Medical Ultrasound
What is Ultrasound
Ultrasound is not much different from the sound waves we are familiar with, except that we can't hear its "sound". When the frequency of sound waves reaches more than 20 kHz, which is beyond the range that normal people can perceive, this kind of sound waves is called ultrasound. Similarly, if the frequency of a sound wave is lower than the range that humans can hear, it is an infrasound wave. So in other physical properties, ultrasonic waves are basically the same as sound waves. Ultrasound/Sound wave is a type of mechanical wave, longitudinal wave and pressure wave. It is propagated by the vibration of particles, and the vibration of particles will continue to generate relatively high and low pressure areas (as shown in the figure below), and its vibration direction is consistent with the propagation direction, so the propagation of ultrasonic waves requires a medium. Under different media, the speed of ultrasonic waves is different. For example, the speed in air is about 340 meters per second, in the human body, it is about 1540 meters per second, and the speed in vacuum is 0. Ultrasound has a wide range of applications, especially in the medical field. As a non-radioactive method, ultrasound can assist doctors in making better diagnosis of patients. It will be expanded in detail later.
How to generate ultrasound
The generation of ultrasonic waves and the generation of sound waves are based on the same principle. For sound waves, we usually use the telephone as an example. When speaking, sound (mechanical energy) is converted into electrical signals (electrical energy) that travel to the other end, and then when listening, the electrical signals are converted back into sound. This is exactly the same as the process of generating and receiving ultrasonic waves, and their principle is the piezoelectric effect. The piezoelectric effect is that certain materials, such as quartz, will generate a certain degree of voltage on its surface when it is subjected to mechanical pressure; and if we apply a voltage to its surface, it will generate a certain degree of mechanical deformation. Then through precise electrical signal control, we can generate and receive ultrasonic waves. At present, PZT is the most common material used in ultrasonic instruments. In the normal operation of the instrument, ultrasonic waves usually appear in the form of pulses rather than continuous waves, so in general, the PZT receives the electrical pulse signal, generates an ultrasonic wave, and then starts to monitor, and the reception returns one after another. The ultrasonic signals are converted into corresponding electrical signals for further data processing, and the cycle is repeated until the scan is completed.
Propagation of ultrasound in the human body
As mentioned earlier, the speed of ultrasonic waves in the human body is about 1540 meters per second, which is actually an average value and is also the calibration speed used by ultrasonic instruments in most cases. As will be mentioned later, ultrasound imaging relies on the estimation of ultrasound velocity, and its accuracy will have a direct impact on image quality. Then, for different organs and tissues, the transmission speed is different. For example, it is about 1510 meters per second in the brain, about 1560 meters per second in the liver and kidney, 1570 meters per second in the muscles, etc. These are not very different from the average. However, the ultrasonic speed in fat is only about 1440 meters per second. This speed difference makes the quality of the ultrasonic image significantly decrease for obese patients, so in this case, the instrument will recalibrate or dynamically adjust the speed.
Since ultrasound is a kind of wave, it will also produce wave-related physical phenomena with various tissues and organs during the propagation of the human body. These phenomena are the basis for imaging with ultrasound. Mainly transmission, reflection, scattering and refraction. When the PZT emits ultrasonic waves and encounters human organs/tissues, part of the waves can penetrate and continue to propagate deep into the human body along the original direction, which is transmission and the energy of the waves will be partially absorbed in the process; the remaining part of the waves Returning in the opposite direction and being received by the PZT, this part is the reflected wave, and the signal of these reflected waves is the main raw material for imaging; the energy of the scattered waves is usually very small, and the refracted waves will interfere with the imaging. Basically, the ability of ultrasound to transmit and the ability of organs/tissues to absorb ultrasound determines how deep the ultrasound can "see". Because the lower the frequency, the stronger the ultrasound penetration, so when doctors need to see deeper, they often A lower frequency detector (Transducer) will be used, but the low frequency usually causes the quality of the image to be degraded. This is a trade-off that needs to be made and will be discussed in detail later when we talk about detectors.
Introduction to Medical Ultrasound Imaging
With the deepening of research and medical needs, ultrasound images have changed from only 1D to 3D/4D. Ultrasound instruments can now support a variety of imaging modes to meet the needs of different patients and doctors. The following introduces several mainstream imaging modes.
A-mode: It is the so-called 1D, which is the simplest mode. The detector emits a wave of ultrasonic waves in a certain direction, and the instrument presents the equation between the reflected signal and depth, and the image is similar to the signal we usually see on an oscilloscope. A-mode was the main mode of early ultrasound instruments and is less used now, but it can also be used to guide high-energy waves to treat tumors during surgery.
B-mode: B is brightness here. In this mode, the detector scans an area and generates a grayscale 2D image. This is one of the most commonly used modes. The lighter the color (white), the stronger the reflected signal, generally the surface of the organ/tissue, and the darker the color (black), the weaker the reflected signal.
M-mode: M is motion here. In motion mode, the instrument performs fast B-mode scanning and imaging, so the doctor can see the movement of the organ, which is especially important for heart-related diagnosis.
Doppler-mode: Doppler mode, named after the use of the Doppler phenomenon to measure the velocity of moving objects. In Doppler mode, doctors can monitor the flow and direction of blood to identify possible lesions in blood vessels.
Color Doppler: This mode can be simply understood as B-mode/M-mode + Doppler, that is, on the basis of 2D grayscale images, the doppler mode and color calibration are used to display the position, blood flow, flow rate and direction of blood vessels.
3D/4D: 3D mode is a 3D image that can present organs/tissues. As for 4D, it is a real-time 3D image. While many advanced ultrasound instruments deploy 3D and 4D modes, they are generally not used much.
