Plain film x-ray is the most common diagnostic radiological modality used in hospitals today. It was first discovered and used for imaging purposes by Wilhelm Röntgen on the 8th day of November 1895, when he took an image of his wife's hand. In this article, we shall look at the basic science underpinning x-rays, and the principles of their interpretation.
X-rays are a type of electromagnetic radiation – just like visible light – and there are three criteria that must be met in order for them to be used for imaging purposes: the ability to create electromagnetic radiation at the wavelength required, the ability to focus the radiation on a particular area, and the ability to detect the radiation once it has passed through the patient.
The radiation is created when an electric current is generated from a high voltage generator. This causes electrons to “boil-off” from the cathode end of an X-ray tube assembly. These electrons are emitted from a filament on the cathode and rush towards a target material known as the anode. This process is known as thermionic emission.
The electrons emitted by the cathode rush towards the anode, which holds a disc made of tungsten. When the electrons collide with the tungsten, several interactions occur at the atomic level. One of these interactions causes electrons to be expelled from the outer orbits of the atoms releasing a X-ray photon. The energy level of the X-ray photon can be adjusted by changing a parameter known as kVP or kilovolts peak.
After being created, the X-rays then travel through a focusing cup which focuses and accelerates the photons towards the area of the body to be imaged. Traditionally, radiographic film was used, known as double emulsion film containing silver nitrate.
The interpretation of an x-ray film requires sound anatomical knowledge and an understanding that different tissue types absorb x-rays to varying degrees: high density tissue, such as bone, absorbs x-rays to a greater degree and appears white on the film; low density tissue, such as the lungs, absorbs x-rays to a lesser degree and appears black on the film; and intermediate density tissue, such as muscle and fat, appears as shades of grey on the x-ray film. It is important to appreciate, however, that x-rays only give a 2D superimposed view of the body part that has been imaged. Therefore, it may be necessary to take multiple views of the same area from different angles (e.g. in cases of suspected fracture) to gain a full understanding of the injury.
The biggest advantage of plain film X-rays is the amount of radiation involved. It offers lower-dosage radiation compared to CT, and certain studies are performed relatively quickly (Chest X-rays). They are often used as an initial screening to rule out anything obvious before an advanced modality such as CT or MR is used. However, plain film X-rays procedures are being replaced by CT and MR due to advancements in technology. There are CT scanners available on the market now that offer radiation dosage levels as low as plain film X-rays.
Given the robustness and versatility of plain film X-rays, it remains a valuable tool in the medical imaging arsenal. It is incredibly useful in the diagnostic process, both for initial screening, and for further investigation of any abnormalities picked up on other modalities. It has saved many lives by allowing physicians to detect and diagnose a variety of conditions quickly and accurately. Plain film X-ray is a procedure that will likely remain in use for many years to come.
Plain film X-ray, discovered by Wilhelm Röntgen in 1895, is a form of electromagnetic radiation. It is produced by an X-ray tube, usually built out of a filament made of tungsten. X-ray images are usually recorded on a radiographic film and interpreted by a radiologist to identify any abnormalities. Plain film X-rays are commonly used for initial screening to rule out any obvious issues before an advanced modality, such as CT or MR, is used. However, due to advancements in technology, plain film X-rays procedures are being replaced by CT and MRI.
For plain film X-ray, a cassette receptor or if newer technology is available, a digital plate receptor is placed behind the patient to capture the X-ray photons that are transmitted through the patient and ultimately form an image. When interpreting an X-ray, the radiologist must take into account the different tissue types that absorb X-rays to varying degrees. High density tissue such as bone absorbs X-rays to a greater degree and appears white on the film, whereas low density tissue such as the lungs absorbs X-rays to a lesser degree and appears black. Intermediate density tissue, such as muscle and fat, is shown as shades of grey on the X-ray film. It is important to be aware that X-rays only provide a 2D superimposed view of the body part that has been scanned, and in cases of suspected fracture, it may be necessary to take multiple views from varying angles to gain a full understanding of the injury.
The main advantage of plain film X-rays is the amount of radiation involved. CT scans are able to offer radiation dosage levels as low as plain film X-rays, but cost, dimensions, soft tissue and bone detail, and radiation must be considered when determining which imaging modality to use.
