TY - JOUR
AU - PhD, Frank G. Shellock,
AB - ABSTRACT The objective of this project was to evaluate magnetic resonance imaging (MRI) issues at 3 T for an armor-piercing bullet and to determine if this item could be identified using a ferromagnetic detection system. An armor-piercing bullet (.30 caliber, 7.62 × 39, copper-jacketed round, steel core; Norinco) underwent evaluation for magnetic field interactions, heating, and artifacts using standardized techniques. Heating was assessed with the bullet in a gelled-saline-filled phantom with MRI performed using a transmit/receive radio frequency body coil at a whole-body-averaged specific absorption rate of 2.9 W/kg for 15 minutes. Artifacts were characterized using T1-weighted spin echo and gradient echo pulse sequences. In addition, a special ferromagnetic detection system (Ferroguard Screener; Metrasens, Lisle, Illinois) was used in an attempt to identify this armor-piercing bullet. The findings indicated that the armor-piercing bullet showed substantial magnetic field interactions. Heating was not excessive. Artifacts were large and may create diagnostic problems if the area of interest is close to this bullet. The ferromagnetic detection system yielded a positive result. We concluded that this armor-piercing bullet is MR unsafe. Importantly, this ballistic item was identified using the particular ferromagnetic detection system utilized in this investigation, which has important implications for MRI screening and patient safety. INTRODUCTION Understanding the effects of magnetic resonance imaging (MRI) on foreign bodies is imperative for the proper management of patients, especially in consideration of situations involving military and civilian gunshot injuries that result in retained ballistic items.1,–7 Although many bullets are made from nonferromagnetic materials, studies have reported that they are often “contaminated” by ferromagnetic materials. In addition, certain bullets are inherently ferromagnetic because of the specific materials used for fabrication,3 including armor-piercing or “cop killer” bullets. Thus, ballistic objects may pose deleterious effects in patients referred for MRI examinations mainly because of magnetic field interactions, although heating and artifacts could also present unwanted problems.2,–5,7 To date, there has been no MRI evaluation of ballistic objects performed at static magnetic field strengths above 1.5 T. Therefore, the primary purpose of this investigation was to evaluate MRI safety issues (i.e., magnetic field interactions, heating, and artifacts) at 3 T for an armor-piercing bullet. Recently, a variety of different ferromagnetic detection systems have been used to supplement MRI screening procedures with the intent of preventing accidents related to external ferromagnetic objects (e.g., oxygen tanks, pocket knives, hearing aids, cell phones, etc.).8 Because it is well known that human tissue is transparent to magnetic fields, a ferromagnetic detection system could also identify ferromagnetic implants or foreign bodies. However, to our knowledge, there has been no report of this application for these screening tools. Importantly, if a ferromagnetic foreign body was missed during routine pre-MRI screening, utilizing a ferromagnetic detection system to discover the object in a patient before entry into the MR system room could potentially avoid a serious injury. In consideration of this scenario, the secondary purpose of this study was to determine if a ferromagnetic detection system could identify the armor-piercing bullet. This has important implications for MRI screening and patient safety. MATERIALS AND METHODS Armor-Piercing Bullet The ballistic sample evaluated in this investigation was a .30 caliber, 7.62 × 39, copper-jacketed round, lead core (referred to as “armor-piercing bullet”; Norinco, Beijing, China; www.Norinco.com) (Fig. 1). This ordnance comprised a lead core with a steel encasement and an outer copper jacket and was selected because it was known to be ferromagnetic (although the degree of ferromagnetism at 3 T was unknown). In addition, this ballistic item was selected for evaluation because it is a round commonly used in an AK-47 assault rifle and thus may be encountered in patients, particularly military personnel, referred for MRI examinations. FIGURE 1 View largeDownload slide Armor-piercing bullet evaluated in this investigation (.30 caliber, 7.62 × 39, copper-jacketed round, steel core). FIGURE 1 View largeDownload slide Armor-piercing bullet evaluated in this investigation (.30 caliber, 7.62 × 39, copper-jacketed round, steel core). Magnetic Field Interactions Tests for magnetic field interactions involved evaluations of translational attraction and torque for the armor-piercing bullet using a short-bore, 3-T MR system (Excite; General Electric Healthcare, Milwaukee, Wisconsin). Translational Attraction The deflection angle test was conducted to assess translational attraction.9,–11 The bullet was attached to a test fixture