Noninvasive methods of bone-mass measurement.

The most widely used noninvasive methods of bone mass measurement include: single photon absorptiometry (SPA), quantitative computed tomography (QCT), dual photon absorptiometry (DPA and DEXA). SPA is used to measure bone mineral content (BMC) of the distal radius. Its advantages are low cost and high precision in vivo (less than 2%), however, correlation between axial and appendicular bone values is poor. Since vertebral fracture is the first complication of osteoporosis, spinal values are the most useful. QCT measurements are performed using available CT scanners and either single or dual energy scanning techniques, computed radiographs for localizing regions-of-interest (ROI) and mineral reference standards for calibration. The advantage of QCT is that it can evaluate pure trabecular bone at the midplane of two to four lumbar vertebral bodies. Single energy in vivo reproducibility is 2-5% in osteoporotics. Radiation exposure with most systems is 100-500 mRem. With DPA scanners, the commonest radiation source used is Gadolinium-153 (44-100keV). The areas of measurement are lumbar vertebrae L2, L3 and L4 and the femoral neck. In vivo lumbar measurement precision is at least 1-2% and radiation exposure is low (less than 10 mRem). DEXA scanners use an X-ray rather than gamma ray source to emit dual energy photons. The advantages of the technique are shorter scan time, lower radiation exposure (less than 3 mRem) and higher precision (less than 1%). In conclusion, measurement of vertebral bone, particularly using DPA and DEXA, is of value in assessing the risk of osteoporosis and in monitoring changes in bone mineral content.