Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 61

The design of a MR magnet (at least for MR safety purposes) can be approximately simulated by two concentric solenoids: the inner one representing the main coil; the outer shield coil has current flowing in the opposite direction. This enables the fringe field to be significantly reduced in extent (Figure 2.8). It also results in a stronger spatial gradient of B₀, dB/dz, close to the bore entrance, but weaker at greater distance. This poses a significantly increased hazard, as the projectile force on a ferromagnetic object may suddenly increase as you approach the bore entrance – and you only notice when it’s too late. Both B and dB/dz drop off with distance more rapidly than for a dipole (FIGURE 2.6).

Figure 2.8 B and dB/dt along the axis of a simulated shielded and unshielded 3 T MR magnet. Distances are from the iso‐centre, with the bore entrance at 0.8m. Simulated data for illustration only.