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AJNR Am J Neuroradiol 24:463–471, March 2003 Aneurysm Clips: Evaluation of Magnetic Field
Interactions and Translational Attraction by Use
of “Long-Bore” and “Short-Bore” 3.0-T MR
Imaging Systems
Frank G. Shellock, Jean A. Tkach, Paul M. Ruggieri, Thomas J. Masaryk, and Peter A. Rasmussen BACKGROUND AND PURPOSE: The use of 3.0-T MR systems is increasing worldwide. We
evaluated magnetic field interactions and translational attraction for 32 aneurysm clips in
association with exposure to “long-bore” and “short-bore” 3.0-T MR imaging systems.
METHODS: Thirty-two different aneurysm clips were evaluated in this investigation. Each
aneurysm clip was qualitatively evaluated for magnetic field interactions and quantitatively
assessed for translational attraction by using the deflection angle test. The deflection angle tests
were performed at the points of the highest spatial gradients for long-bore and short-bore 3.0-T
MR imaging systems.
RESULTS: Seventeen of the 32 aneurysm clips showed positive magnetic field interactions.
Deflection angles for the aneurysm clips were significantly (P < .001) higher on the short-bore
(range, 0 –18 degrees) compared with those recorded on the long-bore (range, 0 –16 degrees)
3.0-T MR imaging system. Aneurysm clips made from commercially pure titanium and titanium
alloy displayed no translational attraction (n ؍ 15), whereas those made from stainless steel
alloy, Phynox, and Elgiloy displayed positive deflection angles (n ؍ 17).
CONCLUSION: The 32 different aneurysm clips passed (angle <45 degrees) the deflection
angle test by using the long- and short-bore 3.0-T MR imaging systems, indicating that they are
safe for patients and other persons in these MR environments (ie, immediate area of MR
imaging systems). However, only clips made from the titanium and titanium alloy are entirely
safe for patients undergoing MR imaging procedures because of the total lack of magnetic field
interactions. The remaining clips require characterization of magnetic field–induced torque.
Because of possible differences in the points of the highest spatial gradients for different 3.0-T
MR imaging systems, the results are specific to the imaging units and bore designs used in this
investigation.
Neurosurgical management of an intracranial aneu- tracranial aneurysm clip in a patient or other person rysm or arteriovenous malformation by application of in the MR environment may present a hazardous a temporary or permanent aneurysm clip is a well- situation (5–10). Although certain aneurysm clips are established procedure (1–4). The presence of an in- a contraindication to the MR environment, others that are classified as “nonferromagnetic” or “weakly ferromagnetic” are deemed safe for patients or other Received April 25, 2002; accepted after revision July 2.
persons exposed to MR imaging systems operating at Supported by The Cleveland Clinic Foundation, Cleveland, OH, and the Institute for Magnetic Resonance Safety, Education, and The use of 3.0-T MR imaging systems for clinical From the University of Southern California, Los Angeles applications is increasing worldwide. Importantly, (F.G.S.), Keck School of Medicine and Institute for Magnetic most previous investigations conducted to assess MR Resonance, Safety, Education, and Research, Los Angeles, CA, imaging safety of aneurysm clips used MR imaging and the Divisions of Radiology (J.A.T., P.M.R., T.J.M.), Neurora- diology Section, and Surgery (P.A.R.), Section of Neurosurgery, systems with static magnetic fields of 1.5 T or less (7, The Cleveland Clinic Foundation, Cleveland, OH.
11–25). In general, the increasing use of MR at 3.0 T Address reprint requests to Frank G. Shellock, PhD, 7511 Mc- requires additional studies to be performed to evalu- Connell Avenue, Los Angeles, CA 90045.
ate metallic implants and devices at this field strength.
Thus, it is necessary to perform ex vivo testing at 3.0 T to characterize magnetic field–related safety for by MR imaging system manufacturers), measurements of de- aneurysm clips before allowing persons with these flection angles may be substantially different. Therefore, in this implants to enter this particular MR environment.
study, long- and short-bore MR imaging systems were used to An important aspect of MR safety testing for me- evaluate translational attraction for the aneurysm clips, as fol- lows: long-bore MR system, actively shielded, head-only, MR tallic implants involves the determination of magnetic imaging system (length, 248 cm; bore inner diameter, 55 cm; field interactions (ie, motion) and translational at- 3-T MR imaging system; General Electric Medical Systems, traction (17, 18, 23–25). Translational attraction is Milwaukee, WI); and short-bore MR system, actively shielded, typically assessed by using the deflection angle test head-only MR imaging system (length, 130 cm; bore inner originally described by New et al (17), modified and diameter, 60 cm; MAGNETOM, Allegra 3-T Headscanner; used by others (18, 23–25), and recommended by the Siemens Medical Systems, Erlangen, Germany).
