Skeletal scintigraphy or bone scanning is a valuable adjunct to standard film radiography (Fig. 1).1 Bone-seeking radionuclides are taken up in areas of increased bone turnover. This occurs normally at the growth plate in children and at abnormal sites in tumors, infections, and fractures; in sites of reactive bone formation in arthritis; and in periostitis regardless of the etiology. Technetium-99m-labeled polyphosphates are the most common radiopharmaceuticals used, particularly technetium-99m methyldiphosphonate (^99mTc-MDP). Fifteen to 20 millicuries (mCi) is injected intravenously, and a scan is obtained 2 hours later. Because the total body dose is 0.009 rad/mCi, the radiation-absorbed dose is very low. The agent is excreted in the kidneys and collects in the bladder. The target organ (i.e., the organ receiving the highest dose) is the wall of the bladder, which is exposed to approximately 0.275 rad/mCi. Bone scanning is more sensitive to areas of bone turnover and destruction than plain film radiography or tomography. The bone scan may be positive despite a normal radiograph (i.e., well before the radiograph becomes abnormal). However, the bone scan is less specific than the radiograph. Areas of increased activity are detected, but the cause of the increase often cannot be stated with certainty, and correlation with plain film radiography, computed tomography (CT), or magnetic resonance imaging (MRI) is necessary to establish a correct diagnosis.

CT is advantageous in the evaluation of the skeletal system. It allows visualization of adjacent soft-tissue structures and also the marrow in the medullary cavity. The positions of surrounding vascular structures can be determined by the use of contrast media. CT has the added advantage of being more sensitive for the detection of bone destruction than plain film radiography or standard tomography. CT images are displayed in the axial or horizontal plane, or even in the sagittal and coronal planes, with image reconstruction. However, image reconstruction degrades the image. Images are displayed with both bone and soft-tissue windows. The soft-tissue window setting allows visualization of surrounding soft tissue but is suboptimal for the bony skeleton (Fig. 1-2A). The bone window setting maximizes the visualization of cortical and medullary bone (Fig. 1-2B).

MRI is of great value in the evaluation of the skeletal system, particularly in the detection and evaluation of joint disorders, tumors, infection, bone infarction, and ischemic necrosis.2,9 Because the image is dependent on the presence of hydrogen, which is abundant in marrow fat, MRI visualizes the bone marrow exquisitely. However, the hydrogen content of cortical bone is very low, and MRI therefore is not as sensitive as CT for the evaluation of cortical bone. Cortical bone is shown as black, or very dark, because of its low signal intensity, contrasting sharply with the bright marrow. Similarly ligaments, tendons, and menisci are also low in signal and are seen as black structures; they often are surrounded by a layer of fat that makes them readily apparent. MRI has the added advantage of displaying the anatomy in any plane, including the coronal and sagittal planes, which are distinctly advantageous for demonstration or visualization of abnormalities within bone and their relationships to surrounding soft tissues (Fig. 1-3). Discrimination between and differentiation of various soft tissues and pathologic processes may be enhanced by varying the technical parameters used for the examination. Furthermore, vascular structures are demonstrated and are clearly visualized without the need for injection of contrast medium.