For in vivo magnetic resonance imaging at high field (.gtoreq.3 T) it is essential to consider the homogeneity of the active B.sub.1 field (B.sub.1+), particularly if surface coils are used for RF transmission. A new method is presented for highly rapid B.sub.1+ magnitude mapping. It combines the double angle method with a B.sub.1-insensitive magnetization-reset sequence such that the choice of repetition time (TR) is independent of T.sub.1 and with a multi-slice segmented (spiral) acquisition to achieve volumetric coverage with adequate spatial resolution in a few seconds.