By Joel Kellogg, ETS-Lindgren and Dave Jordan, West Physics

Designing space for medical imaging equipment can be quite complex and involved as there are many items that must be addressed in order to successfully develop a site. Good site planning must thoroughly evaluate both the impact of the imaging equipment on the surroundings (environmental concerns) and the impact of the space itself on the performance of the imaging equipment and the personnel using the equipment (performance concerns). Depending upon the equipment, concerns may include radiation, magnetic and/or radio frequency (RF) shielding, electromagnetic interference (EMI), vibration, and acoustic requirements. There may also be concerns over co-siting medical imaging equipment as one piece of equipment could have a negative impact on another piece of equipment. As a result, it is critical to develop a site plan and workflow process well in advance that is functional for users, patients, and the planned equipment.

Environmental Concerns
Shielding (Magnetic, Radio Frequency and Radiation) and Acoustics
Magnetic Shielding

Shielding is critical to the proper development of a site for imaging equipment. Some imaging equipment will require radiation shielding and other equipment, mainly Magnetic Resonance Imaging (MRI) systems, will require Radio Frequency (RF) and magnetic shielding. Careful planning that accounts for workflow and surrounding areas can also help reduce the level of shielding required representing a cost savings to the owner while providing a functional, efficient work environment.

When planning for MRI systems, there are some simple things that can be done to avoid excessive magnetic shielding costs. While the majority of MRI systems require an RF shield, the magnetic shielding requirements are driven by specific site selection. For example, in large imaging suites with multiple MRI systems, placing MRI systems side-by-side with limited spacing between MRI suites will drive the requirements for magnetic shielding. In some cases, this may also require expensive modifications to the equipment itself. While many sites will only be concerned with meeting FDA recommendations of 5-Gauss containment in public areas surrounding MRI suites, placing MRI systems next to each other can create a concern for crosstalk. Crosstalk results in MRI systems having an interactive impact on each other. Figure 1 shows crosstalk concerns which result in increased magnetic shielding requirements. To avoid crosstalk in a situation where MRI systems are placed in close proximity, the magnetic shielding requirements may change. For example, a magnet vendor may require that the 3 Gauss fringe fields do not intersect, which means that the magnetic shielding will need to be designed to contain 3 Gauss rather than 5 Gauss. This will result in a heavier magnetic shield and increased shielding costs. A simple solution may be to place the MRI equipment rooms, which house the MRI's electronic systems, between adjacent MRI systems. This may provide enough spacing so that the 3 Gauss lines do not intersect; however, even if the 3 Gauss lines intersect, the amount of magnetic shielding required to separate the 3 Gauss lines will cost much less than having the MRI systems located side by side.




It is also important to consider the areas surrounding imaging equipment that will need radiation and MRI shielding. There are many pieces of equipment that could be adversely impacted by high static magnetic fields similar to those generated by an MRI system. Ultrasound equipment, computerized tomography (CT), cathode ray tube (CRT) monitors, linear accelerators, and electron microscopes are just a few examples of equipment that can be negatively impacted by the one Gauss fringe field. Magnetic shielding costs can be reduced by placing




equipment at distances that are outside the maximum allowable static magnetic field as required by the OEM specifications. Figure 2 shows how providing separation between adjacent MRI systems will reduce the potential for crosstalk and decrease the amount of magnetic shielding required. It is possible to place equipment such as ultrasounds and CTs next to, above, or below an MRI system, but it should be understood that such a placement may change magnetic shielding requirements and increase shielding costs. However, there may be workflow reasons that make it advantageous to place a piece of equipment near an MRI.

Radio Frequency Shielding

Unlike magnetic shielding, Radio Frequency (RF) shielding is required for the majority of MRI applications and consists of a highly conductive material such as copper, aluminum, or galvanized steel surrounding the MRI system. Determining RF shielding requirements is fairly simple due to the fact that all RF shields consist of a six sided structure and the level of attenuation or shielding effectiveness is determined by the field strength of the MRI system to be installed. For example, MRI systems with field strengths of 1.5T or less require an RF shield that provides 100 dB of attenuation at 100 MHz while 3.0T MRI systems require a shielding system that provides a 100 dB of attenuation at 150 MHz. As the field strength of the MRI systems increase, the performance requirements of the RF shielding will typically increase as well.




