Magnetic Resonance Imaging (MRI)

Contributors to this section include Stefan Novosel, Kate Renner, Andrew Petty, Tobias Gilk and Frank Zilm

Newer 1.5T MRI roomMagnetic Resonance is a radiology technique which maps faint radio frequency emissions from electromagnetically manipulated atomic structures. Magnetic Resonance can be used to produce 2D and 3D images (MRI), and spectrographic data (MRS).

Unlike the largely photographic process of conventional X-ray imaging, MRI does not make use of X-rays (or any penetrating ionizing radiation source). By being placed in a high-strength magnetic field, normally randomly aligned unbound protons align themselves with the polarity of the magnetic field. When knocked off-axis by either a radio frequency pulse or a time-varying magnetic field, the proton releases a faint RF energy signal. Specialized antennae plot the frequency and location of each signal and build an image based on the collected data points.

MR systems require very high-strength, and very precisely balanced, magnetic fields. The three principal types of MR magnets, which may be used individually or in concert, are permanent magnets (larger, more powerful versions of what may be stuck to the outside of your refrigerator door), electrically-resistive magnets, and superconducting magnets. The most common of these is the superconducting magnet.

The most predominant type of MR system in clinical use employ magnets measured at 1.5 Tesla in strength. These magnets are approximately 30,000 times the strength of the Earth's magnetic field as experienced in North America. MR magnet strengths in clinical settings are commonly up to 3.0 Tesla in strength, and research settings may have magnets of 9.0 Tesla or greater.

Magnetism has not been shown to have sustained adverse effects on biological function, but injuries and fatalities have occurred because of magnetic field interactions with both internal ferromagnetic materials (pacemakers, implants, aneurysm clips, shrapnel, etc..) and external ferromagnetic materials (wheelchairs, medical gas cylinders, gurneys, chairs, cleaning equipment, etc...).

Facilities designed for MR systems must simultaneously protect the sensitive MR equipment from interference and prevent unscreened persons and equipment from approaching the magnetic field. Structural systems and other components of the MR magnet room construction should minimize, or eliminate, the use of ferromagnetic (magnetizeable) materials. This includes rebar, structural decking, conduit, ductwork, piping, and steel framing.

Suite layouts should make use of the 4-zone screening and access-control protocols identified in the ACR Guidance Document for Safe MR Practices: 2007 document.

Space and Functional Considerations


Based on a research study* that included 17 facilities throughout the United States with a total of 20 MRI imaging exam rooms,
20 rooms/17 facilities = 1.176 rooms/facility on average

For the patient imaging room alone, without control area:
Average area = 433 square feet
Largest area = 567 square feet
Smallest area = 315 square feet

For the patient imaging room with control area:
Average area = 596 square feet
Largest area = 791 square feet
Smallest area = 416 square feet

Significant space planning support, including programmatic values for patient holding, waiting, screening and administrative areas, can be found in the 2006 update to the Department of Veterans Affairs Space Planning Criteria for MRI (.doc format).

Room design and layout considerations:



GE 1.5 MRI ROOM


The illustration on the right show the GE 1.5 MRI specifications recommend 408 square feet (not including the control room and other support spaces), an 8' 9" ceiling height and a 43" wide, 82" high door frame to accommodate the 1.5T Sigma MRI Excite HDx.

GE 1.5 MRI GE Specifications

Commentary:

A principal flaw with nearly all vendor prototypes is that they do not illustrate the sequence of access restrictions and screening that must occur prior to a patient / staff person / visitor arriving at the room with the MRI scanner. These suite design elements, though clearly spelled out in a number of best-practice documents, are not typically included in MRI equipment vendor siting guides.

One significant safety failing of the model illustrated above is the fact that the MRI technologist is 'sequestered' in the control room and is not provided with the capacity to observe persons approaching / entering the MRI room from the corridor door.




GE 3.0 MRI GE Specifications

GE 3.0 MRI ROOMGE 3.0 MRI


The GE 3.0 MRI specifications recommend 408 square feet (not including the control room and other support spaces), an 8' 9" ceiling height and a 43" wide, 82" high door frame to accommodate the 3.0T Sigma MRI Excite HDx.

