In medicine, photon absorptiometry is a "noninvasive method for assessing body composition. It is based on the differential absorption of x-rays (or gamma rays) by different tissues such as bone, fat and other soft tissues. The source of (x-ray or gamma ray) photon beam is generated either from radioisotopes such as 153gadolinium, 126iodine 125, or 241americium which emit gamma rays in the appropriate range; or from an x-ray tube which produces x-rays in the desired range. It is primarily used for quantitating bone mineral content, especially for the diagnosis of osteoporosis, and also in measuring bone mineralization."
|Single-photon absorptiometry (SPA)||Isotope||Single|
|Dual-photon absorptiometry (DPA)||Isotope||Dual|
|Dual-Energy X-Ray Absorptiometry (DEXA)||X-ray||Dual|
Measurement of bone density
Bone density is generally scored by two measures, the T-score and the Z-score. Scores indicate the amount one's bone mineral density varies from the mean. Negative scores indicate lower bone density, and positive scores indicate higher.
Fluid and soft tissue measurement
"Dichromatic absorptiometry allows the measurement of soft tissue and fluid content, and their changes. The technique uses linear transmission scans made across the limbs with a congruent 125I and 241Am dual-photon source and a sodium iodide-thallium crystal with photomultiplier tube detector."Patient measurements could be made at the bedside with the mobile system. Factors affecting the precision and accuracy were evaluated theoretically and experimentally. " 
Fat and muscle
It has been used experimentally to measure abdominal fat,  as well as total body fat and lean body mass.
- Anonymous (2023), Photon absorptiometry (English). Medical Subject Headings. U.S. National Library of Medicine.
- R M Witt and R B Mazess, "Photon absorptiometry of soft tissue and fluid content: the method and its precision and accuracy", Physics in Medicine and Biology (no. 4): 620, DOI:10.1088/0031-9155/23/4/005
- "Direct measurement of abdominal fat by dual photon absorptiometry.", Int J Obes. 14 (7): 603-11., 1990 July