The surgical management of intracranial aneurysms and arteriovenous malformations (AVMs) by the application of aneurysm clips is a well-established procedure. The presence of an aneurysm clip in a patient referred for an MR procedure represents a situation that requires the utmost consideration because of the associated risks.
Certain types of intracranial aneurysm clips (e.g., those made from martensitic stainless steels such as 17-7PH or 405 stainless steel) are a contraindication to the use of MR procedures because excessive, magnetically induced forces can displace these implants and cause serious injury or death. By comparison, aneurysm clips classified as "nonferromagnetic" or weakly ferromagnetic (e.g., those made from Phynox, Elgiloy, austentitic stainless steels, titanium alloy, or commercially pure titanium) are acceptable for patients undergoing MR procedures.
[For the sake of discussion, the term "weakly magnetic" refers to metal that may demonstrate some extremely low ferromagnetic qualities using highly sensitive measurements techniques (e.g., vibrating sample magnetometer, superconducting quantum interference device or SQUID magnetometer, etc.) and as such, may not be technically referred to as being nonmagnetic. All metals possess some degree of magnetism, such that no metal is entirely nonmagnetic.]
MR procedures have been used to evaluate patients with certain types of aneurysm clips. Becker et al., using MR systems that ranged from 0.35 to 0.6-Tesla, studied three patients with nonferromagnetic aneurysm clips (one patient, Yasargil, 316 LVM stainless steel; two patients, Vari-Angle McFadden, MP35N; 316 LVM) and one patient with a ferromagnetic aneurysm clip (Heifetz aneurysm clip) without incident. Dujovny et al. similarly reported no adverse effects in patients with nonferromagnetic aneurysm clips that underwent procedures using 1.5-Tesla MR systems.
Pride et al. performed a study in patients with nonferromagnetic aneurysm clips that underwent MR imaging. There were no adverse outcomes for the patients, confirming that MR procedures can be performed safely in patients with nonferromagnetic clips. Brothers et al. also demonstrated that MR imaging at 1.5-Tesla can be performed safely in patients with nonmagnetic aneurysm clips. This report was particularly important because, according to Brothers et al., MR imaging was found to be better than CT in the postoperative assessment of aneurysm patients, especially with regard to showing small zones of ischemia.
To date, only one ferromagnetic aneurysm clip-related fatality has been reported in the peer-reviewed literature. According to this report, the patient became symptomatic at a distance of approximately 1.2-meters from the bore of the MR system, suggesting that translational attraction of the aneurysm clip was likely responsible for dislodgment of this implant.
This incident was the result of erroneous information pertaining to the type of aneurysm clip that was present in the patient. That is, the clip was thought to be a nonmagnetic Yasargil aneurysm clip (Aesculap Inc., Central Valley, PA) and turned out to be a magnetic Vari-Angle clip (Codman & Shurtleff, Randolf, MA).
There has never been a report of an injury to a patient or individual in the MR environment related to the presence of an aneurysm clip made from a nonmagnetic or weakly magnetic material. In fact, there have been cases in which patients with ferromagnetic aneurysm clips (based on the extent of the artifact seen during MR imaging or other information) have undergone MR procedures without sustaining injuries (Personal communications, D. Kroker, 1995; E. Kanal, 1996; A. Osborne, 2002).
In these cases, the aneurysm clips were exposed to magnetic-induced translational attraction and torque associated with MR systems that had static magnetic fields of up to 1.5-Tesla. Although these cases do not prove or suggest safety, they do demonstrate the difficulty of predicting the outcome for patients with ferromagnetic aneurysm clips that undergo MR procedures. Variables to consider include the size, shape (especially the length of the blade), mass and material of the aneurysm clip.
There is controversy regarding the amount of ferromagnetism that needs to be present in an aneurysm clip to constitute a hazard for a patient in the MR environment. Consequently, this issue has not only created problems for MR healthcare professionals but for manufacturers of aneurysm clips, as well.
