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07 November 2009

Answer to Question #5241 Submitted to "Ask the Experts"

Category: Medical and Dental Equipment/Shielding — Shielding

The following question was answered by an expert in the appropriate field:

What is a reasonable requirement for magnetic shielding at 3 Tesla (T)? I want to protect other people who might walk near the magnetic field of my magnet. I always thought that containment of the 5 Gauss line is sufficient. Are there medical legal requirements to be more strict?

Patients with pacemakers or ferromagnetic aneurysm clips must avoid fringe fields above 0.5 mTesla (or 5 Gauss; 10,000 G = 1 T conversion). Magnetically sensitive equipment such as image intensifiers and color TVs (even some gamma cameras if they're poorly self-shielded) are severely impacted by fringe fields of less than 0.3 mT, as the elctromagnetic focusing in these devices are disrupted.

Administrative controls for magnetic fringe fields is 0.5 mT, requiring controlled access to areas that exceed this level. Magnetic fields below this level are considered to be safe for the general population. Areas above 1.0 mT require controlled and restricted access with warning signs. Also, disruption of the fringe fields can adversely affect the homogenity of the main magnetic field in the imaging volume, leading to degradation in the image uniformity. Any large metallic object (e.g., elevators, automobile, etc.) traveling through the fringe field can produce such an effect.

Kishor M. Patel, PhD, DABR

Answer posted on 7 March 2006. The information and material posted on this Web site is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Web site. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.