Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 33

Whilst MR practitioners commonly refer to their magnets in terms of “magnetic field strength”, this nomenclature is scientifically incorrect. The proper term is magnetic flux density, denoted as B. B is a vector field with components in each direction B_x, B_y and B_z. MRI is only sensitive to B_z and that is what we refer to colloquially as the “field.” Magnetic flux density has the SI (International System) unit of the tesla (T). An older unit is the gauss (G). One tesla equals 10 000 G.

The scientific definition of the tesla is in terms of force. Referring to Figure 1.22, one tesla is the amount of magnetic flux density which exerts a force of one newton (N) on a charged particle of charge one coulomb (C) moving at right angles to the field direction with a velocity of one meter per second (m s⁻¹). It’s not an easy definition, but the fact that it is defined in terms of force is highly apt for MR safety!

Figure 1.22 Definition of the SI unit tesla.