BMD determined by DXA is an areal measurement. The density in grams is divided by the area scanned to determine g/cm². Meticulous detail is necessary to ensure follow-up scans measure the same area and same size region of interest, ensuring that real change has occurred in patients being observed or treated. Changes in measured area can falsely alter the BMD. Guidelines for use of DXA are found in many sources; however, the most commonly used are those of the International Society for Clinical Densitometry, National Osteoporosis Foundation, and Medicare Guidelines.

Quantitative Computed Tomography (QCT)

QCT utilizes software and a phantom with existing CT scanners. The two general techniques employed are single slice and spiral CT acquisition. With either technique BMD is volumetric; a volume is measured with BMD resulting in density as g/cm³.

A phantom is scanned with the patient or before the patient being scanned. The phantom contains several different hydroxyapatite densities along with fat and soft tissue equivalents. A regression curve from phantom data is generated so the density of the patient can be determined using Hounsfield units from the scanner compared with the curve. Each manufacturer of QCT software develops its own normative database to determine the T-score of the patient compared with young normal data. The T-score using QCT is always lower than that seen with DXA because the QCT generally measures density of trabeculae. The WHO definition of osteoporosis (T-score < −2.5) is based on DXA not QCT. QCT can also measure cortical bone alone or cortical and trabecular bone together. Follow-up studies require meticulous detail to ensure precise results. QCT has been used to measure vertebral body, distal radius, and hip density.