Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 70

For diamagnetic and paramagnetic materials where |χ| is very small, we do not have to consider the demagnetizing factors. If we assume only the z‐axis component, then the magnetic force on a paramagnetic object is

(2.10)

For a diamagnetic material the force will be negative, i.e. repulsive. Figure 2.16 shows plots of B, dB/dz and their product B^.dB/dz along the z‐axis for simulations of 1.5 and 3 T shielded magnets. Note that the locations of the maximum values of dB/dz and B^.dB/dz do not necessarily coincide. For a paramagnetic or diamagnetic object the maximum force is exerted at the location of the maximum field‐gradient product, near to the bore entrance. The figure shows B‐field values on‐axis, but in general the spatial gradient and product values are greatest around the edge of the bore circumference.

Figure 2.16 B, dB/dz and product B^.dB/dz along the z‐axis for: (a) a shielded 3 T magnet (b) a shielded 1.5 T magnet with bore length 1.6 m. The horizontal axis is distance from the iso‐centre. The bore entrance is at 0.8 m. Simulation for illustration only.