A nonin-vasive method for creating visual images of internal body structures and organs. During an MRI, the person lies flat on a special table that slides inside a large tubelike device. The outer layer of the device contains magnetic coils that surround the body; the inner layer contains radio frequency coils that send and receive radio signals (waves of electromagnetic energy). The MRI’s powerful magnets surround the body in a magnetic field that polarizes the hydrogen atoms in the body’s tissues, causing them to align uniformly (all in the same direction). Then radio frequency coils send radio signals into the body, to excite the atoms into motion. When the radio signals stop, the atoms realign themselves and in so doing send out electrical signals that transmitters in the radio frequency coils pick up. The coils send these signals to a computer. Various kinds of cells realign at different rates, which the computer translates into visual, and typically three-dimensional, images. MRI produces extraordinarily precise, high-resolution images that are capable of detecting all sorts of problems ranging from muscle and ligament tears to tumors no bigger than a few cells.
MRI’s primary value in Parkinson’s disease is to rule out other possible causes of symptoms such as brain lesions (tumors), structural anomalies, stroke, and other neurodegenerative disorders such as multiple sclerosis, in which the MRI often can show the demyelinization of neurons that is characteristic of that disease. In late Parkinson’s disease MRI sometimes can visualize the loss of DOPAMINERGIC NEURONS in the SUBSTANTIA NIGRA and striatum, visualization serves no therapeutic purpose and is primarily of interest to researchers studying the ways in which Parkinson’s disease progresses.
Researchers have also been looking at ways to obtain more than just information on anatomy (structure) with MRI. In magnetic resonance spec-troscopy (MRS) special equipment and techniques allow researchers to look for concentrations of a certain chemical in each little voxel, the small unit of volume of the brain (sometimes as small as cube a few millimeters in each dimension) that is treated as a single unit for computational purposes, which appears to have much promise in the evaluation of possible brain tumors. Another research technique is functional MRI (fMRI) in which special techniques and equipment are used to estimate the amount of blood flow to each area of the brain; fMRI has been used to study Parkinson’s disease and the changes in blood flow in different areas of the basal ganglia and brain that occur with anti-parkinson’s medications and deep brain stimulation (DBS).
There is no special preparation for MRI, and MRI causes no discomfort, although some people do not like the closed-in feeling of being inside the magnetized tube. Sometimes the radiologist injects a contrast agent into a vein, to improve the details of the image. People with magnetic metals implanted in their bodies, such as stainless steel pins or plates used to repair bone fractures or aneurysm clips, or devices such as pacemakers cannot have MRI. Some dental implants contain magnetic metals as well; the radiologist should know of these to make a determination about whether it is safe to proceed with MRI. The magnets of MRI are so powerful that they can move implanted metals. They also can cause any metal object in the room at the time the magnets are activated to fly around, a very dangerous condition. Radiology staff ask that the person remove any and all metal, including bra straps, watches, eyeglasses, and other items. The MRI magnets also demagnetize credit cards. The radiology department should provide a secure place to leave such items.