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Keywords
Magnetic Resonance Angiography (MRA) «» Magnetic Resonance Spectroscopy (MRS) «» Magnetic Resonance Venography (MRV)
Introduction
Procedure
Technical Details
Risks
Magnetic resonance imaging (MRI) is one of the most versatile medical imaging technologies available. Using this technique, it is possible to obtain highly refined images of the body's interior without any pain or significant physical discomfort. MRI uses strong magnetic fields and pulses of radio waves to utilize the natural magnetic properties in the body in order to produce high-resolution images of the brain and spine as shown below, as well as other body parts.
This modality is particularly useful in the imaging of brain and spine, as well as various other parts of the body such as the soft tissues of joints and the interior structure of bones. Using MRI technology, physicians are increasingly able to make early diagnosis of serious pathology (such as tumors), which often translates into a more favorable outcome for the patient.
Magnetic resonance angiography (MRA) is a variation on standard MRI which is used to visualize blood flow within the arteries. This technique is typically used in combination with a standard MRI in order to obtain images that show abnormalities such as blockage, stenosis (abnormal narrowing) or aneurysms (abnormal dilation) of arteries in the brain and neck, as well as certain other blood vessels in the body. A brain MRA is depicted below.
Magnetic resonance spectroscopy (MRS) is a variation of the MRI technique which allows one to study a particular region within the brain with regard to its chemical composition. This imaging modality is primarily used to detect the presence of brain tumors when traditional MRI scans are inconclusive. This technique is also useful in evaluating metabolic disorders of the muscles and nervous system.
Magnetic resonance venography (MRV) is a similar procedure to an MRA with the exception that the blood vessels being imaged are veins instead of arteries.
The imaging process typically takes place on an MRI machine similar to the one depicted below.
The patient lies back on a narrow cushioned table which slides into a long tube that houses the magnet and is open at both ends. Once the area of the body to be imaged has been properly positioned inside of the magnet tube, a very thin image through the area is obtained after which the table moves down a fraction of an inch and the next image is obtained. Each image acquisition takes several seconds and the entire exam will last anywhere from 30-90 minutes, depending on the area of the body being imaged.
During this time the patient is not allowed to move since this can cause distortions in the image. Furthermore, various probes may be attached to the body which will monitor different vital signs such as pulse, respiration, blood pressure, etc.
The process is typically very noisy and confining. For this reason, music can sometimes be supplied via earphones to make the process more tolerable. If a patient has claustrophobia, the physician may prescribe an anti-anxiety drug prior to the procedure or the MRI may be performed on an open MRI machine (shown below). It should be noted, however, that open MRI machines typically produce images that are lower in quality than traditional MRI machines.
In some cases, a chemical agent, called gadolinium, can be injected into the blood stream to improve the contrast between healthy and diseased tissues.
In many cases, the entire examination will be performed by an MRI technician who is not a doctor. However, the supervising radiologist should be available to consult as necessary during the exam, and will view and interpret the results.
An MRI machine produces a map of hydrogen atoms in the body, which is the most abundant atom within biological tissues. When the patient is placed inside of the strong magnetic field of an MRI machine, the hydrogen atoms in the various tissues in the body will align themselves with the magnetic field, like the needle of a compass. Once a patient's hydrogen atoms have been aligned in the magnet, pulses of very specific radio wave frequencies are used to jolt them out of alignment. The hydrogen atoms will absorb and release these radio waves as they go back and forth between their aligned and non-aligned state. This process does not produce any sensations that are detectable by the person who is being scanned. The released waves are then detected by the MRI equipment, which records the duration, strength, and location of the signals emitted by the atoms and these signals are interpreted (translated) into images that can be viewed on a television monitor.
The machine assigns different colors (in a grayscale spectrum) to each specific type of signal that it receives; and since the state of hydrogen atoms (type of signal) is different in various body tissues, the MRI image can produce sharp-contrast images of different types of tissues (such as bone, muscle, nerve, blood/fluid, etc.). The images are obtained in cross-sectional slices; and a series of these image slices closely spaced (usually less than half an inch) can be obtained, creating a virtual three-dimensional view of the area. A single MRI exposure produces a two-dimensional image of a slice through the entire target area.
MRI does not depend on potentially harmful ionizing radiation (such as standard x rays and CT scans) and there are currently no known risks specific to the procedure itself. The potential side effects of magnetic and electric fields on unborn babies, however, are not well known; and any woman who is, or may be, pregnant should carefully discuss this issue with her physician and radiologist before undergoing a scan.
Also, contrast agents may sometimes be injected into the blood stream during an MRI scan in order to improve the image quality. In rare instances, some patients may be allergic to these chemicals or may have a medical condition or be taking a medication which makes it dangerous to receive these agents. Therefore, if these contrasts materials are are to be used, patients should discuss their medical history, medications, and any other concerns with their physician and radiologist prior to receiving the injection.
Since the MRI machine uses a very strong magnet, it can sometimes cause serious problems for individuals who have certain types of metal in or on their body, such as metallic implants, pacemakers, piercing, or metal shrapnel. Therefore, it is very important to remove all metal objects (such as watches, jewelry, eyeglasses, hair clips, etc.) and to notify the MRI technician of the existence of any attached metal objects prior to being transported into the MRI suite.
