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© Dirk Biddle
Magnetic resonance imaging (MRI) is a unique imaging method because, unlike the usual radiographs (x-rays), radioisotope studies, or even Computed Axial Tomography (CAT or CT) scanning, it does not rely on ionizing radiation. Instead, the magnet creates a strong magnetic field which aligns the protons of hydrogen atoms, which are then exposed to a beam of radiofrequency waves. The protons are first "excited" and then "relaxed," emitting radio signals that are then detected by the receiver portion of the MRI scanner. The receiver information is processed by a computer, and depending on requirements, a 2-D image or 3-D model is then produced. In the body, protons are most abundant in the hydrogen atoms of water (H2O) so that an MR image shows differences in the water content and distribution in various body tissues. Even different types of tissue within the same organ, such as the grey and white matter of the brain, can easily be distinguished. When this scanning method is applied to the blood vessels, it is sometimes referred to as MRA (magnetic resonance angiography).
A typical MRI system consists a giant closed cylindrical magnet enclosed in a unit up to 2m x 2m x 3m in dimension (although new models are rapidly shrinking. For example the so-called "short-bore" systems are wider and shorter and do not fully enclose the patient. Some newer units are open on all sides, however the image quality may vary). There is a horizontal tube (approximately 60cm in diameter) running through the magnet from front to back. A special sliding table allows a patient, lying on his or her back, to slide inside the bore of the magnet, where imaging can take place at the isocentre of the magnetic field. If your feet or knees are being imaged, your head will be outside of the magnet. If your head, shoulder, or chest is being imaged, your feet will be outside of the imager.
All metallic objects on the body are removed prior to obtaining an MRI scan. Patients who have any metallic materials within the body must notify their physician prior to the examination or inform the MRI staff. Metallic chips, materials, surgical clips, or foreign material (artificial joints, metallic bone plates, or prosthetic devices, etc.) can significantly distort the images obtained by the MRI scanner. Patients who have heart pacemakers, metal implants, or metal chips or clips in or around the eyeballs cannot be scanned with an MRI because of the risk that the magnet may move the metal in these areas. Similarly, patients with artificial heart valves, metallic ear implants, bullet fragments, and chemotherapy or insulin pumps should not have MRI scanning.
When you first lie on the bed of the imager, the technologist will position an imaging coil around the anatomy being imaged. The technologist will then move your body to position a specific piece of anatomy at a crossed light beam. This spot on your body will be advanced to the isocenter of the magnet before the scan begins. This process is called landmarking.
Depending on the part of the body being examined (and especially for vascular radiology), a contrast medium (usually gadolinium) may be used to enhance the visibility of certain tissues or blood vessels. A small needle connected to an intravenous line is placed in an arm or hand vein. A saline solution will drip through the intravenous line to prevent clotting until the contrast medium is injected, about two-thirds of the way through the exam. The contrast medium may feel cold in your veins but this is normal and nothing to be concerned about. Rarely, some patients may experience mild nausea.
Typically an MRI examination consists of two to six imaging sequences, each lasting two to 15 minutes, during which time you will be asked to lie completely still. You can however breathe freely during this time. Depending on how many images are needed, the exam will generally take between 15 to 45 minutes, although a very detailed study may take longer. You may, in some cases, be allowed to move slightly between scans, but not so much that your position changes.
Some individuals may become anxious in situations like this and feel “closed in” or claustrophobic. However, be assured that you are in constant intercom contact with the technologists in the control room and can ask get out at any time. Some imaging sites will allow a friend or relative to accompany you through your scan. This individual may sit in the scan room while you are being imaged and talk with you.
The imaging sessions create a series of repetitive loud knocking sounds when the magnetic field gradients are turned on and off. Because of the volume of these sounds, it is recommended that you wear ear protection. Some imaging sites provide an airplane-like audio system for those being imaged. These systems provide some noise suppression, and also mask out the imager sound with music.
You may or may not notice a warm feeling in the area under examination; this is normal but if it bothers you the radiologist or technologist should be notified.
When the exam is over the patient is asked to wait until the computer generated images are examined to determine if more images are needed. A radiologist experienced in MRI will analyse the images and send a report with his or her interpretation to the patient's personal physician. This should take only a few days or less.
Interestingly it has been found that some individuals experience magnetophosphenes when placed in the magnetic field. Magnetophosphenes are a visual sensation of flashes of light on the retina. They were first reported by the French physiologist Jaques-Arsène d'Arsonval in 1896. They are caused by induced electric currents in the retina when moving through a static magnetic field, or when stationary in a changing magnetic field (in either case, the field strength must be greater than ~ 7 mT). They are an interesting but benign phenomenon.
Scientists are developing newer MRI scanners that are smaller, portable devices. These new scanners apparently can be most useful in detecting infections and tumours of the soft tissues of the hands, feet, elbows, and knees. The application of these scanners to medical practice is now being tested.
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