Commentary:

This model is based directly on the GE prototype suite layout and may not provide direct line-of-sight visualization between the MRI technologist at the operator's console and the doorway entering into the MRI scanner room.



Technical Design Considerations

When the MRI scanner is viewed as an 'appliance,' needing little more than a room and a plug, MRI design projects are likely to experience significant problems. MRI equipment has very specific environmental requirements which, when not met, can adversely affect the operation and image-quality of the multi-million dollar scanner.

Shielding: Nearly all clinical MRI scanners require radio frequency (RF) shielding around the entire perimeter of the room, including floor and ceiling. Unlike shielding use in almost every other area of radiology, the RF shield is not present to contain the hazards of the MRI scanner. The RF shield keeps environmental radio signals from entering the room and interfering with the scan, but is essentially 'invisible' to the magnetic energy orbiting the MRI scanner. MRI sites may also have passive magnetic shielding, typically in the form of steel plate, lining walls, floor or ceiling. Unlike RF shielding, passive magnetic shielding may be placed on only those surfaces needed to manage the reach of the magnetic field, either because of safety considerations of unscreened persons on the other side, or operational concerns with magnetic field interference with the operation of other equipment or systems.

Vibration: MRI scanners are carefully calibrated to plot the location of RF emissions of atomic structures and are understandably sensitive to vibration. Vibrations from external sources (trains, roadways, construction) may be transmitted through the soils to the MRI building. Additionally, the assortment of pumps, motors, fans and moving equipment within a building may introduce vibration into the very frame of the structure. The structure supporting the MRI scanner may either be designed to be rigid enough to resist these vibrations, or free-floating to prevent structural transmission.

Shim Tolerance: Because of the very carefully tuned magnetic field of the MRI, ferromagnetic materials in construction can 'de-tune' the MRI scanner. Small quantities of ferromagnetic materials in construction can be compensated for within the MRI scanner, itself. Beyond that, ferromagnetic materials in the construction of an MRI suite may impair the capabilities and image quality of the MRI scanner. Because of the proximity of the scanner to the floor structure, in particular, it is most important to reduce (eliminate, if possible) ferromagentic material in the floor, or below. The introduction of unbalanced quantities of ferromagnetic materials, including passive magnetic shielding, in an MRI room floor construction may exceed the allowable shim tolerance for a particular magnet and may require extensive (and expensive) corrective measures.

Electro-Magnetic Interference (EMI): Similar to an MRI's sensitivity to ferromagnetic materials, an MRI scanner is typically very sensitive to EMI. Sources of EM interference can include high-amperage power lines, transformers or switchgear, even moving metal objects in proximity to the MRI scanner.

An effective MRI suite, specifically designed to support the MRI equipment and patient care process, can have a marked positive impact on both the clinical and financial value of that service. Poorly designed MRI suites, by contrast, can impede efficient operations and can dramatically handicap the capabilities of the MRI scanner. Additionally, the design and construction of an MRI suite has a direct and immediate impact on the safety of patients and staff.


Safety Design Considerations

MRI presents safety hazards, both associated with the image acquisition process and with the general physical environment. The principal hazards that the design and construction of the MRI suite can help attenuate include:

Magnetic Field Related Projectiles
Magnetic Field Related Device Interference
Cryogen Exposure

MRI scanners depend on the presence of magnetic fields 10's of thousands of times stronger than that of the Earth's own magnetic field. The same force that rotates a compass needle, or attracts a magnet to your refrigerator door, is multiplied many thousands of times near MRI equipment. Magnetic energy, or flux, orbits the magnet from one pole to another, passing through all non-magnetic materials as if they weren't even there. The radio frequency (RF) shielding that is typically installed as a part of every clinical MRI does virtually nothing to contain the MRI's magnetic field which may, depending on the design of the MRI scanner's magnet, penetrate walls, ceilings and floor of the MRI scanner room. Ferromagnetic metal shielding can be provided in addition to RF shielding, but this is typically a significant siting expense and may add significantly to structural loading due to the weight of ferromagnetic steel shielding. Additionally, steel shielding must be very carefully engineered so as to not disturb the 'tuned' MRI scanner.

