special subjects
Magnetic Resonance Imaging (MRI)
Disadvantages of MRI
The time needed for MRI is longer than that needed for CT. Also, MRI is usually less likely to be immediately available than CT. Thus, CT may be better in emergencies, such as serious injuries and stroke. MRI is also more expensive than CT. Other disadvantages include Claustrophobia and sometimes difficulty fitting within the MRI scanner because it is a small, enclosed space The effects of the magnetic field on metal devices implanted in the body Reactions to the contrast agent Space in the MRI scanner is small and enclosed, making some people feel claustrophobic, even people who usually are not anxious about confined spaces. Some people with obesity have difficulty fitting within the scanner. Some MRI scanners (called open MRI scanners) have an open side and a larger interior. In them, people may feel less claustrophobic, and people with obesity may fit more easily. The images produced in open MRI scanners may be inferior to those produced by enclosed scanners depending on the magnet strength, but they can still be used to make a diagnosis. People who are anxious about MRI can be given an antianxiety drug, such asalprazolamorlorazepam, 15 to 30 minutes before scanning. Usually, MRI is not used if people have Certain materials (such as shrapnel) in specific parts of their body, especially in the eye Implanted devices that can be affected by powerful magnetic fields These devices include some cardiac pacemakers, defibrillators, cochlear implants, and magnetic metallic clips used to treat aneurysms. The magnetic field used in MRI can cause an implanted device to move, overheat, or malfunction. The device is more likely to be affected if it was implanted within the previous 6 weeks (because scar tissue, which can help hold the device in place, has not yet formed). These devices can also distort MRI images. Some devices, such as common dental implants, an artificial hip, or rods used to straighten the spine, are not affected by MRI. Before MRI is done, people who have any implanted devices should tell their doctor, who can determine whether imaging is safe. The MRI magnetic field is very strong and always on. Thus, if a metal object (such as an oxygen tank or an IV pole) is near the entrance of the scanning room, the object may be pulled into the scanner at high speed. The person being evaluated may be injured, and separating the object from the magnet may be difficult. Gadolinium contrast agents can cause headache, nausea, pain and a sensation of cold at the injection site, distortion of taste, and dizziness. These agents are much less likely to cause severe reactions than the iodinated contrast agents used in conventional and CT angiography. —a severe, life-threatening disorder—has occurred in a small number of people with advanced Chronic Kidney Disease ; however, most of those cases are linked to a type of contract agent called group I gadolinium-based contrast media (GBCM), which is no longer administered in the United States. 