Special consideration must be given to all items that penetrate the exterior of the RF shield into the MRI suite. Specialty shielded penetrations must be provided to maintain the RF shielding integrity. For example, HVAC must pass through HVAC wave guides and plumbing for fire sprinklers, if required, must also pass through a wave guide. Wave guides are designed to specifically limit the frequencies that could pass through the shielding. Wave guides designed for a 100 MHz RF enclosure, for example, will not allow frequencies below 100 MHz to pass through the shielding as some of those frequencies could interfere with the MRI system. Additionally, all electrical items including power for outlets, lighting and specific sensors required for an MRI installation, must pass through electrical filters. Most electrical filters are low pass filters that only allow signals below a "cutoff" frequency to pass through. For example, a filter with a 1 MHz cutoff will only allow signals at frequencies below 1 MHz to pass though the filter into the MRI suite via the wiring of the filter.

Additionally, special attention must be given to the ground isolation of the room. This is a critical, but often overlooked, consideration when site planning. The RF enclosure, when first constructed, should achieve a minimum of 1000 ohms of ground isolation. This is intended to prevent ground loops and other issues, including less than optimal MRI images, which will occur due to poor grounding of an enclosure.

In addition to specialty shielded penetrations, other important aspects of the shielding that should be considered are the doors and windows. The doors and windows are items that the MRI end users will see and use every day. Therefore, functionality should be considered a top priority when designing and specifying products for a site. Typical MRI doors, as an example, use friction to create the RF seal. This works well for creating the RF seal at the door, but can be quite difficult to operate as these doors require a reasonable amount of force to open and close. This raises other concerns and issues. These doors require maintenance and repair of the door's RF fingers to maintain performance. Without proper maintenance, the door can become unreliable. There are doors available that provide mechanical seals to create the RF seal at the door with simple "push button" entry and egress operation. These doors have the look and feel of a standard door adding to user comfort while the ease of operation expedites patient throughput. It is important to keep in mind windows and doors will often have limited acoustic performance. Careful attention should be used when specifying doors and windows if acoustics are a concern.

Another significant issue to consider is the floor. All RF shielding systems have a floor that is between 5/8" and 1-1/2" thick. In some applications, depending upon the vibration and acoustic requirements, the floors may be thicker than 1-1/2". This can create an Americans with Disabilities Act (ADA) issue requiring a ramp into the room; this may be inconvenient and create logistical issues for moving patients. Therefore, when designing a new space, a floor depression for the MRI suite should be included. This will allow the RF shielding vendor to install the RF shielding system with a flat threshold or a threshold that meets ADA requirements, eliminating the need for a ramp into the room. For existing buildings, concrete should be removed to depress the slab.

Radiation Shielding

Imaging procedures that use ionizing radiation pose a health risk to the clinical staff and patients as well as to members of the public in spaces surrounding the imaging suite. Unlike the patient, who derives a medical benefit from the radiation used in the procedure, these individuals must be carefully protected from exposure. Medical radiation comes from two types of sources: X-ray tubes, such as in CT scanners, radiographic rooms, and fluoroscopy suites, and radioactive materials, which are used in procedures such as nuclear medicine, single-photon emission computed tomography (SPECT), and positron emission tomography (PET). There are also hybrid imaging systems such as PET/CT and SPECT/CT which utilize both a radioactive source and an X-ray source.




All sources of ionizing radiation are generally shielded the same way - with layers of lead sheeting applied to the existing structural barriers. Other materials may be used, such as concrete, steel, or gypsum wallboard. A radiation physicist, such as a medical physicist or health physicist, should be consulted to determine these shielding requirements; in many states this consultation is required by law. There are some important differences in the design approach to facilities using X-ray systems and those using radioactive materials. Given the additional cost and weight associated with lead-shielded building systems, facility designers should take prudent steps to reduce the amount of lead needed.




For X-ray imaging systems, the effective radiation source is the machine itself. Figure 3 shows a typical layout while Figure 4 shows a better, more effective layout. Lead shielding requirements can be mitigated by placing equipment in larger rooms, effectively increasing the distance between the source of radiation and other occupied spaces. Care should be taken when locating the wall or chest bucky (the device used to hold film cassettes for chest X-rays taken in a standing position) in a radiographic room, since X-rays will strike this wall directly. In addition to increasing the distance, rooms adjacent to X-ray imaging systems of all kinds should be chosen for low-occupancy uses - storage rooms, janitor's closets, restrooms, and outdoor areas are all good choices to situate next to X-ray and CT rooms. Areas that have high occupancy, such as offices, nurses' stations, and so forth will require more shielding if placed in close proximity to an X-ray source

