The use of magnetic resonance imaging (MRI) by emergency physicians (EPs) is increasing steadily, as new MRI indications arise, technology evolves, and machines become faster and more widely available. It is therefore critically important that EPs understand the basics of this imaging modality, its uses, limitations, cautions, and contraindications.
A full explanation of the physics underpinning MRI is beyond this article’s scope. However, a comprehensive discussion of the topic is available in a 2013 review entitled, "Understanding MRI: basic MR physics for physicians."1 In short, three elements are necessary for an MRI machine to generate images: a strong magnetic field, radio waves, and a computer system. The body’s hydrogen nuclei with their single protons and north-south poles act as mini bar magnets with randomly aligned axes. However, when the body is subjected to the MRI machine magnetic field, these axes line up. When radio waves are applied to the magnetic field, the strength and direction of the magnetic field changes. Then, when the radio waves are turned off, the magnetic field strength and direction return to baseline and a signal is emitted. It is this signal that is interpreted by a computer system to generate images.2
Cautions and Limitations
Although limited availability is often cited as a reason for not obtaining MRI studies in the emergency department (ED), this limitation is institution specific and will likely improve over time. Recent statistics indicate that MRI availability in the United States is second only to that in Japan and climbing. MRI usage in the United States is the highest in the world.3
MRI cost (and the resultant patient bill) exceeds that of other commonly performed ED imaging roughly by a factor of 2:1 when compared to computed tomography (CT). This is unlikely to improve in the near term.
The time to complete an MRI study continues to fall for some indications, but significantly exceeds the time to obtain CT images. MRI scan times range from 20 to 60 minutes depending on test type.
Body habitus, particularly obesity, may limit the ability of certain patients to undergo MRI. Claustrophobia or the inability to lie still for the test’s entire duration may present a challenge for some patients. Be prepared to safely sedate patients with these issues. This is particularly relevant for pediatric patients. Consider a pre-MRI trial of sedation to assess which medication is best suited for individual patients.
Patients with certain medical devices may be unable to undergo MRI. Medical devices and implants from the U.S. and Europe manufactured within the past 30 years are non-ferromagnetic. This generally means they are MR-safe or MR-conditional. Realize, however, that certain non-ferromagnetic implants can heat during MR imaging.4 A free searchable database exists listing MRI-safe devices and implants along with limitations and cautions (http://www.mrisafety.com/TheList_search.asp).5