Assessment of MRI Issues at 7 T for 28 Implants and Other Objects

Transcription

1 Neuroradiology/Head and Neck Imaging Original Research Neuroradiology/Head and Neck Imaging Original Research Adrienne N. Dula 1 John Virostko 1 Frank G. Shellock 2,3 Dula AN, Virostko J, Shellock FG Keywords: 7 T, implants, MRI, safety DOI: /AJR Received February 15, 2013; accepted after revision May 28, Department of Radiology and Radiological Sciences, Vanderbilt University Institute of Imaging Science, Vanderbilt University Medical Center, st Ave S, Medical Center N, AA-1105, Nashville, TN Address correspondence to A. N. Dula (adrienne.n.dula@vanderbilt.edu). 2 Institute for Magnetic Resonance Safety, Education, and Research, Los Angeles, CA. 3 Department of Radiology, Keck School of Medicine, University of Southern California, Los Angeles, CA. AJR 2014; 202: X/14/ American Roentgen Ray Society Assessment of MRI Issues at 7 T for 28 Implants and Other Objects OBJECTIVE. Metallic implants are currently a contraindication for volunteer subjects and patients referred for 7-T examinations because of concerns related to magnetic field interactions and MRI-related heating. Artifacts may also be problematic. Therefore, the purpose of this investigation was to evaluate these MRI issues for 28 implants and other objects in association with a 7-T MR system. MATERIALS AND METHODS. Tests were performed at 7 T using standardized procedures to evaluate magnetic field interactions (translational attraction and torque) for all 28 items. MRI-related heating and artifacts were assessed using spin-echo and gradient-echo pulse sequences, respectively, for two aneurysm clips located within a transmit-receive head radiofrequency coil. RESULTS. Eight of the 28 items showed magnetic field interactions at levels that could pose risks to human subjects. The two aneurysm clips exhibited heating, but the temperature rise did not exceed 1 C. Artifacts were dependent on the material and dimensions of each aneurysm clip. CONCLUSION. These findings show that certain implants and objects may be acceptable for human subjects undergoing MRI examinations at 7 T, whereas others may involve possible risks. This information has important implications for individuals referred for MRI examinations at 7 T. M ajor advances in MRI capabilities have occurred soon after the adoption of higher static magnetic fields: The overall signal-to-noise ratio, sensitivity to soft tissue, quality of nuclear MR spectra, and spatial resolution of images have improved with each advance in field strength [1]. Increases in field strength also have resulted in specialized contrast mechanisms and decreased imaging times. The recent development of 7-T MRI systems has been reported to be advantageous for neurologic applications, including multiple sclerosis [2 4], Alzheimer disease [5], epilepsy [6, 7], movement disorders [8], and stroke [9], and for nonneurologic studies of the breast [10], musculoskeletal system [11], and spine [12]. Each increment increase in the strength of the static magnetic field has impacted sensitivity and contrast and has increased spatial and temporal resolution; these improvements have yielded new structural and functional information that ultimately broadens the applications of MRI. The increase to 7 T poses potential safety concerns, particularly in subjects with metallic implants or other objects (e.g., foreign bodies). Therefore, the presence of metallic items is currently a contraindication for 7-T MRI examinations because of presumed adverse interactions with the static magnetic field and MRI-related heating. Unfortunately, restrictions such as these preclude a large subset of volunteer subjects as well as patients in particular, patients with conditions currently of interest for investigation at 7 T. Thus, there is a need for systematic safety analyses of commonly used metallic implants and other objects in order for 7-T MRI to attain its potential clinical and research utility. Although standardized testing procedures exist to characterize MRI issues for implanted objects in association with MRI, testing a large variety of metallic items has not been performed at 7 T to our knowledge. The purpose of this investigation was to evaluate MRI issues for 28 different metallic implants and other objects in association with a 7-T MR system. Materials and Methods Implants and Other Objects Twenty-eight different implants and other objects were selected for evaluation at 7 T (Table 1). These items were selected to provide a broad range of objects of a variety of sizes, shapes, and mate- AJR:202, February

