in the coil and potential heating or burns for the patient.
Figure 1.20 Receive coil and pre‐amplifier: (a) the coil has inductance L and resistance R; Cd is a detuning capacitor; (b) response of the coil during signal reception (red line) and RF transmission (blue line).
ELECTROMAGNETIC FIELDS
The extent and magnitude of the fields involved in MRI are summarized in table 1.1 and Figure 1.21.
Table 1.1 Magnetic fields in MRI.
| Field | Amplitude | Frequency / Slew rate | Pulse duration |
| Static field B0 | 0.2‐7 T | 0 Hz | Always present |
| Static fringe field spatial gradient dB/dz | 0‐25 T m–1 | 0 Hz | Always present |
| Imaging gradients Gx, Gy, Gz | 0‐80 mT m–1 | 0‐10 kHz 0‐200 T m–1 s–1 | 0‐10 ms |
| RF transmit field B1 | 0‐50 μT | 8‐300 MHz | 0‐10 ms |
Figure 1.21 Relative magnitude of magnetic fields used in MRI.
Static field
Definition of magnetic flux density and the tesla
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 Bx, By and Bz. MRI is only sensitive to Bz 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.
MYTHBUSTER:
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.
B0 fringe field
The most uniform and dense grouping of lines of force for an MR magnet (Figure 1.23b) occurs within the bore. As we move away from the bore the lines diverge and consequently the B‐field decreases. We call this region the fringe field. Your scanner manufacturer provides field maps showing fringe field contours at 0.5, 1, 3, 5, 10, 20, 40, and 200 mT [4] (Figure 1.24). These are important for MRI suite design (Chapter 12). A modern MRI system utilizes self‐shielding in order to reduce the spatial extent of the fringe field (Figure 1.25).
Figure 1.24 Fringe field contours at 0.5, 1, 3, 5, 10, 20, 40 and 200 mT for a 3 T MR magnet. Reproduced with permission of Siemens Healthineers.
Figure 1.25 The magnitude of the B0 fringe field (solid lines, logarithmic LH scale)