Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 89

A question of some significance, especially for implants, is which is stronger, the translational or the twisting force? Figure 2.24 shows the attractive and twisting force (from the torque) for ferromagnetic objects (mass 0.1 kg, density of 8000 kg m⁻³, χ = 1000) approaching a 3 T scanner. We assume that the orientation is such to produce either the maximum torque or translation (note: that these conditions are inconsistent with each other). What is quite surprising is that, in this simulation, the force exerted from twisting is greater than that from translation at most locations. This is bad news for MR safety as the first thing to occur will be the twisting to align with the field direction which is then the optimum orientation for projectiles. Also, for ferromagnetic implants, unless they are perfectly spherical (in which case the torque will be zero) or perfectly aligned with B₀, the twisting forces will persist for the whole time spent in the bore of the magnet. In the figure the forces on a 5 cm long needle (l/d = 50) are shown, where the twisting can greatly exceed the projectile force. Although the attractive force on the needle appears low, remember that items tend to accelerate into the scanner with high velocities, so this situation is very dangerous.

Figure 2.24 Predicted maximum twisting force (solid lines) and translational (dashed lines) from ferromagnetic 0.1 kg cylinders of varying l/d ratios and a 5 cm needle with l/d = 50, with densities 8000 kg m⁻³, χ = 1000 and B_sat = 2 T on the axis of a 1.5 T shielded magnet. The lengths of the cylinders are 11.6, 7.5, 5, and 2.5 cm.