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Aneurysm Clips: Effects of Long-term and Multiple Exposures to a 1.5-T MR System

¹ Department of Radiology, University of Pittsburgh Medical Center, Rm D-132, 200 Lothrop St, Pittsburgh, PA 15213 (E.K.)
² Department of Radiology, University of Southern California School of Medicine, Los Angeles (F.G.S.).

	Abstract

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The authors evaluated the ferromagnetic properties of multiple nonferromagnetic aneurysm clips before and after long-term and multiple exposures to a 1.5-T magnetic resonance system. No alterations in the magnetic properties of these clips was observed. These findings suggest that repeated or chronic exposure of patients with implanted nonferromagnetic aneurysm clips to strong static magnetic fields is unlikely to result in any clinically significant alteration in the magnetic properties of these clips.

Index terms: Aneurysm, cerebral, 17.457, 17.73 • Head, MR, 10.1214, 17.457 • Magnetic resonance (MR), safety, 17.457 • Radiology and radiologists, iatrogenic injury

	Introduction

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There has been much written about the safety of patients with various types of implanted intracranial aneurysm clips who undergo magnetic resonance (MR) imaging procedures (1–27). Specific types of aneurysm clips have been found to be ferromagnetic, and, thus, present safety concerns for one attempting to perform an MR examination of patients in whom these clips are implanted (15,16). Other aneurysm clips have been demonstrated to have little or no forces exerted on them when exposed to strong static magnetic fields and magnetic field gradients and, therefore, are considered to be safe for individuals in the MR environment (15,16).

Magnetic clusters, or groups of similar magnetic elements (including cobalt) that stay together, may exist in a metallic alloy. Since 1960, the existence of magnetic clusters in many different "paramagnetic" alloys has been reported in the literature (28). These clusters can be viewed as microscopic ferromagnetic magnets immersed in a nonferromagnetic (paramagnetic) matrix. These would normally be expected to be randomly aligned with respect to each other, although they would tend to line up relative to a strong external magnetic field. However, such an alignment might take time to accomplish, with the degree of relative alignment of these magnetic clusters (if present) affecting the overall magnetic character of the clip and metal. This alignment may also change with time in a static field.

Recommended testing procedures for aneurysm clips could result in introduction of aneurysm clips into strong MR system–related magnetic fields several times before implantation (1–4,10,12–14,18–20,25). Furthermore, some patients with implanted aneurysm clips that were previously found to be MR compatible have since undergone repeated follow-up MR imaging examinations and the concomitant exposure to strong static magnetic fields. With the potential for the existence of magnetic clusters (some of which have been recently demonstrated to exist in Elgiloy [Ho JC, written communication, 1998]), alterations in the magnetic properties of aneurysm clips before versus after implantation that result from long-term or multiple exposures to strong magnetic fields are a potential concern. Such exposures may grossly "magnetize" aneurysm clips, even if they are made of nonferromagnetic or weakly ferromagnetic materials (Ho JC, written communication, 1998), and could present a potential hazard to an individual in the MR environment.

Therefore, we studied in vitro various intracranial aneurysm clips before and after long-term and multiple exposures to the magnetic fields associated with a 1.5-T MR system to assess the likelihood of such an occurrence and to attempt to quantify the actual incidence of grossly evident alterations in magnetic properties of various intracranial aneurysm clips.

	Materials and Methods

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Aneurysm Clips
This investigation was conducted at two facilities with unshielded 1.5-T MR imaging systems (Signa; GE Medical Systems, Milwaukee, Wis). The aneurysm clips used for this study were divided into two groups.

