x-ray
Meaning of x-ray: The same beam that produces a picture at low dose destroys cancer cells at high dose, and only the amount and aiming differ.
Definition of x-ray
An x-ray is a form of high-energy electromagnetic radiation that passes through the body to differing degrees depending on the density of the tissue in its path. The word also names the image produced this way. Dense tissue such as bone absorbs a large share of the beam and appears white, while air-filled lung absorbs little and appears dark.
How is the image made?
Inside the tube, electrons are accelerated into a metal target and stop abruptly, converting their energy into x-rays. The beam passes through the body toward a detector on the far side. Every tissue removes some of it, a process called attenuation, and the detector records what survives. The image is therefore a shadow, with all the depth flattened into one plane, which is why two views at right angles are usually taken.
| Material | Absorption | Appearance |
|---|---|---|
| Metal, contrast agents | Highest | Bright white |
| Bone | High | White |
| Soft tissue and fluid | Moderate | Gray |
| Fat | Low | Darker gray |
| Air | Lowest | Black |
What builds on the same principle?
Because soft tissues attenuate similarly, plain images distinguish them poorly, and several techniques exist to get past that. Computerized tomography rotates the source around the body and reconstructs cross-sections from hundreds of projections, separating structures that would otherwise overlap. Fluoroscopy runs a continuous low-dose beam to watch movement, such as swallowing or a catheter advancing. Mammography uses a low-energy beam tuned for breast tissue, and bone density scanning uses two energies to isolate mineral content. Swallowed or injected contrast material absorbs strongly, outlining vessels, the bowel or the urinary tract that would otherwise be invisible.
What about the radiation itself?
X-rays are ionizing, meaning they carry enough energy to strip electrons from atoms and damage DNA. Diagnostic amounts are small and the associated risk is correspondingly small, but it is not zero, so imaging is chosen when it will change what happens next. The same physics turned up gives radiation therapy, in which high-dose radiation is aimed precisely at a tumor to damage cancer cell DNA beyond repair. Nothing about the radiation differs between the two uses; the dose, the shaping of the beam and the intent do.
Also known as
Used in a sentence
A chest x-ray was obtained to evaluate the patient's persistent cough.
Good to know
The same underlying radiation is used at very different doses for diagnostic imaging and for therapeutic treatment of cancer, a distinction in dose and purpose rather than a difference in the fundamental type of radiation involved.