2 rials and included the following: two aneurysm clips, one hemostatic clip, seven vascular implants, eight orthopedic implants, eight biopsy tissue markers, and two miscellaneous objects. Each of the 28 items underwent evaluation of magnetic field interactions. The specific implants were selected on the basis of the availability of commonly used implants that have been characterized at lower field strengths. Breast biopsy markers were included because the estimated rate of placement is 1.6 million markers per year. Thus, it was vital to obtain information about these markers to permit MRI examinations to be performed of patients with those implants at 7 T. Because the 7-T MR system currently does not have a transmit radiofrequency body coil, MRIrelated heating and artifacts are not a concern for metallic objects present outside the head. Therefore, MRI-related heating and artifacts were assessed for only two aneurysm clips that would be located within the transmit-receive head radiofrequency coil during an examination at 7 T. Assessment of Magnetic Field Interactions The implants and other objects were assessed for translational attraction and torque using a 7-T MR system (Achieva, Philips Healthcare). Translation attraction Translational attraction of each item was measured using a standardized technique, as previously described [13 16], that is based on the American Society for Testing and Materials (ASTM) International standard [16]. Each item was suspended from a lightweight string (weight < 1% of the weight of the object) and attached to a test apparatus (protractor with 1 -graduated markings mounted to the structure) to measure the deflection angle. Deflection angle recordings were obtained at the point of the highest patient-accessible spatial gradient magnetic field for the 7-T MR system. To properly assess translational attraction for the implants and other objects, it was necessary to characterize the spatial gradient magnetic field associated with the 7-T MR system. Therefore, to accomplish this task, a vector magnetometer (model THM-1176, Metrolab) was placed at regular increments along the scanner axis to determine the magnetic field strength as a function of distance from the magnet isocenter. The test fixture was placed at the determined position of highest spatial magnetic gradient [17], and the deflection angle of each item from vertical was measured. The test fixture was removed and replaced a total of three times for each item, and the average deflection angle was calculated. Torque Magnetic field induced torque was determined using a previously described qualitative assessment technique [13 15] based on the ASTM International standard [18]. Torque was determined at the isocenter of the scanner where torque effects are known to be the greatest. Each item was placed on a plastic board with a millimeter grid and observed for movement. Each item was then rotated in 45 increments through a full 360 of rotation and observed for movement at each orientation. The following qualitative scale was used to describe alignment or rotation: 0, no torque; 1, mild or low torque, the device slightly changed orientation but did not align to the magnetic field; 2, moderate torque, the device aligned gradually to the magnetic field; 3, strong torque, the device showed rapid and forceful alignment to the magnetic field; and 4, very strong torque, the device showed very rapid and very forceful alignment to the magnetic field. The highest qualitative torque values are reported. Assessment of MRI-Related Heating MRI-related heating was determined for two aneurysm clips that may be found in human subjects encompassed within a transmit-receive head radiofrequency coil during a typical brain examination. A gel-saline mixture, referred to as 'gelled saline' in the ASTM standard [19], was used to fill a phantom that was placed in the transmit-receive head radiofrequency coil. Each aneurysm clip was placed in the location of the highest background local specific absorption rate (SAR) as determined experimentally. Fiberoptic temperature probes (model FOT-M, Fiso) were placed at the locations presumed to have the highest radiofrequency energy deposition. For elongated implants, such as these aneurysm clips, these locations were the tips of the clips. The