MRI and the Medtronics SynchroMed Pump

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The following text is reproduced from a Medtronics Product Information Update issued in Australia in July 1998. It came to my attention in February 1999, and the text itself is dated March 5 1998. For further information contact your Medtronics representative.
Medtronics have a simple web based statement on MRI and the Synchromed pump, saying that the scanner will temporarily stop the pump.  They suggest MRI is acceptable if teh patient can do without the drug for the period of the scan, and providing teh pump's programme is checked immediately after the scan.   Their web site also makes some observations about SAR, TVMF, static field and image distortion.   Click here for the link
For some sense of history you should also read a paper concerned with the same pump published in JMRI Vol 1 Jan/Feb 1991 pp 77-81 by von Roemling, Lanning, & Eamas.
Dr Kanal's comments from October 1997 are found at this link.

Posted by Greg Brown   February 1999


Summary of MRI Testing for Synchromed ® Systems

The Medtronic Neurological Division has received numerous inquiries about the risk vs benefit of using a magnetic resonance imaging (MRI) system on patients with an implanted SynchroMed pump who require MRI as part of their medical treatment. As a result, the Medtronic Neurological Division commissioned the University of Minnesota MRI Research center to conduct testing on the SynchroMed pump. Dr. Whit Athey 1, an independent consultant and MRI expert, conducted the analysis of the study data and concluded that no undue risk is present for patients implanted with a SynchroMed pump who require MRI as part of their medical treatment.

This summary of MRI testing will be made available to all physicians and medical staff personnel who inquire about the safety of MRI for patients with an implanted SynchroMed pump. Medtronic will also submit these same study results to the FDA, in order that these safety recommendations can be added to our approved labeling for SynchroMed.

FDA in a co-sponsored workshop on MR safety with the International Society for Magnetic Resonance in Medicine (ISMRM) on June 10-11 1996, suggested that the term "MR Safe" should be applied where there is little or no risk to the patient. Medtronic does not make any "MR Safe Claim" but rather chooses to provide medical practitioners with test results which allow for a determination of patient risk vs benefit when exposed to an MR environment. The following four (4) criteria were identified by ISMRM and FDA as the key criteria for making proper undue risk vs benefit determinations.

(1) When magneto-mechanical effects exert forces and torque's that are no larger than those due to gravity. Larger than gravitational forces shall be considered on a case by case basis.

Study Analysis
The magnetic forces and torque on the SynchroMed pump when it is in a 1.5T MRI system are about half of the values that are due to gravity. The patient may be able to sense these forces, but it is well within the guidelines for safety. For magnetic fields larger than 2T, the magnet-mechanical effects become more pronounced and an elastic garment or wrap may be needed to hold the pump in place.

(2) When the presence of the device does not increase the level of induced low-frequency eddy currents caused by the switching gradient fields beyond accepted guidelines.

Study Analysis
There will be low-frequency and radio frequency currents induced in the patient during MRI. The levels that are induced will be slightly higher around the SynchroMed pump due to the presence of the device. These levels are safe, but care should be taken to keep any monitoring cables away from the pump location during an MRI scan (cables should normally be dressed away from the patient anyway).

(3) When the presence of the device does not increase the local specific absorption rate (SAR) for radio frequency magnetic fields beyond accepted guidelines.

Study Analysis
There will be some increase in the local effective SAR, perhaps two - fold to three - fold near the implanted pump. For comparison it has been shown that factors up to ten in the local SAR from the induced RF current can be caused by the shunting of current around or between naturally occurring structures of low conductivity such as the spine. Therefore it could be said that the implanted pump would have about the same effect as the spine on concentrating radio frequency energy. The result of localized radio frequency energy may result in the patient feeling slight increases in temperature ( 1-2o F ) near the implanted pump.

(4) When the MRI system does not cause damage to the active implant, necessitating revision surgery to repair or replace the implant, or where the MRI system does not cause the active implant to function in an unsafe manner.

Study Analysis
While the SynchroMed pump is in the strong magnetic field of the MRI system, the rotor of the pump (which is magnetic) will align with the field and will not turn, resulting in no drug delivery. AS soon as the pump leaves the MRI system, it will again deliver the programmed amount of drug. There is no "build-up" of undelivered drug - the pump is essentially "off" during the time of exposure to the magnetic field.

IMPORTANT

The physician should determine that the patient can safely be deprived of the drug delivery during the time required for the scan. Upon completion of MRI diagnostics, use the physician programmer to verify pump status. If any change in pump status has occurred a "pump memory error" will be displayed. In the unlikely event that a "pump memory error" is displayed the pump must be reprogrammed and Medtronic Technical services should be notified. Medtronic Technical Services can be reached at (800 328-0810.

1. Dr. Whit Athey, Senior MRI/Regulatory Consultant with C.L. McIntosh & Associates. Dr. Athey has provided scientific consults to FDA's Office of Device Evaluation and Office of Compliance on a wide variety of devices, especially devices employing electric or magnetic fields in their mode of action. He has published extensively in the area of electric or magnetic interactions with biological systems and magnetic resonance (MR) safety. Dr Athey has a broad educational training with degrees in engineering and mathematics from Auburn University and his doctorate in physics and biochemistry from Tufts University

Thursday March 05 1998


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