Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 36

As we move further from the bore of the magnet, the lines of force diverge, and the fringe field decreases (Figure 1.23b). The amount it decreases with distance is known as the fringe field spatial gradient, specified in T m⁻¹. The fringe field spatial gradient is responsible for the attractive force on ferromagnetic objects. Your manufacturer is required to provide you with information about the fringe field gradient. Figure 1.25 shows how the B₀ field and its spatial gradient dB/dz vary along the z‐axis. The fringe field is compressed for the shielded magnet but produces a stronger spatial gradient close to the bore entrance. This is highly significant for projectile safety.