The profound force of contemporary MRI scanners has attracted a plethora of ferromagnetic materials to the MRI magnet when brought into the MRI scanner room. A few examples can be seen at the SimplyPhysics.com website. Some MRI scanners may generate magnetic fields that can cause attractive effects even outside the MRI scanner room.

In 2001, a young boy was killed when a portable oxygen cylinder was brought into the MRI scanner room while he was inside the bore. The extreme power of the MRI attracted the cylinder, pulling it from the grasp of the doctor who was carrying it, and it struck the boy in the head. In response to this accident, formalized screening and access control standards have been implemented which should be accommodated in the design of the MRI suite.

The American College of Radiology's Guidance Document for Safe MR Practices: 2007 includes information on the sequential 4-Zone screening and access control model, which has been adapted by the US Department of Veterans Affairs in their MRI Design Guide's Functional Diagram.

The fundamental design safety objective of the ACR and VA documents is to provide layers of screening and sequential access controls to prevent unauthorized persons or materials from approaching the MRI magnet. These steps include clinical screening for medical contraindications, physical screening, such as with a ferromagnetic (only) detection system and patient gowning, and access restrictions requiring direct supervision by trained MR departmental staff.

Because many clinical devices, including implanted pacemakers, nerve stimulators, insulin pumps, hearing aids, and other devices can be disrupted, even disabled, through exposure to the MRI's magnetic fields, the integrated screening process should include a thorough review of the medical history of each person seeking to approach the MRI scanner. Even 'passive' implants, such as aneurysm clips, may react to the intense magnetic field and damage vital structures inside the body.

Most contemporary MRI scanners are enabled by cryogenic liquids, typically liquid helium, to support superconducting electric coils within the scanner. Under certain fault conditions, the liquid helium can boil at which time it undergoes a dramatic physical expansion. Superconducting MRI scanners should be provided with a helium exhaust, or quench, pipe to the outdoors. There have been events in which quench pipes were improperly installed, or allowed the accumulation of precipitation or debris, which blocked the pipe. If this occurs when the liquid helium inside the MRI boils, it can result in tremendous overpressure. This occurred at a hospital in Birmingham, Alabama, where a blockage in the quench pipe led to a catastrophic failure of the MRI's pressure vessel which lifted the roof off of the building.

Suites designed in support of superconducing MRI scanners should all be provided with three fundamental protections against helium exposure / overpressurization.

1. The helium exhaust pipe (quench pipe) should be engineered to the meet or exceed the MRI manufacturer's standards for pressure drop. Irrespective of the manufacturer's standard detail, all through-the-roof helium exhaust pipes should be turned 180-degrees and discharge downward, providing a complete weatherhead to horizontally-driven precipitation.

2. The MRI scanner room should be provided with an overpressure-relief mechanism that would allow significant volumes of air / gas to escape to an unoccupied area. Outswinging doors from the MRI scanner room, though highly recommended as a redundant safety feature, should not be provided as the primary means of overpressure relief.

3. The MRI scanner room should be provided with an exhaust fan that meets or exceeds the MRI manufacturer's guidelines for air flow volume.

A number of MRI safety design resources are currently available for architects, engineers, and facility / equipment planners. These include:

ACR Guidance Document for Safe MR Practices: 2007 (see, particularly, appendix 2)
US Department of Veterans Affairs MRI Design Guide (2008 Edition)
ASHE Monograph Designing and Engineering MRI Safety (2008)





*Study of imaging department research areas conducted by principal investigators,

David Allison, AIA, ACHA, Professor/Director of Architecture+Health at Clemson University
D. Kirk Hamilton, FAIA, FACHA, Adjunct Professor at Texas A&M University
Frank Zilm, D.Arch, FAIA, FACHA, of Frank Zilm & Associates


with the aid of graduate student investigators,

Megan Gerend of Clemson University
John Grant of Texas A&M University
Scott Weinhoff of Clemson University


and sponsored by,

Academy of Architecture for Health Foundation
American College of Healthcare Architects
Frank Zilm & Associates
McKahan Planning Group, Inc.



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