Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 34

The unit of “magnetic field strength” is not the tesla, but is amperes per meter. B is the magnetic flux density.

So, what is magnetic field strength in actuality? It is given the symbol H and has units of amperes per meter (A m⁻¹). It is defined in terms of a cylindrical electromagnet, just like our scanner – the current in the windings generates an H‐field. In free space

(1.5)

μ₀ is the magnetic permeability in a vacuum, equal to 4π x10⁻⁷ henrys per meter (H m⁻¹).

One way of visualizing magnetic fields is through magnetic field lines. If you have ever done the experiment of introducing iron filings to the proximity of a simple bar magnet you may have observed the pattern shown in Figure 1.23a. These illustrate the magnetic “lines of force”. A small compass needle positioned anywhere will align with these. We can think of the magnetic flux density as being the intensity of grouping of these lines: the more closely grouped together, the stronger the B‐field.

Figure 1.23 Magnetic field lines of force: (a) seen in the pattern of iron filings around a permanent magnet; (b) from an electromagnet.