For example, MR healthcare professionals performing tests on aneurysm clips similar to the method described in the report by Kanal et al. (1996) presumably identified the presence of magnetic field interactions and returned several clips made from Phynox to the manufacturer (Personal Communication, Aesculap, Inc., South San Francisco, CA, 1997). However, the testing method used by Kanal et al. (1996) was admittedly crude and developed to primarily obtain rapid, qualitative screening data for large numbers of aneurysm clips to determine if quantitative assessments were necessary. Importantly, the test technique used by Kanal et al. (1996) may be problematic and yield spurious results, especially if the aneurysm clip has a shape or configuration that is somewhat "unstable" (Unpublished Observations, F.G. Shellock, 1997). For example, aneurysm clips with blades that are bayonet, curved, or angled shapes are less stable on a piece of plate glass (i.e., using the testing method described by Kanal et al.) when placed in certain orientations compared with aneurysm clips with blades that are straight.
A variety of more appropriate testing techniques have been developed and utilized over the years to evaluate the relative amount of ferromagnetism present for implants and devices prior to allowing patients with these objects to enter the MR environment. In 2002, the American Society for Testing and Materials (ASTM) provided recommendations for testing passive implants that involves the use of the deflection angle test, originally described by New et al., to assess translational attraction. Additionally, the U.S. Food and Drug Administration recommends that an evaluation of torque should be performed on aneurysm clips. Thus, procedures such as the deflection angle test and some form of evaluation of torque (qualitative or quantitative) are the most appropriate means of determining which specific aneurysm clip may present a hazard to a patient or individual in the MR environment.
Aneurysm Clips and MRI Procedures: Guidelines. In consideration of the knowledge pertaining to aneurysm clips, the following guidelines are recommended with regard to performing an MR procedure in a patient with an aneurysm clip or before allowing an individual with an aneurysm clip into the MR environment:
1) Specific information (i.e., manufacturer, type or model, and material) about the aneurysm clip must be known, especially with respect to the material used to make the aneurysm clip, so that only patients or individuals with nonferromagnetic or weakly ferromagnetic clips are allowed into the MR environment. The manufacturer provides this information in the labeling of the aneurysm clip. The implanting surgeon is responsible for properly recording and communicating this information in the patient's or individual's records.
2) An aneurysm clip that is in its original package and made from Phynox, Elgiloy, MP35N, titanium alloy, commercially pure titanium or other material known to be nonferromagnetic or weakly ferromagnetic does not need to be evaluated for ferromagnetism. Aneurysm clips made from nonferrromagnetic or weakly ferromagnetic materials in original packages do not require testing of ferromagnetism because the manufacturers ensure the pertinent MR safety or conditional aspects of these clips and, therefore, are responsible for the accuracy of the labeling.
3) If the aneurysm clip is not in its original package and/or properly labeled, it should undergo testing for magnetic field interactions following appropriate testing procedures to determine if it is safe or unsafe for the MR environment.
4) The radiologist and implanting surgeon are responsible for evaluating the information pertaining to the aneurysm clip, verifying its accuracy, obtaining written documentation, and deciding to perform the MR procedure after considering the risk vs. benefit aspects for a given patient.
5) Consideration must be give to the static magnetic field strength that is to be used for the MRI procedure and the strength of the static magnetic field that was used to test magnetic field interactions for the aneurysm clip in question.
MRI at 3-Tesla and Aneurysm Clips. Many aneurysm clips have been tested for magnetic field interactions in association with 3-Tesla MR systems (refer toThe List for information for aneurysm clips tested at 3-Tesla). Findings for these specific implants indicated that they either exhibited no magnetic field interactions or relatively minor or weak magnetic field interactions. Accordingly, these particular aneurysm clips are considered acceptable for patients undergoing MR procedures using MR systems operating at 3-Tesla or less.
Yasargil Aneurysm Clips (Information dated 11/11/09 - Aesculap Inc., Center Valley, PA)
Aesculap currently markets two lines of YASARGIL aneurysm clips, one from a cobalt-chrome alloy (Phynox) and one from a titanium alloy. Phynox clips have been available since 1983 and have catalog numbers that begin with "FE". Titanium clips have been available since 1997 and have catalog numbers that begin with FT. All "FE" and FT model YASARGIL aneurysm clips are non-ferromagnetic and may be safely exposed to MRI. Both implant materials have been tested and proven MR-safe as per ASTM-2052-02 up to 3.0 Tesla*.
Prior to 1985, Aesculap distributed various models of aneurysm clips manufactured from stainless steel. These aneurysm clips were identified with the letters "FD" and have not been proven safe under exposure to MRI. For this reason, Aesculap does not recommend the use of MRI on a patient implanted with a YASARGIL aneurysm clip identified with the letters "FD".
