Читать книгу Essentials of MRI Safety - Donald W. McRobbie - Страница 27

Magnetic field gradients G_x, G_y, and G_z used to spatially select or encode the MR signal during acquisition are generated by three sets of gradient coils. The field generated is always along z. Gradient coils usually require water cooling as they have typically hundreds of amperes (A) of electricity pulsed through them. Specialist hybrid amplifiers and power supplies are used to generate these strong pulses. A consequence of gradient pulsing is the generation of acoustic noise (Chapter 7).

Gradient pulses usually have a trapezoidal waveform (Figure 1.16). The ability of the gradient system to switch rapidly, known as the slew rate (SR), is defined as the maximum amplitude divided by the rise time required to achieve that amplitude:

(1.4)

Figure 1.16 Trapezoidal gradient pulse.

Typical slew rates are 100−200 T m⁻¹ s⁻¹ (tesla per meter per second).