American Society for Testing and Materials (27). Ac- cording to this procedure, the deflection angle for an Qualitative Evaluation of Magnetic Field Interaction implant should be measured at the point of the high- Each aneurysm clip was inspected at the entrance of the est spatial gradient for the specific MR imaging sys- imaging system bore to determine the qualitative presence of tem used for testing (23–27). If the deflection angle magnetic field interactions with the 3.0-T MR imaging systems.
from the vertical is less than 45 degrees, the implant This was defined as any visual observance of directional move- ment, rotation, or alignment to the magnetic field. The results passes the translational attraction test insofar as the were scored as either positive (observable motion, as de- magnetic force acting on the implant is less than the scribed) or negative (absolutely no motion). The entrance of the MR imaging system bore was the position selected for this Various types of magnets exist for commercially assessment because it provided an easy and rapid site for this available 3.0-T MR imaging systems. The magnet evaluation and represented the closest position of the “MR configurations include conventional long- and short- environment” (ie, the immediate area relative to the MR im- bore imaging units used for head-only and whole- body clinical applications. Because of physical differ- ences in the position and magnitude of the highest Assessment of Translational Attraction spatial gradient for different magnets, measurements Translational attraction was assessed for each aneurysm clip of deflection angles for implants by using long- versus by using a standardized procedure known as the deflection angle short-bore MR imaging systems may produce sub- test according to guidelines provided by the American Society for Testing and Materials (27). The aneurysm clip was attached stantially different results. Therefore, the purpose of to a special test fixture to measure the deflection angle in the this investigation was to evaluate magnetic field in- long- and short-bore MR imaging systems at the points of the teractions and translational attraction for 32 different highest spatial gradients (23–25, 27). The test fixture consists of aneurysm clips in association with exposure to long- a sturdy structure capable of holding the aneurysm clip in a and short-bore 3.0-T MR imaging systems. Implica- proper position without deflection of the test fixture. The test tions of the results of this study for patients and other fixture has a plastic protractor with 0-degree graduated mark- ings. The protractor is rigidly mounted to the structure. The persons with aneurysm clips regarding the 3.0-T en- zero-degree indicator on the protractor was oriented vertically.
The test fixture has a plastic bubble level permanently affixed to the top to ensure proper orientation in the MR imaging The aneurysm clip was suspended from a thin, light-weight string (weight, Ͻ1% of the weight of the implant) that was attached at the 0-degree indicator position on the protractor.
Thirty-two different aneurysm clips from various manufac- The length of the string was 20 cm, allowing the aneurysm clip turers were evaluated in this investigation. Each aneurysm clip to be suspended from the test fixture and to hang freely in was representative of the manufactured finished version and space. Sources of forced air movement within the respective was not altered in any manner before testing. These aneurysm 3.0-T MR imaging system bores were shut off during the de- clips were selected for this study because they represent various types of clips made from nonferromagnetic or weakly ferro- Measurements of deflection angles for the aneurysm clip magnetic materials (eg, stainless steel alloy, Phynox, Elgiloy, were obtained at the positions in the 3.0-T MR imaging systems commercially pure titanium, titanium alloy) used for temporary that produced the greatest magnetically induced deflections or permanent treatment of aneurysms or arteriovenous mal- (ie, the points of the highest spatial gradients) (23–25, 27). This formations. The Table lists specific information regarding the position was determined for each 3.0-T MR imaging system by aneurysm clips (ie, the name, material, and manufacturer).
