Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) is a superior technique that uses radio waves and a strong magnetic field to provide remarkably clear pictures. As such, it offers an alternative to patients who react to radiopaque intravenous dye. Because of its ability to show soft tissues in exquisite detail, this technology can detect disease and detail blood vessels or other structures. In the kidney system, for example, an MRI can distinguish a hollow cyst from a solid mass, producing excellent three-dimensional images of any tumor's shape. In particular, its super sensitivity can help urologists identify and measure the spread of kidney cancer into the renal vein and inferior vena cava, the large vessel that returns blood to the heart. But while useful in evaluating kidney transplant donors, MRI has limited applicability for the urinary tract since the non-specificity of its signals makes it ineffective in detecting calcifications and bladder abnormalities.
MRI is unique among imaging methods because, unlike radiographs (X-rays,), CT scan and even radioisotope studies, it does not use ionizing radiation. Instead, MRI uses a strong magnet, radio waves and computers to create detailed images of the body. More specifically, lying inside a massive hollow magnet, a patient is exposed to short bursts of powerful non-ionizing radio wave energy, directed at protons, the nuclei of hydrogen or water atoms, in the body. Radio signals generated by first "exciting" and then "relaxing" those protons, are computer-processed to form digital images, reflecting different types of tissue. Typical MRI examinations consist of multiple imaging sequences, each lasting from two to 15 minutes. While these techniques continue to evolve, the beauty of current MRI is that it can be tailored for any clinical question.
This test is performed in a hospital radiology department or in a health care provider's office by a technician under the supervision of a physician. No patient preparation is necessary prior to this test. The patient will be asked to lie on a narrow table, which slides into a large tunnel-like tube within the scanner. The patient's head will be placed in a padded plastic cradle or on a pillow and the table will then slide into the scanner. The patient will be instructed to breathe quietly and normally but to refrain from any movement, coughing or wiggling. The technician will be able to communicate with the patient during this test through the use of an intercom. While the scanner is taking images, the patient will hear rapidly repeating, loud thumping noises coming from the walls of the scanner, so earplugs are usually provided to the patient to reduce the noise. The entire test usually takes between 30 and 60 minutes to complete. Following the test, the patient may resume their normal daily activities.
For generally healthy individuals, MRI poses no risk. But patients with pacemakers, aneurysm clips, ear implants and metallic pieces in vital body locations cannot be imaged safely.
Reviewed January 2011
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