Group 1 aneurysm clips (n = 34) included the following: Perneczky (Zeppelin Chirurgishe Instrumente, Pullach, Germany; metal, 1.4441/DIN 17443; blades, 7-mm straight and curved upward), Spetzler (Elekta Instruments, Atlanta, Ga; metal, commercially pure titanium; blades, 5-mm straight; 5- and 7-mm 45°; 9- and 11-mm sideward; and 13-mm straight [temporary]), Sugita (Mizuho America, Beverly, Mass; metal, Elgiloy; blades, 6-, 7-, and 10-mm straight; 6-mm straight fenestrated; 4-mm curved; 9-mm slightly curved; 9-mm sideward curved; 6-mm sideward curved bayonet; 7-mm bayonet; 8-mm bent; 8-mm sideward bent), Yasargil (Aesculap, South San Francisco, Calif; metal, Phynox; blades, 4.7-, 8.3-, and 8.6-mm curved; 5-, 9-, and 20-mm bayonet; 5-, 6-, 7-, and 9-mm straight; and 7- and 9-mm angled). Four 7-mm straight Perneczky aneurysm clips and one of all other aneurysm clips were used.

Group 2 aneurysm clips (n = 159) included the following: Perneczky (one 3- and one 12-mm straight), Spetzler (one 9- and two 20-mm straight), Sugita (one 5- and one 7.5-mm bent, one 5-and one 6-mm straight), Yasargil Phynox (six 2.5-, 10 5-, eight 7-, four 7.5-, two 9-, three 9.6-, seven 10-, two 12-, two 20-, and one 25-mm angled; two 3-, four 4-, three 4.2-, one 4.7-, four 5-, one 5.7-, two 6.5-, two 6.8-, eight 7-, three 9-, six 11-, two 14-, eight 15-, two 18.7-, and two 20-mm curved; two 3-, three 5-, seven 6-, six 7-, two 9-, four 11-, five 15-, three 20-, and two 25-mm straight; and two 7-, three 9-, two 10-, two 12-, and one 20-mm bent), and Yasargil titanium alloy (two 3-, four 7-, one 9-, and two 20-mm straight).

We used only previously unused aneurysm clips in their original packages that were all obtained directly from their manufacturers and had not been previously handled in any manner before this investigation. All clips were removed from their packages before they were exposed to the MR systems and tested for response to both translation and torque forces in two distinct tests.

Translation Test
The "deflection angle" test (26,27) was performed to assess the presence of translation force. This test was conducted by suspending each aneurysm clip from a 30-cm-long 4.0 silk thread attached to a plastic protractor (length, 15 cm; radius, 7.5 cm) so the angle of deflection could be measured (12,25,26,27,29,30). The three orthogonal lights were used to facilitate positioning of the protractor apparatus within the MR system. This test was conducted at the longitudinal position where the spatial gradient of the magnetic field had been previously determined to be at a maximum (25,26,29,30,31), namely, 450 G/cm at 35 cm inside the bore (25). This enables determination of worst-case translation forces. The angle of deflection was determined three times, and the results were averaged.

Torque Test
The next assessment of magnetic field interaction was conducted to test for rotation or torque-related forces (25, 29–35). Each aneurysm clip was placed on plate glass or a Petri dish in an orientation perpendicular to the static magnetic field. The plate glass or dish was then positioned in the center of the MR system where the effect of torque from the magnetic field is known to be the greatest (10,12,15,18). Each aneurysm clip was observed for movement with respect to alignment to the magnetic field. Each aneurysm clip was sequentially repositioned at 45° intervals to a full 360°, and the procedure was repeated.

Protocol: Group 1 Aneurysm Clips
Long-term exposure.—The aneurysm clips from group 1 were taped along the inner bore of the 1.5-T MR imaging system approximately 15 inches (38 cm) from the bore isocenter. The clips were positioned within the bore with their long axes aligned with the long axis of the bore. The aneurysm clips were left undisturbed in this position for 31 days, at which time they were removed and retested for torque and deflection forces as described previously. The aneurysm clips were then positioned in the geometric center of the bore of the 1.5-T MR system for 48 hours, at which time they were removed and retested for torque and deflection forces as described previously.

