Section 12 Biologic Hazards of Magnetic Resonance Imaging Answers
1. ANSWER: A
FEEDBACK: Actively shielded magnets are designed to decrease the surrounding
fringe field so that siting concerns are reduced. Nevertheless, a strong
magnetic field is present in the magnet room. Ferromagnetic materials (oxygen
tanks, hemostats, scissors, etc.) brought into the magnet room can become
airborne as projectiles in the static magnetic field. Projectiles pose a
serious risk to the patient and others in the magnet room. Patient screening
and staff training are the most effective methods to exclude ferromagnetic
materials from the magnet room. However, new ferromagnetic detection systems
are capable of detecting small ferromagnetic objects external to the patient
and can now differentiate between ferromagnetic and nonferromagnetic materials.
The ACR recommends ferromagnetic detection systems as an adjunct to screening
of patients and devices. The correct
option is A.
2. ANSWER: B
FEEDBACK: Pregnant health care workers have not been shown to be at increased
risk of adverse outcomes from exposure to static magnetic fields. Although
pregnant health care practitioners are permitted to work in the MR environment,
they should not remain within the magnet room during data acquisition/imaging.
The recommendation to exit the MR magnet room during data acquisition/imaging
is not based on reported studies of potential adverse effects but is a
conservative approach based on arecognition that there are insufficient data
regarding the effects of MR electromagnetic fields. Positioning patients,
imaging, archiving, injection of contrast agent, and entering the MR magnet
room in response to an emergency are considered acceptable activities for the
pregnant health care worker. The
correct option is B.
3. ANSWER: A
FEEDBACK: On the basis of the currently available data, there is no association
of cancer or any other deleterious effect with the use of clinical MR imaging
during pregnancy. The ACR has adopted the policy that MR procedures may be
performed during any stage of pregnancy if medically indicated. The use of ultrasound,
the desired information and effect on the management of the patient, and the
potential for delay are appropriate considerations for the selection of a
pregnant patient for MR imaging. The MR procedure is considered appropriate if
the information requested addresses the clinical problem or is necessary to
manage potential complications for the patient and/or fetus. That is, the
results of the MR examination can potentially affect the care of the patient
and/or fetus during the pregnancy. The risk to the fetus from gadolinium-based
contrast agents is unknown and may be harmful. MR contrast agents should not be
routinely administered to pregnant patients. The ACR makes no recommendation
regarding appropriate pulse sequences or imaging parameters. The correct option is A.
4. ANSWER: A-FALSE, B-TRUE, C-TRUE, D-TRUE
FEEDBACK: The vast majority of patients with NSF were receiving therapy with
dialysis or had stage 4 or 5 renal disease at the time of MR imaging with
gadolinium-based contrast agents. The population at risk
can be identified by an additional question regarding a
history of renal disease or dialysis. Prospective hematologic screening is not
warranted. Option A is false.
No special treatment is recommended for patients with stage 1 or 2 chronic kidney
disease (defined as GFR > 90 mL/min/1.73 m2 or GFR between 60 and 89
mL/min/1.73 m2 ). However, gadodiamide (Omniscan™) is not recommended for a
patient with any level of renal disease. Option
B is true. A risk-benefit assessment for that particular patient with
stage 3, 4, or 5 renal disease should indicate a clear benefit for the
administration of a gadolinium-based contrast agent. The default standard dose
for these patients should be one-half the usual dose. A written order from the
radiologist approving the examination is recommended. Informed consent should
also be provided. Under these conditions, option
C is true. If patients with severe to end-stage renal disease receive
gadolinium-based contrast agents, prompt dialysis following MR imaging should
be considered. Option D is true.
5. ANSWER: C
FEEDBACK: All implanted intracranial aneurysm clips composed of titanium and/or
titanium alloy are acceptable for MR imaging without any additional testing.
However, documentation of the composition in writing is necessary. The correct option is C.
