A magnetic resonance imaging system, wherein the magnetic resonance imaging system comprises: - a memory storing machine executable instructions and MRF pulse sequence commands, wherein the MRF pulse sequence commands cause the magnetic resonance imaging system to acquire MRF magnetic resonance data according to a magnetic resonance fingerprinting protocol, wherein the pulse sequence commands are configured for acquiring the MRF magnetic resonance data in two dimensional slices (410, 412, 414, 416, 418, 420), wherein the two dimensional slices have a slice selection direction, wherein the pulse sequence commands comprise a train of pulse sequence repetitions, wherein the train of pulse sequence repetitions comprises a sampling event where the MRF magnetic resonance data is repeatedly sampled; - a processor for controlling the magnetic resonance imaging system, wherein execution of the machine executable instructions causes the processor to: - acquire the MRF magnetic resonance data by controlling the magnetic resonance imaging system with the MRF pulse sequence commands; and - construct a series of at least one magnetic resonance parameter value for each voxel of the two dimensional slices using the MRF magnetic resonance data, wherein each of the series corresponds to the sampling event of each pulse sequence repetition; and - calculate a composition (502, 504, 506, 508) of each of a set of predetermined substances within two or more sub voxels (306, 308) for each voxel of the two dimensional slices using a sub-voxel magnetic resonance fingerprinting dictionary for each of the two or more sub voxels and the series of the at least one magnetic resonance parameter value, wherein sub voxels divide each voxel in the slice selection direction, wherein the sub-voxel magnetic resonance fingerprinting dictionary comprises separate fingerprints calculated by integrating over distinct parts of an excitation pulse profile for each of the two or more sub voxels, wherein the excitation pulse profile is a flip angle distribution and a phase angle distribution, wherein the composition within the two or more sub voxels is calculated by determining the contribution from each of the two or more sub voxels by solving a linear optimization problem relating the sub-voxel magnetic resonance fingerprinting dictionary, said contributions and the MRF magnetic resonance data, and wherein - the excitation pulse profile has a symmetric flip angle distribution and has an anti-symmetric phase distribution; and/or - the pulse sequence commands specify a radio-frequency pulse, wherein the radio-frequency pulse is asymmetric. - a memory storing machine executable instructions and MRF pulse sequence commands, wherein the MRF pulse sequence commands cause the magnetic resonance imaging system to acquire MRF magnetic resonance data according to a magnetic resonance fingerprinting protocol, wherein the pulse sequence commands are configured for acquiring the MRF magnetic resonance data in two dimensional slices (410, 412, 414, 416, 418, 420), wherein the two dimensional slices have a slice selection direction, wherein the pulse sequence commands comprise a train of pulse sequence repetitions, wherein the train of pulse sequence repetitions comprises a sampling event where the MRF magnetic resonance data is repeatedly sampled; - a processor for controlling the magnetic resonance imaging system, wherein execution of the machine executable instructions causes the processor to: - acquire the MRF magnetic resonance data by controlling the magnetic resonance imaging system with the MRF pulse sequence commands; and - construct a series of at least one magnetic resonance parameter value for each voxel of the two dimensional slices using the MRF magnetic resonance data, wherein each of the series corresponds to the sampling event of each pulse sequence repetition; and - calculate a composition (502, 504, 506, 508) of each of a set of predetermined substances within two or more sub voxels (306, 308) for each voxel of the two dimensional slices using a sub-voxel magnetic resonance fingerprinting dictionary for each of the two or more sub voxels and the series of the at least one magnetic resonance parameter value, wherein sub voxels divide each voxel in the slice selection direction, - the excitation pulse profile has a symmetric flip angle distribution and has an anti-symmetric phase distribution; and/or - the pulse sequence commands specify a radio-frequency pulse, wherein the radio-frequency pulse is asymmetric.