1. Tuesday Case of the Day Category Physics
Authors: David W. Jordan, Ph.D.
University Hospitals Cleveland Medical Center & Case Western Reserve University
Question: An MR-Conditional ventilator (pictured) is used in a 3T MRI suite. The manual states it can be used in magnetic fields up to 100 gauss. At what fringe field should a safety perimeter around the magnet be marked?
0.5 gauss
5 gauss
25 gauss
100 gauss
Figure 1: MR Conditional Ventilator, magnetic fields ≤ 100 gauss

2. Diagnosis: C. 25 gauss

3. Correct Answer: C. 25 gauss
The safety conditions for this type of equipment depend on the threshold for missile (or projectile) effect – the field strength at which the attractive force of the magnet on the equipment exceeds the force of gravity.
If a safety perimeter is established at the 100-gauss line, maneuvering the device close to the line creates an excessive risk of the device becoming a projectile, because the spatial gradient across the 100-gauss line is steep. Moving the device slightly closer to the magnet increases the force sharply over a very short distance.
A 25-gauss safety perimeter is a better choice because when working near the safety line, small differences in positioning will not risk exceeding the projectile threshold for the device.
A 0.5-gauss or 5-gauss safety perimeter for this equipment would be too conservative and would not allow it to be placed close enough to the patient in the scanner to be used for its intended purpose.

4. References/Bibliography:
“iVent 201 Operator’s Manual.” OM Rev. 6. © 2005, VersaMed Medical Systems, Inc.
“Typical Room Plan, Siemens Magnetom Verio.” Cutsheet for Typical #07137 Rev. 0. © 2013, Siemens Medical Solutions USA, Inc.
Ridgway, JP. “The MRI Environment.” In: Cardiovascular MR Manual. S. Plein, et al. (eds.) Springer, DOI / _2