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The application of X-rays in medicine
Publication Time:
2018-03-13
Author:
X-rays are used in medical diagnosis, primarily based on their penetrating power, differential absorption, photosensitivity, and fluorescence.
(1) X-ray Diagnosis
X-rays are used in medical diagnosis, primarily based on their penetrating power, differential absorption, photosensitivity, and fluorescence. Because X-rays are absorbed to varying degrees as they pass through the human body (e.g., bones absorb more X-rays than muscles), the amount of X-rays passing through the body varies. This carries information about the density distribution of different parts of the body. The intensity of fluorescence or photosensitivity on the fluorescent screen or photographic film varies greatly, resulting in shadows of different densities on the fluorescent screen or photographic film (after development and fixing). Based on the contrast of shadow density, combined with clinical manifestations, laboratory results, and pathological diagnosis, it can be determined whether a certain part of the human body is normal. Therefore, X-ray diagnostic technology has become the world's earliest non-invasive internal organ examination technology.
(2) X-ray Therapy
The application of X-rays in therapy is mainly based on their biological effects. When irradiating the diseased parts of the human body with X-rays of different energies, the irradiated cells and tissues can be damaged or inhibited, thus achieving the therapeutic purpose of certain diseases, especially tumors.
(3) X-ray Protection
While utilizing X-rays, people discovered problems such as hair loss, skin burns, visual impairment in staff, and leukemia caused by radiation damage. To prevent X-rays from harming the human body, appropriate protective measures must be taken. The above constitutes the three major aspects of X-ray application in medicine—diagnosis, treatment, and protection.
A Brief History of the Development of Medical X-ray Equipment
Since 1895, X-ray diagnostic and therapeutic technologies have developed rapidly. The main progress can be divided into the following stages:
(1) Ion X-ray Tube Stage (1895～1912)
This was the early stage of X-ray equipment. At that time, the structure of the X-ray machine was very simple, using a gas-filled cold cathode ion X-ray tube with very low efficiency. A heavy induction coil was used to generate high voltage, with exposed high-voltage components and no precise control device. The X-ray machine had a small capacity, low efficiency, weak penetration, low image clarity, and lacked protection. According to records, taking an X-ray pelvic image at that time required an exposure time of 40～60 minutes, resulting in skin burns on the examinee after the photo was taken.
(2) Electron X-ray Tube Stage (1913～1928)
With the development of electromagnetism, high-vacuum technology, and other disciplines, in 1910, American physicist W. D. Coolidge published a report on the successful manufacture of a tungsten filament X-ray tube. It began to be used in practice in 1913. Its main feature is that the tungsten filament is heated to incandescence to provide the electrons required for the tube current. Therefore, by adjusting the heating temperature of the filament, the tube current can be controlled, so that the tube voltage and tube current can be adjusted independently, which is necessary to improve image quality.
The invention of the grid in 1913 partially eliminated scattered radiation and improved image quality. In 1914, a cadmium tungstate fluorescent screen was produced, marking the beginning of X-ray fluoroscopy. In 1923, a double-focus X-ray tube was invented, solving the needs of X-ray photography. The power of the X-ray tube can reach several kilowatts, and the side length of the rectangular focus is only a few millimeters, greatly improving the quality of X-ray images. At the same time, the gradual application of contrast agents has continuously expanded the diagnostic range of X-rays. It is no longer a simple tool for taking skeletal images, but an important medical diagnostic facility that can also examine those with naturally low contrast (small difference in X-ray absorption) such as the gastrointestinal tract, bronchi, blood vessels, ventricles, kidneys, and bladder. At the same time, X-rays began to be used in therapy.
How X-rays are Produced
X-rays can be produced in three ways: Bremsstrahlung, electron capture, and internal conversion. The mechanism by which an X-ray machine produces X-rays is Bremsstrahlung.