using gauss line plots provided by the manufacturer, measure- ments, and visual inspection to identify the location where the spatial magnetic field gradient was the highest. For the long- bore 3.0-T MR imaging system, the highest spatial gradient According to the American Society for Testing and Materi- occurs at a position that is 96 cm from isocenter. The magnetic als (27), translational attraction should be assessed for implants spatial gradient at this position is 3.3 T/m. For the short-bore at the point of the highest spatial gradient for the MR imaging 3.0-T MR imaging system, the highest spatial gradient occurs at system used for testing. This is done to evaluate the magnet- a position that is 78 cm from isocenter. The magnetic spatial related force at an extreme or worst-case position for a metallic gradient at this position is 5.25 T/m. The locations of the object. As previously stated, there are various types of magnets highest spatial gradients were marked by using tape to facilitate used for 3.0-T MR imaging systems, including long- and short- repeated measurements of deflection angles for the aneurysm bore imaging units used for head-only and whole-body clinical applications. Because there are physical differences in the po- Thus, the test fixture was placed at the point of the highest sition and magnitude of the highest spatial gradient for a given spatial gradient for the long- and short-bore 3.0-T MR imaging magnet (based on a review of technical specifications provided systems. The aneurysm clip was held on the test fixture so that Aneurysm clips: evaluation of magnetic field interactions and translational attraction using “long-bore” and “short-bore” 3.0-T MR imaging sys-
tems
straight, 2-mm blade;stainless steel alloy;Zeppelin Chirugishe Instrumente,Pullach, Germany straight, 6-mm blade;stainless steel alloy;Zeppelin Chirugishe Instrumente,Pullach, Germany straight, 7-mm blade;stainless steel alloy;Zeppelin Chirugishe Instrumente,Pullach, Germany straight, 5-mm blade;C.P. titanium;NMT Neurosciences,Duluth, Georgia curved, 7-mm blade;C.P. titanium;NMT Neurosciences,Duluth, Georgia straight, 9-mm blade;C.P. titanium;Elekta Instruments,Atlanta, Georgia curved, 11-mm blade;C.P. titanium;NMT Neurosciences,Duluth, Georgia straight, 11-mm blade;C.P. titanium;NMT Neurosciences,Duluth, Georgia straight, 11-mm blade;C.P. titanium;NMT Neurosciences,Duluth, Georgia Note.—LB indicates long-bore; Mag., magnetic; SB, short-bore; C.P., commercially pure.
Continued
C.P. titanium;NMT Neurosciences,Duluth, Georgia straight, 13-mm blade;C.P. titanium;Elekta Instruments,Atlanta, Georgia bent, 7.5-mm blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts deflected type aneurysm clip forpermanent occlusion; angled, 10-mm serrated blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts straight, 21-mm serrated blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts permanent occlusion;straight, 19-mm non-serrated blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts bent, 8-mm blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts curved, 6-mm blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts bent, 7-mm blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts Continued
straight, 7-mm blade;Elgiloy;Mizuho America, Inc.;Beverly, Massachusetts 45-degree angled, 19-mm serratedblade;titanium alloy;Mizuho America, Inc.;Beverly, Massachusetts bayonet, 7-mm blade;titanium alloy;Aesculap, Inc.;Center Valley, Pennsylvania straight, 9-mm blade;Phynox;Aesculap, Inc.;Center Valley, Pennsylvania straight, 14-mm blade;Phynox;Aesculap, Inc.;Center Valley, Pennsylvania curved, 15.3-mm blade;Phynox;Aesculap, Inc.;Center Valley, Pennsylvania straight, 20-mm blade;Phynox;Aesculap, Inc.;Center Valley, Pennsylvania bayonet, 20-mm blade;Phynox;Aesculap, Inc.;Center Valley, Pennsylvania angled, 7-mm blade;Phynox;Aesculap, Inc.;Center Valley, Pennsylvania Continued
straight, 7-mm blade;titanium alloy;Aesculap, Inc.;Center Valley, Pennsylvania straight, 9-mm blade;titanium alloy;Aesculap, Inc.;Center Valley, Pennsylvania titanium model FT758T;bayonet, 12-mm blade;titanium alloy;Aesculap, Inc.;Center Valley, Pennsylvania straight, 11-mm blade;titanium alloy;Aesculap, Inc.;Center Valley, Pennsylvania straight, 20-mm blade;titanium alloy;Aesculap, Inc.;Center Valley, Pennsylvania the string was vertical and was then released. The deflection to 18 degrees. Aneurysm clips made from commer- angle for the aneurysm clip from the vertical direction to the cially pure titanium and titanium alloy displayed no nearest 0.5 degree was measured three times and averaged translational attraction (n ϭ 15), whereas those made from stainless steel alloy, Phynox, and Elgiloy dis- played positive deflection angles (n ϭ 17).
