Currently, we have multiple systems for obtaining images of the inside of the body to aid diagnosis, such as: X-ray, ultrasound, computed tomography, magnetic resonance imaging, etc., all of which are collectively called Diagnostic Imaging Techniques . However, historically, X-ray was the first diagnostic aid that provided images of the inside of the body.
If you continue reading you will discover what X-rays are and how they are used to generate radiological images.
What are X-rays?
X-rays are a type of electromagnetic radiation , invisible to the human eye, capable of passing through opaque bodies and of printing photographic films. They are a type of ionizing radiation , that is, high-energy, so they are capable of removing an electron (negative particle) from an atom or molecule and causing its ionization. Of the entire electromagnetic spectrum, only X-rays and gamma rays are ionizing radiation capable of interacting with matter, but X-rays differ from gamma rays in that gamma rays are produced in the nucleus of the atom and X-rays come from the outer layers of the atom's electrons.
Because X-rays are ionizing radiation, they cause chemical changes in cells and damage DNA. Exposure to this type of radiation increases the risk of certain diseases, such as cancer. Ionizing radiation comes from natural sources, such as radon and cosmic rays (rays that enter the Earth's atmosphere from outer space), and from medical imaging devices, such as X-ray machines.
X-rays are electromagnetic radiations capable of passing through organic matter and impressing it on a plate with photographic material. Depending on the density of the tissue, the rays reach the plate in greater or lesser quantities, creating an image in black, grey and white tones.
X-rays were discovered by chance on November 8, 1895, by the German physicist Wilhelm Conrad Roentgen while he was studying the penetrating power of cathode rays. He observed that whenever he passed an electric current through a vacuum tube, a fluorescent glow was produced. But there was no source of ultraviolet light in Röntgen's experiment and it was known that cathode rays travel only a few centimeters in the air. Therefore, neither ultraviolet light nor the cathode rays themselves could have caused the fluorescence. Thus, Röntgen established that the fluorescence was due to the presence of a type of ray of unknown nature, so he called them X-rays. Continuing with his experiments, he determined that these unknown radiations could pass through different types of materials such as paper, wood, aluminum, etc. However, they did not pass through lead. To demonstrate that X-rays could pass through materials, he used photographic plates, showing that objects could be more or less transparent to X-rays and that this in turn could depend on the thickness of the object, which led him to take the first human X-rays using his wife's hand. The first X-ray taken by Roentgen caused a great impact throughout the world and is now a historical document, available for consultation in the nuclear forum . With his work on X-rays he won the Nobel Prize in Physics in 1901.
How do X-rays work?
operation of an x-ray machine
X-rays are generated inside a vacuum bulb where a cathode (associated with an incandescent filament that acts as a source of electrons) and an anode (associated with a focus where radiation is generated by the impact of the electrons emitted by the cathode) are aligned. The system is powered by a high voltage source and is located inside an insulating metal structure (usually lead) that has a diaphragm through which the generated X-ray beam exits. The mission of the diaphragm or collimator is to control the width of the X-ray beam produced, so that we use the greatest amount of radiation orthogonal to the object to be x-rayed and reduce non-orthogonal or scattered radiation.
The X-ray beam emerging from the collimator aperture propagates in a straight line in an isotropic manner (in all directions and with equal intensity):
Some of it is scattered into the environment (depending on the degree of collimation).
Another part, direct radiation, passes through the target object of the study (in our case, the area of the patient's body being studied). From direct radiation:
Part of it is absorbed by the target object depending on the applied physical parameters (amperage and voltage). We are interested in keeping this radiation to a minimum, since, as we have said, X-rays are ionizing, which is why they produce alterations in matter, potentially causing alterations at the DNA level.
Another part is reflected outside the object (the so-called Compton effect). To reduce the effect of this radiation, an anti-scattering grid is placed between the body (object) and the X-ray plate.
And another is capable of passing through it with the proper attenuation. This last part is the one that will be useful for the production of the image due to its capacity to more or less hide the radiographic film located behind the object.
The conventional X-ray plate is a base support covered with a gelatin containing silver halides. Since it is altered by visible light, it must always remain in the dark. For use, it is introduced into a chassis that has elements inside called reinforcement plates that produce a certain luminescence that helps to enhance the image created by the X-rays.
To get an X-ray:
The X-ray tube emits a beam of radiation (X-rays)
The radiation passes through the patient's body.
Different fabrics absorb it to different degrees depending on their characteristics.
The radiation that manages to pass through the tissues impresses the X-ray plate.
Silver crystals in the film emulsion absorb X-rays during exposure and store the energy, forming a pattern, an invisible image within the emulsion crystals on the exposed film. The stored energy pattern cannot be observed and is known as a latent image.
To obtain the final image, development and fixation must be carried out .
The developed plate will show a greyscale image, which represents the different structures of the body. The silver halides on the plate turn black when they oxidise, that is, when they are exposed to radiation. The black part that we see on the X-ray will be the area that has received the radiation, due to the action of some reducing chemical substance that gives up its electrons to the silver halides during development. The white part is caused by the action of chemicals that act on the area of the X-ray that has not been exposed to radiation in that area of the X-ray.
In an X-ray, black indicates that the rays have not been attenuated (they pass landline number format philippines through the tissue), which is called radiotransparent or radiolucent . On the other hand, white suggests that the density of the tissue does not allow the radiation to pass through, and the term that describes it is radiodense or radiopaque .
The effect of absorption, dispersion and penetration means that in the human body we can find 5 basic radiological densities , with which we can interpret an X-ray. These densities are identified as different shades, from black to white, in a gray scale and are:
Air – black.
Fat – Darker grey.
Water/Soft Parts – Light Grey.
Calcium / Bone – White.
Metal – opaque white.
Only the first 4 densities are found naturally in the body. Metal always comes from outside, either in the form of foreign bodies, prostheses or other medical devices. Contrast material in radiology contains elements with a high atomic number (barium, iodine) and therefore its density is that of metal.