1 . LA EVICCIÓN Y LOS VICIOS REDHIBITORIOS
B. LA ESTRUCTURA
3. LA RETICENCIA
Once an MRI system has been selected and the site identified, the facility can be designed.
Many design features are independent of the type of MRI system selected. Others must take into account certain characteristics of the type of MRI system. Because many characteristics
AM
Amateur radio CB radio FM radio
TV
Magnetic field strength (T)
Figure 13-9 Radiofrequency allocated by the Federal Communications Commission in the magnetic resonance imaging radiofrequency band. (Courtesy Stanley Hames, Phoenix, AZ.)
are shared by each type of MRI system, general design criteria can be used.
General Design Criteria
Figure 13-12 is a typical plan layout of an MRI facility as part of an imaging center. A similar layout can be proposed for an existing hospital. Within the immediate imaging area, space must be allocated for the imag-ing room, computer room, control room, viewing room, and cryogen storage area. A minimum of approximately 150 m2 is required. In addition, a reception area, an
office space, and a physics/engineering space are necessary.
Reception. Most MRI facilities are not going to be an integral part of an existing radiology department. Therefore a reception and waiting area common with radiology may not be avail-able. In such situations a separate reception area must be provided.
Many MRI examinations are conducted on an outpatient basis, so the patient is examined in street clothes, eliminating the need for change rooms. However, some sort of security Figure 13-10 A cutaway of a magnetic resonance imaging room shielded to exclude envi-ronmental radiofrequency (RF) by a continuous enclosure of sheet or wire mesh of copper or aluminum. The windows and door are specifically designed to be part of the shield.
Penetrations into the room are designed to attenuate RF. (Courtesy ETS Lindgren.)
area is required so that the patient can remove metal objects and valuables before the exami-nation. Often patients are asked to wear scrubs so the technologist can be confident that their clothes are not conductive or magnetic.
Metal Detection. At the threshold of the examination area, space must be reserved for
metal detection. Most facilities choose to sim-ply instruct the patient to remove all metal.
However, a threshold-type metal surveillance device similar to those used for airport security or a wand-type metal detector may be used.
Metal detection is important for not only the patient before imaging but also others who may enter the facility. Physicians, attendants, and custodial personnel must be instructed about potential hazards from metal projectiles.
Magnets and people can be damaged by pro-jectiles from individuals who may not other-wise be adequately advised.
Office Support. Office space for radiologists, imaging technologists, and medical physicists is required. A darkroom may be needed for processing images. However, most facilities now use self-contained processors or dry process printers.
Patient Access. The position of the entrance door to the imaging room must be considered.
The door should be positioned so that a patient in distress can be quickly and easily removed from the room without undue manipulation.
Such a design feature also permits the easy Figure 13-11 The telescoping Faraday cage has
been satisfactorily used by several manufacturers.
(Courtesy Philips Medical Systems.)
Iron shielding
Equipment room Office
Examination room Control room
Dark-room Reading area
Office
Cryogen storage room Hall
Laboratory Iron shielding
Figure 13-12 A representative plan layout for a magnetic resonance imaging facility show-ing the required support areas.
access of a crash cart, whose implements must be confirmed nonmagnetic. The door should also be positioned so that access cannot be gained without knowledge of the staff.
Power Requirements. Regardless of the type of MRI system selected, approximately 10 kW of power is required for computers, operating consoles, and other electronic devices. An additional 10 to 20 kW is required to power the shim coils, gradient coils, and RF network.
Beyond that, a superconducting MRI system requires an additional 20 to 30 kW but only while the imaging system is being brought up to the design B0 field strength. During opera-tion, no power is required for the primary mag-net coils.
Construction Materials and Techniques Because the presence of external ferromagnetic material can degrade the homogeneity of the B0magnetic field, construction materials must be selected carefully. Large existing metal objects such as cast-iron waste water lines and electrical machinery may have to be moved. In general, the site should be metal free and vibration stable. This requires special material and construction techniques.
It is particularly important that the RF shielding of the imaging room remain at least 90-dB attenuation. This presents special problems for all penetrations through the RF shield.
All wires and cables for power or data must be fitted with appropriate RF filters. Heating, ventilation, and air conditioning (HVAC) ducts must be of nonconducting material, such as polyvinyl chloride (PVC), and maintain length-to-diameter ratio in all sections to provide an RF waveguide of infinite impedance.
