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In a 1.5 T MRI system with a gradient amplitude of 10 mT m⁻¹ what is the total magnetic field at a point x = 10 cm from the isocentre?

At a point x = −10 cm, the resultant B‐field is 1.499 T.

The contribution to the overall magnetic field of the gradients is small, but we could not image without them. The strength of the field produced by the gradients decreases rapidly outside the bore of the magnet, and is negligibly small away from the magnet.

As the gradients are switched, they produce time‐varying magnetic fields. The rate of change of field is given by the derivative of B with respect to time, or dB/dt (measured in T s^‐1). For a trapezoidal gradient waveform (Figure 1.16)

(1.7)

where ΔB is the change in B produced by the gradient and Δt is the time over which the change occurs. dB/dt is important when considering acute physiological effects, such as peripheral nerve stimulation (PNS). See Chapter 4 .