The assessment of MR safety or MR conditional safety of medical device implants rely on a number of standard test methods. ASTM F2052, the magnetically induced force test method, is one of these methods required as part of MR safety testing. The purpose of this study was to determine the maximum magnetically induced force of individual materials typically comprising implantable leads. For leads seeking MR conditional labeling that are constructed solely from materials that pass the magnetically induced force testing acceptance criteria (i.e. gravity force), the results of this study may eliminate the need to measure these leads’ magnetic force per ASTM F2052.

The maximum magnetically induced force was measured per ASTM F2052 using a Siemens Magnetom Prisma scanner for each of the commonly used lead materials shown in Table 1. Each material under test was suspended from a test fixture by a string along the center axis of the scanner bore at the location where the magnetic field produced the greatest magnetically induced deflection (Figure 1). Individual material test samples were prepared for testing by laser welding lead components together such that the testing suspension string was <1% of the test sample weight (Figure 2). The angular deflection ($$$\alpha_{L}$$$), magnetic field strength ($$$B_{0,L}$$$), and magnetic spatial gradient ($$$\triangledown B_{0,L}$$$) were then measured and recorded.

For materials that deflect less than 45°, the magnetically induced deflection force is less than the force on the material due to gravity. In this condition, the risk imposed by the magnetically induced force is no greater than the risk imposed by normal daily activity in the Earth’s gravitational field. However, a deflection of less than 45° at the location of the maximum static magnetic field gradient in one MR scanner does not preclude a deflection exceeding 45° in a scanner with a higher field strength or larger static field gradients¹. Therefore, the measurement results were scaled to a maximum magnetic field strength ($$$B_{0,C}$$$) of 3 T and a maximum spatial gradient ($$$\triangledown B_{0,C}$$$) of 19 T/m (in which 19 T/m is greater than or equal to the maximum spatial gradient of the commercially available 3 T cylindrical bores as of 2014¹). The scaled deflection angle ($$$\alpha_{C}$$$) was calculated using the following equation, as specified per ASTM F2052:

$$\triangledown B_{0,C}=\triangledown B_{0,L} \left[\frac{B_{0,L}}{B_{0,C}}\frac{tan(\alpha_{C})}{tan(\alpha_{L})}\right]$$

As shown in Table 2, all seven (7) of the commonly used lead materials tested at a magnetic field strength of 2.3 T and spatial gradient of 4 T/m, exhibited a measured deflection angle that was lower than 45° (i.e. gravity force). Additionally, when these deflection results were scaled to a conservative maximum magnetic field strength of 3 T along with a maximum spatial gradient of 19 T/m, all seven (7) of the materials had a scaled deflection angle that was lower than 45°.Our results demonstrate that the commonly used lead materials tested, when exposed to a 3 T MR scanner, exhibit a maximum magnetically induced force that is less than gravity force.Implantable leads that are constructed solely from the materials passing the magnetically induced force testing acceptance criteria (i.e. gravity force) may not require magnetically induced force testing per ASTM F2052 for MR conditionality with 3 T MR scanners.