Safety of MRI Critical Medical Devices

Safety of MR critical implants Vienna, 09 JUL 2009

mHH

FDA & CDRH, MR Critical Implants,

and

ASTM MR Safety Standards

Wolfgang Kainz, PhD

U.S. Food and Drug Administration - FDA

Center for Devices and Radiological Health - CDRH

Office of Science and Engineering Laboratories - OSEL

Division of Physics - DP


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Purpose is to better understand • FDA and CDRH

• RF induced heating of MR critical implants

• MR labeling of medical devices

Content

• FDA & CDRH

• MR critical implants

• MR critical medical devices

• Factors influencing RF induced heating

• SAR SAR

• The Virtual Family

• SAR distribution in anatomical models

• Safety aspects 1.5T vs. 3T

• B1rms as possible future measure for implant labeling

• ASTM MR Safety Standards

• Labeling


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The U.S. Food and Drug

Administration is

• Scientific, Regulatory, Public Health Agency

• Mission is to protect and promote public health. http://www.fda.gov/

• Authority to regulate medical devices

– Federal FD&C Act

• Established regulatory controls for medical devices (May 28, 1976)

– 21 CFR Parts 800-1299


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Department of Health & Human

Services


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FDA Centers

and Regulated Products• Food

• Drugs

• Medical Devices *

• Biologics

• Animal Feed and Drugs

• Cosmetics

• Radiation-Emitting Products *

• Combination Products (drug-device*, biologic-device*, drug-biologic)

– Primary mode of action

– RFD (Request for Designation)

CDER

Center for Drugs and

Evaluation Research

CVM

Center for

Veterinary Medicine

NCTR

National Center

for Toxicological Research

CFSAN

Center for Food Safety

and Applied Nutrition

CBER

Center for Biologics and

Evaluation Research

CDRH

Center for Devices

and Radiological Health

FDA

*

Office of Regulatory Affairs (ORA)

is the lead office for all field activities.


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Center Director

Dr. Daniel Schultz

Office of Device Evaluation ODE Office of Compliance (OC)

Office of Science and

Engineering Laboratories (OSEL)

Office of Surveillance

and Biometrics (OSB)

Office of Communication,

Education and Radiation Programs

(OCER) *International Affairs

Office of In Vitro Diagnostic

Device Evaluation and Safety (OIVD)

http://www.fda.gov/cdrh/index.html

CDRH


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Regulatory Paradigm: Balancing

Risks and Benefits


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MR critical implants (Guest Editorial in JMRI 26:450–451, 2007)

Definition:

• active implantable medical devices (AIMDs)

• semi-active implants, i.e., implants powered from outside of the body

• elongated metallic structures that are in the range of the critical length

• we currently believe that the critical length is in the range between the half wave

length and the wave length of the RF field inside the body, i.e., 25-50cm for 1.5T and

12-25cm for 3T

• currently no exclusion criteria for small implants exists


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MR critical semi active implant

• Braingates Ischemic Stroke System


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MR critical medical devices

Definition: • active medical devices

• made of conductive material

• have critical masses or dimensions • partially implanted and partially outside of the patient’s body

• are in electrical contact with the patient. • electrically conductive leads (e.g., ECG leads) or probes in contact with the

patient


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• Electrode Arrays Cap


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• AutoLITT Probe from Monteris

RF Head Coil (Clear)

Probe Driver

Commander

Interface Platform

Probe

Head Fixation Device

Probe Driver Follower

PPI


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• AutoLITT Probe from Monteris

RF Head Coil

Probe Driver Commander

Interface

Platform

Probe

MRI Table

Head Fixation Device

Probe Driver Follower PPI


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Factors influencing implant and medical device heating

responsible for implant and medical device heating are the local electric and magnetic

fields in the vicinity of the implant, induced by the radio frequency (RF) field

These local electric and magnetic fields depend on: • scanner type, in particular the type of RF transmitting coil

• patient anatomy

• patient landmark

• implant location and orientation inside the patient; more specifically the implant

location in relation to the RF transmitting coil

• implant shape, implant size, and implant material

• RF exposure or the RF incident field: B1rms and the local electric fields produced by

the RF coil. The RF exposure is indirectly measured by estimating the patient’s whole

body averaged specific absorption rate (WB-SAR), the partial body averaged specific

absorption rate and the local peak (10g) averaged specific absorption rate (only for

local coils)