For additional information, please refer to the following three scientific publications:
1) Dujovny, M., et. al. (1985). Aneurysm clip motion during magnetic resonance imaging: in vivo experimental study with metallurgical factor analysis, Journal of Neurosurgery,. 17(4), 543-548.
2) Shellock, F.G., Kanal, E. (1998). Aneurysm clips: evaluation of MR imaging artifacts at 1.5. Radiology, 209(2), 563-566.
3) Romner, B., et. al. (1989). Magnetic resonance imaging and aneurysm clips, Journal of Neurosurgery, 70(3), 426-431.
*These Aesculap devices were cleared by FDA as MR-Safe per ASTM-2052-02. Due to a change in definition within this standard (F 2503-2005-08), these devices are now termed as MR Conditional by ASTM. The FDA has not mandated a revision to our labeling because the device material and performance have not changed.
Aneurysm Clips, Codman & Shurtleff, Inc., a Johnson & Johnson Company, Raynham, MA
Recently, three different MP35N aneurysm clips (Codman Slim-Line Aneurysm Clip, Straight, Blade length 25-mm; Codman Slim-Line Aneurysm Clip Graft, 5-mm Diameter X 5-mm width; Codman Slim-Line Aneurysm Clip, Reinforcing 30-degree angle, 6-mm X 18-mm; Codman & Shurtleff, Inc., a Johnson & Johnson Company, Raynham, MA) underwent MRI testing that represented the largest mass for 155 additional clips made from MP35N. The clips were evaluated at 3-Tesla for magnetic field interactions, heating, and artifacts. Each aneurysm clip showed relatively minor magnetic field interactions that will not cause movement in situ. Heating was not excessive (highest temperature change, < 1.8 degrees C). Artifacts may create issues if the area of interest is in the same area or close to the aneurysm clip. The results of this investigation demonstrated that it would be acceptable (i.e., MR conditional using current terminology) for patients with these aneurysm clips to undergo MRI at 3-Tesla or less. Notably, in consideration of the sizes of the clips that underwent testing, these findings pertain to 155 additional aneurysm clips made from the same material, which includes the following:
Codman Slim-Line Aneurysm Clip
Codman AVM Micro Clip
Codman Slim-Line Aneurysm Clip Graft
Codman Slim-Line Mini Aneurysm Clip
Codman Slim-Line Temporary Vessel Aneurysm Clip
The following information is the MRI labeling for these aneurysm clips approved by the Food and Drug Administration:
Magnetic Resonance Imaging Information
Non-clinical testing of representative configurations of Codman Slim-Line Aneurysm, Mini, Micro and Graft Clips (Codman Slim-Line Clips) up to 25-mm in blade length has demonstrated that they are MR Conditional. A patient with one such clip up to 25-mm in blade length (i.e., Codman Slim-Line Aneurysm, Mini, Micro and Graft Clips (Codman Slim-Line Clips) can be scanned safely, immediately after placement under the following conditions.
3.0 Tesla Systems:
-Static magnetic field of 3.0 Tesla.
-Spatial gradient magnetic field of 720 Gauss/cm or less.
-Maximum MR system reported, whole-body-averaged specific absorption rate (SAR) of 3.0 W/kg for 15 minutes of scanning (i.e., per pulse sequence).
MRI Related Heating
MRI related heating was assessed for the representative configurations of the Codman clips (up to 25-mm in blade length) following guidelines provided in ASTM F2182-02a. A maximum temperature change equal to or less than 1.8 degrees C was observed during testing (parameters listed below).
-Maximum MR system-reported, whole-body-averaged SAR of 3.0 W/kg (associated calorimetry measured whole body averaged value of 2.8 W/kg).
-15-minute duration MR scanning (i.e., per pulse sequence).
-3 Tesla MR System (EXCITE MR Scanner, Software G3 .0-052B, General Electric Healthcare, Milwaukee, WI) using a transmit/receive RF body coil.
Artifacts were assessed for representative clip configurations using T1-weighted spin echo (T1-SE) and gradient echo (GRE) pulse sequences following methods similar to the guidelines provided in ASTM F2119-07. The 25-mm blade-length aneurysm clip imaged using a GRE pulse sequence produced an artifact that extended approximately 5-cm from the clip in the parallel (long-axis) imaging plane. The void size corresponding to this artifact was approximately 1251-mm2. MR image quality may be compromised if the area of interest is in the exact location or within a few centimeters of the Codman Slim-Line Clip. In general, the GRE pulse sequence produced larger artifacts than the T1-SE sequence for each clip. However, MRI artifacts can be minimized by careful selection of pulse sequence parameters.