Deflection angle measurements obtained for the aneurysm Discussion
clips during exposure to the long-bore MR imaging system were compared with those recorded during exposure to the MR imaging procedures may be unsafe for patients short-bore MR imaging system by using a Wilcoxon Signed with certain implants made from ferromagnetic or Rank Test (StatView; SAS Institute, Inc., Cary, NC).
conductive materials because of problems associated with movement, heating, or induced electrical cur- rents (5–8, 17, 30–36). Regarding aneurysm clips, heating and induced currents are not of concern be- The findings for magnetic field interactions and cause of the physical size and shape of these relatively translational attraction for the aneurysm clips ex- small implants (17, 18, 30–35). Notably, MR imaging– posed to the long- and short-bore MR imaging sys- related heating and induced currents have been re- tems are summarized in the Table. Seventeen of the ported for only those implants or devices that have 32 aneurysm clips showed positive magnetic field in- elongated configurations or that are electronically teractions. Deflection angles for the aneurysm clips activated (eg, neurostimulation systems, cardiac pace- were significantly (P Ͻ .001) higher on the short-bore makers, etc.) (6–8, 31–36). Therefore, from an MR 3.0-T MR imaging system compared with those re- safety consideration, it is primarily important to de- corded on the long-bore 3.0-T MR imaging system.
termine magnetic qualities for aneurysm clips before On the long-bore MR imaging system, deflection an- allowing patients or other persons with these objects gles ranged from 0 to 16 degrees. On the short-bore into the MR environment. Because most previous MR imaging system, deflection angles ranged from 0 testing of aneurysm clips was conducted at 1.5 T, as static magnetic fields of MR imaging systems increase the aneurysm clips in this study because no standard above this level, further investigations are necessary currently exists for the quantification technique for to characterize MR safety for these implants.
torque and no measurement value is available for use From a magnetic field consideration, translational to designate whether an aneurysm clip is unsafe.
attraction or torque may cause movement or dislodg- Therefore, only aneurysm clips that exhibit no mag- ment of a ferromagnetic implant, resulting in injury netic field movements are considered to be safe from (6–9, 15, 17, 18, 23–25, 29, 30). Translational attrac- tion is proportional to the strength of the static mag- Aneurysm clips come in a wide variety of shapes netic field, the strength of the spatial gradient, the and blade lengths and are made from different ma- mass of the object, the shape of the object, and the terials with varying magnetic susceptibilities. Each of magnetic susceptibility of the object (17, 23–25, 29, these factors can influence the MR safety aspects of 30). The effects of translational attraction on ferro- these implants. In the present study, aneurysm clips magnetic objects are predominantly responsible for had shapes that included straight, bent, curved, and possible hazards in the MR environment (ie, imme- angled versions with blade lengths that ranged from 2 diate area around MR imaging system) (5, 8, 30). The mm (Perneczky; Zeppelin Chirugishe Instrumente, deflection angle test is commonly used to determine Pullach, Germany) to 21 mm (Sugita, Large Aneu- magnetic field–related translational attraction for im- rysm Clip for Permanent Occlusion; Mizuho Amer- plants, materials, and devices (17, 18, 23–25, 29).
ica, Inc., Beverly, MA). Materials used to make these The American Society for Testing and Materials aneurysm clips included stainless steel alloy, Phynox, guidelines for deflection angle testing of implants in Elgiloy, commercially pure titanium, and titanium the MR environment, indicate that, “. . . if the im- plant deflects less than 45 degrees, then the magnet- Previous reports investigating magnetic qualities of ically induced deflection force is less than the force on aneurysm clips indicated that every aneurysm clip the implant due to gravity (its weight)” (27). For this made from stainless steel alloy, Phynox, Elgiloy, com- condition, it is assumed that any risk imposed by the mercially pure titanium, and titanium alloy was safe at application of the magnetically induced force is no 1.5 T (6–8, 11–14, 15–26). In consideration of the greater than any risk imposed by normal daily activity current knowledge pertaining to aneurysm clips at 1.5 in the earth’s gravitational field (27). Accordingly, T, the following guidelines have been recommended findings from the deflection angle test permit im- for careful consideration before performing MR im- plants and devices made from nonferromagnetic or aging in a patient with an aneurysm clip and before weakly ferromagnetic materials that display deflec- allowing any person with an aneurysm clip into the tion angles between 0 and 44 degrees to be present in patients or other persons in the MR environment Torque, which tends to align a ferromagnetic object parallel to the magnetic field, is dependent on the Guidelines Regarding Aneurysm Clips and the strength of the magnetic field, the dimensions of the object, and the initial angulation of the object relative 1. Specific information (ie, manufacturer, type or to the static magnetic field (17, 23–25, 30). Torque model, material, lot and serial numbers) regarding effects on ferromagnetic objects are mainly responsi- the aneurysm clip must