M01.12.001 Plain Film X-Ray

Plain film X-ray is the most common diagnostic radiological modality used in hospitals today. Discovered by Wilhelm Röntgen on November 8, 1895, X-rays have become a fundamental tool in medical imaging. This article explores the basic science underpinning X-rays and the principles of their interpretation.

Basic Principles of X-Rays

Creation and Detection

X-rays are a type of electromagnetic radiation, similar to visible light. For electromagnetic radiation to be used for imaging, three criteria must be met:

1. Creation of the Radiation: The radiation must be generated at the required wavelength.
2. Focusing the Radiation: The radiation must be focused on a specific area.
3. Detection of the Radiation: The radiation must be detected after it passes through the patient.

Generation of X-Rays

1. Thermionic Emission: An electric current generated by a high voltage causes electrons to “boil off” from the cathode in an X-ray tube assembly.
2. Collision with Anode: Electrons from the cathode rush towards a tungsten target on the anode. The collision expels electrons from the outer orbits of the atoms, releasing X-ray photons.
3. Focusing: The X-rays are then focused and directed toward the area of the body to be imaged.
4. Detection: Traditionally, a radiographic film containing silver nitrate was used to capture X-rays. Modern technology employs cassette or digital plate receptors.

Interpreting an X-Ray

Interpreting an X-ray film requires sound anatomical knowledge and an understanding of how different tissues absorb X-rays:

• High-Density Tissue (e.g., Bone): Absorbs more X-rays and appears white on the film.
• Low-Density Tissue (e.g., Lungs): Absorbs fewer X-rays and appears black on the film.
• Intermediate-Density Tissue (e.g., Muscle and Fat): Appears in shades of grey.

X-rays provide a 2D superimposed view of the body part imaged. Multiple views from different angles may be necessary to fully understand the injury, such as in suspected fractures.

Comparison to Other Imaging Techniques

The primary advantage of plain film X-rays is the lower radiation dosage compared to CT scans. They are often used for initial screening before advanced modalities like CT or MRI are employed. However, advancements in CT technology have reduced radiation levels to those comparable to plain film X-rays.

Summary of Imaging Modalities

References

• Pulse Radiology Education. (2024). How an X-ray is generated.
• Häggström, M., ZooFari, Stillwaterising, & Gray’s Anatomy. (2024). Illustration of the mediastinal structures in a normal chest radiograph. [CC BY-SA 3.0].
• Zachariou, M. (2022). X-ray imaging in the NHS: five minutes with . . . Jake Knight. The BMJ.