that consisted of a protractor with 1° graduated markings, mounted on an apparatus so that the 0° mark was oriented vertically.9,–11 The bullet was suspended from a 20-cm length of a string (weight less than 1% of the bullet), which was attached to the 0° indicator of the protractor. The protractor device was positioned at the highest “patient accessible” spatial gradient magnetic field for the 3-T MR system, which occurs at an off-axis position that is 74 cm from isocenter of the scanner.9,–12 This point was determined for the MR system using gauss line plots, measurements using a gauss meter (Extech 480823 Electromagnetic Field and Extremely Low Frequency Meter; Extech Instruments, Nashua, New Hampshire) and visual inspection to identify the location where the spatial magnetic field gradient was the greatest. The deflection angle from the vertical position to the nearest 1° was measured three times and the mean value was calculated. Because the deflection angle was found to be 90°, translational attraction was also measured for the body-piercing bullet using a digital force gauge, as previously described.13 The bullet was attached to a lightweight string with the other end attached to the digital force gauge (Model 475040; Extech Instruments, Waltham, Massachusetts). The gauge was positioned more than 10 ft from the MR system to avoid influence from the static magnetic field. The peak translational force was recorded for the bullet at the point of the highest spatial gradient magnetic field, three times, and a mean value was calculated. Torque Magnetic field–induced torque was assessed qualitatively for the armor-piercing bullet, as previously described.12,13 The bullet was placed on the test apparatus (flat plastic material with a millimeter grid on the bottom), which was positioned in the center of the 3-T MR system.11,12 The armor-piercing bullet was observed for alignment relative to the static magnetic field while it was placed in 45° increments to encompass 360° of rotation. This procedure was conducted three times and a mean value of torque was calculated. The following qualitative scale was applied to the results11,12: 0, no torque; +1, low torque (object shifted slightly but did not align to the magnetic field); +2, moderate torque (object aligned gradually to the magnetic field); +3, strong torque (object showed rapid and forceful alignment to the magnetic field); and +4, very strong torque (object showed very rapid and very forceful alignment to the magnetic field). MRI-Related Heating Phantom and Experimental Setup The MRI-relating heating test used a plastic, American Society of Testing Materials head/torso phantom that was prepared to simulate human tissue.11,14 The phantom was filled to a depth of 10 cm with gelled-saline (i.e., 1.32 g/L NaCl and 10 g/L polyacrylic acid in distilled water).11 The bullet was placed in the phantom at the position associated with a high uniform electric field tangential to this object, which ensured a worst-case or extreme condition for heating.11,14,15 MRI Conditions MRI was performed at 3 T/128 MHz (Excite, Software HDx, Software 14X.M5) using the body radio frequency (RF) coil to transmit RF energy. A relatively high level of RF energy was applied to produce an MR system reported, with whole-body-averaged specific absorption rate of 2.9 W/kg (calorimetry value, 2.7 W/kg) for 15 minutes. This specific absorption rate value is above the Normal Operating Mode but below the First Level Controlled Mode of operation for an MR system. Temperature Recording System and Placement of Thermometry Probes Temperature recordings were obtained using a fluoroptic thermometry system (Luxtron Model 3100; LumaSense, Santa Clara, California). Fiber-optic probes were applied to the bullet and calibrated before obtaining temperature measurements. Placement of the probes was as follows: Probe 1, placed in contact with one end of the bullet; Probe 2, placed in contact with the other end of the bullet; and Probe 3, placed in contact with the middle of the bullet. The thermometry probes were visually inspected immediately before and after the heating experiment to ensure proper positioning. In a separate heating experiment, “background” temperatures were recorded in the American Society of Testing Materials head/torso phantom at the same temperature probe positions used for the evaluation of MRI-related heating for the armor-piercing bullet.11 In each case, baseline (pre-MRI) temperatures were recorded at 4-second intervals before MRI (5 minutes), during MRI (15 minutes), and after MRI (2 minutes). The highest temperature changes were recorded for the armor-piercing bullet and for those obtained during the background heating assessment. This is the standard test methodology applied to study MRI-related heating for a metallic object.10,11,14 Artifacts Artifacts were characterized for the armor-piercing bullet with this object placed in a gadolinium-doped, saline-filled plastic phantom and conducting 3-T MRI