In imaging operations using radioactive material, the patient actually becomes the radiation source once injected with the radiopharmaceutical. This requires a different approach, as the source of radiation moves, occupies, and exposes different areas of the human body. For SPECT and general nuclear medicine imaging, this is less of a concern because the emitted radiation is not very penetrating. However, for PET and PET/CT suites, the site planner should consult closely with the radiation physicist at the layout stage. For these suites, the PET/CT scan room is typically not the most critical shielding design element. Rather, the "uptake" or "quiet" rooms, where the patients wait for 30-60 minutes between the radiopharmaceutical injection and the scanning procedure, are the major shielding concern. In a facility not laid out with this in mind, the uptake room walls can require several inches of lead thickness to provide adequate shielding to an adjacent office or waiting room. Of course, for a PET/CT suite, the scan room must be analyzed and shielded as a CT scanner room in addition to the shielding required due to the PET radioactive material.

In either case, care should be taken to configure the imaging suite so that the imaging technologist will have full control to prevent members of the public from inadvertently coming into contact with sources of radiation. This can mean simply configuring the direction of a door swing so that a technologist will have an unobstructed view of a doorway when seated at his or her workstation, or a more difficult approach in providing for proper security of areas where radioactive materials are stored.

Acoustics

Acoustics is an important issue that should be investigated when designing a site for medical imaging systems. Unlike EMI and vibration concerns, acoustic issues are often created by the imaging equipment itself. In particular, MRI systems can be quite disruptive to the surrounding environment. MRI systems can create airborne noise, which is the propagation of acoustic noise through the air, and structure-borne noise, which is the propagation of acoustic noise through the building structure. As a result, acoustic solutions typically need to address both airborne and structure-borne noise in order to be effective. Typical airborne acoustic solutions involve detailed wall, ceiling and floor construction to meet predetermined acoustic criteria. The solution should also detail how penetrations, HVAC ducts, and gaps in construction around the imaging suite should be treated. Recommended wall and ceiling construction usually involves some combination of gypsum board, stud placement, sound batt insulation, air gaps, and isolation clips to address both the transmission and reflection of acoustic noise.




Structure-borne acoustic solutions can be more complicated. Typically, a structure-borne solution involves some combination of weight and isolation material. For example, with proper site planning, a vibration slab that is isolated from the surrounding structure and is placed on spring isolators could also be a cost-effective solution for structure-borne acoustic noise.

Performance Concerns
Electromagnetic Interference (EMI)

Medical imaging systems can be sensitive to electromagnetic interference (EMI). EMI may be caused by electrical equipment, motors, moving metal objects such as cars, trucks and elevators, and by transportation systems that run on electrical power such as subways and trains. Proper site layout based upon an awareness of the impact this existing equipment may have on the medical imaging system can avoid many of these issues. For example, avoiding the placement of EMI sensitive imaging equipment near electrical rooms, large transformers and motors, parking garages, roadways, and elevators can aid in the prevention of potential EMI issues (as well as the significant vibrations associated with many of these elements).

Unfortunately, EMI issues cannot always be avoided especially in large urban areas where an owner may have limited placement options. This does not mean that imaging equipment cannot be installed in an EMI environment that exceeds manufacturer specifications. In fact, there are solutions to EMI issues. EMI shielding consultants are available that can survey existing buildings to quantify the EMI environment to determine if a facility meets the equipment EMI criteria. If the building is under construction, a consultant can approximate the source(s) of EMI based upon the electrical layouts and the proximity of EMI sensitive equipment to items such as moving metal found in elevators or subways or electrical sources. Ideally, addressing performance considerations prior to or during the construction period results in the most cost effective solution for optimal performance of the medical imaging equipment.

EMI issues also may be endemic to the facility itself in the case of a retrofit of an MRI suite. MRI scanners can permanently magnetize steel and other ferromagnetic components within the building structure, and the resulting magnetic field can cause EMI within the room after the MRI system is removed. Steel shielding from the old MRI system, steel beams, and corrugated metal deck and rebar inside concrete floors and ceilings are common culprits when this type of interference exists.

In the event that a site does not or will not meet the manufacturer's requirements for EMI, a shielding consultant could propose shielding options that will reduce the EMI in the area around EMI sensitive equipment. These solutions typically come in two forms; passive shielding and active shielding.

