Bone scanning: principles, technique and interpretation

Bone scanning is most useful in the detection of bone metastases. The recent introduction of new radiopharmaceuticals and instrumentation has reduced the time needed to perform the study and its relative cost, while increasing the usefulness of the study in detecting roentgenographically occult diseases. Metastatic disease is used as the pathophysiologic model for understanding the principles of bone scanning. When a tumor invades bone, in addition to causing bone destruction, it also causes reactive bone formation or repair. It is here that radioisotopes are of considerable value, since some radionuclides are incorporated into the hydroxyapatite crystals of reactive bone. Bone repair is described as occurring in three phases. In Phase I, the roentgenogram shows no change in bone density, but the scan is abnormal. In Phase II, both scintigraphic and roentgenographic abnormalities increase, and in Phase III, when the osteoid has calcified completely, the roentgenogram shows radiodensities and the scan appears almost normal. Fewer than 5 per cent of patients have a normal scan in the presence of an abnormal roentgenogram. Presently, most bone scans are performed with phosphate compounds labeled with -99m-Tc. In the past, 85-Sr, 87M-Sr, and 18-F were more broadly used. Scanning may be performed on either a rectilinear scanner or a scintophoto (gamma) camera. Areas which are abnormal on bone scan should be interpreted with current roentgenograms in the light of clinical findings.