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. Major clinical uses include assessing the risk of osteoporosis and the need for preventive therapy in postmenopausal women, and in bone growth disorders in children.

Many of the methods in use are approximations that do not correct for bone thickness, so essentially give an "area" or two-dimensional measurement, which can be inaccurate in children or people of short stature.

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 aluminium 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".

X-ray computed tomography

The advantage of X-ray computed tomography is that it provides true three-dimensional imaging, and allows close examination of a clinically interesting area. It is more expensive than DXAD, gives the patient a much larger radiation dose, and must be performed with extreme care and standardization to be reproducible.

This precision, however, is especially useful in the childhood diseases such as osteochondrosis and its knee-specific subclass, Osgood-Schlatter disease.

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.

Clinical interpretation

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.