Design of the hottest ultrasonic imaging system an

2022-08-24
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Abstract: this application note introduces the design considerations of ultrasonic imaging system, discusses the development trend of miniaturization, low cost and portability of imaging system, and expounds the important conditions for realizing the performance and diagnostic ability of large vehicle mounted system in small system. This paper discusses the sub functions of ultrasonic system and the requirements for electronic components, focusing on sensors, high-voltage multiplexers, high-voltage transmitters, imaging channel receivers, digital beamforming, beamforming digital signal processing and display processing

overview

by transmitting ultrasonic energy into the human body, receiving and processing the returned reflected signals, the phased array ultrasonic system can generate images of organs and structures in the body, map blood flow and tissue movement, and provide high-accuracy blood flow velocity information at the same time. In the traditional design, building such an imaging system requires a large number of high-performance phased array transmitters and receivers, which makes the on-board equipment bulky and expensive. In recent years, with the progress of integration technology, designers can obtain small-size, low-cost and highly portable imaging system solutions, and can achieve performance indicators close to large-scale imaging equipment. However, new design challenges still exist, that is, to further improve the integration of the scheme while improving the system performance and diagnostic capability

sensor

the key component of the imaging system is the ultrasonic sensor. Typical ultrasonic imaging systems need to use various sensors to support specific diagnostic requirements. Each sensor is composed of a group of piezoelectric sensor unit arrays, which concentrate energy and transmit it to the interior of the human body, and then receive the corresponding reflected signal. Each unit is connected to the ultrasonic system through a thin coaxial cable. Generally, the sensor is composed of 32 to 512 units, and the operating frequency is 1MHz to 15MHz. Most ultrasonic systems provide two to four sensor adapters, and clinicians can easily replace sensors according to different detection types

high voltage multiplexer switch

a typical phased array ultrasonic system is equipped with 32 to 256 transmitters and receivers. In most cases, the system is equipped with fewer transmitters and receivers than the number of sensor units. In these cases, it is necessary to install a high-voltage switch in the sensor or system for signal multiplexing, and the switch is connected between a specific sensor unit and a transmitter/receiver (tx/rx) pair. Thus, the system can dynamically change the effective sensor aperture in the provided sensor array

the requirements of imaging system for high-voltage switch mainly include several aspects: it must be able to withstand the emission pulse with voltage swing up to 200vp-p and peak current up to 2A; The switch must be able to switch quickly to quickly adjust the effective aperture and meet the requirements of image frame rate; Finally, these switches must also have minimal charge injection to avoid spurious transmission and related false images

functional block diagram of ultrasonic imaging system. For the ultrasonic scheme recommended by Maxim, please refer to:

high voltage transmitter

digital transmission beamformer is used to generate the required digital transmission signal and generate the focused transmission signal at the correct time and phase. High performance ultrasonic system can produce complex emission waveforms through arbitrary waveform generator, so as to optimize image quality. In these cases, the transmit beamformer generates 8-10 digital characters at a rate of about 40MHz, thereby generating the required transmit waveform. The digital to analog converter (DAC) converts the digital waveform into an analog signal, which is amplified by a linear high-voltage amplifier to drive the sensor unit. Because this launch technology takes up a large volume, is expensive and requires high energy consumption, this architecture is limited to expensive non portable devices. Most ultrasonic systems do not use this transmit beamforming technology, but use multistage high-voltage pulse generator to generate the signal that needs to be transmitted. In this alternative, a highly integrated, high-voltage pulse generator is used to quickly switch the sensor unit to an appropriate programmable high-voltage power supply to generate the emission waveform. In order to generate a simple two pole emission waveform, the pulse generator needs to alternately switch the sensor unit to the positive and negative emission voltage controlled by the digital beamformer. A more complex design allows the sensor unit to switch to multiple power supplies and ground, resulting in more complex and better performance multiple waveforms

in recent years, with the wide application of second harmonic imaging, high-voltage pulse generator has higher and higher requirements for slope and symmetry. Second harmonic imaging utilizes the nonlinear acoustic characteristics of the human body. These nonlinear characteristics tend to convert the sound energy of frequency fo into 2fo frequency. Many reasons make it possible to obtain higher image quality by receiving second harmonic signals. Therefore, second harmonic imaging has been widely used

there are two basic methods to realize second harmonic imaging. One is called standard harmonic imaging, which suppresses the second harmonic of the transmitted signal as much as possible, so that the received second harmonic mainly comes from the nonlinearity of the human body. This mode requires the emission energy of the second harmonic to be at least 50dB lower than the fundamental energy. Therefore, the duty cycle of the transmitted pulse is required to be accurate by 50% and the error is less than ± 0.2%. Another method is called pulse inversion, which uses the inverted transmission pulse to generate two opposite phase received signals of the same image path. Sum the two inverse received signals in the receiver to recover the harmonic signal generated by human nonlinearity. This pulse inverse Z - the perimeter length of the corrugated box (CM); The method must offset the inverse component of the transmitted pulse as much as possible during superposition. Therefore, the rise time and fall time of the high-voltage pulse generator must be strictly consistent