using a transmit/receive RF head coil.11,12 The following pulse sequences were used, as previously described:11,12 (1) T1-weighted spin echo (SE) pulse sequence—repetition time, 500 milliseconds; echo time, 20 milliseconds; matrix size, 256 × 256; section thickness, 10 mm; field of view, 28 cm; number of excitations, 2; and bandwidth, 16 kHz. (2) Gradient echo (GRE) pulse sequence—repetition time, 100 milliseconds; echo time, 15 milliseconds; flip angle, 30°; matrix size, 256 × 256; section thickness, 10 mm; field of view, 28 cm; number of excitations, 2; and bandwidth, 16 kHz.11,12 The frequency encoding direction was parallel to the plane of imaging. We recognize that there are other possible MRI parameters that may be used to evaluate artifacts; however, this particular methodology has been applied to characterize artifacts for many different metallic implants and devices and thus it permits comparison to other objects that have undergone similar assessments for artifacts.7,11,12 The imaging planes were oriented to encompass the long axis and short axis of the armor-piercing bullet. Section locations were selected from several scout images to represent the largest or worst-case artifacts. Planimetry software was used to measure the maximum cross-sectional areas for the artifacts (accuracy ±10%) for each MRI condition, ensuring that the sizes of the artifacts were not underestimated.11,12 Ferromagnetic Detection In addition to investigating MRI safety issues for the armor-piercing bullet, a secondary goal was to determine if this object could be identified using a ferromagnetic detection system. The device (Ferroguard Screener; www.Metrasens.com) that was utilized in our study is configured as a post or pillar that may be installed in a variety of possible places in the MRI environment, outside of the MR system room (Fig. 2). Unlike a “portal” or handheld device, this particular ferromagnetic detection system was selected for use in this study because its configuration permits its potential utilization to effectively screen patients with implants or foreign bodies (see description below). FIGURE 2 View largeDownload slide Ferromagnetic detection system used in this investigation. Ferromagnetic detection system shown mounted on the wall to facilitate screening before entry into the MR system room. This is a pillar that permits the individual to stand in front and rotate 360° and, thus, screen for ferromagnetic objects. FIGURE 2 View largeDownload slide Ferromagnetic detection system used in this investigation. Ferromagnetic detection system shown mounted on the wall to facilitate screening before entry into the MR system room. This is a pillar that permits the individual to stand in front and rotate 360° and, thus, screen for ferromagnetic objects. The ferromagnetic detection system uses fluxgate sensors, which are the most sensitive type of solid-state magnetometer that can be used conveniently for the application in such a device.8 This screening system incorporates several fluxgate sensors to provide head-to-toe coverage of the individual or patient. The sensors are electronically configured to remove large unwanted magnetic signals, such as the Earth's geomagnetic field and power line fields. Thus, this ferromagnetic detection system is only sensitive to changing magnetic fields to distinguish magnetic objects from the Earth's magnetic field. The combination of high sensitivity to magnetic objects and high immunity to environmental noise is essential to reliably detect small ferromagnetic objects.8 Because the magnetic field of an object can be static (i.e., no movement) with respect to the item, the object needs to be in motion to make the field at the sensor change so that it can be detected. This is achieved by having the individual or patient rotate in front of the ferromagnetic detection system. To screen a patient with possible implants or foreign bodies, the individual would stand at a predetermined position (i.e., in consideration of the fact that distance is a known factor that may affect the sensitivity of detecting a ferromagnetic object) in front of this pillar and rotate slowly for a 360° rotation. (Note: This procedure is specific to the use of this particular ferromagnetic detection system.) The following protocol was utilized: a test subject without implants was confirmed to be free of ferromagnetic objects by standing two inches away from the ferromagnetic detection system and rotating 360°. A negative finding was, thus, verified. The armor-piercing bullet was then secured to this test subject. The placement of the bullet was selected to simulate its presence in a mid-thorax position. (Note: Human tissue is transparent to magnetic fields and, therefore, it is not necessary to have the bullet internalized for this evaluation.) The test subject approached the front of the ferromagnetic detection system to a distance of 2 inches (5 cm) and then rotated 360° (note that with the subject rotating 360°, the distance of the