Electron Capture:
Beta decay includes three types: β- decay, β+ decay, and electron capture (EC). Electron capture (EC) can be expressed as the parent nucleus capturing one extranuclear orbital electron, causing one proton in the nucleus to transform into a neutron and releasing one neutrino. Therefore, the charge number of the daughter nucleus becomes Z-1, while the mass number remains unchanged. In general, electrons in the K-shell are mostly captured by the atomic nucleus because the K-shell is closest to the atomic nucleus and has the highest probability of being captured. However, the probability of electrons in the L-shell being captured also exists. After the atomic nucleus captures an electron, there will be an electron vacancy in the K-shell or L-shell of the daughter nucleus atom. When an outer electron fills this vacancy, one of the following two situations may occur: either the excess energy is released in the form of characteristic X-rays, or the excess energy is transferred to other electrons in another layer, and this electron gains energy and leaves the atom, becoming an Auger electron. The emission of characteristic X-rays or Auger electrons is a hallmark of the K-capture process.
Internal Conversion:
An atomic nucleus can reach an excited state through some method (such as β decay). An atomic nucleus in an excited state can transition to a lower excited state or ground state by emitting γ-rays. This phenomenon is called γ decay or γ transition. The photons emitted by nuclear level transitions are essentially no different from the photons emitted by atomic level transitions. The difference is that the energy of photons emitted by atomic level transitions is only on the order of eV～keV, while the energy of photons emitted by nuclear level transitions is on the order of MeV. Without considering nuclear recoil, the photon energy Eg can be expressed as Eg = Es - Ex. Sometimes, the transition of an atomic nucleus from an excited state to a lower energy state does not emit photons, but directly transfers the energy to an extranuclear electron, causing the electron to leave the atom. This phenomenon is called internal conversion (IC), and the electron that leaves the atom is called an internal conversion electron. An atomic nucleus in an excited state can return to the ground state by emitting γ photons or by producing internal conversion electrons. Which process occurs depends entirely on the energy level characteristics of the nucleus. The study of internal conversion is an important means of obtaining knowledge about nuclear energy levels. Of course, characteristic X-rays of the atom can also be produced through internal conversion.
Basic Principles of X-ray Machines
X-rays were discovered by German Professor Röntgen in 1895. This radiation emitted from a vacuum tube can penetrate objects. On the electromagnetic spectrum, it has higher energy than visible light, shorter wavelength, and higher frequency. Similar radiation includes cosmic rays and X-rays.
Generating X-rays requires an X-ray tube, whose basic structure must include:
Cathode
Anode
Evacuated glass envelope
And of course, a power supply.
X-ray Characteristics
Penetrates objects; invisible light; wide wavelength range within the electromagnetic spectrum; linear scattering; travels at the speed of light; causes fluorescence in certain materials; exposes film; causes scatter radiation.
When X-rays enter an object, three things can happen:
Absorption
Scattering
Penetration
Four factors influencing image quality:
Density - mAs
Contrast - kVp
Sharpness - motion, geometric parameters
Distortion - position, angle
Relationship between X-ray wavelength and film contrast
When X-rays pass through a patient, the penetration rate is mainly related to the patient's tissue structure and the X-ray wavelength.
Short wavelength X-ray (high kV)
Higher energy, good penetration, resulting in lower contrast on the film (low contrast).
Long wavelength X-ray (low kV)
Lower energy, easily absorbed by the human body, poor penetration, and higher contrast on the film (High contrast).
Applications
X-ray machines are widely used in medical, scientific, educational, and industrial fields. For example, X-ray machines can be used in hospitals to assist doctors in diagnosing diseases, in industrial non-destructive testing, and for security checks at train stations and airports.
Medicine, radiology, electronics, energy, capture, diagnosis, atom, atomic nucleus, photon, human body
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The application of X-rays in medicine
X-rays are used in medical diagnosis, primarily based on their penetrating power, differential absorption, photosensitivity, and fluorescence.
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