  Purpose: Safety limits for the permitted Specific Absorption Rate (SAR) place
restrictions on pulse sequence design, especially at ultra-high fields ($\geq
7$ tesla). Due to inter-subject variability, the SAR is usually conservatively
estimated based on standard human models that include an applied safety margin
to ensure safe operation. One approach to reducing the restrictions is to
create more accurate subject-specific models from their segmented MR images.
This study uses electromagnetic simulations to investigate the minimum number
of tissue groups required to accurately determine SAR in the human head.
  Methods: Tissue types from a fully characterized electromagnetic human model
with 47 tissue types in the head and neck region were grouped into different
tissue clusters based on the conductivities, permittivities, and mass densities
of the tissues. Electromagnetic simulations of the head model inside a parallel
transmit (pTx) head coil at 7T were used to determine the minimum number of
required tissue clusters to accurately determine the subject-specific SAR. The
identified tissue clusters were then evaluated using two additional
well-characterized electromagnetic human models.
  Results: A minimum of 4 clusters plus air was found to be required for
accurate SAR estimation. These tissue clusters are centered around gray matter,
fat, cortical bone, and cerebrospinal fluid. For all three simulated models the
pTx 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 accurate
personalized SAR simulations of the human head when using pTx at 7T.
