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