# Magnetic Resonance Imaging Safety

Filed under Cardiovascular

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CHAPTER 19 Magnetic Resonance Imaging Safety

Safety of MRI is essential for any patient evaluation, and is especially critical for cardiovascular MRI, where high-risk patients, procedures, and implanted devices are common. Safety of patients is a priority for the American Medical Association and The Joint Commission. Safety in MRI has been a source of controversy, however, among health care providers and patients. Growth in medical technology has resulted in more patients receiving implanted devices, many of which were not specifically designed to be safe for MRI. Many devices lack definitive evidence to support their MRI safety.

This chapter provides an overview of MRI safety as it pertains to patients with cardiovascular disease, with suggestions of supplemental information and references. MRI safety is an enormous topic that is constantly changing, and a host of Web-based resources are available. Questions of MRI safety need to be evaluated with the most up-to-date information possible, which is why providers need to research the specific devices and scanning circumstances carefully. Box 19-1 lists useful websites for the reader’s reference. MRI safety guidelines are updated on a regular basis. Failure to follow current safety guidelines puts patients at increased risk for injury and, in some instances, results in unnecessary avoidance of clinically necessary MRI examinations.

# GENERAL CONCERNS

## Physics of Electricity and Magnetism

Maxwell’s equations are the building blocks of our understanding of electricity and magnetism. The four equations known as Gauss’ law for electricity, Gauss’ law for magnetism, Faraday’s law, and Ampere-Maxwell law help in understanding many of the important elements of MRI safety.5 The mathematical predictions for safety cannot be calculated easily. Ensuring the safety of a device in a patient undergoing an MRI examination requires knowledge of the device, the specific MRI system, the type of RF coils used, the sequences employed for imaging, the way the patient loads the scanner and transmit RF coil, and whether or not monitoring or gating equipment is also being used. It is nearly impossible to predict all of the possible ways for heat deposition or current induction that may occur with a given implant in a patient—hence the safety dilemma.

## Static Magnetic Field Strength

Currently, most clinical scanners for cardiac use are operating at a static magnetic field of 1.5 T or 3 T. Table 19-1 outlines the FDA significant risk recommendations. Magnetic fields have the potential for translational and rotational (or torque) effects. Basically, magnetic field interactions increase as the static magnetic field strength increases. If a device is considered to pose a potential safety concern, it is prudent to consider carefully the potential options for such scanning at the lowest field strength appropriate for the imaging needs (e.g., coil selection, imaging pulse sequence selection). Imaging time is also important because the increase in scan time may prolong the requirements for monitoring a child or a sedated claustrophobic patient. Discussion with the patient on what to expect and the need for the patient to communicate any unusual sensations immediately is imperative. Devices and scanners vary considerably, so many factors must be considered. Substances that are attracted to the magnetic field are often referred to as having ferromagnetic properties. Devices that are ferromagnetic have the potential to interact with the magnetic field in a translational or rotational manner (or both), possibly causing great harm to the patient or equipment or both. The plethora of MRI accident pictures that are available via the Internet provide some idea of the danger of the static magnetic field of the MRI scanner. Many pictures of objects flying into scanners can be found at sites such as Simply Physics (http://www.simplyphysics.com/flying_objects.html).

Population Main Static Magnetic Field Greater than (tesla)
Adults, children, and infants >1 month old 8
Neonates (<1 month old) 4

From United States Food and Drug Administration, Center for Devices and Radiological Health. Guidance for Industry. Criteria for significant risk investigations of magnetic resonance diagnostic devices. Available at: http://www.fda.gov/cdrh/ode/guidance/793.pdf. Accessed June 5, 2008.