Assessment of magnetic field interactions and radiofrequency-radiation-induced heating of metallic spinal implants in 7 T field

Abstract

The safety of metallic spinal implants in magnetic resonance imaging (MRI) performed using ultrahigh fields has not been established. Hence, we examined whether the displacement forces caused by a static magnetic field and the heating induced by radiofrequency radiation are substantial for spinal implants in a 7 T field. We investigated spinal rods of various lengths and materials, a screw, and a cross‐linking bridge in accordance with the American Society for Testing and Materials guidelines. The displacement forces of the metallic implants in static 7 T and 3 T static magnetic fields were measured and compared. The temperature changes of the implants during 15‐min‐long fast spin‐echo and balanced gradient‐echo image acquisition sequences were measured in the 7 T field. The deflection angles of the metallic spinal materials in the 7 T field were 5.0–21.0° [median: 6.7°], significantly larger than those in the 3 T field (1.0–6.3° [2.2°]). Among the metallic rods, the cobalt–chrome rods had significantly larger deflection angles (17.8–21.0° [19.8°]) than the pure titanium and titanium alloy rods (5.0–7.7° [6.2°]). The temperature changes of the implants, including the cross‐linked rods, were 0.7–1.0°C [0.8°C] and 0.6–1.0°C [0.7°C] during the fast spin‐echo and balanced gradient‐echo sequences, respectively; these changes were slightly larger than those of the controls (0.4–1.1°C [0.5°C] and 0.3–0.9°C [0.6°C], respectively). All of the metallic spinal implants exhibited small displacement forces and minimal heating, indicating that MRI examinations using 7 T fields may be performed safely on patients with these implants. © 2016 The Authors. Journal of Orthopaedic Research Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 35:1831–1837, 2017.

MRI following scoliosis surgery? An analysis of implant heating, displacement, torque, and susceptibility artifacts

“…Despite the considerable construct length and the large amount of metal, the maximum RF-induced temperature increase of the spinal implants in the worst case conditions (SAR 2 W/kg, 15 min MR acquisition) was only 1.3 K. The RF-induced heating was even less when two cross-links were used, although electrically closed loops (coils) can favor heating of the surroundings [19]. These results are in agreement with the findings of Tsukimura et al [1] for a 200-mm implant at 7 T. A review [20] demonstrated that low heating (up to 43°C) for several hours causes no damage and that heating to 44°C for 200 min leads only to transient erythema. Therefore, it can be assumed that adverse biological effects due to RF-induced heating of the investigated pedicle-screwrod system in scoliosis patients, examined with a 1.5 to 3 T MRI in a supine position, become unlikely.…”

“…When the patient is lying supine, the implant is located in the middle of the scanner and therefore has the lowest risk of heating. Tsukimura et al [1] did not find any significant difference between Ti rods with different alloys. Moreover, the Ti rods with CoCr alloy have the ability to produce higher correction rates in adolescent idiopathic scoliosis (AIS) and provide significant and stable spinal correction compared to Ti rods with TiCP alloy of the same diameter [27].…”

“…The use of metallic implants such as pedicle screws and rods is currently standard in spinal surgery to treat deformity, degeneration, destruction, and trauma [1,2]. Especially in patients with deformity, the implants may be long and the constructs may have a high density of screws.…”

“…The compatibility of various orthopedic implants with magnetic resonance imaging (MRI) has been studied [2][3][4], but the modern metallic pedicle-screw-rod systems have not been sufficiently investigated in this regard. A previously published study [1] on a pedicle-screw-rod system focused mainly on shorter implant constructs and used a field strength of 7 T. However, the pedicle screw density used here was lower and the 7-T magnetic field strength was higher than in routine clinical care because usually the patients are examined at 1.5 T and 3 T. Furthermore, not all aspects of MRI compatibility were examined; no attention was paid, for example, to magnetically induced torque or susceptibility artifacts. Patients with long metallic pedicle-screw-rod constructs who need a tomographic examination, e.g., in the presence of postoperative complications or in scoliosis secondary to tumors such as neurofibromas, often undergo computed tomography (CT) despite the exposure to ionizing radiation.…”

Quantification of metal‐induced susceptibility artifacts associated with ultrahigh‐field magnetic resonance imaging of spinal implants

“…In our previous study, we investigated whether the displacement forces caused by a static magnetic field and the heating induced by radiofrequency (RF) radiation are substantial for spinal implants in a 7‐T field and reported on its safety in accordance with the American Society for Testing and Materials (ASTM), which sets MRI compatibility standards…”