MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging (MRI) generates images by applying a varying magnetic field to the body. The magnetic field aligns atoms. When the field is released, radio waves are generated. The frequency of the emitted radio waves is related to the chemical environment of the atoms as well as to their location. With computer analysis of these data, MR images (which are essentially hydrogen maps) can be generated.

Although many MRI techniques exist, the two basic types of images are T1 and T2. T1 images show fat as a white or bright signal, whereas water (or cerebrospinal fluid [CSF]) is dark. On a T2 image, fat is dark, and blood, edema, and CSF appear white. Unfortunately, calcium and bone are difficult to see on MR images. What people think are the (white) bones is really visualization of fat in the marrow. Computer manipulation of MR images allows slices similar to those of computed tomography orientation to be used. An intravenous contrast agent (gadolinium) is often used in conjunction with MRI. Not many significant patient reactions occur with this agent.

The primary advantages of MRI are that it obtains exquisite images of the central nervous system and stationary soft tissues (such as the knee joint). It also does not use ionizing radiation. Recent developments and shorter imaging times have allowed images of the heart and blood vessels to be generated without the need to inject anything into the patient.

Disadvantages of MRI have been artifacts due to patient motion, the inability to bring ferrous objects near the magnet, and cost. The major safety problem with these magnets is that they are so strong that if you bring a ferromagnetic object (such as a wrench) into the room, it can accelerate to 150 miles per hour as it is ripped out of your hand and flies into the bore of the magnet. Large floor polishers have been sucked into magnets. If a patient is in the machine at the time, lethal consequences will result. Be aware that some "sandbags" used for neck stabilization actually contain small BBs and can destroy magnets.