Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 108

The fields and forces associated with MRI and MRI equipment are not simple. They are all consequences of Maxwell’s equations. Forces on objects may be:

 purely magnetic, relating to the shape and ferromagnetism of the material

 related to electrical current flowing in the object

 related to induction

 or movement within a field gradient.

For ferromagnetic objects the attractive or projectile magnetic force is proportional to the spatial gradient of the B₀ fringe field. The torque on an unsaturated object is proportional to the square of B₀. Most ferromagnetic objects will be saturated close to the scanner in which case the attractive force is related to B_sat ^.dB/dz and the torque to B_sat ².

Time‐varying magnetic fields from the gradients, RF, or movement within the static fringe field gradient will induce electric fields in tissue. These can lead to the stimulation of excitable tissues or to tissue heating. The RF field can exhibit unpredictable wave‐like behavior. The next three chapters will look at the biological effects of the static field, gradient fields, and RF field.