MRI at 8.0-Tesla and Aneurysm Clips. Ex vivo testing has been conducted to identify potentially hazardous implants and devices using an 8-Tesla MR system. The first investigation of this type was conducted by Kangarlu and Shellock.
Twenty-six different aneurysm clips were tested for magnetic field interactions using previously-described techniques. These implants were specifically selected for this investigation because they represent various types of clips used for temporary or permanent treatment of aneurysms or arteriovenous malformations. Additionally, these aneurysm clips were reported previously to be safe for patients undergoing MR procedures using MR systems with static magnetic field strengths of 1.5-Tesla or less.
According to the results, six aneurysm clips (i.e., type, model, blade length) made from stainless steel alloy (Perneczky) and Phynox (Yasargil, Models FE 748 and FE 750) displayed deflection angles above 45 degrees (i.e., referring to the guideline stated by the American Society for Testing and Materials, ASTM) and relatively high qualitative torque values. These findings indicated that these specific aneurysm clips may be unsafe for individuals or patients in an 8.0-Tesla or higher MR environment.
Aneurysm clips made from commercially pure titanium (Spetzler), Elgiloy (Sugita), titanium alloy (Yasargil, Model FE 750T), and MP35N (Sundt) displayed deflection angles less than 45 degrees (i.e., referring to the ASTM guideline) and qualitative torque values that were relatively minor. Accordingly, these aneurysm clips are considered to be acceptable for patients or individuals exposed to an 8.0-Tesla MR system.
As previously indicated, at 1.5-Tesla, aneurysm clips that are considered to be acceptable for patients or others in the MR environment include those made from commercially pure titanium, titanium alloy, Elgiloy, Phynox, and austentic stainless steel. By comparison, findings from the 8.0-Tesla study indicated that deflection angles for the aneurysm clips made from commercially pure titanium and titanium alloy ranged from 5 to 6 degrees, suggesting that these aneurysm clips would be safe for patients or individuals in the 8.0-Tesla MR environment. However, deflection angles for aneurysm clips made from Elgiloy ranged from 36 to 42 degrees, such that further consideration must be given to the specific type of Elgiloy clip that is present. For example, an Elgiloy clip that has a greater mass (e.g., due to a longer blade length) than those tested in this study may exceed a deflection angle of 45 degrees (i.e., referring to the ASTM guideline of 45 degrees or less being acceptable for translational attraction) in association with an 8.0-Tesla MR system.
Depending on the actual dimensions and mass, an aneurysm clip made from Elgiloy may or may not be acceptable for a patient or individual in the 8.0-Tesla MR environment. Notably, the results of this investigation are specific to the types of intracranial aneurysm clips that underwent testing (i.e., with regard model, shape, size, blade length, material, etc.) as well as the spatial gradient magnetic field associated with the 8.0-Tesla MR system.
Effects of Long-Term and Multiple Exposures to the MR System. MR testing procedures used for aneurysm clips over the past few years would result in the potential for reintroduction of aneurysm clips into strong magnetic fields several times prior to implantation into the patient. Furthermore, there are patients with implanted aneurysm clips previously tested and designated as MR-safe or MR-conditional that have undergone repeated exposures to strong magnetic fields during follow-up MR examinations.
A concern has emerged that a potential alteration in the magnetic properties of pre-or post-implanted aneurysm clips may occur that results from long-term or multiple exposures to strong magnetic fields. Long-term or multiple exposures to strong magnetic fields (such as those associated with MR imaging systems) have been suggested to grossly magnetize aneurysm clips, even if they are made from nonferromagnetic or weakly ferromagnetic materials. This could present a substantial hazard to an individual in the MR environment. Therefore, an in vitro investigation was conducted to study intracranial aneurysm clips prior to and following long-term and multiple exposures to 1.5-Tesla MR systems. This was done to quantify possible alterations in the magnetic properties of these aneurysm clips.
