special subjects
Magnetic Resonance Imaging
Uses of MRI
MRI is preferred to CT when soft-tissue contrast resolution must be highly detailed (eg, to evaluate intracranial or spinal cord abnormalities, inflammation, trauma, suspected musculoskeletal tumors, or internal joint derangement). MRI is also useful for the following: Vascular imaging:Magnetic resonance angiography (MRA) is used to image arteries with good diagnostic accuracy and is less invasive than conventional angiography. A gadolinium contrast agent is sometimes used. MRA can be used to image the thoracic and abdominal aorta and arteries of the brain, neck, abdominal organs, kidneys, and lower extremities. Venous imaging (magnetic resonance venography, or MRV) provides the best images of venous abnormalities, including thrombosis and anomalies. Hepatic and biliary tract abnormalities:Magnetic resonance cholangiopancreatography (MRCP) is particularly valuable as a noninvasive, highly accurate method of imaging the biliary and pancreatic duct systems. Masses in the female reproductive organs:MRI supplements ultrasonography to further characterize adnexal masses and to stage uterine tumors. Certain fractures:For example, MRI can provide accurate images of hip fractures in patients with osteopenia. Bone marrow infiltration and bone metastases:MRI is particularly useful for evaluating patients with positive bone scintigraphy and normal x-rays and for characterizing bone marrow abnormalities as metastases versus nonmalignant lesions. MRI can also be substituted for CT with contrast in patients with a high risk of reactions to iodinated contrast agents. With MRI, contrast agents are often used to highlight vascular structures and to help characterize inflammation and tumors. The most commonly used agents are gadolinium derivatives, which have magnetic properties that affect proton relaxation times. MRI of intra-articular structures may include injection of a diluted gadolinium derivative into a joint. 