Musculoskeletal Imaging

Radionuclide bone imaging

a diagnostic imaging technique in which bone scans are produced using radiopharmaceutical agents. For a general description, see radionuclide bone imaging. The method utilizes the increased uptake of the radionuclide agent at sites of bone abnormalities. Bone scans are extremely sensitive but relatively nonspecific, since any process involving changes in bone production and resorption can cause abnormalities on bone scans. The current state of the art technique is the single photon emission computed tomography (SPECT) camera, which acquires multiple planar image projections from which tomographic slice images that can be viewed in coronal, sagittal and transaxial projections are generated. Some of the indications for radionuclide bone imaging are given in Table 1.

Radionuclide bone imaging, Table 1. Some indications for radionuclide bone imaging.

Screening for bone metastases

Localization of metastsaes for diagnostic biopsies

Diagnosis of osteomyelitis before radiographic changes are evident

Aid in differentiating cellulitis from osteomyelitis

Aid in evaluating painful prostheses for infection and loosening

Detection and evaluation of the extent of articular involvement in various forms of arthritis

Aid in the characterization of benign bone lesions

Aid in the workup of compression fractures

Evaluation of bone pain of any causes in the presence of normal radiographs

Aid in diagnosis and management of ischaemic necrosis of bone

Aid in management of myositis ossificans

Aid in the selection of amputation site or level

Most cases of skeletal metastasis lead to foci of increased accumulation of the radionuclide ("hot" lesions). Another scintigraphic pattern that may be associated with widespread metastatic bone disease is the superscan seen on whole body imaging, in which the kidneys may not be visualized and the blood background may be markedly free of radioactivity, with the generalized bone-to-background ratio of counts being unusually high. In some cases skeletal metastases may appear as "cold" or photon-deficient areas on bone scans.

The bone scan may also be of value in assessing either fatigue or insufficiency fractures and shin splints (tightness and aching in the lower legs on exercise, commonly encountered in athletes), especially when radiographic results are ambiguous. In addition to showing focal abnormalities after a fracture, scintigraphy has some potential in assessing the course of fracture healing and nonunion. Two patterns of radionuclide concentration are seen:

intense homogeneous activity at the fracture site, which indicates a good healing response; and

a line of decreased activity (negative defect) at the fracture site surrounded by increased uptake on both sides, which is associated with a poor response.

Other conditions in which scintigraphy is helpful are child abuse, metabolic bone disease (osteomalacia, renal osteodystrophy, Pagets disease, Legg Calv Perthes disease, ischaemic necrosis of bone, osteoarthritis, rheumatoid arthritis, seronegative spondyloarthropathies, heterotopic ossification and soft tissue lesions.