MRI Safety

MRI Safety Requirements

  • Designed to meet annual MRI safety requirements by the Joint Commission.
  • Upon completion of this module, one should be able to:
    • List screening criteria for:
      • Ferromagnetic materials
      • Medical implants
      • Medical equipment
      • Patients at risk for nephrogenic systemic fibrosis (NSF)
    • Demonstrate proper patient positioning to avoid thermal burns.
    • Explain emergency procedures for patients and equipment.
    • Describe methods to protect patients from acoustic damage.
    • Explain how to manage claustrophobic and anxious patients in the MRI environment.

Introduction to MRI

  • Mid-1977: First human scan using magnetic resonance (MR) imaging.
  • Early 1980s: MR scanners installed across the country.
  • MRI is now a common term, but safety precautions are crucial.
  • Superconducting and permanent magnets in open MR scanners cannot be powered off, even if the electrical supply is shut down.

Main Safety Considerations

  • Force and torque of the magnetic field on ferromagnetic materials.
  • Potential for injury from metallic objects being pulled into the magnet.
  • Possible biological effects from magnetic fields and radio frequency pulses.
  • Potential injuries in the event of a superconducting magnet quench.

Fringe Field

  • Invisible magnetic force extending outside the magnet.
  • MR technologists must understand the strength and reach of the fringe field.
  • Magnetic fringe field extends 360 degrees around the scanner, including floors above and below.
  • Factors affecting the fringe field:
    • Main magnet's field strength
    • Amount of shielding inside the MR scanner

Translational Force

  • Primary safety concern involving the static magnetic field.
  • Ferromagnetic objects can be pulled into the scanner bore.
  • Objects accelerate to high speed, becoming dangerous projectiles.
  • Example: Incident in July 2001 at Westchester Medical Center where an oxygen tank became a projectile, resulting in fatality.

Torque

  • Certain implanted devices are subject to torque (twisting force) from the magnetic field.
  • Examples: Ferromagnetic aneurysm clips, implanted devices labeled MR unsafe or MR conditional.
  • Torque can cause serious complications.
  • If a patient with an implanted device is in the MR scanner room:
    • Back the patient out of the room without turning to minimize torque.
    • If a scan has started, move the table and patient slowly out of the room.
    • Follow facility policies for evaluating the patient and notifying healthcare providers.

Screening

  • Best protection method: Thorough screening of patients and others entering the MR suite.
  • Use a ferromagnetic detection system or handheld magnet (minimum 1,000 gauss) to test equipment and objects taken into the scanning room.
  • Testing should supplement, not replace, screening forms and verbal screening.

Safety Zones

  • American College of Radiology (ACR) has established four safety zones.
  • Zones are determined by the increasing safety risk posed by the magnetic field.
    • Zone 1: Safe for the general public, fringe fields lower than 5 gauss (0.5 millitesla).
      • The 5 gauss line is the border where electromagnetic devices like pacemakers start to be affected.
    • Zone 2: Safe for the general public, area where MR patient safety screening occurs.
    • Zone 3: Stronger magnetic fields require screening by an MR technologist.
      • Includes hospital or clinical personnel not working in the MR department, along with all patients.
    • Zone 4: Area of the strongest magnetic field, most dangerous zone.
      • Ferromagnetic objects are not permitted due to torque and projectile risks.

Radiation Exposure

  • RF waves are non-ionizing radiation.
  • MR imaging is considered safer than radiography and computed tomography (CT), which use higher-end radiation.

RF Energy Deposition

  • RF waves deposit energy into the patient, increasing tissue temperature.
  • Factors affecting energy deposition:
    • Patient's body weight
    • Imaging pulse sequence
    • Repetition time
    • Number of slices
    • Amount of tissue scanned
    • Type of MR imaging coil
  • More RF pulses lead to higher energy deposition and temperature rise.
  • The body attempts to reduce temperature through perspiration.
  • MR scanners have fans and internal temperature gauges.
  • If temperature exceeds threshold limits (based on patient weight and specific absorption rate), the equipment stops scanning.

RF Field and Tissue Heating

  • RF coil produces an oscillating electromagnetic field.
  • RF pulses excite tissues to produce a signal for image conversion.
  • Metallic implants, devices, and objects can overheat when exposed to the RF field.
  • Sudden temperature increases can cause complications for patients with poor thermal regulation (cardiac conditions, high blood pressure, diabetes).
  • Thermal skin burns can occur if the patient's skin contacts the coil, bore sides, or cords.

Preventing Thermal Burns

  • Use manufacturer-approved MR padding.
  • Remind patients to keep arms by their sides and not cross arms or feet.
  • Wires should not touch the patient's anatomy because of the potential for thermal burns.
  • Ensure wires are not looped, as this can create an arc.
  • MR safe ECG leads are made of silver and silver chloride.
  • Check expiration dates of leads.
  • Plug all MR safe imaging coils into the appropriate slot.
  • Never send an unplugged coil into the bore.

