Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 35

The most uniform and dense grouping of lines of force for an MR magnet (Figure 1.23b) occurs within the bore. As we move away from the bore the lines diverge and consequently the B‐field decreases. We call this region the fringe field. Your scanner manufacturer provides field maps showing fringe field contours at 0.5, 1, 3, 5, 10, 20, 40, and 200 mT [4] (Figure 1.24). These are important for MRI suite design (Chapter 12). A modern MRI system utilizes self‐shielding in order to reduce the spatial extent of the fringe field (Figure 1.25).

Figure 1.24 Fringe field contours at 0.5, 1, 3, 5, 10, 20, 40 and 200 mT for a 3 T MR magnet. Reproduced with permission of Siemens Healthineers.

Figure 1.25 The magnitude of the B₀ fringe field (solid lines, logarithmic LH scale) and its spatial gradient dB/dz (dashed lines, linear RH scale) along the z‐axis simulated for shielded 1.5 and 3 T MRI magnets. The vertical lines indicate the locations of the 0.5 mT contour. The iso‐centre is located at z=0, and the bore entrance at 0.8 m.