How do X-rays work?

How do X-rays work?

Nearly everyone has had an X-ray image taken of their teeth at the dentist, or of their chest at a medical clinic.

X-rays are a type of electromagnetic radiation, as are radio waves and visible light. Medical X-rays have a wavelength about 1/20,000 that of light, or energy per photon of about 20,000 times that of a visible photon.

X-radiation is produced by X-ray tubes in which energetic electrons smash into a tungsten target (sometimes it is molybdenum or other material). When the electrons are stopped by the target, they very occasionally emit an X-ray photon. This is called bremsstrahlung (German for “braking radiation”). (The same process occurs, with lower efficiency, in computer CRT monitors. The radiation from these is blocked by lead in the glass. The lead is why disposal of old CRT monitors is a problem.)

X-rays are useful to medicine because they can pass right through the body to cast shadow pictures. They best portray large differences in atomic number (e.g., bone versus soft tissue) and differences in density (air versus anything else). They don’t do as well at distinguishing soft tissues from each other except for specialized exams like mammography for breast cancer detection. Often special “contrast agents” are injected or swallowed, such as iodine compounds to visualize blood vessels, or barium for the stomach.

X-ray imaging systems have higher spatial resolution than MRI or ultrasound, and are the modality of choice for assessing fractures. X-ray images can be acquired and displayed in real time on a TV monitor which makes them very useful for following motion such as swallowing or of blood flow through an organ.

The CT (computed tomography) scanner is a special type of X-ray imager in which a cross sectional image of the patient, rather than a shadow picture, is computed from precise X-ray measurements.

In short, X-rays are highly useful in providing a new perspective, and as a result, can help solve a number of medical mysteries.

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