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SAR SAR

• local peak SAR

• spatially averaged SAR

– averaged over certain mass of tissue or phantom material without specifying the

shape of the averaging volume; ICNIRP guidelines average over any 10g of

contiguous tissue

– averaged over certain mass of tissue or phantom material with specifying the

shape of the averaging volume; usually a cube

– averaged over parts of the body or parts of the phantom

– averaged over the whole body or the whole phantom = whole body averaged

SAR (WB-SAR); conservative WB-SAR estimate displayed on MR console

• temporally averaged SAR

– ICNIPR Guidelines and IEC 60601-2-33 average over any 6-min period

– IEC 60601-2-33 allows 3 fold increase of SAR within 10 seconds


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Virtual Family

• Duke: male, 34yrs, 1.76m, 74kg

• Ella: female, 26yrs, 1.60m, 58kg

• Billie: female, 8yrs, 1.34m, 26kg

• Thelonious: male, 6yrs,1.07m, 17kg

• models are available for free

[email protected]


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Virtual Population

• 1 baby (coming in 2010)

• 5 children of both genders

(5 - 14 years; 13.5 - 18.4kg/m2)

• 1 men (23.1kg/m2)

• 1 female (22.7kg/m2)

• 1 obese male (35kg/m2)

• 1 pregnant female (24kg/m2)

• others on request


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SAR distribution in different anatomical models at 1.5T


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Sufficient Modeling Capabilities for AIMD Safety


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Poser


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SAR and MR critical implants - Conclusions:

• the SAR distribution in a patient is highly inhomogeneous and depends on the anatomy, landmark and RF coil type

• the SAR distribution in the ASTM is also inhomogeneous and depends on the landmark and RF coil type; however, the distribution can be calculated for each landmark

• SAR distribution in ASTM phantom must be considered for placing the implant

• anatomical equivalent positioning of the implant in the ASTM phantom does not reliable predict the implant heating in the patient

• worst case position in the ASTM must be guaranteed for conservative implant heating assessment

• unresolved (IEC/ISO JWG AIMD MRI):

– standardized worst case implant positioning for 1.5T and 3T in the ASTM phantom

– how to accurately relate the worst case heating in the ASTM phantom to the possible heating in the patient; for the whole patient population Virtual Family


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Safety aspects 1.5T versus 3T

• Force testing at higher field strengths is sufficient as long as the scanner with lower

field strength does not have a higher spatial static magnetic field gradient

• Torque testing needs to be done at the highest static field strength.

• The field distribution and the wavelength inside the patient at 3T are substantially

different than at 1.5T or at any higher or lower field strength.

• Therefore, RF induced heating can be substantially different at 3T and 1.5T.

• Important: RF induced heating testing at 3T, and subsequent 3T MR Conditional

labeling, does not necessarily guarantee safe scanning at 1.5T.

• The same is true for testing and labeling at 1.5T and then scanning at 3T.


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B1rms could replace WB-SAR in the future for implant scanning

• The whole body averaged specific absorption rate (WB-SAR) displayed on MR

scanner consoles are conservative estimates intended to give an upper bound of the

WB-SAR induced in patients.

• The WB-SAR is intended only for patients and not for phantoms.

• This is supported by publications from Baker et al. and Nitz et al. and by the results of

the FDA initiated SAR Intercomparison protocol.

• The RF incident field, called the B1rms is the driving factor for the in the patient induced

electric and magnetic fields.

• B1rms will be displayed on the scanner console as required by IEC 60601-2-33 3rd Edt.

• B1rms will probably be used for labeling of implants in the future.


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ASTM MR Test Methods

• ASTM F2052-02 for Measurement of Magnetically Induced Displacement Force on

Medical Devices in the MR Environment

• ASTM F2119-01 for Evaluation of MR Image Artifacts from Passive Implants

• ASTM F2182-02a for Measurement of Measurement of Radio Frequency Induced

Heating Near Passive Implants During MRI

• ASTM F2213-04 for Measurement of Magnetically Induced Torque on Medical

Devices in the MR Environment

• ASTM F2503-05 Standard Practice for Marking Medical Devices and Other Items for

Safety in the Magnetic Resonance Environment

• JWG TS on AIMDs


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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

• MR Safe

• MR Conditional

• MR Unsafe


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FDA’s MR Conditional Labeling Suggestions

• Non-clinical testing has demonstrated that the MedDevABC up to a total length of XX mm is MR

Conditional. It can be scanned safely under the following conditions:

– Static magnetic field of X.X Tesla and Y.Y – Tesla (if applicable)

– Spatial gradient field of XXXX Gauss/cm or less

– Maximum whole-body-averaged specific absorption rate (SAR) of XX W/kg for XX minutes of

scanning. For landmarks (if applicable) XXXX (specify landmarks, if needed add drawing to

describe landmarks), the maximum whole-body-averaged specific absorption rate must be

less than XX W/kg.

– In a configuration where XXXX (describe the configuration for MR conditional labeling; e.g.,

legs apart, padding, maximum length of MedDevABC, etc).

