• Section 12 Biologic Hazards of Magnetic Resonance Imaging Answers and Feedback

  • Section 12  Biologic Hazards of Magnetic Resonance Imaging Answers and Feedback 

    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. 

    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. 

    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 C. 

    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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