A DEXA bone scan is a medical imaging test used to measure bone mineral density, especially in the hip and spine. DEXA stands for dual energy X-ray absorptiometry because the scanner uses two different X-ray energies. The test helps doctors estimate fracture risk and diagnose bone loss before a serious break occurs.
It is important in screening for osteoporosis, monitoring treatment, and studying how bone strength changes with age or disease.
During the scan, a patient lies still on a padded table while an X-ray source and detector move across the body. Soft tissue and bone absorb low and high energy X-rays differently, so the machine can subtract the soft tissue signal and estimate the mineral content of bone. The result is usually reported as bone mineral density in g/cm^2, along with a T-score that compares the patient to a healthy young adult reference group.
Lower T-scores indicate weaker bones and a higher risk of fracture.
Key Facts
- DEXA means dual energy X-ray absorptiometry.
- Bone mineral density is often reported as BMD = bone mineral content / scanned bone area.
- Typical BMD units are g/cm^2.
- A T-score compares a patient's BMD to the average BMD of healthy young adults: T-score = (patient BMD - young adult mean BMD) / standard deviation.
- A T-score of -1.0 or above is usually considered normal, between -1.0 and -2.5 suggests low bone mass, and -2.5 or below is consistent with osteoporosis.
- DEXA uses very low radiation dose compared with many diagnostic X-ray or CT procedures.
Vocabulary
- DEXA
- DEXA is a bone density imaging method that uses two X-ray energy levels to estimate bone mineral density.
- Bone mineral density
- Bone mineral density is the amount of mineral in a measured area of bone, commonly reported in grams per square centimeter.
- T-score
- A T-score tells how many standard deviations a patient's bone density is above or below the average value for healthy young adults.
- Osteoporosis
- Osteoporosis is a condition in which bones become less dense and more likely to fracture.
- X-ray attenuation
- X-ray attenuation is the reduction in X-ray intensity as the beam passes through tissue and is absorbed or scattered.
Common Mistakes to Avoid
- Confusing DEXA with a regular X-ray image is wrong because DEXA is mainly a quantitative density measurement, not just a picture of bone shape.
- Thinking a higher T-score means weaker bones is wrong because lower, more negative T-scores indicate lower bone density and greater fracture risk.
- Ignoring the scan site is wrong because hip, spine, and forearm measurements can give different BMD values and may have different clinical importance.
- Assuming DEXA measures total bone strength perfectly is wrong because fracture risk also depends on bone structure, age, falls, medications, and other health factors.
Practice Questions
- 1 A DEXA scan measures 12.6 g of bone mineral in a projected bone area of 18.0 cm^2. Calculate the BMD in g/cm^2.
- 2 A patient's hip BMD is 0.72 g/cm^2. The young adult mean is 1.00 g/cm^2 and the standard deviation is 0.12 g/cm^2. Calculate the T-score and classify the result as normal, low bone mass, or osteoporosis.
- 3 Explain why using two different X-ray energies helps a DEXA scanner separate the effects of soft tissue from the effects of bone.