DEXA Bone Densitometry
Dual energy X-ray absorptiometry measures the mineral density of bone. Two X-ray beams of different energy pass through the body, and the difference in how much each is absorbed separates bone from soft tissue.
The result is the bone mineral density in g/cm2, which is compared with the mean of young healthy adults (the T value) and with people of the same age and sex (the Z value). Both are expressed as the number of SD units from the mean.
The sites measured are the lumbar spine (L1 to L4), the total hip and femoral neck. The one third radius of the forearm is added when the spine or hip cannot be measured, or in hyperparathyroidism.
A scan takes 5 to 15 minutes, and the radiation dose is very low (1 to 10 microsieverts), less than a day of background radiation.
Osteoporosis affects about 1 in 3 women and 1 in 5 men over 50 in their remaining lifetime for a fragility fracture. Hip fracture carries a mortality of 20 to 30% in the first year.
Bone density explains about 60 to 70% of bone strength, and each fall of 1 SD unit increases fracture risk 1.5 to 2.5 times. DEXA is the accepted method for diagnosis and for monitoring treatment.
Indications.
- Women aged 65 and over and men aged 70 and over.
- Postmenopausal women and men aged 50 to 69 with risk factors: previous fragility fracture, parental hip fracture, low body weight, smoking, heavy alcohol use.
- Long term glucocorticoid therapy (over 3 months at 5 mg of prednisolone or equivalent daily).
- Conditions causing bone loss: hyperparathyroidism, hyperthyroidism, hypogonadism, early menopause, coeliac disease, inflammatory bowel disease, rheumatoid arthritis, chronic kidney disease, chronic liver disease, eating disorders and diabetes.
- Treatment with aromatase inhibitors, androgen deprivation therapy, anticonvulsants or antiretroviral therapy.
- Height loss of over 4 cm or a vertebral fracture seen on imaging.
- Monitoring response to treatment for osteoporosis.
- Assessment before and after bariatric surgery or transplantation.
- Children and adolescents with fragility fractures or with conditions affecting bone (limited, with paediatric interpretation).
Contraindications and precautions.
- Pregnancy: avoid, because of radiation to the foetus.
- Recent barium or contrast studies and nuclear medicine studies within 7 days affect the result and should be avoided.
- Weight over the table limit (usually 130 to 200 kg) may prevent the scan.
- Metal in the scan field, such as a spinal implant, hip prosthesis or jewellery, gives errors, and another site is used.
- The patient must lie still for 5 to 15 minutes.
- Calcium supplements should be omitted for 24 hours before the scan, since a tablet in the gut can alter spine readings.
- Severe scoliosis or contracture of the hip limits positioning.
- Scans are repeated on the same machine, with the same operator technique.
Preparation and technique.
No fasting is needed. Clothing with metal is removed and a gown is given. Height and weight are measured, and the history of fractures, medicines and risk factors is recorded.
For the spine, the patient lies supine with the legs raised on a block, to flatten the lumbar curve. For the hip, the leg is rotated inward so that the femoral neck is seen in full.
The scanning arm passes over the body, and the software finds the bone edges. Regions of interest are checked for correct placement, and vertebrae with fractures, artefacts or large osteophytes are removed.
Vertebral fracture assessment, a lateral image of the spine, can be added at a very low dose. A trabecular bone score may be derived from the spine image.
