X-ray machines send X-ray beams through the body. Dense tissue like bone absorbs more and shows up white on the detector, while soft tissue shows darker.
How it works
An X-ray tube produces X-ray photons. They pass through the body and hit a detector. Dense tissues, like bone, absorb more and appear white. Soft tissues appear gray and air black.
Discovery
Wilhelm Röntgen discovered X-rays in 1895 and won the first Nobel Prize in Physics (1901).
Dose
A chest X-ray gives about 0.1 mSv, similar to 10 days of natural background radiation.
Uses
Fractures, chest and teeth.
Safety
Pregnant women should tell staff.
Physics of the image
X-ray machines generate X-rays by accelerating electrons from a heated cathode to strike a tungsten anode in a vacuum tube at 40 to 150 kilovolts. About 99 percent of the energy becomes heat, and 1 percent becomes X-rays. Dense tissue such as bone absorbs more X-rays, so it appears white on the detector, and air-filled lungs appear dark.
- Wilhelm Roentgen discovered X-rays in November 1895 and received the first Nobel Prize in Physics in 1901.
- A chest X-ray delivers about 0.1 mSv, comparable to roughly 10 days of natural background radiation (about 3 mSv per year in the US).
- A CT scan of the abdomen is about 8 to 10 mSv.
Safety
Doctors use the ALARA principle (as low as reasonably achievable). Lead aprons protect areas not being imaged. Pregnant women should tell the technologist, as fetal exposure is avoided where possible. Digital detectors have cut doses compared with film. Uses include fractures, pneumonia, dental imaging and mammography, which uses lower-energy X-rays.
Physics of the image
X ray machines generate X rays by accelerating electrons into a metal target, usually tungsten. X rays pass through soft tissue and are absorbed more by dense bone, so a detector records a shadow image. Wilhelm Roentgen discovered X rays in 1895 and won the first Nobel Prize in Physics in 1901.
Safety
A chest X ray delivers about 0.1 mSv, equal to about 10 days of natural background radiation. Pregnant patients should inform staff. Shielding and dose limits keep exposure low.
Imaging physics
X rays have wavelengths of 0.01 to 10 nanometers. In the tube, electrons accelerated by 40 to 150 kV strike a tungsten anode, producing X rays by bremsstrahlung and characteristic emission. Digital detectors convert X rays to electrical signals with sensitivity far better than film.
Dose and indications
Typical effective doses: chest X ray 0.1 mSv, mammogram 0.4 mSv, CT abdomen 8 mSv. Average annual natural background in the US is about 3 mSv. ALARA principle means exposure is as low as reasonably achievable. Pregnant women should tell staff, and shielding is used when needed. X rays detect fractures, pneumonia and dental decay.
Based on data from the Radiological Society of North America and the US Food and Drug Administration