Implanted Medical Devices

  • Can cause permanent injury or serious complications if exposed to static, RF, or gradient magnetic fields.
  • Know device name and model number to determine safety at the facility's magnet strength.
  • Devices that can cause excessive heating:
    • Cardiac pacemakers and implantable cardiac defibrillators (ICDs)
    • Neurostimulation systems
    • Halo vests and cervical fixation devices
    • Transdermal medication patches
    • Damaged ECG leads
    • Pulse oximeters
    • Jewelry, body piercings, tattoos, and permanent makeup should be screened for.

Burns and Metallic Objects

  • RF waves can heat metallic objects, even nonferrous materials.
  • Body piercing jewelry is often stainless steel and can heat up.
  • Even 14-karat gold jewelry can overheat.
  • Permanent makeup and tattoo ink may contain metallic flakes that heat up.
  • Warn patients about potential skin irritation and burning sensations.
  • Patients having orbital scans should remove all eye makeup.
  • Instruct patients to report warming, burning, or pulling sensations immediately.

Gradients

  • Used to change the static magnetic field to produce parameters for various MR examinations.
  • MR scanner has three pairs of gradient coils.
  • Rapidly changing gradient fields can produce:
    • Acoustic noise
    • Peripheral nerve stimulation
    • Induction of electric current into nearby conductors (e.g., coils) in contact with the patient's skin, potentially resulting in burns.

Acoustic Noise

  • Loud noises and vibrations created when gradient coils are turned on and off.
  • The sequence and duration of the sounds depend on the specific pulse sequence.
  • The FDA has established specific guidelines on acceptable noise levels for MR scanning.
  • Provide patients with hearing protection, such as earplugs or headphones (at least 29 decibels of attenuation).
  • Acoustic noise can cause:
    • Lowered patient tolerance
    • Fright in individuals, especially children
    • Headaches
  • Tell patients what to expect during the examination.
  • Encourage patients to use the call device if they experience discomfort.

Emergency Procedures

  • MR facilities should have policies and procedures and conduct regular emergency drills.
  • During a patient emergency:
    • Remove the patient from the MR scanning room to a safe area as soon as possible using an MR conditional stretcher.
    • Activate the facility's emergency response team.
  • Emergency response team members may not be MR safety-trained and may carry ferromagnetic materials.
  • All potential members of the emergency response team should receive annual training regarding MR safety concerns.

Equipment/Environmental Emergencies

  • Move the patient from the MR Scanner Room to a safe area.
  • Close and lock the scanner room door.
  • Activate the facility's emergency response code.
  • Participate in the decision whether to quench the magnet if fire department personnel need to enter.

Quench

  • Cryogen gases cool superconducting magnet systems.
  • A quench is a sudden loss of the magnetic field involving the release of liquid helium at negative 452 degrees Fahrenheit or liquid nitrogen at negative 320 degrees Fahrenheit.
  • Can occur as a controlled or uncontrolled event.

Controlled Quench

  • Rapidly reduce the magnetic field in response to an emergency (e.g., a metallic object pins a person against the magnet).
  • Start a quench by pushing the emergency quench button.
  • If no immediate danger, call the MR service engineer to slowly release the cryogen gases and gradually dissipate the magnetic field.
  • Can also occur during magnet maintenance or decommissioning.
  • A controlled quench by a qualified service engineer is always the preferred method when possible.

Uncontrolled Quench

  • System components fail without warning, causing the temperature of the cryogen gases to rise.
  • A hissing sound may be heard just before a full quench occurs.
  • Cryogen gases are released into the MR scanning Room.
  • An exhaust tube should vent the gases to the outside air.
  • If the vent flap becomes stuck, the cryogen gases can fill the room, causing:
    • Frostbite or asphyxiation
    • Increased pressure that can rupture a person's tympanic membrane
  • Doors to the MR suite should open outward.
  • Facilities should have emergency evacuation plans.
  • Remain calm and evacuate patients and personnel.
  • Contact the MR service engineer to inspect the room and equipment.
  • This process may take several days or weeks.
  • A properly designed room should have an exhaust grill in the ceiling.
  • The room also should have a passive backup system.
  • Technologists should be careful when opening an outward swinging door.
  • The ACR cautions against breaking control room glass to vent the scanning room.

Ethical and Legal Responsibilities

  • Ensure anyone entering the MR department undergoes a thorough safety screening.
  • Ideally, the patient is screened three times before entering the MR scanner.
  • Identify patient contraindications before scheduling to minimize patient no-shows and unfinished examinations.

Pre-Screening

  • The referring physician should prescreen the patient before ordering the MR examination.
  • Check for factors that might contraindicate the examination, such as:
    • Non-MR safe or MR conditional pacemakers
    • Abandoned or fractured pacemaker wires
    • Ferromagnetic aneurysm clips
  • Verify that the patient's size and weight do not exceed MR table specifications.
  • Ask whether a woman is pregnant.
  • If a patient is claustrophobic or has excessive pain, the referring physician should prescribe medication.
  • If intravenous contrast is required, lab tests to check blood urea nitrogen, creatinine, and estimated glomerular filtration rate levels should be performed.
  • Patient care and safety should be the top concern of all MR personnel.

