Ultrasonic imaging needs three steps: transmitting sound waves, receiving reflected sound waves, and signal analysis and processing to obtain images.

Ultrasonic probes emit ultrasonic waves through piezoelectric ceramic transducers, and different probes can emit different sound waves with different frequencies. The frequency of medical ultrasound is generally 2- 13MHz. The higher the sound frequency, the weaker the diffraction and the higher the imaging resolution. But at the same time, the higher the frequency, the faster the sound wave attenuation and the smaller the penetration depth. Therefore, when we detect the heart, we can only use low-frequency sound waves, otherwise the detection depth is not enough, although the imaging effect is poor; When detecting subcutaneous blood vessels such as carotid artery and femoral artery, high-frequency sound waves are used, and the imaging is much clearer. In the experiment, we used a 2-4MHz cardiac probe and a 10MHz vascular probe.

The reflected wave is still received by the same ultrasonic probe. Piezoelectric ceramic transducer converts acoustic signals into electrical signals, and then the computer system processes the signals and images them.

B-mode ultrasound shows the two-dimensional gray image of tissue slice facing the probe. As we know, three pieces of information are needed to determine each point on the two-dimensional gray scale map, namely, abscissa, ordinate and gray scale. How did you get these? Because ultrasound will be reflected when it hits the tissues in human body, different tissues have different acoustic impedances, so the acoustic impedance can be calculated according to the reflectivity of the received echo, which corresponds to the gray level on the map (for example, if the acoustic impedance of the tissues on the blood vessel wall is similar and the gray level on the image is similar, the shape of the blood vessel can be seen). Assuming that the probe is one-dimensional, the position of each probe on the probe corresponds to an abscissa. The ordinate is determined by the time difference between transmitting and receiving sound waves. Assuming that sound waves travel at the same speed in the human body, the longer the time, the deeper the position of the reflective tissue. Finally, we can see the outline of the tissue and measure it, such as the diameter and area of blood vessels.

Of course, the specific imaging process is far more complicated than this, because B-ultrasound is real-time. How to distinguish transmitted waves from reflected waves and how to denoise and amplify signals is very complicated, and I don't know. But the above simple description is enough to give us a general understanding of the imaging process.

We have all studied the Doppler effect in middle school physics. Whether the transmitter or receiver moves relative to the sound wave propagation medium, it will cause the observed sound wave frequency change.

Doppler effect is used to measure blood flow velocity, as shown in the figure below. The included angle between the direction of sound wave emitted by the probe and the direction of blood flow is \ θ, the frequency of sound wave emitted is f_0, the frequency of reflected sound wave is f', the Doppler frequency is f_D, the propagation speed of sound wave in human tissue is c, and the blood flow speed is v.

Then the blood flow velocity can be calculated from the Doppler frequency, and the formula is as follows

Its derivation process is mainly a set of two Doppler effect formulas. When transmitting, the receiver (blood) is considered to move relative to the acoustic medium (human tissue), and when recovering, the transmitter (blood reflects sound waves) is considered to move relative to the medium. Then, the sum term is approximately constant at two frequencies to obtain 2f_0 of the denominator.

Before doing the uterine color ultrasound examination, I asked the nurse sister what color ultrasound was, because I found that the examination results and their display screens were all black, and I didn't know where the color was.

Compared with B-ultrasound, color Doppler ultrasound measures blood flow velocity through Doppler effect, which is represented by color in the image. So this color does not directly reflect the color of human tissues, which is quite disappointing. Generally speaking, the red color in the image indicates that the blood flow direction is coming, and the blue color indicates that the blood flow direction is leaving you. At the same time, the darker the color, the faster the blood flow.

I don't know much about the principle of pulse Doppler. I checked the difference between color Doppler and pulse Doppler on the Internet. Probably, the methods are different, and each has its own advantages and disadvantages. In the experiment, what we get by pulse Doppler is the spectrum of blood flow velocity, that is, the change diagram of blood flow velocity with time (waveform diagram), rather than the imaging diagram of human tissue. By measuring the horizontal distance (time difference) between two blood flow velocity pulses, the heart rate can be calculated. If the diameter of a blood vessel is measured in a color Doppler image (B-ultrasound image is also acceptable), the area of the blood vessel can be calculated, and then the blood flow (blood flow flowing in one minute) can be obtained by multiplying the area (integral) under the curve of the blood flow velocity waveform in one period.

The following picture shows my carotid artery color Doppler imaging (upper picture) and pulse Doppler imaging (lower picture), and measured the peak blood flow velocity, heart rate (double heart rate), blood vessel diameter and blood flow (VolFlow).

To sum up, the physical principle of medical ultrasonic instrument: transmitting and receiving ultrasonic waves through piezoelectric transducer, obtaining tissue contour imaging through reflectivity, receiving time and probe position, and measuring blood flow velocity through Doppler effect. B-ultrasound imaging is a two-dimensional gray image, which reflects the outline of tissue. Color Doppler ultrasound is a two-dimensional gray image with blood flow velocity information, and pulse Doppler obtains the waveform of blood flow velocity changing with time.

Think of an interesting place, when using pulsed Doppler, the instrument will make a beating sound, whether it is measuring blood vessels or heart. I don't know if this sound is the amplification of my heartbeat or the sound of blood pulse, or the sound of the instrument itself, which plays with my heartbeat.

Some questions and answers:

1. How to measure blood flow velocity: Doppler effect

2. How to get blood flow: the integral of vascular area multiplied by blood flow velocity.

3. How to get the heart rate: In pulsed Doppler, the interval between two maximal blood flows is the period.

4. How to get the heart volume: automatically track the area.

5. How to get the blood vessel area: Draw or measure the blood vessel radius.

6. How to obtain cardiac function: the ratio of left ventricular systolic and diastolic volume.

7. The difference between color Doppler and pulse Doppler: one is two-dimensional imaging and the other is spectrum.

References:

1. Wikipedia: medical ultrasound examination

related articles

I wrote several blogs to introduce and record our Level 4 physics experiment: using medical ultrasonic instruments to study the influence of exercise on human blood flow distribution.

(1) Why do you want to do the level 4 big experiment in the school hospital?

② the principle of medical ultrasonic imaging

③ Experimental design of the influence of exercise on blood flow distribution.

④ The experimental results of the influence of exercise on human blood flow distribution.