Aneurysm clips made from Elgiloy, Phynox, titanium alloy, commercially pure titanium, and austenitic stainless steel were tested in association with long-term and multiple exposures to 1.5-Tesla MR systems. The findings indicated that there was a lack of response to the magnetic field exposure conditions that were used, such that long-term or multiple exposures to 1.5-Tesla MR systems should not result in significant changes in their magnetic properties.
Artifacts Associated with Aneurysm Clips. An additional problem related to aneurysm clips is that artifacts produced by these metallic implants may substantially detract from the diagnostic aspects of MR procedures. MR imaging, MR angiography, and functional MRI are frequently used to evaluate the brain or cerebral vasculature of patients with aneurysm clips. For example, to reduce morbidity and mortality after subarachnoid hemorrhage, it is imperative to assess the results of the surgical treatment of cerebral aneurysms.
The extent of the artifact produced by a given aneurysm clip will have a direct effect on the diagnostic aspects of the MR procedure. Therefore, an investigation was conducted to characterize artifacts associated with aneurysm clips made from nonferromagnetic or weakly ferromagnetic materials. Five different aneurysm clips made from five different materials were evaluated in this investigation, as follows:
1) Yasargil, Phynox (Aesculap, Inc., Central Valley, PA),
2) Yasargil, titanium alloy (Aesculap, Inc., Central Valley, PA),
3) Sugita, Elgiloy (Mizuho American, Inc., Beverly, MA),
4) Spetzler Titanium Aneurysm Clip, commercially pure titanium (Elekta Instruments, Inc., Atlanta, GA), and
5) Perneczky, cobalt alloy (Zepplin Chirurgishe Instrumente, Pullach, Germany).
These aneurysm clips were selected for testing because they are made from nonferromagnetic or weakly ferromagnetic materials. These aneurysm clips have been previously reported to be acceptable for patients in the 1.5-Tesla MR environment and, as such, are often found in patients referred for MR procedures.
MR imaging artifact testing revealed that the size of the signal voids were directly related to the type of material (i.e., the magnetic susceptibility) used to make the particular clip. Arranged in decreasing order of artifact size, the materials responsible for the artifacts associated with the aneurysm clips were, as follows: Elgiloy (Sugita), cobalt alloy (Perneczky), Phynox (Yasargil), titanium alloy (Yasargil), and commercially pure titanium (Spetzler). These results have implications when one considers the various critical factors that are responsible for the decision to use a particular type of aneurysm clip (e.g., size, shape, closing force, biocompatibility, corrosion resistance, material-related effects on diagnostic imaging examinations, etc.).
An aneurysm clip that causes a relatively large artifact is less desirable because it can impact the diagnostic capabilities of the MR procedure if the area of interest is in the immediate location of where the aneurysm clip was implanted. Fortunately, aneurysm clips exist that are made from materials (i.e., commercially pure titanium and titanium alloy) that created minimal artifacts.
Burtscher et al. conducted additional artifact research with the intent of determining the extent to which titanium aneurysm clips could improve the quality of MR imaging compared to stainless steel aneurysm clips and to assess whether the associated artifacts could be reduced by controlling MR imaging parameters. The results of this investigation indicated that the use of titanium aneurysm clips reduced MR artifacts by approximately 60% compared to stainless steel aneurysm clips. MR imaging artifacts were further reduced by using spin echo pulse sequences with high bandwidths or, if necessary, gradient echo pulse sequences with a low echo times (TE).
American Society for Testing and Materials (ASTM) Designation: F 2052. Standard test method for measurement of magnetically induced displacement force on passive implants in the magnetic resonance environment. ASTM, West Conshohocken, PA.
Becker RL, Norfray JF, Teitelbaum GP, et al. MR imaging in patients with intracranial aneurysm clips. Am J Roentgenol 1988;9:885-889.
Brothers MF, et al. MR imaging after surgery for vertebrobasilar aneurysm. Am J Neuroradiol 1990;11:149-161.
Brown MA, Carden JA, Coleman RE, et al. Magnetic field effects on surgical ligation clips. Magnetic Resonance Imaging 1987;5:443-453.
Burtscher IM, et al. Aneurysm clip MR artifacts. Titanium versus stainless steel and influence of imaging parameters. Acta Radiology 1998;39:70-76.
Dujovny M, et al. Aneurysm clip motion during magnetic resonance imaging: In vivo experimental study with metallurgical factor analysis. Neurosurgery 1985;17:543-548.
