MRI is synonymous with proton imaging. It provides detailed images of gross anatomy and pathology owing to the excellent soft-tissue contrast, signal void of flowing blood, versatile geometry, and freedom from streak artifacts, as well as other advantages summarized in Table 8-2. In the CNS, MRI has emerged as the most sensitive imaging modality in virtually all pathologies--some reservations remaining concerning acute hemorrhage, focal calcifications, and bone detail. Hence, it should be considered the premier noninvasive examination in the evaluation of the cancer patient with any suspicion of CNS pathology. Economics and availability must, of course, be considered when evaluating MR's role relative to CT. MR clearly provides the best means of excluding pathology, particularly in the posterior fossa, and must be considered after a negative CT examination with persistent clinical suspicions. MRI must also be considered in routine surveillance, if the earliest possible detection of metastasis, demyelination, and other pathologies is to be achieved. MRI should be considered in the evaluation of vertebral metastases, spinal cord compression, and back pain because of its ability to depict CSF, spinal cord, disk, and vertebral body as distinct structures and its sensitivity to marrow disease. In the extremities and pelvis, clearer depiction of soft tissues, vessels, and marrow is a proven advantage. Hence, MRI is indicated in the evaluation of prostate/bladder/rectal carcinoma, uterine/cervical carcinoma, soft tissues/bony sarcomas, and bone metastasis/infarction. In the abdomen, MRI's display of the retroperitoneum and sensitivity to liver lesions indicates its use in the evaluation and staging of renal/adrenal carcinoma, retroperitoneal sarcomas, primary liver tumors, and metastases. Moreover, MRI is also indicated in the evaluation of liver or adrenal masses of uncertain histology owing to a limited specificity of the MR signal for adenoma, carcinoma, and hemangioma. In the chest, MRI's advantages are currently limited owing to the excellent quality of CT images of mediastinum and lung parenchyma and the deleterious effects of respiratory motion. MRI's primary indications in the chest are for the distinction of mediastinal and hilar masses from vessels and aneurysms; evaluation of lumenal patency and superior vena cava syndrome; detection and display of pericardial effusion and the relationship of tumor to the pericardium; and evaluation of internal cardiac anatomy, thrombi, and tumor. Because of rapid technological advances, statements concerning MRI's limitations must be guarded.(ABSTRACT TRUNCATED AT 400 WORDS)