The invention provides for a magnetic resonance imaging (MRI) system (100) that comprises a memory (134) for storing machine executable instructions (140) and MRF pulse sequence commands (142). The MRF pulse sequence commands cause the MRI system to acquire MRF magnetic resonance data (144) according to a magnetic resonance (MR) fingerprinting protocol. 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 comprises a train of pulse sequence repetitions. The train of pulse sequence repetitions comprises a sampling event where the MRF magnetic resonance data is repeatedly sampled. The MRI system further comprises a processor for controlling the magnetic resonance imaging system. Execution of the machine executable instructions causes the processor to: acquire (200) the MRF magnetic resonance data by controlling the magnetic resonance imaging system with the MRF pulse sequence commands； and construct (202) a series (148) 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 (204) 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 (150) 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.
