Osteoporosis: radiologic and nuclear medicine procedures.

A number of radiographic and nuclear medicine techniques are available for the assessment of patients suspected of having osteoporosis or at risk for the development of osteoporosis. Spinal radiographs are insensitive indicators of osteoporosis. They can document the presence of metastatic tumors or other lesions that may produce compression fracture. The Singh index assesses the trabecular pattern of the proximal femur. As bone loss occurs, the trabeculae disappear in a definite sequence. Radiogrammetry refers to the measurement of bone and cortical widths in the peripheral skeleton, usually the second metacarpal. The method is low cost and most useful for population studies. It does not reflect early osteoporosis. Photodensitometry entails radiography of a part of a bone with inclusion of a standard reference wedge on the radiograph. The density of the bone is compared with the wedge. In some laboratories, precision is good (1.5 percent) and the radiation dose is low. Peripheral cortical bone is measured primarily, which is a disadvantage. Single photon absorptiometry uses the transmission of 27.5 keV photons emitted from an iodine-125 source to assess bone density. In most instances, measurements of the radial shaft are made, which reflect primarily cortical bone rather than the more reactive trabecular bone. The distal end of the radius and the calcaneus may also be measured; these areas contain primarily trabecular bone. Radiation dose is low (less than 10 mrad), but when cortical bone is evaluated, the results correlate poorly with spinal bone mineral. Thus, the test is an insensitive indicator of spinal osteoporosis. Dual energy photon absorptiometry uses a gadolinium-153 isotope source to emit photons of two different energies. The spine, hip, or total bone mineral can be evaluated. The radiation dose is low(<20 mrem). Although the axial skeleton can be assessed, the scan reflects the density of both the cortical and the trabecular bone and any adjacent calcification.Quantitative computed tomography (QCT) uses a reference phantom that is scanned adjacent to the patient to correct for scanner variations. The study allows the trabecular bone of the spine to be evaluated separately from the cortical bone. Precision is 1-3 percent. Accuracy is decreased by the presence (in older individuals) of vertebral marrow fat that makes the measured values of bone mineral spuriously low.CT scanning, using two X-ray energies, can improve accuracy, but decreases precision.