bullet from the pillar varies). Data were collected by having the test subject rotate four times at a rate of 4 seconds per rotation, with a pause of 3 seconds between each trial. The ferromagnetic detection system was observed to determine if there was evidence of a positive (i.e., as indicated by illumination of amber- or red-colored lights) or negative finding during each trial. RESULTS The deflection angle was 90°, and the qualitatively measured torque was +4 for the armor-piercing bullet. Translational attraction measured using the digital force gauge yielded a value of 3.30 N (329 g). The highest temperature change for the bullet was 1.7°C, whereas the background temperature rise was 1.5°C. The ferromagnetic detection system gave a positive alarm (i.e., positive alarm in four/four trials) when used to screen the study subject with the armor-piercing bullet. Artifact test results are displayed in Table I. The artifacts associated with the armor-piercing bullet were seen as large signal voids. The GRE pulse sequence produced larger artifacts than the T1-weighted SE pulse sequence. Figure 3 presents artifacts shown on images obtained using the GRE sequence. TABLE I Summary of MRI Artifacts at 3 T for the Armor-Piercing Bullet Pulse Sequence  T1-SE  T1-SE  GRE  GRE  Signal Void Size  36,933 mm2  12,808 mm2  62,395 mm2  61,658 mm2  Imaging Plane  Parallel (Long Axis)  Perpendicular (Short Axis)  Parallel (Long Axis)  Perpendicular (Short Axis)  Pulse Sequence  T1-SE  T1-SE  GRE  GRE  Signal Void Size  36,933 mm2  12,808 mm2  62,395 mm2  61,658 mm2  Imaging Plane  Parallel (Long Axis)  Perpendicular (Short Axis)  Parallel (Long Axis)  Perpendicular (Short Axis)  View Large TABLE I Summary of MRI Artifacts at 3 T for the Armor-Piercing Bullet Pulse Sequence  T1-SE  T1-SE  GRE  GRE  Signal Void Size  36,933 mm2  12,808 mm2  62,395 mm2  61,658 mm2  Imaging Plane  Parallel (Long Axis)  Perpendicular (Short Axis)  Parallel (Long Axis)  Perpendicular (Short Axis)  Pulse Sequence  T1-SE  T1-SE  GRE  GRE  Signal Void Size  36,933 mm2  12,808 mm2  62,395 mm2  61,658 mm2  Imaging Plane  Parallel (Long Axis)  Perpendicular (Short Axis)  Parallel (Long Axis)  Perpendicular (Short Axis)  View Large FIGURE 3 View largeDownload slide MRI artifacts associated with the armor-piercing bullet: (A) long-axis view; (B) short-axis view (GRE pulse sequence: repetition time/echo time, 100/15 milliseconds; flip angle, 30°; long-axis imaging plane). FIGURE 3 View largeDownload slide MRI artifacts associated with the armor-piercing bullet: (A) long-axis view; (B) short-axis view (GRE pulse sequence: repetition time/echo time, 100/15 milliseconds; flip angle, 30°; long-axis imaging plane). DISCUSSION MRI Findings The armor-piercing bullet exhibited substantial magnetic field interactions (90° deflection angle, +4 torque, 3.30 N digital force gauge reading), minor heating, and large artifacts in association with the 3-T MRI conditions used in this investigation. Because of the excessive magnetic field interactions, this ballistic item is considered to be “MR unsafe” according to the current terminology used to characterize objects relative to the MRI environment.16 These results contribute to the existing peer-reviewed literature insofar as there has been no prior evaluation of ballistic objects above 1.5 T. Ferromagnetic Detection System Comprehensive screening procedures are used to ensure safety for patients, staff members, and other individuals in the MRI environment.7 Permitting a patient or individual to enter the MR system room without identifying a ferromagnetic foreign body or implant could have serious consequences. The utilization of ferromagnetic detection systems has been proposed as a means of augmenting the MRI screening process, with the primary intent of preventing accidents related to external ferromagnetic objects.8 There has been no prior suggestion to use these devices as a method to find internal ferromagnetic objects. The magnetic field of an object within the human body is unaffected by the surrounding tissues because the relative magnetic permeability, μr, of tissue is close to 1. Accordingly, the tissues of the human body would be “transparent” carriers of magnetic objects relative to a ferromagnetic detection system. Therefore, we hypothesized that a ferromagnetic detection system could identify a ferromagnetic foreign body or implant in an individual. The present study indicated that the armor-piercing bullet evaluated in this study was detected by the ferromagnetic detection system, and this is the first report of such a finding. Further investigation is warranted to determine the overall utility of this device by investigating more ferromagnetic objects and implants, with attention directed toward studying items with different sizes (i.e., masses) and various levels of magnetic susceptibility. Although a ferromagnetic detection system should not be used to replace current MRI screening practices, it is possible that the utilization