Oncologic imaging: interactions of nuclear medicine with CT and MRI using the bone scan as a model

There are many radionuclides currently used in oncologic imaging including technetium 99m diphosphonates, gallium 67, thallium 201, technetium 99m sestamibi, and others. The specific interactions of each of these agents with computed tomography (CT) and magnetic resonance imaging (MRI) are extensive. The radionuclide bone scan using 99mTc diphosphonate is the most frequently performed nuclear medicine examination in oncologic imaging. The bone scan can be used as a model to generalize the interactions of nuclear medicine with CT and MRI. The applications for the bone scan and many other nuclear medicine procedures in oncologic imaging include evaluating for metastases, assessing the response to therapy, and guiding radiation therapy planning. Bone scan findings that are equivocal for metastases can be evaluated with other imaging modalities. Areas of abnormal uptake in the axial skeleton can be evaluated with CT or MRI, whereas those in the appendicular skeleton can be evaluated with plain radiographs, followed by CT or MRI if necessary. The bone scan is valuable in oncologic imaging because of its high sensitivity for lesion detection, its ease in whole body imaging, and its low cost. The major disadvantage of the bone scan is that it lacks fine anatomic detail, which is of particular importance in the cancer patient with local back pain, radiculopathy, or myelopathy. Because local back pain with or without radiculopathy is the earliest symptom of spinal cord compression in 90% of patients, an MRI is the study of choice because of its exquisite depiction of anatomy. A myelogram followed by a postmyelogram CT can be performed if there are contraindications to an MRI. The basic principle of high sensitivity for lesion detection and ease in whole body imaging provided by nuclear medicine and fine anatomic detail provided by CT and MRI can be applied also to the use of other radionuclides in oncologic imaging.