Safety Concerns in the MR Environment

Magnetic resonance (MR) imaging is a diagnostic and therapeutic imaging technology for soft tissue and organs that uses strong magnetic fields and radio waves rather than ionizing radiation. Compared with other imaging technologies, it provides better three-dimensional images of internal anatomy and better contrast resolution. The technology has been in use since the 1980s, and today about 11,900 machines are in use in the United States (Rinck). Larger healthcare facilities may have multiple fixed diagnostic units as well as mobile units that visit off-site locations (Edmonson). In 2017, approximately 111 MR exams were performed for every 1,000 people in the United States (OECD).

The most common type of MR scanner is the horizontal-bore cylindrical type, but others have been developed in which patients can stand up to reduce claustrophobia. MR imaging systems have been designed for image-guided interventional or surgical procedures, sometimes referred to as interventional MR imaging; systems with open-bore magnets permit surgery with real-time concurrent imaging, and portable systems can be set up to allow easy and rapid interchange between scanning and surgery (Association of Anaesthetists of Great Britain and Ireland et al.).

When first introduced, MR imaging was used for general diagnostic imaging; now many MR units are used only for specific anatomic applications, with the spine and brain accounting for over half of all scans. The upper extremities are the next most commonly scanned areas (11%), followed by blood vessels (angiography) and the lower extremities (both 9%); the abdomen and pelvis (8%); and other body areas (Rinck). Interventional MR imaging is also used to provide guidance during procedures such as biopsy and aspiration; thermal tumor ablation; open surgical procedures, such as deep brain stimulation with electrode placement; and catheter-based procedures such as cardiac catheterization (ACR-ASNR).

MR imaging is safer than technologies that use ionizing radiation, but risks still apply, associated with the large static and changing magnetic fields and with the high-powered radiofrequency (RF) coil systems. To understand the nature of these risks to patients, family members who may enter the MR suite, and medical personnel, it is necessary to understand the basics of the MR imaging process. Other safety concerns involve reactions to the use of contrast agents and monitoring concerns during sedation and anesthesia, discussed in Ensure Safe Anesthesia.

The human body is mostly water, and different organs and tissues have different amounts of water and fat. Water molecules contain protons that are sensitive to magnetic fields. Normally, these protons are randomly arranged, but the powerful magnetic field created by the MR scanner's main magnet causes the protons to line up with that field. A second magnetic field is then quickly turned on and off, causing the protons to change their alignment and producing a radio signal in the process. Because protons in different body tissues resonate at different rates, the RF coils can distinguish between different organs or tissues and send this information to a computer that creates the image.

MR imaging units consist of three primary systems: the main magnet, the gradient set of magnets, and the RF transmit-receive system (units also include a patient table, computer, and monitoring station). The three electromagnetic fields created by these systems cause...