All MR imaging systems are equipped with a set of resistive wire windings known as gradient coils, which produce the gradient magnetic field. The gradient magnetic field is a time varying magnetic field, and this property requires that different safety precautions be taken as opposed to the static magnetic field. Gradients provide position-dependent variation in magnetic field strength and are pulsed on and off during and between RF excitation pulses. The purpose of these gradients is to spatially encode information contained in the emitted RF signal. The amplitudes of these magnetic fields may seem small in comparison with the main static magnetic field (e.g. a maximum amplitude of 80 mTm-1), but the salient feature is not the strength of the field but the rate of change of field generated when the field is switched on or off. A quantity called the slew rate, which is the rate of change of gradient amplitude is usually quoted in evaluating gradient performance and potential physiological effects (e.g. a maximum slew rate of 200 Tm-1s-1). You may calculate it by taking the gradient strength used and dividing it by the rise time (the time taken to reach that strength).
Further reading on this topic:
Books: MRI From Picture to Proton p170, 174