Each year, HGA’s integrated team of healthcare architects and engineers designs many successful MRI projects. MRI suites can cost between $3 to $5 million for construction and equipment purchase. Because of this investment, owners should be aware of critical structural considerations that can influence a good design.
MRI, or Magnetic Resonance Imaging, is a diagnostic tool that uses magnetic fields and radio waves to generate soft tissue images. Today, typical MRI field strength varies from 1.5 to 3.0 Tesla. Within the past few years, 7.0 Tesla magnets are emerging. These machines provide amazing scan quality and can generate magnetic fields that are nearly 140,000 times stronger than the magnetic field of earth itself.
Weight Matters
Let’s start with weight, as MRI equipment is heavy, typically weighing 8,000 to 10,000 pounds with the load concentrated over a relatively small footprint of approximately 4.5-foot square. Besides the magnet self-weight, associated control-room electrical equipment can weigh between 2,500 to 3,000 pounds. Years ago, most imaging suites for healthcare facilities were located on ground floor slab on grade conditions because the MRI equipment was heavy. That is not the case any-more; imaging suites can be located anywhere in the facility, including elevated floors above grade
For instance, an existing clinic building may have been designed for a floor live load of 40 to 50 pounds per square foot; supporting a 10,000 pound MRI and associated equipment will be challenging. The structural engineer will need to evaluate the floor capacity and stiffness to determine if reinforcing is required. The design team also will review the path of travel for the MRI and how the machine is brought into the building. Typically, an 8-foot-wide by 8-foot-high zone is required to move the magnet through the building. The structural engineer will perform a general capacity check of the floor, and the rigging contractor installing the MRI will be responsible to design shoring if required.
Building Vibration
Building vibrations must be evaluated because vibrations can reduce the image quality of the scans. When MRI suites are located on grade-level conditions, isolation joints in the floor slab can help limit the transfer of floor vibrations entering the suite. Vehicular traffic in proximity to the suite can cause ground vibrations to enter the suite. External building vibrations caused by mechanical equipment or human foot traffic also can affect the imaging quality, especially on elevated floors. Vibrations can move through the structure over multiple floors. New MRI technology has made magnets lighter and faster, but also more sensitive to vibrations.
On a recent HGA project for Hennepin County Medical Center in Minneapolis, for instance, a vibration/acoustical consultant determined that a mechanical pump two floors above was transmitting a low-frequency energy through the structure and would have potentially affected the imaging quality. In this case, spring isolators were used to isolate the motors, reducing the vibration threat.
For new building structures with the MRI suite located on an elevated floor, the structural engineer can design a floor system with the appropriate required stiffness. Typically cast-in-place concrete pan and joist floors are stiffer than steel beams and composite floor slabs. Many variables will be evaluated by the structural engineer during the design phase. If the new building has long bay spans, it may make sense to introduce intermediate column lines to reduce the long bays, which can stiffen the floor at the MRI suite.
Equipment Selection
Today, MRI manufacturers/vendors provide vibration design criteria on the equipment cut sheets. The design team can benefit when the owner makes final equipment selection early in the design phase. When equipment selection is not finalized, the design team must make assumptions about the design criteria. Hiring a vibration consultant is recommended, particularly for MRI installations in existing buildings. The consultant will visit the site and place sensors at various locations around the proposed suite. The sensors will monitor vibrations over 48 to 72 hours. If troubled vibrations are detected, the consultant will provide recommendations to remedy the specific issues. Finally, MRI manufacturers typically install isolation pads below the support feet of the magnet. While this will provide some vibration protection, the structural engineer should confirm with the manufacturer specific information regarding the isolation pad material and thickness.
MRI electronics are sensitive to distortions in the electromagnet field. The proximity to high-amperage power lines, electrical switchgear and transformers are crucial placement considerations. Best practice is to not place an electrical room near an MRI suite. The design team will consult with the MRI vendor on appropriate distances required early in the design phase.
Sound Interference
MRI scanners are capable of producing sound pressure levels in excess of 110 dBA. Sound from MRI equipment can be extremely disruptive to other occupants in the building. Just as vibration can travel through a building structure to an MRI, acoustical frequency vibration can be telegraphed through building components to surrounding spaces. Construction details and material selections should be carefully considered to maximize absorption and dissipation of acoustic noise from MRI equipment. Avoid placing patient and conference rooms directly adjacent to MRI suites. Acceptable noise ratings are typically in the range of 45 dBA and should be confirmed with the owner during the design phase. It is recommended that the design team engage an acoustical consultant to provide sound dampening guidelines and recommendations.
MRI machines produce high magnetic fields that can interfere with other instruments and effect pacemakers. The 5 Gauss line must not project outside the room into public spaces. If it does, thin layers of steel plates located in the floor, walls and ceiling structure are used to redirect the magnetic field and keep it in the room. The location and thickness of the plates is determined from a physicist either employed by the hospital, MRI vendor, or shielding supplier. The weight of the steel plates must be reviewed by the structural engineer and verified that the building structure can support these loads.
Another type of shielding that is required for every MRI suite is Radio Frequency (RF) Shielding. Interfering RF noise that can distort the MRI image comes from a variety of common electronic devices, such as transformers, motors, and computers. RF shielding material can come in the form of many different types of material, the most common type is thin layers of copper attached to plywood sheets. Floors are typically recessed 1.5 inches for the RF shielding, providing a smooth transition at the door. The magnetic and RF shielding is provided by a single shielding supplier/installer and is a delegated design. The shielding engineer must also design the system to support lights, diffusers and ACT grid below the RF shielding. The supplier also will test the completed RF system to insure it works properly.
Conclusion
While this article primarily touched on some of the structural and architectural design considerations of MRI suites, many other design aspects should be considered by your design team, including mechanical, electrical and plumbing systems. The most successful MRI suites result from a knowledgeable, integrated architecture and engineering planning process.