special subjects
Magnetic Resonance Imaging
Disadvantages of MRI
MRI is relatively expensive, requires longer imaging times than CT and may not be immediately available in all areas. Other disadvantages include problems related to The magnetic field Patient claustrophobia Contrast reactions MRI is relatively contraindicated in patients with implanted materials that can be affected by powerful magnetic fields. These materials include Ferromagnetic metal (ie, containing iron) Magnetically activated or electronically controlled medical devices (eg, pacemakers, implantable cardioverter defibrillators, cochlear implants) Nonferromagnetic electrical wires or materials (eg, pacemaker wires, certain pulmonary artery catheters) Ferromagnetic materialmay be displaced by the strong magnetic field, injuring a nearby organ; for example, displacement of vascular clips can result in hemorrhage. Displacement is more likely if the material has been in place<6 weeks (before scar tissue forms). Ferromagnetic material can also cause imaging artifacts. Magnetically activated medical devicesmay malfunction when exposed to magnetic fields. Magnetic fields may induce current in any conductive materials strong enough to produce enough heat to burn tissues. Whether a specific device is compatible with MRI depends on the type of device, its components, and its manufacturer (seeMRIsafety.com). Patients with an implantable device should not be placed in the MRI magnetic field until examiners are sure that MRI is safe with such a device in place. Also, MRI machines with different magnetic field strengths have different effects on materials, so safety in one machine does not ensure safety in another. The MRI magnetic field is very strong and may always be on. Thus, a ferromagnetic object (eg, an oxygen tank, a metal pole) at the entrance of the scanning room may be pulled into the magnet bore at high velocity and injure anyone in its path. The only way to separate the object from the magnet may be to turn off (quench) the magnetic field. The imaging tube of an MRI machine is a tight, enclosed space that can trigger claustrophobia even in patients without preexisting phobias or anxiety. Also, some patients with obesity do not fit on the table or within the machine. Premedication with an antianxiety medication (eg,alprazolamorlorazepam1 to 2 mg orally) 15 to 30 minutes before scanning is effective for most anxious patients. MRI scanners with an open side can be used for patients with claustrophobia (or those with severe obesity). Images obtained during open MRI may be inferior to those of enclosed scanners depending on the field strength of the magnet, but they are usually sufficient for making a diagnosis. Patients should be warned that the MRI machine makes loud, banging noises during scanning. Gadolinium-based contrast agents injected IV can cause headache, nausea, pain, and distortion of taste, as well as sensation of cold at the injection site. Serious contrast reactions are rare and much less common than with iodinated contrast agents. However, nephrogenic systemic fibrosis (NSF) has been reported in a small number of patients with advanced chronic kidney disease, and most of these cases are linked to group I gadolinium-based contrast media (GBCM). NSF is a rare but life-threatening disorder that involves fibrosis of the skin, blood vessels, and internal organs, resulting in severe disability or death. Group I GBCM is no longer advertised in the United States or Europe (seeEuropean Medicines Agency, November 2009 press release). The likelihood of developing NSF or nephrotoxicity is extremely low in patients who received group II GBCM, even in high-risk patients. The consensus statement from the American College of Radiology and the National Kidney Foundation indicates that kidney function screening is not mandatory for any Group II GBCM ( Reference ). Gadolinium-based contrast agents should be used only when necessary and at the lowest possible dose. 1.Weinreb JC, Rodby RA, Yee J, et al: Use of Intravenous Gadolinium-based Contrast Media in Patients with Kidney Disease: Consensus Statements from the American College of Radiology and the National Kidney Foundation.Radiology298(1):28-35, 2021. doi:10.1148/radiol.2020202903 