Even though magnetic resonance imaging (MRI) equipment has been installed in the nation’s hospitals for more than 20 years, there is still very little information available to assist facility planners, designers, architects and engineers in siting these multimillion-dollar scanners. This is due, in large part, to the unique way in which MRIs operate and the complexity inherent in such a project.

Contrary to the siting ease that is implied through cut-and-paste suite layout templates, MRIs are not simply appliances needing a room and a plug. They interrelate and interact with their surroundings far more than other radiology modalities, such as computed tomography (CT) or conventional X-ray. Poorly sited magnets can impair patient safety as well as the clinical and technical performance of the modality.

Siting considerations

Most MRI systems are operational 24/7 so the magnetic fields, which can be as much as 60,000 times more powerful than the Earth’s, aren’t turned off after hours or on weekends. The magnetism from these devices is so strong that it can magnetize steel elements of the building and draw loose iron- or steel-containing objects from across the room into the MRI at speeds of up to 40 miles per hour. And while it presents no direct biological hazards, magnetism can pose a significant threat to pacemakers and other types of implants and can cripple adjacent modalities. Most conventional MRI machines also depend on liquid helium, at around -450 degrees Fahrenheit, to enable the magnet. These factors all come into play when designing facilities to support MRI equipment.

Most designers and contractors are familiar with shielding for radiology equipment. Conventional X-ray, CT, positron emission tomography (PET) scanners and even linear accelerators all have lead or concrete shielding to keep the ionizing radiation within the room. So when people learn of the hazards associated with magnetic fields, and that MRI suites are all provided with shielding, they naturally assume that the shielding contains the magnetism. Unfortunately, this isn’t the case. The radio frequency (RF) shield is installed to protect the MRI from stray radio waves that can interfere with the scan process, not to protect people from the magnetic energy.

In fact, the magnetic field generated by the MRI will easily penetrate all conventional building materials in its path. Concrete, masonry, wood and even copper, which is the most conventional RF shield material, are transparent to magnetic fields. This means that the hazards presented by the magnetic field aren’t limited to the magnet room. The three-dimensional volume of the magnetic field may penetrate outside the magnet room, to spaces above, below, adjacent or perhaps even outside the building.

The Food and Drug Administration recognizes the Occupational Safety and Health Administration’s standard of a maximum allowable incidental exposure of 5 gauss (gauss is a measurement of magnetic field strength equal to 1/10,000th of a Tesla). Any person entering a magnetic field stronger than

5 gauss is required to have successfully undergone appropriate clinical screening for devices or implants to assure that they aren’t at significant risk of injury. Unfortunately, for many MRI installations, the 5 gauss line is not wholly contained within the magnet room, but projects into adjacent spaces.

While a conventional RF shield does nothing to contain the magnetism, there is the option of adding a shield that will contain the magnetic field. The only material that can do this is steel. So, should the 5 gauss line extend into areas where unscreened people might be, a facility is obliged to either restrict the area or provide passive steel plate shielding to contain the magnetic field. Depending upon the degree to which the magnetic field must be attenuated, a steel shield could be just a few millimeters thick or up to several inches. Clearly, the more steel shielding required impacts siting costs and the overall structural load on the building.

Additionally, many MRIs are highly sensitive to vibration. Disruptive or crippling vibrations can be transmitted through the ground from cars, trucks and trains, sometimes from distances over 1,000 feet away. Similarly, vibrations from pumps, fans or motors that are commonly used throughout buildings can be transmitted across a building’s structural frame. Regardless of the origin, vibrations can impair an MRI’s image quality, particularly for many of the latest magnet systems, including 3.0 Tesla models and new high-field open MRIs. At the extreme end, harmonic or high-amplitude vibrations can cause quenches, running the risk of permanently crippling a $1 million magnet.

Finally, many contemporary magnets are also very loud when scanning, some comparable to a jet aircraft at takeoff, during certain sequences. The noise generated by powerful electromagnets  in the MRI can reach painful levels inside the bore, damaging the unprotected ear. These sounds can also be transmitted outside the magnet room to other spaces, either through the air or communicated structurally through the building itself. MRI suites, particularly those with adjacent functions that are sensitive to high-volume noise, need to be designed to absorb and block as much sound as possible.

Where to turn?

So, where does a planner or designer turn for objective information on shielding, magnetic fields, magnetic field distortion, vibration and sound transmission? The most commonly referenced source for information on MRI siting are the guides provided by the equipment vendors.

However, vendor siting guides are geared toward assuring that the minimum technical criteria are met. Optimal system performance, life-cycle costs, top-notch image quality, operational safety issues and site selection criteria receive minimal attention in vendor documents.

With few vendor-neutral sources of information on MRI siting, it is left largely to each facility and their design team to discover for themselves the siting and design criteria that will facilitate their desired clinical, operational and financial outcomes.

While the way in which planning criteria are applied to different MRI suites should be unique to the setting, patients, scans and MRI equipment, there are a number of other considerations that ought to be reviewed for any MRI upgrade, expansion, replacement or new installation.

All MRI suites, whether proposed or existing, should comply with the four-zone guidelines as established by the American College of Radiology (ACR). The ACR’s four-zone principle marries access controls with risk and patient screenings to create suites that maximize patient safety (see diagram and chart above).

Zone I / While areas outside the MRI suite don’t present hazards from the magnetic field, activities outside the MRI suite can have a direct impact on the operation of the MRI equipment. Vibration from cars, helicopters, trains, even rolling carts can disrupt MRI operations. Construction activities can also disrupt MRI systems from vibrations (such as from jack-hammering, pile-driving and excavation) and also through the introduction of significant quantities of additional ferrous material (usually steel) near the magnet. Other design and construction considerations for Zone I include the following:

• In addition to work directly associated with the MRI suite, consider the impacts of any future construction work;
• Alert the MRI/radiology staff of potential impacts; and
• Benchmark MRI system performance and image quality during and after construction activities.