Foundation. The weight of most imaging sys-tems requires a substantial concrete pad with reinforcing. Instead of iron reinforcing rods and corrugated iron sheets, some of the avail-able fiberglass-impregnated reinforcing rods and epoxy concrete should be used.
A sufficient structural foundation is required not only in the imaging room but also along the route for installation. Posttension or other techniques may be necessary to ensure that the foundation is vibration free. Even sub-tle vibration can encourage cryogen to boil off and degrade image quality.
Normal construction techniques for walls and ceilings are generally acceptable. Unreinforced concrete or wood stud construction with stan-dard nails is acceptable.
Electrical Service. Electrical conduits in the MRI room should be made of either PVC or aluminum. Electrical receptacles and fixtures should be aluminum or ceramic. Electrical dis-tribution transformers should not be located within the 1-mT fringe magnetic field.
Lighting in the imaging room must be incandescent; no fluorescent lamps are allowed. The supply should be direct current or properly filtered. Dimmer controls should not be mounted within the room. Fixtures should be brass or ceramic.
Plumbing. Supply lines, floor drains, and soil pipes should be nonferrous. Copper or PVC is acceptable. If building codes require a sprinkler system, only brass or copper compo-nents should be used. All sprinkler heads that penetrate the RF shielding must be completely electrically grounded.
Patient Viewing. The ability to view the patient during the examination is mandatory.
Although closed-circuit television capable of operating in the magnetic field of the room has been developed, it is expensive and not totally satisfactory. Most facilities find that a direct-view window incorporating a wire mesh as an RF shield is better.
HVAC. Heating, ventilation, and air condi-tioning are important engineering considera-tions for an MRI site. The HVAC design must deal with not only the normal space-occupying activities of a conventional office or laboratory
but also the special requirements of the MRI system.
Constant temperature is essential for the stability of the magnet and associated elec-tronic components. The B0 field of a perma-nent magnet increases approximately 0.1% per degree. A 10˚ drift could cause an electronic frequency mismatch that would destroy the tuned response at the Larmor frequency.
Any computer that accompanies MRI must be in a cool, dry environment. Temperature must be maintained between 18˚ C and 20˚ C at a relative humidity of not more than 40%.
Magnet Cooling. Permanent magnet imaging systems have no special cooling requirements beyond those normally needed for electronic and computer components. This feature con-tributes to the relatively low capital cost and site preparation requirements.
A superconducting magnet requires cryo-gens (liquid helium and sometimes liquid nitrogen). Up to 0.5 l/hr of helium and 2 l/hr of liquid nitrogen may be required to maintain the low temperature to support superconduc-tivity. Superconducting magnets require alu-minum venting, usually through the ceiling, for cryogen exhaust. It is desirable to have the liquid nitrogen piped in from a storage tank.
Additional Features. In addition to the pre-ceding design features that are appropriate for any MRI facility, there are special considera-tions attendant to each type of imaging system concerning the design of the facility.
Permanent magnets are small but heavy. A 0.3-T whole-body imaging system can weigh
40,000 kg. Such a mass will probably preempt its placement anywhere but on the ground level. Smaller head and neck imaging systems weigh no more than 5000 kg and can be located on any level.
Superconducting magnet imaging systems require cryogenic support. Loading, handling, and storage space for cryogens must also be provided. The loading dock for cryogen dewars should be outside the 1.0-mT isomagnetic line and easily accessible to the magnet room.
CHALLENGE QUESTIONS
1. What is the main consideration given to the proper location of a permanent magnet MRI system?
2. What is the recommended isotesla exclu-sion line for cryogenic dewars?
3. What is the principal advantage to siting a permanent magnet MRI system?
4. When constructing the facility to house an MRI system, what are the principal con-siderations to heating, ventilation, electri-cal, and plumbing installations?
5. What are the principal advantages and the principal disadvantage to a supercon-ducting MRI system?
6. Why is metal detection essential in an MRI suite?
7. What are the two principal concerns to be considered for siting an MRI system?
8. What is a Faraday cage?
9. What is the difference between a passive shield and an active shield?
10. What is the range of the RF band accord-ing to the FCC?