6. ANSWER: A, B, C
FEEDBACK: Quenching is the loss of superconductivity of the magnet coils, which
is accompanied by the rapid escape of helium as evidenced by clouds or fog
around the MR imager. Helium will displace oxygen in the room. All magnet rooms
should have helium-venting equipment. The ACR, citing reliability concerns,
does not currently recommend oxygen monitors in the magnet room. An
uncontrolled quench is an emergency situation requiring the rapid removal of
all personnel/patients from the magnet room. Site access must be restricted
until the magnetic field has dissipated. The
correct option is A, B and C.
7. ANSWER: B
FEEDBACK: The need for controlled access is based on the potential risk from
the attraction of objects containing ferromagnetic materials, torque on
metallic materials, and the possible dysfunction of medical implants for
persons with such devices inadvertently entering the area. Control may be
established by barriers or signs based on the manufacturer’s recommendations. The correct option is B.
8. ANSWER: A-TRUE, B-TRUE, C-TRUE, D-TRUE
FEEDBACK: Significant risk criteria were developed by the FDA to include static
magnetic field over 4 T, RF exposure sufficient to produce a core temperature
increase of 1°C, time-varying magnetic fields (dB/dt) sufficient to produce
severe discomfort or painful stimulation, and peak acoustic noise over 14 0 dB.
All options are true.
9. ANSWER: B
FEEDBACK: During MR imaging, the patient absorbs a portion of the transmitted
RF energy, which results in tissue heating. This could cause an elevation of
core body temperature or a skin burn by localized heating. The specific
absorption ratio (SAR) is the dosimetric means by which RF power absorbed per
unit mass is characterized. The FDA has set limits for SARs to limit whole-body
and local temperature rise. The SAR should not exceed 4 W/kg for the whole body
for 15 minutes, 3 W/kg averaged over the head for 10 minutes, 8.0 W/kg in 1 g
of tissue (head or torso) for 15 minutes, and 12 W/kg in 1 g of tissue
(extremities) for 5 minutes. Alternatively, RF heating is considered acceptable
if the core temperature increase is less than 1°C and local heating is no
greater than 38°C in the head, 39°C in the trunk, and 40°C in the extremities.
The SAR is proportional to the number of images acquired per unit of time and
depends on patient dimensions, RF waveform, tip angle, and coil type. Patient
heating also depends on ambient temperature, relative humidity, airflow rate,
blood flow, and patient insulation. The
correct option is B.
10. ANSWER: C
FEEDBACK: Gradient magnetic fields are rapidly applied by passing currents
through the gradient coils. In the presence of the static magnetic field, a
current in the gradient coil creates a force on the coil, which causes the coil
to vibrate. The mountings for coils also flex and vibrate in response to
movement of the gradient coils. The result is acoustic noise in the form of
loud knocking. The correct option is
11. ANSWER: C
FEEDBACK: Rapidly switched gradients allow improved image quality and faster
acquisition. The potential physiologic response includes peripheral nerve
stimulation, muscle movement, and discomfort. The thresholds for cardiac
stimulation and brain stimulation are much greater than for peripheral nerve
stimulation and are unlikely to occur with current MR systems. The correct option is C.
12. ANSWER: E
FEEDBACK: Focal heating and thermal injuries have been induced by time-varying
RF magnetic fields during MR examination. The presence of a conductive loop
(crossed arms, ECG leads or unconnected surface coil leads in contact with
patient’s skin, etc) can produce induced electrical currents, which can cause
thermal injury. Near the inner bore of the magnet, high levels of RF standing
waves may form. The patient must be positioned so that there is no direct
contact between the patient’s skin and the inner bore of the magnet. Absorption
of RF energy by the metallic object creates a heated surface, which can cause
local temperature elevations. All
options are correct.
13. ANSWER: D
FEEDBACK: At magnetic field strengths below 4 T, no deleterious biologic
effects (either acute or chronic) have been identified. At magnetic field
strengths above 4 T, magnetophosphenes and vertigo have been reported. The two
major concerns associated with the static magnetic field are forces exerted on
ferromagnetic objects (either rotational or attractive) and magnetically
sensitive equipment whose function may be disrupted. The correct option is D.
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