gelledsaline phantom containing the clip and temperature probe was wrapped in insulating material and placed in the transmit-receive head coil. Fluoroptic thermometry was used to continuously measure MR-related temperature changes during application of a pulse sequence eliciting 100% of the allowed SAR to create extreme MRI-related heating conditions. Thus, a radiofrequency-intensive turbo spin-echo (factor = 4) sequence was implemented with a TR of seconds, initial TE of 8 ms, subsequent TE of 100 ms, and flip angle of 180. An imaging matrix of was used with an FOV equal to 220 mm 2, which resulted in a total imaging time of 18 minutes 29 Fig. 1 Determination of spatial gradient magnetic field ( B 0 ) for 7-T MR system. Measured static magnetic field (dashed line) and calculated static gradient field (solid line) are shown. Highest spatial gradient was found to be 708 G/cm at 138 cm from isocenter. db = change in measured static magnetic field (G), Dz = change in distance from magnet isocenter (cm). Measured B 0 (G [ 10 4 ]) seconds. The positions of the thermometry probes were inspected and verified immediately before and after the MRI-related heating experiment. The highest temperature rises are reported. Assessment of Artifacts The two aneurysm clips that would be visible on an MRI scan of the brain were tested for artifacts using a previously described protocol [13 15] based on ASTM standard protocols [20]. Each clip was attached to a plastic frame and placed in a CuSO 4 -doped (1.14 g/l) water-filled phantom and inside the transmit-receive head radiofrequency coil. Imaging consisted of gradient-echo sequences (TR/TE, 166/3.5; FOV, mm; matrix, 72 70; 30 slices) performed with the imaging planes aligned with the long axis of the implant and then repeated with the imaging planes aligned with the short axis of each clip. The resulting image artifacts were quantified using planimetry to calculate the area of the induced artifact. Results According to static magnetic field measurements, the highest patient-accessible spatial gradient of the magnetic field is 708 G/ cm and is located at the edge of the bore at 137 cm from the magnet isocenter (Fig. 1). Table 1 summarizes the findings for the deflection angles and torque values for the 28 items in association with the 7-T static magnetic field. In general, items that showed high translational attraction also exhibited high torque. The deflection angles ranged from 7 to 49 for the aneurysm clips, 90 for the hemostatic clip, 1 49 for the vascular implants, 0 55 for the orthopedic implants, 0 18 for the biopsy tissue markers, and for the miscellaneous objects. Qualitative torque measurements ranged from 0 to 4 for the aneurysm clips, 4 for the hemostatic clip, 0 3 for the vascular implants, 0 2 for the orthopedic implants, 0 1 for the biopsy markers, and 3 4 for the miscellaneous objects Distance From Isocenter (cm) B = db/dz (G/cm) 402 AJR:202, February 2014

3 TABLE 1: Biomedical Implants and Devices Tested for Magnetic Field Interactions at 7 T Type of Implant or Device and No. Brand Name (Description) Vendor (Location) Deflection Angle ( ) Aneurysm clips 1 Yasargil FE 863 K b (permanent long clip, 40 mm) Aesculap (Center Valley, PA) Yasargil FT 790D c (permanent standard clip, Aesculap (Center Valley, PA) mm blade) Hemostatic clip 3 Resolution Clip (hemostatic clip) Boston Scientific (Natick, MA) 90 4 Vascular implants 4 TMR (coronary artery stent, 4 28 mm) Biocore Biotechnologia (Canoas, Brazil) Luminexx (vascular stent) Bard Peripheral Vascular (Tempe, AZ) Valeo (biliary Y stent) Bard Peripheral Vascular (Tempe, AZ) corvcd (conduit coupling device) corlife GBR (Hanover, Germany) VenaTech LP b (vena cava filter) B. Braun (Bethlehem, PA) VenaTech LGM b (vena cava filter) B. Braun (Bethlehem, PA) Celsite (vascular access port) B. Braun (Bethlehem, PA) 1 0 Orthopedic implants 11 PEEK Power HTO Plate (tibial fixation device) Arthrex (Naples, FL) PEEK Power Distal Radius Plate (radial fixation Arthrex (Naples, FL) 0 0 device) 13 PyroCarbon Implant Replacement (knee implant) Moirai Orthopaedics (Metairie, LA) Krackow HTO staple (cobalt chrome staple) Smith & Nephew (London, UK) Oxinium femoral component (oxidized zirconium Smith & Nephew (London, UK) 5 1 knee femoral component) 16 Synergy Hip System (compression hip screw, Smith & Nephew (London, UK) 55 2 plate, lag screws) 17 Summit Hip Stem c with CoCrMo Head (stem) Smith & Nephew (London, UK) Orthofusion