Staff seminar on 29 Feb 2008

Safety of MRI in patients with CV devices

By Dr S. Sanyal

MBBS, MS, MSc (UK), ADPHA, ADHRD

Consultant Surgeon & Associate Professor

Seychelles

Study highlights

• CV devices• MRI mechanisms• MRI risk determinants• FDA classification of CV devices• Recommendations• Conclusions• Reference • Addendum

Examples of CV devices

• Coronary stents

• Peripheral stents

• Aortic stent grafts

• Embolization coils

• IVC filters

Examples of CV devices

• Prosthetic heart valves

• Annuloplasty rings

• Cardiac closure / occluder devices

• Loop recorders

• Retained fractured intravenous leads

Examples of CV devices

• Pacemakers

• Implantable cardioverter defibrillators (ICD)

• Hemodynamic support devices

– Intra-aortic balloon pumps (IBP)

– Right ventricular (RV) assist devices

– Left ventricular (LV) assist devices

Recommendations for CV devices in general

• Screening: Patients with CV device requiring MRI should undergo screening to accurately identify the implanted device

• Temporary devices: Inpatients should be examined for temporary devices

MRI safety

• No ionizing radiation

• Non-invasive procedure

• No iodinated contrast agents

MRI risk mechanisms

• Static magnetic field

• Radiofrequency (RF) energy

• Gradient magnetic fields

Static magnetic field

• 1.5 to 3 Tesla, which corresponds to 30,000 - 60,000 times the strength of earth's magnetic field

• Scanners are typically superconducting

• Thus, they are always ‘on’.“

• Attraction of a ferromagnetic object into the scanner

Recommendation for static

• Devices should be non-ferromagnetic or weakly ferromagnetic (most are)

• Lowest field strength available to minimize ferromagnetic risk

RF energy

• RF energy pulsed into body to generate the MR image → ohmic heating.

• Metallic devices (e.g. lead) act as antennae → concentrate RF energy → excessive local heating at device tip

• Electrical currents created in wires / leads → arrhythmias.

Gradient magnetic field

• Rapidly changing magnetic fields

• Excite peripheral nerves

• Induce electrical currents in conductive devices.

• Flow of electricity in conductive blood produce ECG changes / arrhythmias

Risk determinants

• MRI emergency / elective

• Material of CV device

– ferromagnetic / weakly ferromagnetic / non-ferromagnetic

• Specific type of device

MRI elective vs. emergency

• Knee pain for 5 years and needs an MRI but has just had a device implanted 2 days ago

Vs.

• Admitted to hospital after being hit by a car; acute knee pain; surgeons want urgent MRI

Material of CV device

• Non-metallic safer than metallic

• Among metallic

– Non-ferromagnetic < weak ferromagnetic < strong ferromagnetic

Recommendations based on material

• Non-ferromagnetic ‘passive’ implant (e.g. titanium, titanium alloy, nitinol)

– MRI immediately after implantation

• Weakly ferromagnetic devices – MRI 6 weeks after implantation

Specific type of device

• Obsolete FDA definitions –

– ‘MR safe’ / ‘MR compatible’

• Current FDA definitions –

– ‘MR safe’

– ‘MR conditional’

– ‘MR unsafe’

MR safe

• Non-metallic

• Non-conducting

• Nonmagnetic

MR conditional

• No known hazards in a specified MR environment with specific conditions

MR unsafe

• Known to pose hazards in all MR environments

Specific recommendations

• Those pacemakers that have been tested have been labelled “MR unsafe”

• Pacemaker / ICD – strong relative contraindication to MR examination

– Scan only at experienced centres

– Written informed consent.

– Clinician with pacemaker / ICD expertise– Crash cart including defibrillator

Specific recommendations

Known retained fractured intravenous leads Poses a high risk for thermal injury. MR examination should not be

performed

Specific recommendations

• Hemodynamic support devices (IBP, RV / LV assist devices) with variable ferromagnetic / electrical components – Absolute contraindications to MR

examination

Specific recommendations

Coronary / peripheral vascular stents, cardiac occluder devices, embolization coils Most considered ‘MR safe’ Some considered ‘MR conditional’ MRI can be conducted with non-

ferromagnetic stents

Specific recommendations

Most aortic stent grafts that have been tested are labelled ‘MR safe’

Only Zenith AAA endovascular graft (Cook Vascular, Inc) is labelled ‘MR unsafe’.

Specific recommendations

Most prosthetic heart valves / annuloplasty rings that have been tested are labelled ‘MR safe’

Some are ‘MR conditional’

Specific recommendations

Loop recorders that are considered ‘MR conditional

May experience slight movement of the device during MRI

Should be warned to avoid undue concern

Conclusion

• MRI risks arise from 3 mechanisms

– Static magnetic field

– RF energy

– Gradient magnetic fields

Conclusion

• Hemodynamic support devices (IBP, RV / LV assist devices)

– Absolute contraindications to MR examination

Conclusion

ICDs and pacemakers

Relative contraindications to MRI

Conclusion• For most devices, if there is good clinical

indication for MRI scan, then benefits probably outweigh the risks

– American Heart Association

– American College of Cardiology

– North American Society for Cardiac Imaging

– Society for Cardiovascular Magnetic Resonance

Journal source• Levine GN, Gomes AS, Arai AE, et al.

Safety of magnetic resonance imaging in patients with cardiovascular devices. Circulation 2007 doi: 10.1161 / CIRCULATION AHA.107.187256

• New Statement on Safety of MRI With CV Devices. URL: http://www.medscape.com/viewarticle/566881

Addendum

• Next 15 slides give some examples of cardiovascular devices that are implanted in patients.

• The images have been culled from various sources, which are obvious from the caption, where relevant.

• This is meant as an APPENDIX to the main presentation.

Coronary stent

Coronary stent – Drug eluting

PTCA stent – Enlarged view

Peripheral stent

Aortic stent-graft

Tornado® Embolization coil

Gunther-Tulip IVC filter

IVC Filter – University of Texas Health Science Centre

Heart valve – TTK Chitra

Annuloplasty ring

Septal occluder

Loop recorder

Implantable Cardioverter Defibrillator

Intra-aortic balloon catheter

Ventricular assist device