be known, especially with ble for possible hazards during an MR imaging pro- respect to the material used to make the aneurysm cedure, when the patient is positioned at the center of clip, so that only patients or other persons with non- the MR imaging system (ie, the position where torque ferromagnetic or weakly ferromagnetic clips are al- lowed into the MR environment. The manufacturer A variety of techniques have been used to qualita- provides this information in the labeling of every tively or quantitatively determine magnetic field-re- aneurysm clip. The implanting surgeon is responsible lated torque for implants and devices (17, 23, 25, 29, for properly communicating this information in the 30). To date, a test procedure and acceptable mea- surement value for torque imposed on implants has 2. An aneurysm clip that is in its original package not been defined by the American Society for Testing and made from Phynox, Elgiloy, MP35N, titanium and Materials. However, according to the American alloy, commercially pure titanium, or other material Society for Testing and Materials (27), a torque value known to be nonferromagnetic or weakly ferromag- for an implant “that is less than that produced by netic does not need to be evaluated for ferromag- normal daily activities (which might include rapidly netism. Aneurysm clips made from nonferromagnetic accelerating vehicles or amusement park rides) is or weakly ferromagnetic materials in original pack- assumed to be safe.” Notably, the amount of torque ages do not require testing of ferromagnetism be- necessary to displace an aneurysm clip is unknown, cause the manufacturers ensure the pertinent MR particularly because counter forces (eg, related to the safety aspects of these clips and, therefore, should be closing force of the clip, granulation of tissue, and held responsible for the accuracy of the labeling.
other factors) may be present that require additional 3. If the aneurysm clip is not in its original package characterization, possibly by using in vivo techniques.
and properly labeled, it should undergo testing for Therefore, torque was not specifically determined for 4. The radiologist and implanting surgeon should rysm clips made from commercially pure titanium or be responsible for evaluating the available informa- titanium alloy seem to be entirely safe because of the tion pertaining to the aneurysm clip, verifying its total lack of magnet-related movements. The remain- accuracy, obtaining written documentation and decid- ing aneurysm clips made from stainless steel alloy, ing to perform the MR procedure after considering Phynox, and Elgiloy require characterization of mag- the risks versus the benefits of the examination.
netic field-induced torque to determine whether they Of note is that Brothers et al (37) evaluated pa- are safe for patients during MR imaging procedures.
tients after surgery for vertebrobasilar aneurysms Notably, these results are specific to the 3.0-T MR with nonferromagnetic Sugita aneurysm clips at 1.5 T imaging systems used for this evaluation or with com- and reported that no ill effects occurred. In addition, Pride et al (26) conducted a study of patients with nonferromagnetic aneurysm clips who underwent MR imaging. No objective adverse outcome occurred Long-Bore versus Short-Bore Deflection in these patients, further confirming that MR imaging can be performed safely in patients with nonferro- An interesting finding of this study is that there were significantly (P Ͻ .001) higher deflection angles However, as previously discussed, few studies have measured for the aneurysm clips during exposure to been performed to evaluate magnetic field interac- the short-bore versus the long-bore 3.0-T MR system.
tions of implants in association with MR imaging To our knowledge, this is the first description of such systems operating above 1.5 T (28, 29). A study con- an important phenomenon that is obviously due to ducted at 8.0 T by Kangarlu and Shellock (29) re- the higher spatial gradient associated with the short- ported that all aneurysm clips, even those made from bore imaging unit. Although this did not impact the titanium or titanium alloy, displayed positive transla- MR imaging safety aspects of the aneurysm clips in tional attractions (deflection angles ranged from 5 to this study, it is conceivable that other metallic im- 53 degrees). Importantly, several aneurysm clips re- plants may be found to be safe on the long-bore MR ported to be safe at 1.5 T (6–8, 17, 18, 23) were found imaging system and unsafe on a short-bore MR im- to be potentially unsafe at 8.0 T because they showed aging system. Therefore, further study of this issue is excessive deflection angles and relatively high quali- tative torque values (29). In view of the findings at 8.0 T and because of the proliferation of 3.0-T MR im- aging systems, it was considered important to deter- Acknowledgments
mine magnetic field–related safety for comparable Special thanks to Dr. Mark Cohen, Dr. Susan Bookheimer, and Dr. John Mazziotta at the University of California, Los Findings from the present study indicated that only Angeles Brain Mapping Division, Los Angeles, CA, for per- the aneurysm clips made from commercially pure mitting use of the 3.0-T MR imaging system and, thus, facili- titanium or titanium alloy are definitely safe because tating the performance of this research project.
they exhibit no magnet-related movements in associ- ation with exposure to 3.0-T MR imaging systems.
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