  The diffuse high-energy gamma-ray emission of the Milky Way arises from
interactions of cosmic-rays (CRs) with interstellar gas and radiation field in
the Galaxy. The neutral hydrogen (H I) gas component is by far the most massive
and broadly distributed component of the interstellar medium. Using the 21-cm
emission line from the hyperfine structure transition of atomic hydrogen it is
possible to determine the column density of H I if the spin temperature (Ts) of
the emitting gas is known. Studies of diffuse gamma-ray emission have generally
relied on the assumption of a fixed, constant spin temperature for all H I in
the Milky Way. Unfortunately, observations of H I in absorption against bright
background sources has shown it to vary greatly with location in the Milky Way.
We will discuss methods for better handling of spin temperatures for Galactic
diffuse emission modeling using the Fermi-LAT data and direct observation of
the spin temperature using H I absorption.
