Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 78

What if the object is already magnetized, i.e. is a permanent magnet? Such a situation may arise in MR if it is your institution’s policy to scan patients with cochlear implants. To start, we are not reliant on the external B₀ to magnetize the object. Whether an increase occurs close to the magnet will depend upon the hysteresis properties of the object – it may even become demagnetized‐ so it is virtually impossible to predict. We can say, however, that the initial translational (attractive or repulsive depending upon orientation) and twisting forces are likely to exceed those of a soft ferromagnetic object. Cochlear implants are discussed in Chapter 10.