The Science Journal of the American Association for Respiratory Care

2006 OPEN FORUM Abstracts

BEHIND THE BLACK LINE: EFFECT OF MRI SHIELDING ON THE POSITIONING OF MECHANICAL VENTILATORS DURING MRI

Sherwin E. Morgan, RRT, Avery Tung M.D.

Director, Respiratory Therapy, Associate Professor of Anesthesia and Critical Care, University of Chicago , Chicago , IL   60637

Introduction:  RF interference from MR imaging can complicate the delivery of mechanical ventilation.  Ventilators used in the MRI suite must be both resistant to electromagnetic interference and sufficiently non-magnetic to avoid being dragged into the magnet.  One issue that may play an important role is the difference in shielding strategies used by different MRI manufacturers.  While some manufacturers apply secondary magnets to modify the magnetic field, others use steel plates.   These variations may cause the relationship between magnetic force and distance from the magnet to differ (1).  To examine this possibility, we tested ventilator performance in the actively shielded 1.5 T GE LCC MRI scanner and the passively shielded 1.5 T Phillips Achieva machine.  Materials and

Methods:
We modified a Siemens SV300 ventilator for MRI use by removing ferromagnetic components identified with a strong, handheld magnet.  The ventilator was then used to ventilate a plastic test lung in both the GE and Philips machines.  Function was tested on both sides of the 5 gauss safety line.  We performed a phantom scan, and then tested the ventilator for proper function.  Finally, we compared magnetic field mapping diagrams for each of the MRI environments.

Results:
The waveform screen, wheels, and pin-screw connector for the waveform monitor cable were identified and removed.   We then tested the SV300 in the GE scanner.  Immediately behind the 5 gauss line, no MRI artifact was noted and the ventilator functioned normally.  Positioning the ventilator 1-2 feet in front of the line resulted in scan artifact, but no movement of the ventilator itself.  When the ventilator was tested in the Philips unit, however, the ventilator failed to function when placed 1 foot in front of the 5 gauss line.  Moreover, we observed physical movement of the ventilator towards the magnet.  When placed behind the 5 gauss line, the ventilator functioned normally with no MRI artifact.  Comparisons of magnetic field maps revealed the distance between the 5 gauss and 30 gauss lines to be 3 feet for the GE machine vs. 1 foot for the Philips device, suggesting a dramatically steeper increase in magnetic field at the 5 gauss line in the Philips than in the GE scanner.  

Conclusion: Our finding has two important safety implications for respiratory care providers:  Since objects deemed safe with a handheld magnet may still become magnetized in the stronger magnetic field of a MRI device, all potentially ferromagnetic equipment should be tested in the magnet room before being used for patient care.  We also found that the 5 gauss safety line does not provide information on the change in magnetic field with distance.  Such changes can differ dramatically between manufacturers, and be clinically significant.  References:  Radiology Today 2005;6:22-3

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