Cannulated Screw (orthopedic Silver Bullet Therapeutics (San Jose, CA) 8 0 screw) Breast biopsy markers 19 UltraClip II Tissue Marker d (marker) Bard Biopsy Systems (Tempe, AZ) MicroMark II (marker) Ethicon Endo-Surgery (Cincinnati, OH) UltraClip Dual Trigger (marker) Bard Biopsy Systems (Tempe, AZ) GelMark Ultra GMU 11 T d (marker) Bard Biopsy Systems (Tempe, AZ) Mammotome CorMARK Tissue Marker (marker) Devicor Medical Products (Cincinnati, OH) HydroMark Breast Biopsy Marker (marker) Biopsy Sciences (Clearwater, FL) Altec TriMark Titanium Biopsy Marker (marker) Hologic (Bedford, MA) GelMark Ultra MK 2011 d (marker) Bard Biopsy Systems (Tempe, AZ) 6 0 Miscellaneous 27 Port-A-Cath Vascular Access Port (19-gauge SIMS Deltec (St. Paul, MN) 70 3 needle, metal hub, 1.5 inches) 28 Armor-Piercing Full Metal Jacket (bullet) Norinco (Beijing, China) 90 4 a The following qualitative scale was used to describe alignment or rotation; 0, no torque; 1, mild or low torque, the device slightly changed orientation but did not align to the magnetic field; 2, moderate torque, the device aligned gradually to the magnetic field; 3, strong torque, the device showed rapid and forceful alignment to the magnetic field; and 4, very strong torque, the device showed very rapid and very forceful alignment to the magnetic field. b Material: Phynox (cobalt-chromium-nickel alloy), Alloy Wire International. c Material: titanium alloy. d Material: titanium. Torque a AJR:202, February

4 For the aneurysm clips, the highest temperature rises were 0.8 C for the aneurysm clip with the 20-mm-long blade and 0.6 C for the clip with the 40-mm-long blade (Fig. 2). The local SAR values were calculated as follows: SAR = c T t where c is the specific heat capacity of the phantom, ΔT is change in temperature, and Δt is change in time. The calculated SAR values were 2.99 and 1.50 W/kg for the clips with the 20- and 40-mm-long blades, respectively. The largest artifact was observed for the aneurysm clip with the 40-mm-long blade in the long-axis orientation, and the volume of signal dropout was measured to be 25,037 mm 3. The largest artifact was found with the aneurysm clip with the 20-mm-long blade in the long-axis orientation and was measured to be 1850 mm 3. Figure 3 shows examples of the artifacts for the aneurysm clips. Fig. 2 Graph shows radiofrequency-induced heating of two aneurysm clips clip with 40-mmlong blade (dashed line) and clip with 20-mm-long blade (solid line) that underwent radiofrequency-intensive spin-echo MRI at 7 T. Temperature Change ( C) Fig. 3 Artifacts induced by two aneurysm clips acquired using gradientecho pulse sequence at 7 T. A and B, MR images obtained of aneurysm clip with 40-mm-long blade in short axis (A) and long axis (B). C and D, MR images obtained of aneurysm clip with 20-mm-long blade in short axis (C) and long axis (D) Time (s) Discussion The adoption of 7-T MR systems necessitates a better understanding of the potentially adverse interactions with metallic implants and other objects to ensure the safety of volunteer subjects and patients undergoing MRI. Currently, a human subject with a known metallic implant is not permitted to undergo MRI at 7 T. This policy is notable because it means that a large population of individuals with conditions amenable to study at 7 T are excluded from undergoing MRI at this very high static magnetic field strength. Thus, this research has important implications because it identifies field interactions with common biomedical implants for patients under specific conditions at 7 T. It is well known that magnetic field interactions can be hazardous to human subjects with metallic implants and foreign bodies. Because the translational attraction is proportional to the static magnetic field strength, results for metallic objects reported at lower field strengths [21] are not applicable to metallic objects at 7 T. The guideline from the ASTM International [16] states that if the deflection angle for an object is less than 45, it is safe because the force from the magnetic field attraction is less than the force of gravity. Thus, deflection angle results above 45 have potential safety implications for metallic objects exposed to the 7-T MRI environment, which may be of concern for the following items: the aneurysm clip with a 40-mm-long blade (Yasargil FE 863 K, Aesculap [rated MR conditional at 3 T]); Resolution Clip (Boston Scientific [rated MR conditional at 3 T]); VenaTech LP and