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Purpose: Safety limits for the permitted Specific Absorption Rate (SAR) placerestrictions on pulse sequence design, especially at ultra-high fields ($\geq7$ tesla). Due to inter-subject variability, the SAR is usually conservativelyestimated based on standard human models that include an applied safety marginto ensure safe operation. One approach to reducing the restrictions is tocreate more accurate subject-specific models from their segmented MR images.This study uses electromagnetic simulations to investigate the minimum numberof tissue groups required to accurately determine SAR in the human head.  Methods: Tissue types from a fully characterized electromagnetic human modelwith 47 tissue types in the head and neck region were grouped into differenttissue clusters based on the conductivities, permittivities, and mass densitiesof the tissues. Electromagnetic simulations of the head model inside a paralleltransmit (pTx) head coil at 7T were used to determine the minimum number ofrequired tissue clusters to accurately determine the subject-specific SAR. Theidentified tissue clusters were then evaluated using two additionalwell-characterized electromagnetic human models.  Results: A minimum of 4 clusters plus air was found to be required foraccurate SAR estimation. These tissue clusters are centered around gray matter,fat, cortical bone, and cerebrospinal fluid. For all three simulated models thepTx maximum 10gSAR was consistently determined to within an error of <12%relative to the full 47-tissue model.  Conclusion: A minimum of 4 clusters plus air are required to produce accuratepersonalized SAR simulations of the human head when using pTx at 7T.

An investigation into the minimum number of tissue groups required for  7T in-silico parallel transmit electromagnetic safety simulations in the  human head