Biomedical Engineering Reference
In-Depth Information
Fig. 3.3
Schematic MRI-coil assembly
Fig. 3.4
Variation of the total magnetic field strength B(z, t) indicated by (red line) along the z-
axis
magnetization into the transverse plane, and serve as detector coils for MR-signal
detection (surface rf-coils).
Shim coils are utilized to improve the homogeneity of the static field B
0
. These
are required due to magnet inhomogeneities and/or presence of external ferro-
magnetic objects disturbing the field homogeneity. The use of shim coils, referred
to as active shimming, employs low currents to balance the external field. Iron
sheets are used to cover the magnet, employing passive shimming.
Exemplarily, the field z-gradient generated by z-gradient coils is depicted in
Fig.
3.4
.
