(From Kanal E, Barkovich AJ, Bell C, et al; ACR Blue Ribbon Panel on MR Safety. ACR guidance document for safe MR practices: 2007. AJR Am J Roentgenol 2007; 188:1447-1474.)

Zone II is accessible by the general public, but has controlled access to zones III and IV. Zone II is typically where reception and general waiting are located. Initial MRI screening, medical histories, and insurance issues are attended to in zone II. Safety guidelines recommend that the patient preparation and holding be included in zone II to protect the patients and other individuals. The design of the facility must also take into consideration the need to have the patient holding area well thought out to ensure that patients can still be monitored as needed by health care personnel.

Zone III is the area in which all non-MRI personnel should be escorted and supervised at all times. All individuals and equipment entering zone III should be screened. A physical restriction such as a locking mechanism needs to be in place for zone III to prevent entrance by non-MRI personnel. Key locks, pass card systems, or other types of security locks are recommended. Combination locks are not recommended because lock codes are easy to disseminate. Only MRI personnel who have been properly trained in MRI safety should have free access to zone III.

Zone IV is the MRI system room, which is inside zone III. This room has the highest restrictions and should be clearly marked as being hazardous. Only MRI personnel, screened patients, and MRI approved equipment may enter zone IV. Zone IV needs to be locked and to have otherwise controlled access. MRI system operators need to have direct visual control (direct sight or camera) over zone IV. In the event of a patient emergency, such as a “Code Blue,” a plan must be in place to remove the patient from zone IV and into a safe area in zone II or III for emergency personnel to manage the patient safely.

No patient should undergo an MRI examination without proper identification of an implanted device and the potential risk to the patient ascertained relative to the intended MRI conditions. The potential hazards associated with MRI procedures are affected by, but are not limited to, the static magnetic field, gradient switching, transmit RF coil, imaging sequences, RF energy heating and induction, device specifications, body part to be imaged, and patient. The safety of the patient and the device must be considered with all potential hazards in mind. Many devices have been tested by scientists and manufacturers so that the risk/benefit ratios can be more confidently evaluated. Not all devices have been tested under all MRI scanning conditions, however, plus new devices are constantly being introduced into clinical care. MRI scanners and procedures have also evolved over time and are very complex, making MRI safety a particular challenge.

Cardiovascular MRI has extra considerations beyond the general MRI safety requirements and recommendations because cardiovascular imaging often includes advanced imaging techniques, such as ECG gating, cardiac stress imaging with pharmacologic stress agents, and interventional procedures. Advanced vascular work may include an MRI “runoff” or “bolus chase” angiogram, in which a dynamic injection of contrast agent is administered while moving the table and patient through the MRI system to visualize the vessels in the chest, abdomen, pelvis, legs, and feet. Many of these techniques require extra care of the patient, availability of specialized personnel for advanced cardiac care, and specialized equipment to maintain safety and image quality. More patients with implanted devices are being requested to be scanned with MRI. Improving MRI facilities and safety training is imperative for imaging suites and procedures that are based on the latest information to prevent negative outcomes, while providing the best possible health care.

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).

TABLE 19-1 Static Main Magnetic Field Limit Recommendations by the U.S. Food and Drug Administration *

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

* The FDA considers an MRI scanner to be of significant risk when it is operating above the levels listed.

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.

Gradient Magnetic Fields

MRI scanners use time-varying magnetic fields called gradients to encode for field of view, slice thickness, and other imaging parameters, and in certain aspects of pulse sequence image contrast. Stronger and faster gradients and more powerful RF transmission coils are being used in MRI scanners.⁶ The current FDA limit for gradient magnetic field rates of change (dB/dt) varies depending on operating conditions. Significant risk for dB/dt is classified as when painful nerve stimulation or severe discomfort is produced.⁷ Gradient fields are magnetic fields that are much weaker than the main magnetic field, but they are switched on and off very quickly and, according to Maxwell’s equations, can induce time-varying electrical currents.⁸ Gradients can induce currents in conductive materials⁹ and potentiate peripheral nerve stimulation, but are typically not painful on current FDA-approved MRI systems.¹⁰ This is a safety concern because these gradient currents have the potential also to induce currents in certain cardiac devices, which could induce arrhythmias.⁹ Tandri and coworkers⁸ reported that gradient currents may be strong enough under certain conditions to distort pacing pulses, which is significant enough to interfere with myocardial capture or loss of capture when patients with pacemakers are scanned in MRI. This finding reinforces the need for prudent screening and risk/benefit considerations with patients with any kind of implanted cardiac device.

Radiofrequency Fields

The transmit RF coils of the MRI scanner emit RF energy that is specifically tuned to the tissue of interest at a given static magnetic field strength. RF energy is absorbed by the tissue and released, producing signals that receiver coils can detect. RF energy may cause heating of biologic tissues, however, when the tissues are exposed to excessive RF energy levels. To ensure patient safety, levels of RF energy used for MRI procedures indicated in the units of specific absorption rate (SAR), are recommended by the FDA. Generally, MRI scanners calculate or estimate SAR based on the patient’s weight and report values in W/kg. In 2003, the FDA relaxed the SAR limits as follows: 4 W/kg—whole body for 15 minutes, 3 W/kg—head for 10 minutes, 8 W/kg—peak 1 g of tissue (head or torso) for 15 minutes, 12 W/kg—peak 1 g of tissue (extremities) for 15 minutes.⁷

SAR increases with increasing static magnetic field strength, flip angle, duty cycle of the equipment, and patient size.¹¹ The SAR experienced at 3 T is approximately four times the absorption rate as 1.5 T,¹² making some of the cardiac imaging at 3 T potentially challenging. The SAR can be reduced, however, by modifying parameters such as flip angle, repetition time, slice thickness, imaging matrix, and echo train length, depending on the pulse sequence and the desired image contrast. Parallel imaging schemes have been a successful strategy for SAR reduction at 3 T.