Multiple exposures.—After the long-term exposure tests were completed, the aneurysm clips were introduced 1.5 ft (45 cm) into the bore of the MR system on plate glass and aligned parallel to the bore (ie, parallel to the magnetic lines of force) and then removed from the MR imaging room. This was repeated 50 times to attempt to reproduce what some of these clips might experience on repeated introduction into an MR system bore for retesting or reimaging purposes. This was followed by retesting for torque and deflection forces as described previously.

Protocol: Group 2 Aneurysm Clips
Long-term exposure.—To test long-term exposure to the static magnetic field of the MR system, the aneurysm clips were positioned in the geometric center of the bore for 48 hours. This was followed by retesting for torque and deflection forces as described previously.

Multiple exposures.—After the long-term exposure tests were completed, all the clips were positioned on the patient table at the entrance to the bore and were slowly moved into the MR system until they reached the center of the long axis of the bore. After approximately 10 seconds, they were moved slowly back to the entrance. This entire procedure was repeated 50 times. This was followed by retesting for torque and deflection forces as described previously.

	Results

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Group 1 Aneurysm Clips
None of the clips tested positive on initial baseline tests. In all cases, the deflection angle was consistently less than 2° (ie, within the visual margin of error) and there were no grossly detectable rotation forces. Testing after the long-term exposure and multiple-exposure phases of the study yielded the same findings.

Group 2 Aneurysm Clips
Before long-term exposure to the 1.5-T MR system, no clip showed a magnetic field interaction with respect to translation forces (as determined with the deflection angle test, deflection angles were 0°) and torque forces (as determined with the Petri dish test, no movements were detected). After long-term exposure and multiple exposures, again, no clip showed magnetic field interaction (deflection angles were 0° and no movements were detected).

	Discussion

TOP Abstract Introduction Materials and Methods Results Discussion References

 
Interest in the safety of exposing patients with implanted intracranial aneurysm clips to the strong static magnetic fields of MR systems is keen. Variability in response to strong magnetic fields by supposedly nonferromagnetic clips has spurred performance of magnetization characterization tests before and after MR imaging (10). Furthermore, a concern has surfaced that repeated in vitro or in vivo exposure of these clips to MR system–related magnetic fields might result in their developing some degree of gross magnetization over time. This might then alter the assessment of MR compatibility and the safety of repeated imaging of patients with such aneurysm clips. Therefore, we directed this investigation to specifically address these issues by exposing multiple aneurysm clips of various types and from various manufacturers to strong 1.5-T magnetic fields associated with MR systems with higher field strength. According to the American Society for Testing and Materials (26), if an aneurysm clip deflects less than 45° in the deflection test, it is judged to be nonferromagnetic and suitable for implantation. Therefore, on the basis of this standard, the lack of noticeable response (<2°) to these exposures suggests that long-term or multiple exposures to MR systems in vivo for diagnostic MR examinations or in vitro for testing purposes should not result in clinically significant changes in their magnetic properties or MR safety.

In conclusion, findings in this investigation demonstrate that long-term and multiple exposures to the strong magnetic fields associated with 1.5-T MR systems produce no grossly detectable effects on aneurysm clips made from Elgiloy, Phynox, titanium alloy, commercially pure titanium, and austenitic stainless steel of the types tested. Repeated exposures to MR examinations and their attendant magnetic fields should not increase the risk of subsequent exposure to MR environments for individuals with intracranial aneurysm clips.

	Footnotes

 
Address reprint requests to E.K.

Author contributions: Guarantor of integrity of entire study, E.K.; study concepts, E.K.; study design, E.K., F.G.S.; definition of intellectual content, E.K., F.G.S.; literature research, E.K., F.G.S.; experimental studies, E.K., F.G.S.; data acquisition and analysis, E.K., F.G.S.; manuscript preparation, E.K.; manuscript editing and review, E.K., F.G.S.

Received February 19, 1998; revision requested April 27, 1998; revision received June 12, 1998; accepted August 24, 1998.

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