imaging channel receiver

the receiver of ultrasonic imaging channel is used to detect two-dimensional (2D) signals and pulse Doppler (PWD) signals and spectral PWD required for color Doppler fluid imaging. The receiver includes tx/rx switch, low noise amplifier (LNA), variable gain amplifier (VGA), anti aliasing filter (AAF) and analog-to-digital converter (ADC)

tx/rx switch

tx/rx switch can protect the low-noise amplifier from the influence of high-voltage transmission pulse, and isolate the low-noise amplifier input and transmitter during reception interval. The switch is generally realized by a group of correctly biased diode arrays. When there are high-voltage transmitting pulses, they will automatically close or open. Tx/rx switch must have a fast recovery time to ensure that the receiver can be turned on immediately after transmitting a pulse. These fast recovery times are critical for shallow buried imaging and providing low on resistance to ensure receiving sensitivity

low noise amplifier (LNA)

the LNA in the receiver must have excellent noise performance and sufficient gain. For a properly designed receiver, LNA will determine the noise performance of the whole receiver. The sensor unit is connected to the input of the corresponding low impedance LNA through a long coaxial cable. If there is no proper cable terminal matching, the cable capacitance and the source impedance of the sensor unit will greatly restrict the bandwidth of the signal received from the broadband sensor. The sensor cable is matched to low resistance, which helps to reduce the impact of this filter and effectively improve the image quality. Unfortunately, this termination also reduces the input signal of LNA, thus reducing the receiving sensitivity. It can be seen that it is very important to provide active input termination for LNA, which can provide the necessary low input impedance termination and excellent noise performance under the above conditions

variable gain amplifier (VGA)

vga is sometimes called time gain control (the testing force of TGC double column tensile testing machine is much larger than that of single column tensile testing machine) amplifier, which can provide sufficient dynamic range for the receiver in the whole receiving cycle. Ultrasonic signals transmit about 1540 meters per second in the body, and the round-trip attenuation rate is 1.4db/cm-mhz. After transmitting an ultrasonic pulse, you can immediately receive an echo signal up to 0.5vp-p at the LNA input, which will quickly fall to the thermal noise base of the sensor unit. The dynamic range required for receiving this signal is about 100dB to 110dB, which is beyond the input range of the actual ADC. Therefore, it is necessary to use VGA to convert the signal into a signal amplitude equivalent to the ADC range. In typical applications, 12 bit ADC is used, and VGA is required to provide 30dB to 40dB gain. The gain is adjusted with time (i.e. "time gain control") to achieve the required dynamic range

the transient dynamic range of ultrasonic receiver is also critical, which will affect the quality of 2D image and the ability of the system to detect Doppler shift (blood or tissue movement). Especially in the second harmonic imaging system, the interested second harmonic signal is significantly lower than the fundamental wave of the transmitted signal. The same is true for small Doppler signals. The Doppler signal frequency may be within 1kHz, and the amplitude is far lower than the reflected signal of tissue or blood vessel wall. Therefore, special attention should be paid to the bandwidth and near carrier SNR of variable gain amplifiers, which are usually the key parameters that restrict the performance of receivers

anti aliasing filter (AAF) and ADC

anti aliasing filter AAF are placed in the receiving channel to filter out high-frequency noise and signals beyond the normal maximum imaging frequency range, and prevent these signals from being converted and aliased to baseband through ADC. In order to suppress aliasing and ensure the time-domain response of the signal, the filter needs to attenuate the signal beyond the first Nyquist frequency. Therefore, Butterworth filter or higher-order Bessel filter is often used

in typical applications, 12 bit ADC is used, and the sampling rate is usually between 40msps and 60msps. ADC provides the necessary transient dynamic response range with appropriate cost and power consumption. In a properly designed receiver, ADC will limit the transient SNR of the receiving channel. As mentioned above, VGA with poor performance will limit the SNR index of the whole receiving channel

digital beamformer

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the output signal of ADC is transmitted to the digital receiving beamformer through the high-speed LVDS serial port. This transmission method reduces the design complexity of PCB and the number of interface pins. The beamformer has built-in up conversion low-pass filter or band-pass digital filter, which increases the effective sampling rate by 4 times and improves the accuracy of system beamforming. The up conversion signal is stored in memory. After appropriate delay, it is superimposed by the delay coefficient adder to obtain the appropriate focus. The signal is also properly weighted or "apodized". Apodizing before stacking can adjust the receiving aperture, reduce the influence of sidelobe on the receiving beam, and improve the image quality

digital signal processing of beamforming

the received beamforming digital ultrasonic signal is processed by DSP and PC based equipment to obtain video and audio output signals. This process can usually be divided into B-ultrasound or 2D image processing, and Doppler processing with color ultrasound fluid imaging information. Doppler processing is divided into pulse Doppler (PWD) processing and continuous wave Doppler (CWD) processing

B-ultrasound processing

in B-ultrasound processing, RF beamforming digital signals are filtered and detected. The detection signal has a very wide dynamic range, and the B-ultrasonic processor must digitally compress these signals to make them reach the dynamic range specified by the display

color Doppler fluid signal processing

in color Doppler fluid signal processing, RF digital beamforming signal and positive

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