Passive shielding involves using materials with magnetic shielding properties. Common materials that are used for passive shielding are aluminum, silicon steel, and low carbon steel. Passive shielding can be very effective in resolving 60 Hz and higher frequency EMI issues, but has limited effectiveness in resolving lower frequency disturbances that can be created by moving metal objects such as subways and trains that operate on DC electric power. Passive shielding also requires no maintenance after installation and can often be the most cost effective solution for higher frequency (frequencies of 50 Hz and greater) EMI issues. However, this solution is dependent upon the level of the EMI and the amount of material required to resolve the issue.

Active shielding, on the other hand, utilizes electronics and coil systems to create a cancelling magnetic field over a predetermined volume. Active shielding, unlike passive shielding, can be very effective at lower frequencies (frequencies below 100 Hz). There are several benefits associated with active shielding solutions. First, an active system has the ability to respond to a changing environment. Therefore, if the EMI environment becomes worse, a well designed active system will be able to respond to those changes and maintain an EMI environment that meets manufacturer specifications for EMI sensitive equipment. Second, an active system requires considerably less construction. An active system typically requires the installation of coils unlike passive shielding, which often requires the installation of a six sided structure that requires interior finishes. Additionally, since passive shielding is a much less effective solution for lower frequency applications, active shielding is often a more cost effective solution to EMI issues generated by subways and moving metal. In the event that a particular site does or will experience EMI issues, solutions should be evaluated based upon the effectiveness of the solution and the cost to implement the solution.

Vibration

ETS-Lindgren uses isolators under the magnet to perform vibration surveys
Imaging equipment, particularly CT and MRI scanners, can be impacted by building vibrations. Fortunately, most vibration concerns can be addressed through careful design and construction of the area to house sensitive pieces of equipment. In existing buildings, it is critical to perform a vibration survey. A survey will quickly determine whether the existing structure meets the vibration requirements of the equipment being sited. If the equipment does not meet the vibration criteria, a good survey will analyze the cause of the vibration and provide general recommendations to resolve vibration issues. Many vibration issues can be resolved through inexpensive solutions. For example, a nearby mechanical room may not include isolation pads or isolators for mechanical equipment that induce vibrations into the structure. This may simply require placing vibration isolators under such equipment. It may also be possible to use vibration isolators on some imaging equipment such as MRIs, which would allow for the attenuation of vibrations and decoupling of the equipment from the structure due to the benefits of the isolators. However, in some situations, it may be necessary to stiffen the structure under the imaging equipment or build an isolated slab to meet the equipment's vibration criteria. The unfortunate aspect is that isolated slabs and modifications to the structure can be quite expensive. However, such a building retrofit is much less costly to perform during the process of outfitting a space and installing imaging equipment. Performing a retrofit after imaging equipment has been installed and found to be adversely affected by vibrations can be extremely expensive.




In the case of new construction, a vibration consultant can aid in the development of the site by reviewing equipment specifications and site plans to determine the vibration response of the structure due to vibration sources such as mechanical, other imaging equipment, and traffic in the building. Further, a vibration consultant can aid in the development of isolated slabs as necessary, which is much less expensive to implement at the time of construction and can have the added benefit of providing structure-borne sound attenuation due to high noise levels that are produced by some pieces of imaging equipment, in particular MRI systems.

Summary
Informed site planning when designing space for medical imaging equipment, especially prior to construction, results in a cost effective and successful site - one that is completed on time and on budget. An experienced shielding design team with an experienced acoustic and/or vibration consultant will aid the user in this often complex process. The upfront investment in an experienced consultant for site planning will result in construction and operational savings that more than return the initial investment. To protect your investment, ask any consultant or firm you may hire for customer references and a list of recently completed projects. Follow up on the references and visit the project if prossible to assess the capability of the consultant and value-added services provided to the customer.

Acknowledgement

The authors would like to acknowledge and thank Rick Ottolino, AIA, of Ottolino Winters Huebner for his invaluable contributions to and review of this article.

About the Authors

Joel Kellogg is Manager of Technical Engineering and Consulting with ETS-Lindgren in Glendale Height, IL. His experience includes design of MRI suites to meet various acoustic, vibration, EMI, and radio frequency shielding requirements. He may be reached at 630-307-7200 or joel.kellogg@ets-lindgren.com

Dave Jordan is Senior Medical Physicist with West Physics Consulting in Atlanta, GA. His experience includes acceptance, accreditation, and safety evaluations of medical imaging systems and design of radiation shielding. He may be reached at (770) 435-9186 or dave@westphysics.com