VenaTech LGM (B. Braun [rated MR conditional at 3 T]); compression hip screw, plate, and lag screws (tested as assembly) (Synergy Hip System, Smith & Nephew [rated MR safe, according to terminology used before 2005, at 3 T]); Summit Hip Stem (Smith & Nephew [rated MR safe, according to terminology used before 2005, at 3 T]); Port-A-Cath Vascular Access Port (SIMS Deltec [rated MR conditional at 3 T]); and bullet (Armor-Piercing Full Metal Jacket, Norinco [rated MR unsafe at 3 T]). However, it is important to consider the nature of the implant including its physiologic purpose, anatomic location, date implanted, and surrounding critical anatomy. The torque associated with the powerful static magnetic field of an MR system also poses additional safety concerns. Similar to translational attraction, torque is proportional to the magnetic field strength; thus, results for torque on devices reported at lower field strengths cannot be translated to 7 T. Qualitative torque measurements correlated with translational attraction measurements: All eight implants with deflection angles exceeding 45 also displayed high qualitative torque values. Implants located within the transmit radiofrequency coil are subjected to MRI-related or radiofrequency-induced heating during an MRI examination [13 15, 19, 21]. To date, 7-T MR systems do not have a transmit radiofrequency coil; thus, MRI-related heating will impact only those metallic objects that are located within the volume of the transmit radiofrequency coil used for the MRI procedure. Therefore, in this investigation, the aneurysm clips underwent evaluation of heating using a 404 AJR:202, February 2014

5 transmit-receive head radiofrequency coil and a tissue-mimicking phantom. The findings indicated that both aneurysm clips displayed heating under high-sar conditions; however, the temperature rises did not exceed 1 C, a threshold typically considered to preclude health effects in human subjects [19]. The presence of a passive metallic implant is known to cause distortion and signal loss artifacts in MR images. In this study, we found that the presence of aneurysm clips within the radiofrequency coil resulted in signal losses that were proportional to the size and shape of each clip. The clip made from cobalt-chromium-nickel alloy (Phynox, Alloy Wire International) produced larger artifacts than those for the clip made from titanium (i.e., even in consideration of the fact that the Phynox clip had a longer blade), which is understandable given the magnetic susceptibility of these particular materials. These artifacts may be problematic for brain MRI procedures performed at 7 T. However, when a metallic object is located in the FOV during an MRI examination, the imaging parameters may be optimized or MR techniques to minimize the extent of metal-related artifacts may be used [20]. Possible Limitations As we stated earlier, the heating and artifact assessments were applied to only two of the 28 implants and other objects that underwent evaluation in this investigation because the other items would be located outside the region of the radiofrequency transmission area. Accordingly, it was necessary to assess only magnetic field interactions for the remaining 26 items. However, once a transmit body radiofrequency coil or other type of anatomy-specific radiofrequency coil becomes available for 7-T MR systems, it will be necessary to repeat the MRI-related heating test for the implants and other objects that would be subjected to radiofrequency energy to ensure the safety of MRI examinations of patients with those items. Additionally, because most current 7-T work is in neuroimaging, future work will include evaluation of ventricular shunt valves as well as other neurosurgical implants. Conclusions The results of testing 28 implants and other items for MRI issues resulting from magnetic field interactions at 7 T are reported. Eight of the items exhibited substantial magnetic field interactions that may pose risks to human subjects undergoing MRI on a 7-T scanner. Several of the implants we tested that were known to be MR conditional at 3 T proved to be potentially unsafe at 7 T. However, the other 20 implants did not exhibit significant magnetic qualities at 7 T and, thus, are considered to be acceptable for subjects referred for 7-T MRI procedures as long as they are not located within a transmit radiofrequency coil. 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