7.1 Principle of bone densitometer

A bone densitometer measures the amount of mineral in bone, expressed as bone mineral density (BMD), to diagnose osteoporosis, estimate fracture risk and monitor treatment. The standard method is dual-energy X-ray absorptiometry (DXA, also written DEXA), in which the bone is scanned with X-rays at two different energies. Bone and soft tissue absorb the two energies in different proportions, so by comparing the two measurements the machine can remove the effect of soft tissue and calculate the bone mineral alone. The examination is quick (a few minutes), painless and uses a very low radiation dose. Example: The Nairobi Hospital's bone densitometry service scans post-menopausal women referred by gynaecologists to decide who needs osteoporosis treatment.

Why bone density is measured

• Osteoporosis diagnosis: Osteoporosis is a disease of low bone mass and weakened bone structure that leads to fractures of the hip, spine and wrist after minor falls.
• Fracture risk assessment: BMD, combined with clinical risk factors in tools such as FRAX, estimates the ten-year probability of a major fracture.
• Treatment monitoring: Repeat scans, usually one to two years apart, show whether BMD is improving on treatment.
• Other uses: Assessing bone loss from long-term steroid use, chronic kidney disease, and body composition (fat and lean mass).

Basic principle of DXA

1. Two energies: The X-ray beam is produced at two effective energies, a low energy (around 30 to 50 keV) and a high energy (around 70 to 100 keV), either by switching the tube voltage or by filtering the beam with a K-edge filter (such as cerium or samarium).
2. Attenuation: As each beam passes through the body, it is attenuated by both bone and soft tissue. Bone attenuates the low-energy beam much more strongly than the high-energy beam; soft tissue shows a smaller difference.
3. Detection: Detectors measure the transmitted intensity at each energy for each point scanned.
4. Calculation: The machine solves two equations (one for each energy) for two unknowns, the mass of bone mineral and the mass of soft tissue per unit area (topic 7.2).
5. Bone mineral density: The software outlines the bone (for example a vertebra) and calculates areal BMD = bone mineral content (g) ÷ projected bone area (cm²), in g/cm².
6. Comparison with reference data: The result is compared with a reference population to give T-scores and Z-scores.

T-score and Z-score

• T-score: The number of standard deviations (SD) by which the patient's BMD differs from the mean of healthy young adults of the same sex. T-score = (patient BMD − young adult mean BMD) ÷ young adult SD.
• Z-score: The number of SD from the mean of people of the same age and sex. It is used for children, pre-menopausal women and men under 50.

Worked example: patient BMD = 0.760 g/cm², young adult mean = 1.000 g/cm², SD = 0.110 g/cm².

• T-score = (0.760 − 1.000) ÷ 0.110
• T-score = −0.240 ÷ 0.110
• T-score = −2.2, which is in the osteopenia range

WHO diagnostic categories (post-menopausal women and men aged 50 and over)

Sites measured

• Lumbar spine (L1 to L4): Rich in trabecular bone, responds early to treatment; affected by degenerative changes in older patients.
• Proximal femur (hip): Total hip and femoral neck; the best predictor of hip fracture.
• Forearm (one third radius): Used when the spine or hip cannot be measured, and in hyperparathyroidism.
• Whole body: For body composition and in children.

Other bone densitometry methods

Main parts of a DXA scanner