03-05 Gradient Coils

Magnetic field gradients are necessary to create MR images. The reasons for this will be explained in detail in Chapter 6. As shown in Figure 01-04, two wires with electric current moving in the opposite direction can create a linear change of the magnetic field between each other when placed at the correct distance. This is also known as a magnetic field gradient.

Three sets of gradient coils are necessary to be able to create a weak magnetic field in any direction in space (Figure 03-08). With mid- and high-field systems, the strength of the magnetic field created by gradient coils is approximately 100 times lower than the main field.

The performance of gradients coils is measured in mT/m, their peak am­pli­tu­des. Common peak amplitudes are 10 mT/m; high performance systems require up to 30 mT/m. A second important property of gradients is their rise time (ms) or slew rate (mT/m/ms). The faster the rise time or the higher the slew rate, the better the system performance will be, i.e., the faster image data can be ac­qui­red.

Figure 03-08: Schematic depiction of gradient coils in a su­per­con­duc­ting or resistive magnet sys­tem. To cover all three spatial dimensions (x, y, and z), three sets of gradient coils are in­stal­led in the bore of the MR machine. By changing the current flow relative to each other, planes in any direction can be laid through the patient's body.

03-05-01 Eddy Currents

These currents are introduced by the gradients’ time varying magnetic field. They can both degrade the homogeneity of the static magnetic field as well as distort the gradient pulse profiles.

If they are not compensated for, image quality will significantly decrease. One way to counteract their influence is by shielding the gradient so that their fields are restricted to the interior of the patient bore.