Contacting Patients

  • Let patients know what to expect and how to better prepare for the examination.
  • Remind patients of necessary preparations to prevent cancellations.
  • Patients with claustrophobia can be instructed to contact their referring physician for medication.
  • Minimize patient wait times.
  • Prescreening helps MR technologists and radiologists prepare for the MR examination.

Screening Questions

  • Inquire whether the patient has had a previous MR examination.
  • Ask whether the patient has ever had a reaction to a contrast agent or to iodinated contrast media.
  • Ask about prior medical imaging.
  • Inquire whether the patient has a history of allergies or asthma.
  • Inquire whether the patient has a history of significant cardiac disease.
  • Ask the patient about any history of renal disease, including renal disease in relatives.
  • Explain the procedure and answer any questions.

Gadolinium-Based Contrast Agents (GBCA) and Nephrogenic Systemic Fibrosis (NSF)

  • Most patients tolerate gadolinium chelates well.
  • The U.S. Food and Drug Administration provides guidance concerning the development of nephrogenic systemic fibrosis (NSF) in patients with moderate to end-stage renal disease, following exposure to gadolinium-based contrast agents or GBCAs.
  • Give the patient a medication guide.
  • Risk factors include patients with stage three through five chronic kidney disease and acute kidney injury.
  • Patient assessment should begin with a medical history, but providers should not rely solely on this information.
  • The patient's glomerular filtration rate also should be tested no more than six weeks before administering contrast.
  • Contrast media most associated with NSF include gadodiamide, gadopentatate dimethylamine, and gadovecetamide.

High-Risk Patients

  • If the pre-examination assessment shows that the patient is at high risk for NSF or other contrast reaction, consider alternative imaging examinations that do not require a gadolinium-based agent.

Checklist for Reducing Safety Incidents

  • Ensure the patient has the correct order from the referring physician.
  • Prescreen patients for contraindications.
  • Surgically implanted devices in details such as device name and model number to verify that the apparatus is safe for the facility's magnet field strength.
  • Ensure that prescreening radiographs are completed, reviewed, and approved by a radiologist.
  • If contrast is indicated, ensure the appropriate lab work is completed, reviewed, and approved.
  • Ask whether a female patient of childbearing age is pregnant or breastfeeding.
  • Verify that the patient's weight does not exceed the MR scanner's table weight limits.
  • Remind patients with claustrophobia to discuss pre-examination medication with their referring physician and that they might need a ride home.
  • Inquire whether patients who will receive a contrast injection have any known difficulties with venipuncture.

Final Screening

  • MR technologists who are trained in MR safety do the final patient screening.
  • Review an MR safety questionnaire with the patient or the patient's legal guardian.
  • If the patient is unable to respond because of dementia or other condition, a qualified person should complete the safety interview.
  • If the patient or patient's representative is hearing or vision impaired, not fluent in English, or has problems with literacy, follow facility's policies and procedures regarding interpreter services.
  • If a patient is unconscious and no family member or representative is available who knows the patient's medical history, physicians must determine if the MR examination is absolutely indicated.

Education

  • Education is a key component to ensuring the cooperation of the patient.
  • Remaining still during the examination helps produce motion-free images and minimizes the time.
  • Demonstrate the intercom system and squeeze ball, as well as the levels of bore lighting and bore fan speed.
  • Remind the patient about the loud noises that occur during the examination.
  • Informing the patient about the expected exam length may help the patient better tolerate any discomfort.
    *Technologists encounter situations that are not textbook perfect, such as claustrophobic patients, and at times conventional coils might not fit some patients.

Special Needs Patients

  • Extra care should be taken with special needs patients to ensure patient comfort and avoid movement during scanning.
  • Some patients might require additional padding.
  • Others might have drains, IVs, Foley catheters, and other medical devices.
  • MR technologists should remain aware of a patient's breathing tube and any other lines when placing coils.
  • Be creative for lateral decubitus positioned patient and perhaps use a flexible coil or other coil for the patient's brain scan.

Patient Monitoring

  • Special patient monitoring may be needed for unconscious patients, those who are hard of hearing or who have weak voices, pediatric patients, special needs patients, or those under sedation.
  • Typical monitoring equipment includes an MR safe pulse oximeter, blood pressure cuff, and electrocardiogram or ECG equipment.
  • Never attempt to use any monitoring equipment that is not specifically designed for the MRI environment.
  • When monitoring a patient's ECG findings, it's important to understand the magnetic hemodynamic effect on ECG waveforms in a magnetic magnetic field.
  • The ECG waveform may not be sufficient for accurate patient monitoring when running certain sequences.
  • Some patients are extremely anxious and cannot complete a scan even after sedation is attempted.
  • These patients may require anesthesia.
  • Anesthesia personnel who plan to be in the MR Room must be screened for safety before entering.