– Use only, e.g. whole body coils, no transmitting local coils are allowed, receiving local coils

can be used.

• Add the MR conditional symbol to the label.

• The MedDevABC has not been evaluated for stent migration and heating in MR systems with field

strengths other than specified above. The heating and migration effect in the MR environment for

the MedDevABC in XXXX (specify other device configurations if applicable) is not known.


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FDA’s MR Conditional Labeling Suggestions – Additional

Information

In an analysis based on non-clinical testing the MedDevABC was determined to

produce a potential worst-case temperature rise of XX°C for a whole body

averaged specific absorption rate (SAR) of 2 W/kg for XX minutes of MR

scanning in a XX Tesla, whole body MR system for a landmark in XXXX.

Temperature rises of the MedDevABC were measured in a non-clinical

configuration using a XXXX Whole Body active shield MR scanner using

software version XXXX and a phantom designed to simulate human tissue. The

phantom average SAR calculated for this non-clinical testing using calorimetry

was XX W/kg. When the MedDevABC was placed in a worst-case location within

the phantom, the maximal temperature rise was XX°C when the local SAR was

scaled to 2 W/kg.


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FDA’s MR Conditional Labeling Suggestions – Additional

Information

• (For image quality labeling see also FDA Guidance on Establishing Safety and Compatibility of Passive Implants in the Magnetic Resonance (MR) Environment)

• MR image quality may be compromised if the area of interest is in the exact same area or relatively close to the position of the MedDevABC. The image artifact extends approximately XX mm from the MedDevABC, both inside and outside (if applicable) the device when scanned in non-clinical testing using echo and gradient echo sequences as specified in ASTM F2119-01 in a XX Tesla, XXXX system, with XXXX software, MR system with body radiofrequency coil. Therefore, it may be necessary to optimize MR imaging parameters for the presence of this metallic implant. The device lumen was always obscured during scanning (if applicable).

• Scanning at X.X Tesla and Y.Y Tesla may be performed immediately following the implantation of the MedDevABC.

• We (name of the sponsor) recommend that the patient register the MR conditions disclosed in this IFU with the MedicAlert Foundation or equivalent organization.

The MedicAlert Foundation can be contacted in the following manners:

Mail: MedicAlert Foundation International

2323 Colorado Avenue

Turlock, CA 95382 Phone: 888-633-4298 (toll free)

209-668-3333 from outside the US

Fax: 209-669-2450

Web: www.medicalert.org


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FDA’s MR Conditional Labeling Suggestions – Implant Card

Non-clinical testing has demonstrated that the MedDevABC up to a total length of XX mm is MR Conditional. It can be scanned safely under the following conditions:

• Static magnetic field of X.X Tesla and Y.Y – Tesla (if applicable)

• Spatial gradient field of XXXX Gauss/cm or less

• Maximum whole-body-averaged specific absorption rate (SAR) of XX W/kg for XX minutes of scanning. For landmarks (if applicable) XXXX (specify landmarks, if needed add drawing to describe landmarks), the maximum whole-body-averaged specific absorption rate must be less than XX W/kg.

• In a configuration where XXXX (describe the configuration for MR conditional labeling; e.g., legs apart, padding, maximum length of MedDevABC, etc).

• Use only whole body coils, no transmitting local coils are allowed, receiving local coils can be used.

• Scanning at X.X Tesla and Y.Y Tesla may be performed immediately following the implantation of the MedDevABC. The MedDevABC has not been evaluated for stent migration and heating in MR systems with field strengths other than specified above. The heating and migration effect in the MR environment for the MedDevABC in XXXX (specify other device configurations if applicable) is not known.


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Take home messages

• CDRH: getting safe and effective medical devices to market as quickly as possible

• Recognize MR critical implants and MR critical devices and scan only according labeling

• Factors influencing implant heating: scanner type, RF coil, patient anatomy, patient landmark, scan parameters, implant

• SAR SAR

• system estimated SAR is for patient safety only and not for phantoms

• SAR distribution in a patient is highly inhomogeneous

• MR safety testing in 1.5T and 3T scanners is complete different and cannot be related to each other

• large variation of implant heating in different scanners needs to be considered

• B1rms could, and probably will, replace whole body averaged SAR for implant labeling

and scanning procedures

• Labeling: FDA guidance for passive implants, labeling for passive implants might not

be applicable to MR critical devices, labeling is an evolving subject

• although peer-reviewed and white papers published scanning procedures for

various active implantable medical devices (AIMDs), scanning of all patients

with implants should only be done according to approved labeling procedures


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Thank you for your brain work ...

Documents

Safety of MRI Critical Medical Devices