Magnetic Resonance TIP - MRI Database : Safety Guidance

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'Safety Guidance'

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5 Gauss Limit

The national regulatory boards decided to limit the threshold for access to MRI areas to 5 gauss.

MRI Safety Guidance
It is of special interest for the observer of bureaucratic procedures that the 5 gauss safety limit is ten times higher than the average earth magnetic field, but lower than the magnetic field in electric trains such as subways (up to 7 gauss). For example, the fields measured on the surface of the receiver of a telephone are 35 gauss and of an audio headset 100 gauss.

Further Reading:

Basics:

Magnetic Sensitivity of MRI Systems to External Iron: The Design Process

by www.integratedsoft.com

News & More:

	A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems by www.fda.gov

	Modern Implantable Heart Devices Safe For Use In MRI Scans Wednesday, 16 March 2005 by www.sciencedaily.com

Cardiac Pacemaker

A pacemaker is a device for internal or external battery-operated cardiac pacing to overcome cardiac arrhythmias or heart block. All implanted electronic devices are susceptible to the electromagnetic fields used in magnetic resonance imaging. Therefore, the main magnetic field, the gradient field, and the radio frequency (RF) field are potential hazards for cardiac pacemaker patients.
The pacemaker’s susceptibility to static field and its critical role in life support have warranted special consideration. The static magnetic field applies force to magnetic materials. This force and torque effects rise linearly with the field strength of the MRI machines. Both, RF fields and pulsed gradients can induce voltages in circuits or on the pacing lead, which will heat up the tissue around e.g. the lead tip, with a potential risk of thermal injury.
Regulations for pacemakers provide that they have to switch to the magnet mode in static magnetic fields above 1.0 mT. In MR imaging, the gradient and RF fields may mimic signals from the heart with inhibition or fast pacing of the heart. In the magnet mode, most of the current pacemakers will pace with a fix pulse rate because they do not accept the heartsignals. However, the state of an implanted pacemaker will be unpredictable inside a strong magnetic field. Transcutaneous controller adjustment of pacing rate is a feature of many units. Some achieve this control using switches activated by the external application of a magnet to open/close the switch. Others use rotation of an external magnet to turn internal controls. The fringe field around the MRI magnet can activate such switches or controls. Such activations are a safety risk.
Areas with fields higher than 0.5 mT (5 Gauss Limit) commonly have restricted access and/or are posted as a safety risk to persons with pacemakers.

MRI Safety Guidance
A Cardiac pacemaker is because the risks, under normal circumstances an absolute contraindication for MRI procedures.
Nevertheless, with special precaution the risks can be lowered. Reprogramming the pacemaker to an asynchronous mode with fix pacing rate or turning off will reduce the risk of fast pacing or inhibition. Reducing the SAR value reduces the potential MRI risks of heating. For MRI scans of the head and the lower extremities, tissue heating also seems to be a smaller problem. If a transmit receive coil is used to scan the head or the feet, the cardiac pacemaker is outside the sending coil and possible heating is very limited.

Further Reading:

Basics:

	A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems by www.fda.gov

	Magnetic Resonance Imaging and Cardiac Pacemaker Safety at 1.5-Tesla(.pdf)

	Magnetic Resonance Imaging in patients with ICDs and Pacemakers (.pdf) 2005 by www.bbriefings.com

News & More:

	Newer Heart Devices Safe During MRI Monday, 23 August 2004 by www.medinews.com

	Modern Implantable Heart Devices Safe For Use In MRI Scans Wednesday, 16 March 2005 by www.sciencedaily.com

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Cardiac Risks

During the MRI scan an augmentation of T waves is observed at fields used in standard imaging but this possible MRI side effect is completely reversible upon removal from the magnet. A field strength dependent increase in the amplitude of the ECG in rats has been observed during exposure to high homogeneous stationary magnetic fields, but this side effect is not transferable to standard imaging situations for humans.

MRI Safety Guidance
The minimum level at which augmentation can be observed is 0.3 T and increases by higher field strength. An augmentation in T-wave amplitude can occur instantaneously and is immediately reversible after exposure to the magnetic field ceased. There should be no abnormalities in the ECG in the later follow-up. Augmentation of the signal amplitude in the T-wave segment may result from superimposed electrical potential. No circulatory alterations coincide with the ECG changes. Therefore, no biological risks are believed to be associated with them.
For more MRI safety information see also Contraindications and MRI Risks.

Further Reading:

Basics:

A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems

by www.fda.gov

News & More:

MRI Safety

by imaging.ugent.be

Cardiac Stent

The MRI safety of cardiac stents is dependent of the material, the examined part of the body and the used field strength. A susceptibility artifact is expected also in low magnetic fields, but less.

MRI Safety Guidance
Most of the used materials are non-magnetic, for this case there is no risk for movement caused through the magnetic field. If the cardiac stent is outside the region of the radio frequency pulse, also the risk of e.g. heating is low.

Further Reading:

News & More:

	Cell therapy offers repair hope Monday, 7 January 2008 by www.channel4.com/

	Medtronic Receives FDA Approval for Endeavor® Zotarolimus-Eluting Coronary Stent System Friday, 1 February 2008 by wwwp.medtronic.com

Clips

MRI Safety Guidance
Cerebral (aneurysm) clips are at first contraindicated for MRI examinations unless specifically approved.
Other type of clips: Examinations may be done a few weeks after an operation. Movement of clips or staples placed in a body cavity can present a hazard, but this is often reduced due to the formation of fibrosis around the clips.

Further Reading:

Basics:

MRI Safety

by imaging.ugent.be

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