Standards for Safetyof Medical Devices in MRI

Terry O. Woods, Ph.D.Center for Devices & Radiological HealthU. S. Food and Drug Administration

May 12, 2006

IntroductionFirst MRI scanners approved in 1984Safety Concerns produced by• Large Static Field and Spatial Gradients,

dB/dxCurrent clinical scanners: 3T, >500 gauss/cm >50,000 times Earth’s magnetic field

• Pulsed RF fields 128 MHz for 3T scannerused to elicit MR signal from tissue

IntroductionSafety concerns also produced by:• Pulsed gradient magnetic fields dB/dt

used for signal localization »Can cause peripheral nerve and

eventually cardiac stimulation• Cryogens

»Can cause asphyxiation or burns during a quench or spill

MRI Safety Concerns –Static Field EffectsMagnetically induced displacement force • Causes magnetic objects to become

projectiles• Has caused fatal accidents • Scissors, forklift tines, IV poles, “sandbags,”

gas cylinders, spring pillow, iron filings…For ferromagnetic materials : Greatest where |∇B| is a maximumFor paramagnetic materials: Greatest where |B| |∇B|, is a maximum

MRI Safety Concerns –Static Field Effects

Location of maximum gradient generally near the entrance to the magnet boreObjects that reach the “creep point” can begin to accelerate very suddenlyWith increased field strength and shielding, gradients are becoming largerObjects that were safe at 1.5T may not be safe at 3T or at 1T

Schenck, Med. Phys. 23(6), 1996.

Chaljub, et. al. AJR:177, July 2001

http://www.simplyphysics.com/flying_objects.html

http://www.simplyphysics.com/flying_objects.html

MRI Safety Concerns –Static Field Effects

Magnetically induced torque - causes objects to rotate to align with the magnetic field • Is greatest in the center of the magnet

bore where |B| is maximum • Has contributed to fatal accidents

MRI Safety Concerns –RF EffectsRF Heating - heating produced by currents induced by RF imaging gradients applied during scanningPotential for RF heating increased by presence of implants or other medical devices, particularly ones containing electrical leads• Parkinson’s patient with implanted

neurostimulator left with a severe permanent disability; has produced burns severe enough to require amputations

MRI Safety Concerns –Image Artifact

Image Artifact – appears as voids or geometric distortionsAt least one surgery performed based on an MR image artifact

MRI Safety Concerns –Magnet Quenches

At least one death due to asphyxiationBurns from supercooled cryogens

http://www.bioreg.kyushu-u.ac.jp/vsb/Fun/icons/beri750quench.GIF

Safety Standards for MR Systems

IEC 60601-2-33, Part 2 defines operating limits for MR scanners• safety limits for dB/dt• safety limits for SAR in head, whole body

and local tissue• currently being revised

NEMA standard test methods• MS-4 for Acoustic Noise (1990)• MS-7 for measurement of dB/dt (1993)• MS-8 for measurement of SAR (1993)

Standards for Implants and Other Medical Devices

FDA asks for information demonstrating MR safety for finished devicesNeeded test methods did not existIn 1997, FDA requested ASTM International consider developing MR safety/compatibility standards

ASTM InternationalDevelops voluntary, consensus standards for materials, products, systems & services Has approximately 11,000 standards used internationallyOver 31,000 members from 110 countries participate on ASTM International committeesCommittee F04 on Medical & Surgical Materials & Devices

Standards for Implants and Other Medical Devices

ASTM task group F04.15.11 on MR Safety and Compatibility of Materials and Medical Devices • Completed 5 standards addressing the

principal issues that produce safety concerns for implants and other devices in the MR environment

ASTM MR Test MethodsASTM F2052-06 for Measurement of Magnetically Induced Displacement Forceon Medical Devices in the MR EnvironmentASTM F2119-01 for Evaluation of MR Image Artifacts from Passive ImplantsASTM F2182-02a for Measurement of Measurement of Radio Frequency Induced Heating Near Passive Implants During MRIASTM F2213-06 for Measurement of Magnetically Induced Torque on Medical Devices in the MR Environment

Newest ASTM MR StandardASTM F2503-05 Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment

ASTM F2052 - Test Method for Displacement Force

Acceptance criterion: Magnetically induced force less than object weight

Device holder

Turning Knob

Torsional spring

ASTM F2213 - Test Method for Torque

Acceptance Criterion: Torque less than worst case torque due to gravity, defined as (Weight)(Length)

ASTM F2119 – Test method for Image Artifact

Defines standard sequences for determining artifact so the amount of artifact for different devices can be comparedNo acceptance criteria: Depending on region of interest, different amounts of artifact are acceptableIn some cases, artifacts are desirable (biopsy needles, image guided surgery)

ASTM F2182 - for Measurement of RF Induced Heating

11.50 in.

