What Is Vascular Doppler?

Doppler radar is a radar that uses the Doppler effect to perform positioning, speed measurement, and ranging. The so-called Doppler effect is that when sound, light, radio waves and other vibration sources move relative to the observer at a relative speed V, the vibration frequency received by the observer is different from the frequency emitted by the vibration source. Because this phenomenon was first discovered by Austrian scientist Doppler, it is called the Doppler effect.

Doppler principle

The Doppler Principle ^[1]

The frequency change caused by the Doppler effect is called a Doppler frequency shift, which is proportional to the relative velocity V and proportional to the frequency of the vibration.

The working principle of pulse Doppler radar can be expressed as follows: When the radar transmits a fixed-frequency pulse wave to scan the air, if it encounters a moving target, the frequency of the echo and the frequency of the transmitted wave will be different. frequency. According to the size of the Doppler frequency, the radial relative movement speed of the target to the radar can be measured; according to the time difference between the transmitted pulse and the received time, the target distance can be measured. At the same time, the Doppler frequency line of the target is detected by the frequency filtering method, and the spectral line of the interference clutter is filtered, so that the radar can distinguish the target signal from the strong clutter. Therefore, the pulse Doppler radar has better anti-clutter interference ability than ordinary radar, and can detect moving targets hidden in the background.

Pulse Doppler radar was successfully developed and put into use in the 1960s. Since the 1970s, with the development of large-scale integrated circuits and digital processing technology, pulse Doppler radar has been widely used in airborne early warning, navigation, missile guidance, satellite tracking, battlefield reconnaissance, shooting range measurement, weapon fire control and weather. Detection and other aspects have become important military equipment. Early warning aircraft equipped with pulse Doppler radar have become effective military equipment against low-altitude bombers and cruise missiles. In addition, this radar is also used for meteorological observations, and Doppler velocity resolution of meteorological echoes can be used to obtain the distribution of various air turbulences in the atmosphere at different heights.

The main use of airborne fire control system is pulse Doppler radar. For example, the APG-68 radar equipped by American fighter aircraft represents the advanced level of airborne pulse Doppler fire control radar. It has 18 working modes, which can search and track targets in the air, on the ground and at sea, and has good anti-jamming performance. When the aircraft is flying at low altitude, it can also guide the aircraft to track terrain fluctuations to avoid collision with the ground. This radar is small in size, light in weight, and highly reliable.

Airborne pulse Doppler radar is mainly composed of antenna, transmitter, receiver, servo system, digital signal processor, radar data processor and data bus. Airborne pulse Doppler radar usually adopts a coherent system, has extremely high carrier frequency stability and spectral purity, and extremely low antenna side lobes, and adopts advanced digital signal processing technology. Pulse Doppler radar usually adopts higher and multiple repetition frequencies and multiple transmitted signal forms to use algebraic methods in data processors, and can apply filtering theory to further filter target coordinate data in data processors or prediction.

Pulse Doppler radar has the following characteristics: The programmable signal processor is used to increase the processing capacity, speed and flexibility of the radar signal, and to improve the reusability of the equipment, so that the radar can search and track while tracking. It can change or increase the working state of the radar, so that the radar has the ability to cope with various interferences and the ability to identify targets beyond line-of-sight; The programmable grid-controlled traveling wave tube is used to enable the radar to work at different pulse repetition frequencies. The ability of adaptive waveforms, which can select low, medium or high pulse repetition frequency waveforms according to different tactical states, and can obtain the best performance in various working states; Doppler beam sharpening technology to obtain high resolution In the air-to-ground application, it can provide high-resolution map mapping and high-resolution local magnification mapping. In the air-to-air enemy situation judgment status, it can distinguish group targets in dense formations.

Doppler Ultrasound (D ultrasound)

An ultrasound diagnostic apparatus made based on the Doppler effect is called a Doppler ultrasound diagnostic apparatus (D-type ultrasound diagnostic apparatus). It is used in medical clinical diagnostics to diagnose heart, blood vessels, blood flow, and fetal heart rate.

There are many types of ultrasound Doppler. According to the different display methods, it can be roughly divided into two categories: spectrum Doppler and ultrasound Doppler.

Spectral Doppler is divided into continuous spectrum Doppler and pulsed Doppler according to the way the signal is generated.

Ultrasound Doppler imaging includes ultrasound Doppler angiography and color Doppler blood flow imaging.

In the past few decades, the research work of ultrasonic spectrum Doppler detection of blood flow has made great achievements, and the appearance of color Doppler has made it more perfect. The detection of blood flow by spectral Doppler is not intuitive. The change of the frequency spectrum and then the change of blood flow are expressed. For the quantitative measurement of blood flow, spectral Doppler is an indispensable tool; As the display of blood flow is intuitive, it has become the most reliable method for qualitative diagnosis.