FDA stresses the need for caution during MR scanning of patients with aneurysm clips. In: Medical Devices Bulletin, Center for Devices and Radiological Health. March, 1993;11:1-2.
Johnson GC. Need for caution during MR imaging of patients with aneurysm clips [Letter]. Radiology 1993;188:287.
Kakizawa Y, et al. Cerebral aneurysm clips in the 3-tesla magnetic field. Laboratory investigation. J Neurosurg 2010;113:859-69.
Expert Panel on MR Safety, Kanal E, Barkovich AJ, Bell C, et al. ACR guidance document on MR safe practices: 2013. J Magn Reson Imag 2013;37:501-30.
Kanal E, Shellock FG. MR imaging of patients with intracranial aneurysm clips. Radiology 1993;187:612-614.
Kanal E, Shellock FG. Aneurysm clips: Effects of long-term and multiple exposures to a 1.5-Tesla MR system. Radiology 1999;210:563-565.
Kanal E, Shellock FG, Lewin JS. Aneurysm clip testing for ferromagnetic properties: Clip variability issues. Radiology 1996;200:576-578.
Kangarlu A, Shellock FG. Aneurysm clips: Evaluation of magnetic field interactions with an 8.0-T MR system. J Magn Reson Imag 2000;12:107-111.
Khursheed F, et al. Artifact quantification and tractography from 3T MRI after placement of aneurysm clips in subarachnoid hemorrhage patients. BMC Med Imaging 2011;11:19.
Klucznik RP, et al. Placement of a ferromagnetic intracerebral aneurysm clip in a magnetic field with a fatal outcome. Radiology 1993;187:855-856.
Lauer UA, et al. Radio frequency versus susceptibility effects of small conductive implants-a systematic MRI study on aneurysm clips at 1.5 and 3 T. Magnetic Resonance Imaging 2005;23:563-9.
McFadden JT. Magnetic resonance imaging and aneurysm clips. J Neurosurg 2012;117:1-11.
New PFJ, et al. Potential hazards and artifacts of ferromagnetic and nonferromagnetic surgical and dental materials and devices in nuclear magnetic resonance imaging. Radiology 1983;147:139-148.
Olsrud J, et al. Magnetic resonance imaging artifacts caused by aneurysm clips and shunt valves: Dependence on field strength (1.5 and 3 T) and imaging parameters. J Magn Reson Imag 2005;22:433-7.
Pride GL, et al. Safety of MR scanning in patients with nonferromagnetic aneurysm clips. J Magn Reson Imag 2000;12:198-200.
Shellock FG, Crues JV, Editors. MRI Bioeffects, Safety, and Patient Management. Biomedical Research Publishing Group, Los Angeles, CA, 2014.
Shellock FG. Biomedical implants and devices: Assessment of magnetic field interactions with a 3.0-Tesla MR system. J Magn Reson Imag 2002;16:721-732.
Shellock FG, et al. Aneurysm clips: Evaluation of magnetic field interactions and translational attraction using “long-bore” and “short-bore” 3.0-Tesla MR systems. American Journal of Neuroradiology 2003;24:463-471.
Shellock FG, Crues JV. High-field strength MR imaging and metallic biomedical implants: An ex vivo evaluation of deflection forces. Am J Roentgenol 1988;151:389-392.
Shellock FG, Crues JV. Aneurysm clips: Assessment of magnetic field interaction associated with a 0.2-T extremity MR system. Radiology 1998;208:407-409.
Shellock FG, Kanal E. Aneurysm clips: Evaluation of MR imaging artifacts at 1.5-Tesla. Radiology 1998;209:563-566.
Shellock FG, Kanal E. Magnetic Resonance: Bioeffects, Safety, and Patient Management. Second Edition, Lippincott-Raven Press, New York, 1996.
Shellock FG, Kanal E. Yasargil aneurysm clips: Evaluation of interactions with a 1.5-Tesla MR system. Radiology 1998;207:587-591.
Shellock FG, Shellock VJ. MR-compatibility evaluation of the Spetzler titanium aneurysm clip. Radiology 1998;206:838-841.
Shellock FG. Valencerina S. In vitro evaluation of MR imaging issues at 3-T for aneurysm clips made from MP35N: Findings and information applied to 155 additional aneurysm clips. Am J Neuroradiol 2010;31:615-619.