Skeletal Scintigraphy

Introduction

Skeletal scintigraphy has become an important tool in the early detection of certain diseases and for quick whole body scanning. Its applications are many but its non-specificity means both the referring and reporting physician need to be provided with as much information as possible when making a diagnosis. The main radiopharmaceuticals used in skeletal scintigraphy are diphosphonates labeled with technetium 99m. The mineral phase of bone has an affinity for these compounds resulting in the uptake and localization of the technetium tracer. In scintigraphy, tracer concentration or distribution is the mechanism by which contrast is produced. When interpreting bone scan images, areas of increased and decreased tracer uptake (hot and cold spots) are of clinical significance. The non-specificity of skeletal scintigraphy arises because a number of different diseases or disorders are manifested as hot or cold spots.

Clinical Applications

It is important for physicians requesting bone scans to understand that a bone scan is a non-specific exam. A complete patient history including the results of preceding tests (e.g. PSA, radiographs) are of great importance in the final diagnosis.

Metastatic Disease

Skeletal scintigraphy is an important tool in the evaluation of patients with extracellular primary malignancies for the presence of bone metastases. Breast, prostate, lung and kidney cancers are the main types with tendencies to metastasize to the bone. Bone scans show abnormalities, due to increased metabolism, before they become evident on radiographs. For this reason bone scans have become an important tool for physicians in the decision making process of patients with the above mentioned cancers. After the initial diagnosis of cancer a prompt metastatic survey aids in the determination of the most suitable treatment course for the patient. Once the results of the metastatic survey are returned, the physician can then advance to the next stage in the patient's treatment. A bone scan is a very simple procedure, it should be requested immediately along with other tests (CT, radiographs) and the results should be returned as soon as possible. In patients already diagnosed with metastatic disease serial bone scanning is of particular value in the detection of lesions in critical weight bearing areas (e.g. femur Figure 2.1). Early detection permits surgical intervention or radiotherapy to prevent pathologic fractures which can disable and decrease the quality of life of a patient. Figures 2.2 and 2.3 show the whole body images of a patient diagnosed with prostate cancer. This patients first bone scan was normal (Figure 2.3) but a significant rise in PSA prompted the doctor to request another bone scan 8 months later (Figure 2.3). The second bone scan shows extensive metastases in the axial skeleton.

Figure 2.1 Whole body images of breast cancer patient. The lesion on the femur is at a critical weighting bearing point.

Figure 2.2 Normal bone scan. No metastases detected in this cancer patient

Figure 2.3 Abnormal bone scan. An elevated PSA was the reason to request a follow up bone scan 8 months after the previous one.

Trauma

Radiographs are the first step in the diagnosis of fractures and bone trauma however some fractures not apparent on radiographs may be readily detected with either MRI or bone scintigraphy. Bone scans tend to be more effective when multi-focal trauma is suspected but if cost or availability of MRI is an issue, a bone scan will suffice.

Stress Fractures

For athletes a stress fracture can adversely affect their ability to perform and in some cases cause the premature end to a season or career. In the early stages bone scintigraphy is more sensitive than radiographs and typically shows abnormalities 2 or more weeks before they become evident on radiographs. Early detection of a stress fracture can be the difference between several weeks and several months of healing. Figures 2.4 and 2.5 show spot view images of athletes who complained of injuries but their radiographs were normal. Bone scans were able to confirm the doctor's suspicions that there was some bone trauma not yet evident on radiographs. A MRI scan would also detect these injuries.

Figure 2.4 Spot view: Legs

Figure 2.5 Spot view: Feet

Osteomyelitis and Cellulitis

Osteomyelitis and cellulitis both have high levels of incidence in diabetic patients. The differential diagnosis of the two is of great clinical importance because of the therapeutic implications of a diagnosis of osteomyelitis. A normal bone scan is not specific enough, but a modification of the procedure to include blood flow and blood pool images along with the delayed skeletal images, makes a differential diagnosis possible.