Ex Vivo testing of a Ferromagnetic Detector for Evaluation of Bullets and Aneurysm Clips: A Potential Screening Device Prior to MR of Patients with Metal of Unknown Magnetic Properties.

Division of Neuroradiology, Department of Radiology
University of Pennsylvania Hospital, Philadelphia, PA

Purpose
MR imaging is presumed hazardous in many patients with retained bullets, shot, and some aneurysm clips. Even though most bullets 1 and aneurysm clips are safe to scan, radiologists should be hesitant to perform MR in patients with metal when the composition is unknown. The purpose of this study was to test the effectiveness of a commercial ferromagnetic metal detector, currently used for routine screening of patients who may be inadvertently carrying metal objects into the scanner, to discriminate nonferrous metal from ferrous ex vivo with the consideration of its potential use to screen for implanted metal.

Methods
Test objects consisted of various metallic objects from a personal collection which included nine aneurysm clips, two unfired bullets, one bullet found on a street, a penny, a small steel bolt, a Star-Edwards heart valve, and a paperclip. To determine ferrous properties of each object a magnet was placed over each device and objects demonstrating attraction were considered ferrous. Testing then was performed with a FerrAlert Halo Prescreen (Kopp Development, Jensen Beach, FL) ferromagnetic detector which was set to a sensitivity sufficient to detect a standard small paperclip. A deflection angle test was performed on the aneurysm clips in a 1.5 Tesla and 3.0 Tesla MR magnet according to the American Society for Testing and Materials guidelines2 to determine their magnetic properties. Objects were passed through the FerrAlert Halo Prescreen detector in a subject’s armpit to simulate an imbedded object.