X-ray imaging
1. Basic principles
Medical X-ray examination mainly uses the strong penetrating power of X-rays in human soft tissues to achieve the purpose of "seeing" the internal state. The nature of X-rays, like the visible light we see, are electromagnetic waves. However, the wavelength range of visible light band is 380~780nm, and the wavelength of X-ray is much smaller than that of visible light band, which is 10~10-³nm.
Since the energy of a photon is defined as E=hv=hc/λ, which is inversely proportional to the wavelength, the photon energy of X-rays is much larger than that of visible light, making it highly penetrating. While visible light cannot transmit even the thin layer of our eyelids, a considerable fraction of X-ray photons can easily penetrate our body and be picked up by detectors on the other side. Of course, gamma rays with shorter wavelengths are more penetrating. But in front of gamma rays, our bodies are almost transparent. It's like you wanted to see what's going on in the clothes of the person on the other side, but the penetration is too strong. You can directly see the building behind it, which is also a cup. In addition, we can't guarantee that you can get out of bed after being irradiated by gamma rays once. Come down; if you can still come down, maybe become the Hulk.
2. Interaction with matter
As we mentioned earlier, X-rays will interact with different substances in the body, so that part of the energy is absorbed by different tissues of the human body, and the other part is received by the detector at the other end through the human body.
After the X-rays are emitted from the transmitting end, they pass through different parts of human tissue, and are then received at the corresponding positions on the detector. By analyzing the results on the detector, we can obtain the internal information of the corresponding body part. So what interactions do X-rays have in the human body, how they work, and what tissues they interact with? These are the questions we need to study.
We know that matter is made up of atoms. When X-rays pass through the human body, they also interact with the atoms in our body and cause attenuation. There are three main forms of interaction between X-rays and atoms:
1. Photoelectric effect
2. Compton Scattering
3. Pass through without reaction
Because in matter, the distance between atoms is very large, not only does the nucleus occupy a very small volume, but it is not easy for a photon to collide with an electron. So a considerable part of the photons will pass through the human body unaffected to the detector. For details, refer to Rutherford's gold foil experiment.
The following is to focus on the analysis of the photoelectric effect and Compton scattering
2.1 Photoelectric effect
The photoelectric effect refers to the interaction of photons with the inner electrons of atoms, and the photons are absorbed. After absorbing the photon energy, the electron breaks free from the atomic bond and forms a photoelectron.
The photoelectric effect is more obvious on metals, and photoelectrons can even converge into photocurrents. The probability of occurrence of the photoelectric effect is inversely proportional to the cube of the photon energy ([formula]) α=1/E³, E=hv, that is, the higher the photon energy, the less it will be absorbed and the higher the penetration; The cube of the ordinal number is proportional (α Z³, Z: atomic number), so lead (atomic number: 82) is often used for X-ray protection. Compared with metals, the human body is mainly composed of carbon, hydrogen, oxygen, nitrogen and other elements. It has a low atomic number and a low density of atomic distribution. Therefore, there is no need to worry about being electrocuted by self-generated electrons when taking X-rays.
The photoelectric effect is the main attenuation form of X-rays in clinical practice, and it is also the attenuation form we need. As mentioned above, in the soft tissue mainly composed of organic matter, the attenuation of X-rays is very low, and most of them can pass through directly. However, in the bone part, because the bone is mainly composed of calcium phosphate and also contains atoms such as potassium, magnesium, sodium, and strontium, the attenuation of X-rays in the bone is relatively high.
Therefore, exploring the situation of bones is one of the most important clinical applications of X-rays. This is why basically all orthopedic patients are asked to take a film.
2.2 Compton Scattering
Well, the next step is the scattering of Compton children's shoes.
Different from the photoelectric effect, Compton scattering refers to the interaction of photons with the outer electrons of atoms, causing the photon's energy to weaken and change the direction of motion (scattering), while exciting the outer electrons.
Of course, you don't need to panic, you don't need to calculate the energy of the scattered photons and the scattering angle θ, and the energy and angle Ø of the excited electrons.
It's annoying when Compton scattering occurs. Because in geometric optics, we all think that light travels in straight lines. Therefore, the signal received by the detector and the final result displayed on the film should be in one-to-one correspondence with the anatomical structure of our human body. The signal intensity of each pixel point on the detector should reflect the attenuation of X-rays by the human body passing through the connection between this point and the light source. But when Compton scattering occurs at a point, the scattered photons are likely to randomly hit other pixels of the detector, which will not only weaken the light intensity received by the point, but also cause random other A little light boost. Moreover, a little understanding of atomic energy levels shows that, unlike the photoelectric effect, the energy required to excite the outer electrons is not on the same order of magnitude as the energy to excite the inner electrons:
This results in an incident X-ray photon that remains within the spectral range of the X-ray source even if it has undergone Compton scattering and has reduced energy. As the main optical noise of X-ray imaging, Compton scattering has a great influence on the signal-to-noise ratio of the image. Generally, in order to suppress the noise caused by Compton scattering, we will add a lead grid in front of the detector to suppress X-ray photons from other angles:
3. Generation of X-rays
Knowing X-rays is not enough, we should be able to emit X-rays like Ultraman, that's cool
Of course, when you take X-rays, there won't be an Ultraman hiding at you biubiubiu, but an X-ray tube.
The basic principle is that we pressurize the cathode and shoot out a beam of electrons that bombards the anode (usually a metal like tungsten, rhodium, etc.). The electrons are slowed down in the anode, and the lost kinetic energy is converted into photons. When the voltage across the cathode is high (measured in kV), the photon energy we obtain is in the wavelength range of X-rays. X-ray GET!
This principle of generating photons is called Bremsstrahlung, which is pronounced [ˈbʁɛmsˌʃtʁaːlʊŋ] in German. You can listen to Bremsstrahlung here. Don't look at me, I definitely won't read it to you. It roughly means deceleration radiation, which is almost the meaning of "deceleration radiation".
Except for the characteristic radiation of tungsten atoms in the middle several peaks, it is due to the self-emission generated by the high-energy electrons bombarding the inner electrons, making the atoms in the excited state.
Then the problem comes, in the X-rays we get, a large part of the photon energy is relatively low. We have already mentioned in 2.1 Photoelectric effect that the lower the photon energy, the weaker the penetration. This means that a considerable part of the X-rays will be almost completely absorbed by the body, which is not only unhelpful for detection, but also greatly increases the radiation dose to the patient. So generally speaking, we will now add a filter in front to filter out these low-energy X-rays. That way you don't have to worry about cancer after you finish filming.
4. Application
As we mentioned earlier, because bones contain more calcium phosphate and other metal elements, they have a larger attenuation rate compared with other soft tissues, so most X-ray applications are mostly used to check fractures and analyze bone density. and many more. So what about other parts that don't have any metallic elements?
The answer is very simple, if you don't add it~
Such as barium meal. Through gastrointestinal barium meal angiography, or barium enema (don't ask me what an enema tastes like, I won't tell you), place a barium sulfate contrast agent in the digestive tract, and then use X-rays to check for lesions in the digestive tract. The main component of barium meal is barium sulfate, which has obvious absorption of X-rays, and is insoluble in water and insoluble in acid. It will not be absorbed by the digestive tract and is harmless to the human body.
And angiography. By injecting iodine-containing contrast agent into the blood vessels of the corresponding parts, the distribution and lesions of blood vessels can be displayed.