of this apparatus, combined with conscientious screening protocols, may help to optimize screening procedures. Possible Limitations A possible limitation in this investigation is that only 1 armor-piercing bullet was tested. As previously stated, this ballistic item was selected for evaluation because it is a round commonly used in an AK-47 assault rifle. Notably, this particular ballistic object was selected because it represents a worst-case scenario with respect to its high magnetic qualities. Importantly, this type of ordnance is often encountered in military personnel, and even though it is illegal to possess this type of bullet in the United States, it may be encountered in patients referred for MRI examinations. Clinical Implications Any retained ballistic item that exhibits substantial magnetic field interactions, either due to occult impurities or materials used in the fabrication process, poses a possible risk to soft tissue, vascular, neural, or other sensitive anatomic structures because of the displacement in association with the powerful static magnetic field of an MR system. Therefore, when a patient with a bullet is referred for an MRI examination, the standard of care to manage this situation involves an appropriate analysis of the risk versus benefit for the patient, with various factors that must be taken into consideration. One of these factors includes knowing the type of bullet that is present, although this information may not be available, and, as previously stated, ballistic items are frequently “contaminated” with ferromagnetic materials. Therefore, in these cases, it must be assumed that the bullet in question is ferromagnetic, unless information is otherwise available to support the presence of a nonferromagnetic bullet. Additional consideration must be given to whether the foreign body is located near or in a vital anatomic area. Furthermore, the time span related to the injury by the bullet and the existence of possible “counterforces” must be considered because these may result in retention of a ferromagnetic object (e.g., the bullet may be lodged in bone or encapsulated by fibrous tissue).6,7 With regard to the armor-piercing bullet (.30 caliber, 7.62 × 39, copper-jacketed round, steel core; Norinco) evaluated in this study, it is apparent that this ballistic object is likely to involve a substantial risk associated with an MRI procedure. Therefore, this information has important implications when judging the risk versus benefit for the patient. Consideration should also be given to the location of this bullet relative to the area undergoing MRI because of the large signal void created by this highly magnetic object. CONCLUSIONS MRI testing indicated that the tested armor-piercing bullet exhibited substantial magnetic field interactions, little heating, and large artifacts at 3 T. This information must be considered as part of the risk versus benefit when managing patients with gunshot injuries referred for MRI examinations. Performing screening of a patient with this armor-piercing bullet gives a positive alarm when using a particular type of ferromagnetic detection system (Ferroguard Screener), resulting in verification that a potentially unsafe foreign body may be present. ACKNOWLEDGMENT The authors thank Dean Williams for obtaining the armor-piercing bullet used in this investigation. REFERENCES 1. Walker JJ, Kelly JF, McCriskin BJ, Bader JO, Schoenfeld AJ Combat-related gunshot wounds in the United States military: 2000–2009 (cohort study). Int J Surg  2012; 10: 140– 3. Google Scholar CrossRef Search ADS PubMed  2. Eshed I, Kushnir T, Shabshin N, Konen E Is magnetic resonance imaging safe for patients with retained metal fragments from combat and terrorist attacks? Acta Radiol  2010; 51: 170– 4. Google Scholar CrossRef Search ADS PubMed  3. 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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. In: Annual Book of ASTM Standards, Section 13, Medical Devices and Services, Volume 13.01 Medical Devices; Emergency Medical Services , pp 1576– 80. ASTM International, West Conshohocken, PA, 2001. 10. Shellock FG, Valencerina S In vitro evaluation of MR imaging issues at 3T for aneurysm clips made from MP35N: findings and information applied to 155 additional aneurysm clips. AJNR Am J Neuroradiol  2010; 31: 615– 9. Google Scholar CrossRef Search ADS PubMed  11. Weiland JD, Faraji B, Greenberg RJ, Humayun MS, Shellock FG Assessment of MRI issues for the Argus II retinal prosthesis. Magn Reson Imaging  2012; 30: 382– 9. Google Scholar CrossRef Search ADS PubMed  12. 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TI - Armor-Piercing Bullet: 3-T MRI Findings and Identification by a Ferromagnetic Detection System
JF - Military Medicine
DO - 10.7205/MILMED-D-12-00374
DA - 2013-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/armor-piercing-bullet-3-t-mri-findings-and-identification-by-a-DI6EncEbBQ
SP - e380
EP - e385
VL - 178
IS - 3
DP - DeepDyve
ER -