Bone densitometry

Bone densitometry encompasses a range of medical imaging methods used to detect weakening of bone, to detect incipient or actual osteoporosis, and to predict the risk of fractures.

Methods of measurement

Ionizing radiation

Traditional X-ray is not useful for screening, because bone density loss is only visible at a severe 40% loss level. There are, however, ways to use X-ray measurement if the preferred methods are not available:

• "The Singh index describes the trabecular patterns in the bone at the top of the thighbone (femur). X-rays are graded 1 through 6 according to the disappearance of the normal trabecular pattern. Studies have shown a link between a Singh index of less than 3 and fractures of the hip, wrist, and spine.
• Radiographic absorptiometry was developed during the late 1980s as an easy way to determine BMD with plain X-ray. An X-ray of the hand is taken, incorporating an aluminum reference wedge. The X-ray is then analyzed, and the density of the bone is compared to the density of the reference wedge.

After X-ray, the next techniques developed used an external radioisotope source. While single-photon absorptiometry (SPA) and dual-photon absorptiometry are accurate with a fresh isotope, the methods were logistically problematic because the isotope was short-lived and the source would become less reliable over time.

In SPA, a single-energy photon beam is passed through bone and soft tissue, usually at the distal radius of the wrist, and the amount of radiation-absorbing material is calculated. The wrist is used because there is relatively little soft tissue in the area, which confuses the measurement. DPA uses two photon energy levels, one more absorbed by bone and the other more by soft tissue, which allows correction for soft tissue in the beam.

By returning to an X-ray source, Dual Energy Xray Absorptiometry (DEXA) did away with the isotope decay problem, and is the current "gold standard".

Computed X-ray tomography

Quantitative computed tomography of the spine must be done following strict protocols in laboratories that do these tests frequently; in community settings the reproducibility is poor. The QCT measurements decrease more rapidly with aging, so the "T scores" in older individuals will be much lower than DEXA measurements.

Ultrasound

Ultrasound is cheaper than radiation based methods. Measurements are usually performed at the calcaneous, and it is not feasible to measure at likely fracture sites such as spine or hip.

Magnetic resonance imaging

Magnetic resonance imaging is a new method for bone densitometry, and experience is being gained.