Dual Energy X-ray Absorptiometry (DEXA) and Quantitative Computed Tomography (QCT) are both reimbursed for measurement of bone density to assess fracture risk. Both have advantages and disadvantages. In general however, DEXA is the most widely used. The reasons for this are several fold.
Firstly, the radiation dose with QCT is much larger than with DEXA, about a 10 fold difference.
Secondly and perhaps more importantly, most of the data on fracture prediction by bone density has been generated by DEXA measurement.
The T score threshold of -2.5 for diagnosing osteoporosis has been carefully validated for DEXA, but this has never been done in the same way for QCT. Because the appropriate T scores for intervention with QCT are less well defined, many patients will appear to be very osteoporotic on QCT but less so when measured on DEXA. Some QCT scanners only generate values for the lumbar spine not the hip, unlike with DEXA where two sites are always measured.
On the other hand, QCT has the advantage that in those patients with severe osteoarthritis in the spine, a better assessment of their trabecular bone density, uninfluenced by osteoarthritis, can be made. However, this information can often be obtained from the DEXA hip measurement in a patient with osteoarthritis of the spine when a DEXA scan is performed.
In summary, although both methods can be used to assess osteoporosis risk, DEXA is the more widely used and preferred because of the robust nature of the T score interpretation with this method.
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