Numerical approach to investigate MR imaging artifacts from orthopedic implants at different field strengths according to ASTM F2119

Abstract

Objective: This study presents an extended evaluation of a numerical approach to simulate artifacts of metallic implants in an MR environment.

Methods: The numerical approach is validated by comparing the artifact shape of the simulations and measurements of two metallic orthopedic implants at three different field strengths (1.5 T, 3 T, and 7 T). Furthermore, this study presents three additional use cases of the numerical simulation. The first one shows how numerical simulations can improve the artifact size evaluation according to ASTM F2119. The second use case quantifies the influence of different imaging parameters (TE and bandwidth) on the artifact size. Finally, the third use case shows the potential of performing human model artifact simulations.

Results: The numerical simulation approach shows a dice similarity coefficient of 0.74 between simulated and measured artifact sizes of metallic implants. The alternative artifact size calculation method presented in this study shows that the artifact size of the ASTM-based method is up to 50% smaller for complex shaped implants compared to the numerical-based approach.

Conclusion: In conclusion, the numerical approach could be used in the future to extend MR safety testing according to a revision of the ASTM F2119 standard and for design optimization during the development process of implants.

Keywords: Artifacts; Human model artifact simulation; Magnetic field strength; Metallic implants; Numeric simulations; Orthopedic implants.

Fig. 1

Simulated and measured test objects (Königsee Implantate GmbH, Allendorf, Germany). 3D renderings of CAD models of A titanium distal radius plate with eleven angle-stable cortical screws (Ø 2.4 mm, length 20 and 24 mm) and B titanium acromioclavicular joint hook plate with five angle-stable cortical screws (Ø 3.5 mm, length 20 mm). Phantom setup for MRI measurements of both implants (C, D). The implants were fixated with a fishing line in the middle of an oil filled Plexiglas phantom

Fig. 2

The figure compares the ASTM-based and numerical-based artifact evaluation method of the distal radius plate at 3 T. A The simulated magnitude artifact image from which the artifact mask (green) in (B) was calculated. C The calculation of the artifact size was performed according to ASTM F2119. In D, the TO (distal radius plate, yellow) was placed in the artifact mask and in figure E, the distance map is shown. Here, the distance from each pixel within the artifact to the test object was calculated. Note that dark colors indicate proximity to the implant while light green colors indicate larger distances to the implant

Fig. 3

The simulated (A) and measured (B) magnitude images at 7 T and the corresponding artifact masks C, D of the distal radius plate are shown. Image E exemplarily shows the overlay of the simulated and measured artifact. The dice similarity coefficient (DSC) of this overlay was 0.77

Fig. 4

The simulated and measured magnitude images of the distal radius plate (A, B, E, F) and the acromioclavicular joint hook plate (C, D, G and H) are shown in the center slice of X–Y plane for a gradient echo sequence (A–D) and for a spin echo sequence (E–H) at 1.5 T

Fig. 5

The placement of the TO (distal radius plate) within the distance map at 3 T. A Illustration of the artifact with the static magnetic field (B₀) parallel and the frequency-encoding gradient perpendicular (G_f) to the plate. B Identical TO but this time B₀ oriented perpendicular and G_f parallel to the plate. Note how shape and size of artifact change relative to the TO just due to changes in orientation of B₀ and G_f

Fig. 6

The two graphs show the artifact area as a function of the echo time (A) and as a function of the bandwidth (B), separated by spin echo (orange) and gradient echo sequence (blue) averaged over 1.5 T, 3 T, and 7 T

Fig. 7

The simulated artifact of the distal radius plate in the context of the hand of the human body model. A Reference simulation of the hand without the test object, B Simulated artifact at 7 T. C Artifact masks at 1.5 T (light blue), 3 T (blue) and 7 T (darker blue). D Overlap area of the artifact masks of human model (A_HM) and phantom (A_PHA) simulation (red), artifact area from the phantom simulation, not covered by the human model simulation (yellow), and artifact area from the human model simulation, not covered by the phantom simulation (blue). The DSC between phantom simulation and human model simulation is 0.86