24.50 in.

17.50 in.

5.25 in.

7.00 in.

F2182 – Test Method for RF Heating

Place device in gelled saline phantomSubject to RF field and measure worst case temperature rise during scan

Current Work on F2182Method uses SAR displayed on consoleRecent research shows large variations in SAR for similar scans between different systemsDue to differences in estimation of SAR by different systemsASTM conducting SAR IntercomparisonFor MR Systems• Collect data on variation in reported SAR

with F2182 phantom for a simulated implant with specified scan conditions for an array of MR systems

Current Work on F2182Revise to include measurement of SAR using calorimetryRevise to cover active devices

ASTM F2503 - Practice for Marking Items for Safety

Intent:• TO PREVENT MR RELATED

ACCIDENTS • To correct problems with the use of

historical terminology• To introduce a new set of terms and

MR icons consistent with current international safety signs

New Terminology

MR Safe

MR Conditional

MR Unsafe

MR SafeMR Safe– an item that poses no known hazards in all MR environments.

• MR safe items include nonconducting, nonmagnetic items such as a plastic Petri dish. An item may be determined to be MR Safe by providing a scientifically based rationale rather than test data.

MR ConditionalAn item that has been demonstrated to pose no known hazards in a specified MR environment with specified conditions of use. Field conditions that define the specified MR environment include field strength, spatial gradient, dB/dt (time rate of change of the magnetic field), radio frequency (RF) fields, and specific absorption rate (SAR). Additional conditions, including specific configurations of the item, may also be required.

MR UnsafeMR Unsafe – an item that is known to pose hazards in all MR environments.

• MR Unsafe items include magnetic items such as a pair of ferromagnetic scissors.

Where do we go from here? Standards for MR Systems

IEC 60601-2-33, Part 2 defines operating limits for MR scanners• currently being revised

NEMA standard test methods• MS-4 for Acoustic Noise (1990)

currently being revised• New standards being developed:• MS-10 for determination of Local SAR• MS-11 for determination of Gradient-

Induced Electric Fields

Where do we go from here? RF heating test method• Revise to add SAR determination using

calorimetry for measurements with phantoms

• Continue revision to expand it to cover active devices

Consider additional torque methodsMore fully address large active devices used in interventional procedures

Where do we go from here?Working with ISO TC 150, Implants for Surgery, to reference new terminology and icons First in the fundamental standard for implants, ISO 14630 Non-active surgical implants – General requirementsMove on to fundamental standard for active implants and to implant specific standards as neededGoal – avoid duplication of effort and have one set of terms and icons used world-wide

ASTM F2213 - Test Method for Torque

Place device on holder suspended on torsional spring with 1 principal axis verticalPlace fixture in MR scanner & measure angular deflection; calculate torqueRotate device 360° to get torque as function of angleRepeat for other 2 principal axesAcceptance Criterion:Torque less than worst case torque due to gravity, defined as (Weight)(Length)

F2182 – Test Method for RF HeatingPlace device in gelled saline phantomSubject to RF field with SAR ≥ 1 W/kg over phantom volumeMeasure worst case temperature rise during scan ≥ 15 minReport temperature rise for test scan sequence and time

SummaryMR Safety/Compatibility Issues:Force, Torque, RF Heating, Image ArtifactASTM Standards for Force, Torque, RF Heating, Artifact, & MarkingNew Terminology & Icons

Historical DefinitionsMR safe—the device, when used in the MR environment, has

been demonstrated to present no additional risk to the patient or other individuals, but may affect the quality of the diagnostic information. The MR conditions in which the device was tested should be specified in conjunction with the terms MR safe and MR compatible since a device which is safe or compatible under one set of conditions may not be found to be so under more extreme MR conditions.

MR compatible—the device, when used in the MR environment, is MR safe and has been demonstrated to neither significantly affect the quality of the diagnostic information nor have its operations affected by the MR device. The MR conditions in which the device was tested should be specified in conjunction with the terms MR safe and MR compatible since a device which is safe or compatible under one set of conditions may not be found to be so under more extreme MR conditions.

DiscussionIf historical terms are used properly, there should be no confusionIf historical terms are used improperly – without list of test conditions – there could be a problem with or without the new terms