal and lateral resolution of ultrasound images is linearly related to the wavelength of the sound waves in the medium.

Table 1 shows the properties of selected biological tissues, water, and air. Strong reflection signals appear in two situations where the acoustic impedances are extremely different. Bone has a high density and a high speed of sound; therefore, it is always a strong reflector in ultrasound images. On the other hand, the acoustic impedances of blood and liver are similar, so the reflection between these two tissues is weak. Only a highly sensitive transducer can pick up weak signals. As shown in Table 1, the signal will attenuate during propagation. The cumulative attenuation increases with the propagation distance.

Table 1: Acoustic properties of typical tissues and media

Ultrasonic signals have an extremely wide dynamic range to characterize the differences in physiological structures from the skin surface to internal organs. Therefore, complex electronic circuits are required to provide sufficient dynamic range, which is not easy to achieve under a limited power budget.

When the transducer receives the echoes, an appropriate processing unit is needed to convert these signals into understandable image information for the sonographer or other end user. Ultrasound imaging uses several imaging modalities to study tissue characteristics, body fluid distribution and flow, organ function, etc.

In the earliest ultrasound systems, clinical diagnosis was guided by displaying the amplitude of the echo and its time domain information. This was the A-mode (amplitude mode) ultrasound imaging system. When the transducer's sound beam scans fast enough, real-time images can be achieved. These images are called B-mode (brightness mode) images, which create a cross-sectional image parallel to the scanning direction.

More and more novel imaging modes (such as 3D and 4D imaging) have recently been introduced on the latest commercial ultrasound systems, which are extensions of B-mode imaging. 3D imaging is a superposition of multiple cross-sectional B-mode images acquired by scanning the acoustic beam in two dimensions, as shown in Figures (c) and (d) above. In addition, 4D imaging is defined as real-time 3D imaging.

Figure 4. Scanning mode:

(a) A-mode scanning line, (b) B-mode image, ( c) 3D acoustic beam scanning,

(d) B-mode (sub-images 1, 2, 3) and 3D (sub-image 4) clinical images

Most clinical ultrasound systems include another essential feature: Doppler ultrasound to show blood flow information. Doppler ultrasound has been used in medical applications as early as the 1950s. Today, it can evaluate blood flow and tissue motion. Over the past 60 years, a variety of Doppler technologies have provided different diagnostic information, including continuous wave (CW) Doppler, pulsed wave (PW) Doppler, and color Doppler. There are large differences in the applications between these Doppler modes.

Color Doppler remains an active area of ​​research. It is well known that autocorrelation and cross-correlation processing techniques require strong computing power. New algorithms are being developed to analyze blood flow at a lower computational cost. At the same time, thanks to recent advances in semiconductor technology, digital signal processors with lower power consumption and higher computing power are being applied in this field.

B-mode, CW Doppler, PW Doppler and Color Doppler are the main imaging modes in ultrasound systems. Other imaging modes are also often used in daily diagnosis to obtain more comprehensive clinical information.

The following block diagram represents a typical ultrasound system. The main components include high-voltage transmit circuitry, low-noise analog front end, transmit and receiver beamforming circuitry, digital signal processing unit, image display and storage unit, and other supporting circuits.

Figure 5. Block diagram of a typical ultrasound electronics circuit

Before any AFE design, semiconductor process selection is always the first key consideration based on the design goals. CMOS and BiCMOS processes are the most commonly used processes in ultrasound analog front-end design. Each of them has its own advantages and is suitable for the corresponding circuit blocks.

The BiCMOS (bipolar CMOS) process is currently more popular than pure bipolar processes because it contains high-performance bipolar transistors for analog design and CMOS components for digital design. Bipolar transistors are suitable for low-noise amplifier design, with ultra-low 1/f noise, wide bandwidth, and good power/noise efficiency. The bipolar process also reduces circuit capacitance to obtain good total harmonic distortion. Therefore, amplifiers based on bipolar or BiCMOS processes can achieve the same performance in a much smaller area and lower power consumption than amplifiers based on CMOS processes.

Texas Instruments' 0.35um BiCMOS process was used to study the performance impact of amplifier designs between bipolar and CMOS devices. The figure below (a) shows that bipolar transistor-based amplifiers achieve lower noise at the same bias current; it also shows that bipolar transistors have ultra-low 1/f noise characteristics, which is critical for Doppler applications with modulation and demodulation circuits; (b) the bipolar design significantly reduces the area compared to a similar CMOS design. Of course, as the feature size of semiconductor processes decreases, the area difference between the 0.35um BiCMOS process and the <0.35um CMOS process becomes smaller. However, in general, the 0.35um BiCMOS process is still extremely suitable for amplifier design due to the advantages mentioned above.

Figure 6: Comparison of CMOS and BiCMOS process designs

CMOS technology is more suitable when the circuit has more digital content and switching elements (such as medium-speed ADC). The frequency of medical ultrasound signals is in the range of 1~20MHz, and its ADC sampling rate is usually less than 100MSPS, which can be easily processed by most current CMOS processes. Using 0.18um~65nm CMOS technology, ADC design can achieve better integration and power consumption reduction. In addition, compared with comparable BiCMOS processes, CMOS processes are generally lower in cost and achieve shorter manufacturing cycles. All of this shows that CMOS technology is suitable for ADC design in ultrasound AFE.

In summary, when reducing noise/power consumption is the main goal, BiCMOS process is suitable for TGC amplifier design in ultrasound AFE, that is, voltage-controlled amplifier (VCA) design. On the other hand, CMOS process is a good choice for achieving low power consumption and high integration in ADC design. Especially at the 0.18um to 65nm node, CMOS process with a complete low-voltage digital library can achieve higher integration at a competitive cost compared to 0.35um BiCMOS process.

In the past decade, the process technology of ultrasonic AFE has moved from 0.5um to 90nm, from CMOS only to BiCMOS and CMOS, and from single chip to multiple chips with passive components in the package. As shown in the figure, all these technologies have greatly reduced power consumption, improved performance and reduced chip size.

Figure 7: Evolution of AFE integration

Ultrasonic signals have their own characteristics. As we discussed in the previous section, dynamic ranges exceeding 100dB are often observed in systems. Low-frequency audio circuits, high-frequency digital circuits, low-noise amplifiers, and low-noise clock circuits exist in the same system, on the same board, or even on the same chip. AFE design and system design must address these challenges.

Ultrasound imaging is a safe medical imaging modality with great potential. An increasing number of bedside applications for on-site examinations require low power, low noise and compact systems. In order to fully utilize the advantages of ultrasound signals, the right process must be selected to achieve low power, low noise and small size. The BiCMOS process is suitable for low noise amplifier design, with ultra-low 1/f noise, wide bandwidth and good power/noise efficiency; while the CMOS process achieves high digital density at low power consumption. The combination of the two using advanced packaging technology can provide the most advanced analog front-end solutions. In order to achieve the required ultrasound parameters, such as fast and consistent overload recovery, low IMD3 and PSMR, precise I/Q matching, odd harmonic suppression in continuous wave Doppler mixers, etc., it is necessary to consider the various parameters in the chip to achieve comprehensive optimization of the design.