In the examination of a subject with a magnetic resonance tomography apparatus, data for a slice of the subject to be examined are obtained with a sequence of a fast MRT imaging method that includes at least three phase correction scans and measurement signals of the respective phase correction scans as well as of the slice are obtained. The phase difference of corresponding data points of two phase correction scans are calculated point-by-point, the average phase difference between the phase correction scans is evaluated, and the frequency offset between the actual resonance frequency relative to the adjusted resonance frequency is calculated based on the average phase difference and the echo time difference between the phase correction scans used. A B0 field map is calculated dependent on the frequency offset and, the measurement data for the slice are corrected using the calculated B0 field map.