DEXA Scan | DNB-Orthopaedics & MRCS: Questions & Answers

Dual-energy X-ray absorptiometry (DXA or DEXA) is a special type of X-ray that measures bone mineral density (BMD). Two X-ray beams, with different energy levels, are aimed at the patient’s bones. When soft tissue absorption is subtracted out, the bone mineral density (BMD) can be determined from the absorption of each beam by bone.
It can also be called as: a DXA scan, a bone density scan, or a bone densitometry scan
DXA/DEXA scans allow for accurate assessment of bone mineralization in both the axial and appendicular skeleton.
Over the last decade, DEXA has become a safe, cost-effective and reliable method to quantify BMD. The World Health Organization (WHO) has adopted DEXA derived BMD measurements to define normal bone, osteopenia, and osteoporosis in the adult and paediatric population.

Physics: TypeI- DXA scanner with a cerium filter & a tube voltage of 80 kV, resulting in effective photon energies of about 40 and 70 keV.
TypeII-DXA scanner type with a samarium filter & a tube voltage of 100 kV, resulting in effective energies of 47 and 80 keV.
The combination of DXA and laser uses the laser to measure the thickness of the region scanned.

Uses:

Measurement of Bone mineral density in individuals with risks of Osteoporosis- to diagnose Osteoporosis;
e.g. a)post-menopausal woman not on estrogen,
b)personal or maternal history of hip fracture or smoking,
c)a man with clinical conditions associated with bone loss, like rheumatoid arthritis, chronic kidney or liver disease,
d)use of drugs causing bone loss- corticosteroids, anti-epileptic medicines, high-dose thyroid replacement drugs,
e) type 1 diabetes, liver disease, kidney disease or a family history of osteoporosis, hyperthyroidism, hyperparathyroidism,
f)history of fracture after trivial trauma,
g)X-ray evidence of vertebral fracture/ other signs of Osteoporosis
to assess an individual’s risk for developing osteoporotic fractures.
also effective in tracking the effects of treatment for osteoporosis and other conditions that cause bone loss.
to measure skeletal maturity.
to measure total body composition and fat content with a high degree of accuracy

Scoring: A DEXA scan compares one’s bone density with the bone density expected for a young healthy adult or a healthy adult of the same age, gender and ethnicity.

The difference is calculated as a standard deviation (SD) score.

T score- the difference between pt’s measurement and that of a young healthy adult
Z score- the difference between pt’s measurement and that of someone of the same age

The World Health Organization (WHO) classifies T scores as follows:

above -1 SD is normal
between -1 and -2.5 SD is defined as mildly reduced bone mineral density (BMD) compared to peak bone mass (PBM)- Osteopenia
at or below -2.5 SD is defined as osteoporosis

If an pt’s Z score is below -2, his/her bone density is lower than it should be for the same age.

Limitations: -not easy to interpret the results of DXA scan of the spine with a degenerative condition, e.g. spondylosis.
-spinal abnormalities/a previous spinal fracture can give a false result.
-A DEXA scan won’t show if low BMD is due to osteoporosis, or osteomalacia.

Radiation Exposure: The radiation dose of current DEXA systems is small, as low as 0.001 mSv, much less than a standard chest X-ray.
-not recommended for pregnant women.

The amount of radiation used during a DEXA scan varies depending on the area of the body being examined, but is very low and less than two days’ exposure to natural background radiation (NBR), (By comparison, a chest X-ray uses the equivalent of about three days’ exposure to NBR, and a flight from Britain to North America is equivalent to approximately a week’s exposure to NBR).