Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 59

The field on the z‐axis from a circular loop of radius a is directed along the z‐direction and is proportional to the current I. At the centre of the loop B is proportional to 1/a, so the field generated depends upon the radius of the coil. At long distances from the loop, it acts like a magnetic dipole. On‐axis the field only has a B_z component with a 1/z³ dependence. This is often cited to represent the dropping off of the B₀ fringe field, but modern shielded MRI magnets do not exactly follow this behavior; they are not simple dipoles. Nevertheless, the 1/z ³ dependence serves as a useful approximation of the nature of the fringe field. The spatial gradient from a dipole varies with 1/z⁴ along the z‐axis. This is a very rapid decrease with distance from the iso‐centre and is intensely significant for projectile safety. The magnitude of B_z for a long straight wire, a loop and a dipole are plotted in Figure 2.6.

Figure 2.6 Relative magnitude of B_z along the z‐axis for a long straight wire, simple loop, magnetic dipole, solenoid, and simulated self‐shielded magnet with radii of 0.4 m. The iso‐centre is at z = 0.