In a magnetic resonance slice multiplexing method and apparatus, measurements are repeated with additional phase amounts being applied, wherein in each repetition, the additionally applied phase amounts are changed such that at least a central k-space region is fully sampled in the course of the repeated acquisitions. A calibration dataset, which is used in reconstructing image data for the simultaneously excited slices from the acquired measurement data, is determined from the measurement data that have been fully acquired in the central k-space region. The calibration dataset is updated in further measurements.