Zone II / Because patients and visitors must review their medical history with staff prior to entering the magnet room, a space must be provided to allow for private conversations in conformance with current Health Insurance Portability and Accountability Act (HIPAA) requirements. Even when patients are prescreened, it is vital that the clinical questionnaire be reviewed prior to the exam by trained MRI staffers.

Because of the tremendous power of MRI magnets and their ability to rip steel objects out of people’s grasp, it is vital that all unnecessary ferrous materials be kept away from the magnet. It is recommended that incidental steel, which is often found in items like key chains, nail clippers, cell phones, hair pins and steel-reinforced boots, be interdicted in Zone II, before Zone III is entered. Other design and construction considerations for Zone II include the following:

• Construction materials and building assemblies with low sound transmission classifications and high noise reduction coefficient acoustic values can be very helpful in creating auditory privacy for patient interview areas.
• For facilities that don’t provide fully enclosed interview areas, “white noise” generators can be installed to help mask private conversations.
• Provide changing rooms and storage lockers for patient belongings, enough to accommodate patients “gowning-in” and “gowning-out.”
• In addition to screening patients, facilities must screen all visitors, staff and equipment for contraindications or hazards before they are permitted to pass to Zone III.

Zone III / Zone III should be secured to prevent the access of any unscreened person or object. Often this means that the MRI suite is on a separate security system and that staff who are accustomed to having free access,  such as security, transport and housekeeping, are restricted unless trained or under the direct supervision of a qualified staff person.

Magnetic fields are three-dimensional, extending beyond the confines of the magnet room. Thus, the 5 gauss line and the hazards that it can present can be on floors above or below the MRI, and even outside the building. As a result, access controls and warning signage may be needed outside the immediate MRI suite. Equipment rooms, rooftops and areas surrounding magnet rooms are often locations that require additional protection.

MRI suites must be provided with equipment and incidental appliances that have been tested safe for the MRI systems installed. These materials include wheelchairs, fire extinguishers, hand tools, cleaning supplies and oxygen cylinders. For patients who bring their own ferrous walkers or oxygen cylinders, imaging facilities need to provide “loaners” that are safe for the MRI environment, and also have a secured area to lock away any devices that could be dangerous if inadvertently brought into the magnet room. An infamous 2001 MRI fatality occurred when a steel oxygen cylinder, which was left out in the control room, was accidentally rushed to a young boy who was having difficulty breathing in the MRI.

No matter how determined a facility’s resolve to keep potential threats out of the MRI suite, ferromagnetic materials will find their way into the area. To find these objects before they’re sucked into an MRI, facilities are advised to consider the new breed of ferromagnetic detectors developed specifically for screening patients and objects entering the MRI suite. These specialized metal detectors alert on only magnetic materials that may become missile-threats in proximity to the MRI magnet. Other design and construction considerations for Zone III include the following:

• Provide unique access controls for the MRI suite other than magnetic swipe cards or conventional keys, which could be either erased or attracted to the magnet.
• To identify all areas of potential hazard, map magnetic fields in sections as well as in plan, paying particular attention to the location of the 5 gauss line. Be aware that many “open” magnets project magnetic fields much farther vertically than “bore” magnets, which are typically more powerful.
• Equip MRI suites with appropriate  MRI-safe materials and equipment. Provide ample storage for safe materials as well as a secured “ferrous quarantine” area for potentially unsafe materials brought into the suite.

Zone IV / The door to the magnet room is the very last opportunity for the staff to intervene and prevent potentially dangerous objects or unscreened personnel from entering Zone IV.

Once inside the MRI room, added to the threats of flying steel and disabled pacemakers are concerns about cryogens. Many contemporary MRIs use super-cold liquid helium to enable electromagnets. Under certain failure conditions, these cryogens blast from the MRI and can potentially escape into the magnet room. As the cryogenic gases escape, they warm and expand dramatically (by the time it warms to room temperature, a single liter of liquid helium expands over 700 times its original volume), and can displace breathable air and pressurize the magnet room. Other Zone IV considerations:

• The technologist, while seated at the console, should have a commanding view of the door to the magnet room and the approach to it.
• Magnet room designs must include cryogen vent pipes, emergency exhaust and pressure relief systems.

While there are many other considerations facilities should address in the evaluation or planning of MRI facilities, these recommendations represent some of the most overlooked components of effective and efficient MRI suites.

Clinical, financial necessity

The technical, clinical and financial demands of MRIs have shifted dramatically in the last several years, yet a surprising number of contemporary suites are developed with cut-and-paste floor plans that predate interventional applications, HIPAA, higher patient acuities and the ACR guidelines.

Given the cost of MRI equipment and the potential hazards introduced by the high-strength magnetic fields, health facilities are well-advised to carefully consider design and construction issues for MRI or multimodal imaging suites.

For most facilities that have these scanners, the MRI has become both a clinical and financial necessity. Facilities managers who have questions about their design team’s MRI portfolio or working knowledge would be well-advised to enlist the assistance of consultants with specific expertise in planning and designing for MRI and multimodal imaging facilities. 

Tobias Gilk is an associate architect for Junk Architects in Kansas City, Mo., a firm specializing in MRI and radiology design consulting. He also is a member of the American College of Radiology’s MR (magnetic resonance) Safety Committee charged with authoring the 2006 “Update and Revisions” to the ACR’s white paper on MR safety. He can be reached via e-mail at tgilk@junkarchitects.com.

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