Photon absorptiometry

In medicine, photon absorptiometry, also called x-ray densitometry, dual-energy x-ray absorptiometry (DEXA Scan or DXA Scan), dual photon absorptiometry (DPX Scan), 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 gadolinium 153, iodine 125, or americum 241 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."

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.

T-score
The T-score is a comparison of a patient's bone density to that of a healthy thirty-year-old. The criteria of the World Health Organization are :
 * Osteoporosis is defined as -2.5 or lower, meaning a bone density that is two and a half standard deviations below the mean of a thirty year old woman.
 * Osteopenia is defined as less than -1.0 and greater than -2.5
 * Normal is a T-score of -1.0 or higher

Z-score
The Z-score is a comparison of a patient's bone density to the average bone density of their, sex, and race. This value is used in premenopausal women, men under aged 50, and in children.