Bone Densitometry - DEXA scans by Michael Broder, MD

by Michael Broder, M.d. Radiologist and patient

Bone densitometry for evaluation of osteoporosis.
DEXA (Dual Energy X-RAY Absorptiometry) compares the relative transmission of two different low energy xray beams through an area of bone in order to calculate the approximate amount of calcium present in bone. Bone densitometry is actually NOT an imaging technique. The images are low resolution, and are just used for positioning so that the data can be collected, and compared to a database of test individuals which the manufacturer of the machine has developed.
There are also methods for CT scan bone densitometry, expensive & not commonly used.  Ultrasound methods of bone densitometry have been developed for gross screening, but have significant limitations. Not used for followup & treatment.
Each Brand of Bone Densitometry (DEXA) machine has its own data bases. There are separate ones for males and females in American Caucasian, Black and Asian populations. The results are stated as the statistical standard deviation (SD) of the result from the mean in each database, also correlated with the age of the patient. The accuracy of these machines is limited. If the patient is tested several times, on the same day, on the same machine, the results will NOT be exactly the same. The results are affected by the exact position of the patient, and certain settings of the machine which are adjusted by the technologist.
The result on an individual patient on one machine Brand CAN NOT be accurately compared to another because of technical differences, and the fact that the data bases are not the same.

Bone densitometry of the hip is determinined by averaging measurements in a large area. One of these is in the femoral neck. The other includes the entire femoral head. Post OP, neither of these measurements can be used since the prosthesis blocks transmission of the X-ray beam.

Therefore, Bone densitometry CAN NOT be performed on the POST OP HIP. The measurement of a non operated hip, and the spine can be used. In most patients, the most important information is the change in the individual patients data over time. It is rarely noted to change much in less than 1 year, and the 2 year figure is more useful. The main purpose in all cases is to establish a baseline regarding the degree of osteoporosis, and relative risk of fracture in an individual patient.
NOTE: A Nuclear Medicine "Bone Scan": is an entirely different IMAGING procedure which does not have anything to do with evaluating osteoporosis.

Michael (MD in NC) L C+ 3/31/03