What is MR Environment
The MR environment presents unique safety hazards for patients with implants, external devices, and accessory medical devices.
Medical device risk management should ensure the effectiveness and safety of implants and other medical devices expected to be used in the MR environment.
What is MR safety?
Magnetic resonance (MR) safety and image compatibility are important issues when using medical devices in the environment of magnetic resonance imaging (MRI). MR testing of medical devices is required for device approval by the FDA. 510K Submission not provided without proper justification of MR safety.
Magnetically Induced Displacement Force (ASTM F2052)
The electric and magnetic fields used by the MR system to create MR images produce hazards that must be considered to determine whether a medical device or other object may safely be used in or near an MR system. Magnetically induced deflection force and torque are two of the hazards that must be addressed to determine the MRI safety of any device or object that will be used in or near an MR system.
Magnetically Induced Torque (ASTM 2213)
The magnetically induced torque may cause tissue damage by inducing unwanted movement or dislodgement of the medical device. The magnetically induced torque is proportional to the static magnetic field strength and is the greatest inside the MR system bore where the static magnetic field strength is greatest.
According to ASTM F2213, the static field in the MR environment can cause the medical device to align with the magnetic field. The mass and length of the device determine the device’s response to the magnetic field. We can measure the magnetically induced torque and help determine an appropriate acceptance criterion to maintain patient safety.
Heating
RF-induced heating:
When implanted medical devices constructed of electrically conductive materials are subjected to an oscillating magnetic field, electric currents are produced according to Faraday’s Law of Induction. These electric currents cause potentially damaging Radio Frequency (RF) induced heating near the device. For fully implanted passive medical devices, ASTM F2182 provides a method for the measurement of RF-induced heating.
Heating induced by time-varying magnetic field gradients, (dB/dt):
Exposure to time-varying magnetic fields (gradient pulses) can induce eddy currents on conductive surfaces of conductive/metallic implants and in the internal conductive components of AIMDs placed inside the bore of the MR system.
ISO/TS 10974 includes test methods for the assessment of gradient-induced heating for AIMDs. There are no standard test methods for the assessment of gradient-induced heating for passive medical devices.
Gradient Induced Vibration (ISO/TS 10974):
During magnetic resonance imaging (MRI), pulsed gradient magnetic fields are sent out from the MR scanner to the object inside the scanner. This can induce eddy current magnetic moments in conductive objects, even if these are non-magnetic. This gradient-induced vibration may lead to device malfunction or tissue damage. This hazard should be addressed for all AIMDs.
Gradient-Induced Extrinsic Electrical Potential (Unintended Stimulation) (ISO/TS10974):
The time-varying gradient magnetic fields associated with an MR exam can induce an electric potential at the electrodes of a lead.
Rectification of RF pulses from MR Exams (Unintended Stimulation):
Unintended tissue stimulation can occur if the rectified voltages are generated at the active medical device electrodes. Concerning active medical devices in the MR environment, rectification refers to the conversion of RF waveforms to slowly varying voltages which in turn can lead to unintended tissue stimulation.
The tests outlined in ISO/TS 10974 measure the levels of rectified voltages generated by the AIMD during RF exposure.
Medical Device Malfunction:
ISO/TS 10974 provides standardized test methods for assessing AIMD malfunction in the MR environment. These include potential malfunctions induced by MR fields, including:
· Static magnetic field (B0)
· RF fields
· Time-varying gradient magnetic fields (dB/dt)
· Combined fields
The extent of Image Artifact: Artifacts in the obtained MR images may be caused by metallic implants or other medical equipment. An active medical device’s functioning could also cause distortions or artefacts in the obtained MR images. Both may result in visually confusing or non-diagnostic images or artefacts that mimic diseases.
A warning in the MR Conditional labelling to alert healthcare professionals to the possibility of medical device-induced image artefacts should be included for all medical devices that are expected to be used in the MR environment.
Reporting Results in a 510k Application
Test Report summaries of complete test reports should include the following:
o List the hazards addressed by the test.
o List the test equipment used.
o Results should be reported as per the contents of the test report defined in the consensus standard.
MRI Safety Labelling in 510k Submission
• MR Safe: An item that poses no known hazards resulting from exposure to any MR environment.MR Safe items are composed of materials that are electrically nonconductive, non-metallic, and nonmagnetic. By definition, MR Safe items contain no metal.
MRI Safety Information and/or a statement such as “The <insert medical device name> is MR Safe. A mandatory requirement forFDA 510k
• MR Unsafe: If a device is proven to pose unacceptable risks to the patient, medical staff, or other persons within the MR environment.
MRI Safety Information And/or a statement such as “The < insert medical device name > is MR Unsafe.” and, if appropriate, the statement “The device presents a projectile hazard.” For non-implanted medical devices, the MR Unsafe labelling should appear directly on the medical device if possible. For example:
• MR Conditional: These are the devices that may safely enter the MRI scanner room only under the very specific conditions provided in the labelling. Patients should not be scanned unless the device can be positively identified as MR Conditional and the conditions for safe use are met. An item with demonstrated safety in the MR environment within defined conditions including conditions for the static magnetic field, the time-varying gradient magnetic fields, and the radiofrequency fields.
Conditions of safe use should be identified in the labelling. The conditions for safe use will be different based on the intended use of the device. Example MR Conditional labelling for a passive medical device:
Safety in MRI Not Evaluated
This applies to a very limited number of medical devices anticipated for use in the MR environment, such as certain passive implants:
· That has historically not provided any information about MRI safety.
· With no known adverse events due to exposure to the MR environment
The labelling should include the following information:
“The (DEVICE NAME) has not been evaluated for safety in the MR environment. It has not been tested for heating or unwanted movement in the MR environment. The safety of the MR environment is unknown. Performing an MR exam on a person who has this medical device may result in injury or device malfunction.” Contact I3CGLOBAL 510k consultantsand US Agents for guidance.
FDA Recognized Voluntary Consensus Standards Relevant to MR Safety. ASTM F2052 Standard Test Method for Measurement of Magnetically Induced Displacement Force on Medical Devices in the Magnetic Resonance Environment.
· ASTM F2119 Standard Test Method for Evaluation of MR Image Artifacts from Passive Implants.
· ASTM F2182 Standard Test Method for Measurement of Measurement of Radio Frequency Induced Heating Near Passive Implants During Magnetic Resonance Imaging
· ASTM F2213 Standard Test Method for Measurement of Magnetically Induced Torque on Medical Devices in the Magnetic Resonance Environment
· ASTM F2503 Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment
· ISO TS 10974 Assessment of the safety of magnetic resonance imaging for patients with an active implantable medical device
