Limitations and Hazards of MRI
MRI scanning has a number of limitations. Imaging is time consuming,
and individual scans may take up to 20 minutes, with an entire examination lasting
more than 1 hour. Any patient movement, even that resulting from physiologic motion
(e.g., cardiac and vascular flow pulsations, cerebrospinal fluid flow and pulsation,
respiratory excursion, and peristalsis in the gastrointestinal tract),
can produce artifacts on the image. To optimize information when imaging the cardiovascular
system, the signal acquisition is synchronized or "gated" with phases of the cardiac
cycle (electrocardiographic [ECG] R wave) to virtually freeze cardiac motion. The
magnet generating the large static magnetic field has a small-diameter hollow bore,
typically 50 to 65 cm in diameter. Obese patients cannot be examined in this small
magnetic bore, and patients otherwise suitable for examination may not tolerate long
periods inside the confines of the magnet. Switching on and off of the RF generators
produces loud noises (>90 dB). Hearing protection is mandatory for both the patient
and health cre personnel who must be present in the scanning room. Heating resulting
from the RF energy of nonferromagnetic prosthetic devices has not proved to be a
problem.[41]
Body surfaces do absorb this RF energy,
but it is unlikely that the patient's temperature will increase by more than 1°C.
Direct adverse biologic effects of the MRI magnetic field are
not believed to exist. Nonetheless, a number of serious hazards are attributable
to the intense magnetic fields necessary for MRI. Although heating of the whole
patient is not generally a concern, potential exists for heat generation within monitoring
wires because of an "antenna effect," especially ECG lead/electrode systems. Thermal
injury in the area of ECG electrodes has been reported.[42]
Prevention of such injury may be accomplished by not monitoring the ECG, placing
ice packs on the ECG electrode sites, or repositioning the ECG leads after each scanning
sequence to limit local thermal load on the skin of the patient.
The most significant risk posed in the MRI suite is the effect
of the magnet on ferrous objects.[36]
[40]
[41]
[42]
Dislodgement
and malfunction of implanted biologic devices or other objects containing ferromagnetic
material are also real possibilities. Such items include shrapnel, vascular clips
and shunts, wire spiral endotracheal tubes, pacemakers, automatic implantable cardioverter-defibrillators
(ICDs), mechanical heart valves, and implanted biologic pumps. Potential problems
with cardiac pacemakers and ICDs include lead heating by induced current, inhibition
of pacemaker output of rapid pacing, reed switch malfunction, ICD malfunction, or
torque on the pacemaker pack.[43]
[44]
Patients with such devices should not undergo MRI examinations. Tattoo ink may
contain high concentrations of iron oxide. Burns at tattoo sites have been reported
after exposure to MRI magnetic fields, but such incidents are very rare and the presence
of, for example, permanent eyeliner should not exclude the patient from MRI examination.
[45]
[46]
Intraocular
ferromagnetic foreign bodies may migrate within the MRI magnetic field, and MRI examination
of patients with suspected ferromagnetic ocular foreign bodies should not be undertaken.
[47]
Death from torque of a vascular clip within the MRI magnetic field
has been reported.[48]
Manufacturers of some implantable
devices, such as the clips used in vascular surgery and neurosurgery, are now trying
to use alloys that have low ferromagnetic properties and are safe in the MRI suite.
Clips with low ferromagnetic properties are safe in MRI systems with magnetic fields
of 1.5 T or less. MRI systems of up to 3 T, which are increasingly being used, mandate
that the clips be nonferrous (e.g., titanium) to avoid clip movement within the scanning
magnet.[49]
Before MRI examination, the radiologist
and the surgeon who placed the vascular clip must confer, and a positive determination
of the MRI safety of the particular clip implanted in the patient must be made.
In the absence of positive knowledge of the exact composition of the clip in question,
MRI examination should not be performed.
Though potentially significant, relatively small movements of
implanted ferromagnetic devices and local damage caused by heat generation within
lead systems are overshadowed in the popular and medical literature by the large,
rapid movements possible when iron-containing items are brought into the vicinity
of the MRI magnetic field. Typical objects include scissors, pens, keys, and gas
cylinders. When such objects are brought into the vicinity of the MRI scanner, the
intense magnetic field may attract them into the scanner bore at high velocity.
The result when a patient is present in the scanner bore may be fatal.[50]
[51]
Even in the absence of a patient, health personnel,
maintenance workers, or visitors to the MRI suite might be injured by objects flying
at high velocity toward the scanner.[51]
For this
reason, ferromagnetic items must never be allowed in the vicinity of the MRI magnet.
Because of the numerous hazards just discussed, some of which
may entail severe consequences for both the patient and hospital personnel in the
MRI suite, recommendations regarding MRI safety have been developed by the American
College of Radiology.[42]
To summarize, recommendations
include the development of formal policies and procedures regarding MRI, which should
be present at every MRI site. These policies should be under the direction of formally
appointed competent personnel, and review mechanisms should be in place to ensure
quality care. Importantly, the recommended policies include site access restriction
within the MRI suite. The MRI suite should be divided into four zones. Zone I is
the public zone with free access. Zone II is the interface between the public zone
and the MRI suite, where all movement by non-MRI personnel is supervised by MRI personnel.
Zone III is the area within which the introduction of ferromagnetic objects may
form a hazard. Zone II should be separated from Zone III by physical restriction
(locked doors). MRI personnel are responsible to control all
movement into zone III by any personnel, including
patients, the public, health care workers, maintenance workers, hospital administration,
security, and fire department personnel. Note that this restriction specifically
includes non-MRI-trained physicians. Zone IV is the scanner room itself, which should
be further separated from zone III and clearly marked with lighted signs. Formal
training programs are to be put in place for MRI personnel. No one is to be considered
"MRI personnel" until they have completed the formal training program. MRI personnel
will screen all non-MRI personnel who request access
to zone III and beyond and supervise their activities. It is important that anesthesiologists
caring for patients understand both the restrictions in place on access to the MRI
suite and the reasons for those restrictions. Anesthesia personnel who may be called
on to provide patient care in the MRI suite should complete the formal training program
to become recognized MRI personnel. At the Mayo Clinic, these guidelines have been
adopted, and
web-based training and testing programs have been put in place to qualify personnel
for MRI activities.
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