Clinical application

Continuous ultrasound Doppler diagnostic instrument

The continuous ultrasound Doppler diagnostic instrument obtains information about moving targets by transmitting and receiving continuous Doppler signals. This type of instrument has a simple structure and low price, and it can be used to observe the motion of the heart wall, valves, and fetal body. There are also limitations in the measurement of such instruments. For example, the direction of movement of an object cannot be determined, and the state of blood flow cannot be detected. Because it has no depth resolution, it cannot detect the depth of a moving object, so it has been rarely used clinically except for fetal detection.

2. Continuous ultrasound Doppler flowmeter

Continuous ultrasonic Doppler flowmeter can detect the size and direction of blood flow velocity, especially when measuring high-speed blood flow. Continuous ultrasonic Doppler flowmeter has its unique advantages. Such instruments are still unable to distinguish the distance between the probe and the moving target. The measurement results are greatly affected by the angle between the sound beam and the direction of movement, and it is impossible to understand the location of the abnormal blood flow.

3. Pulsed ultrasound Doppler flowmeter

The pulsed ultrasound Doppler flowmeter emits ultrasound pulses and has a delay circuit to control the receiver, making this instrument capable of distance gating. If different delay times are used, blood flow velocities at different depths in the direction of the sound beam can be obtained, thereby forming a blood flow profile. At present, the pulse Doppler flowmeter is combined with a B ultrasound imaging device. The former is used to check the blood flow state, and the latter is used to detect the anatomical structure. Therefore, it is possible to diagnose valve stenosis and vascular stenosis, valve insufficiency and congenital septal defects Good results have been achieved with respect to the diversion. This type of instrument also has its shortcomings. The magnitude of the blood flow velocity it measures, that is, the Doppler frequency shift, is limited by the pulse repetition frequency. When the frequency shift value exceeds the Nyquist frequency, the high-speed blood flow spikes cannot be displayed normally, and the frequency inversion appears. In addition, because the sampling volume range is small, it needs to be repeatedly moved on the cross section, and the detection time is longer.

4.Color Doppler blood flow imaging device

The color Doppler blood flow imager uses phase detection of the scattered echo Doppler information and undergoes autocorrelation processing. The color gray scale coding classifies the average blood flow velocity into color display. It is related to B-type images and M-type images. The combination of echocardiography can provide temporal and spatial information of blood flow in the heart and large blood vessels. It can simultaneously display the distribution and number of all blood flow bundles on a certain section of the heart, the shape and size of the chamber; show the blood flow path and direction; identify laminar flow, turbulence or eddy current; measure the area, contour length and width of blood flow bundles; Clearly suggest the relationship between vascular structural abnormalities and hemodynamic abnormalities. It is clinically used for non-invasive diagnosis of heart valve disease, congenital heart disease, cardiomyopathy, and cardiac tumors. Color Doppler blood flow imaging is intuitive, visual, and rapid detection. The diagnosis is sensitive and accurate. Of course, color Doppler blood flow imaging also has its limitations. It is more used as a qualitative diagnostic method, and the quantitative analysis of hemodynamics must rely on spectral Doppler. 1. Pulse Doppler ultrasound system How it works

The pulsed Doppler ultrasound system transmits pulse waves. The number of ultrasonic pulses transmitted per second is called the pulse repetition frequency (PRF), which is generally 5 to 10 kHz. At present, the commonly used range-selective pulse Doppler ultrasound system consists of a transducer, a high-frequency pulse generator, a master oscillator, a frequency divider, a sampling pulse generator, a receiving amplifier, a phase detector, a low-pass filter, and fv converter and other components. The transducer (probe) uses a separate transmitting and receiving type. The transmitting piezoelectric crystal is excited by a high-frequency pulse with a very short duration and transmits ultrasonic pulses. The receiving piezoelectric crystal receives the high-frequency echo scattered by the red blood cells, which is amplified and input to the phase detector for demodulation. The low-pass filter removes the high carrier wave and allows Doppler echo signals of different depths to pass. By adjusting the delay time between the sampling pulse and the high-frequency transmitting pulse, the echo signal from a certain depth can be gated and sampled to detect the blood flow in the blood vessel at that depth. According to the sampling theorem, the repetition frequency of the sampling pulse must be greater than 2 times the maximum Doppler frequency shift. The delay time between the sampling pulse and the transmitting pulse can be generated by a simple monostable delay circuit. The dial indicating the strobe distance is directly mounted on the axis of the potentiometer that adjusts the delay time. For each 13 s delay time change, the distance scale on the distance dial changes exactly 1 cm. The signal output by the sample-and-hold circuit contains a control pulse signal component, which is filtered by a low-pass filter and then sent to fv to be converted into a voltage output.

2. Characteristics and limits of pulsed wave Doppler and continuous wave Doppler ultrasound

Pulse wave Doppler is transmitted and received by the same (or a group of) wafers. It transmits in less time and receives in more time. Due to the use of depth gating (or distance gating) technology, fixed-point blood flow measurement can be performed, so it has high distance resolution, and can also make accurate analysis of the nature of noisy blood flow. Because the maximum display frequency of pulse wave Doppler is limited by the pulse repetition frequency, aliasing is easy to occur when detecting high-speed blood flow. This is very unfavorable for the examination of diseases such as mitral stenosis and aortic stenosis.