Infections of bones and joints are severe diseases that can result in debilitating joint destruction, joint fusion, or bone and soft tissue defects. Early diagnosis is the key to successful therapy and prevention of complications. Commonly, infection is not limited to one compartment. Osteomyelitis and septic arthritis may be associated with soft tissue infections including cellulites, abscess formation, or fasciitis. Conversely, soft tissue infections may involve bones and joints. A good example is diabetic foot, which typically starts as soft tissue infection and then proceeds to bone and joint infection. The ideal imaging method should not only be sensitive and specific in the diagnosis of infection but also be topographically precise. For assessment of soft tissue abnormalities, ultrasound and magnetic resonance imaging (MRI) are commonly used; for joints standard radiographs, ultrasound, and MR imaging; and for bones, standard radiographs and MR imaging. Both ultrasound and MRI are excellent in identifying effusion and soft tissue abscess formation. The cited imaging methods have many advantages, including topographic information, early detection of disease (MR imaging), or extent of bone destruction (standard radiographs). However, these methods tend to be nonspecific, and it may be difficult to differentiate infection from nonspecific edema, trauma, or neoplasm.

In this situation, radionuclide imaging can play an important role. Bone scans are sensitive in diagnosing bone and joint infections. Many institutions use labeled leucocytes as the standard of reference to identify infectious foci in the musculoskeletal system. Major weaknesses of this method are in imaging chronic infections and the axial skeleton (see Chapter 58). Similarly, antigranulocyte antibody imaging has a definitive role in imaging osteomyelitis of the peripheral skeleton. Integrated single-photon computed tomography (SPECT) and computed tomography (CT) may assist in the differentiation of increased uptake by using image coregistration. This is particularly promising in skeletal imaging.

Fluorodeoxyglucose (FDG) positron emission tomography (PET) has shown encouraging results for diagnosing both acute and chronic infection of the trunk and the extremities and seems to be more accurate in detecting chronic osteomyelitis than conventional functional imaging, including bone scan, antigranulocyte antibody imaging, and labeled leucocyte imaging (see Fig. 60.1). Whether PET has the potential to replace conventional scintigraphy completely depends on several factors, including cost and availability. FDG-PET makes use of the fact that there is no physiologic FDG accumulation in white blood cells not actively fighting inflammation. Therefore, granulocytes residing in the hematopoietic marrow do not demonstrate activity, which permits imaging of the axial skeleton. FDG-PET may be superior to MRI in distinguishing disc space infection from erosive osteochondrosis. Data with FDG-PET/CT concerning osteomyelitis in diabetic foot are controversial.

Experience with FDG-PET-CT and SPECT-CT in the musculoskeletal system is still limited, and more data have to be gathered before a definitive role in the imaging algorithms for infection can be assigned to them. Combined PET-CT improves the limited anatomic information of FDG-PET.