We developed an efficient, depth- and energy-dependent Monte Carlo model for columnar CsI detectors. The
optical photon, electron/positron Monte Carlo package MANTIS developed by our group, was used to generate
optical photon response and collection efficiency as a function of the x-ray/electron interaction depth for a
realistic scintillator geometry. The detector geometry we used for the simulations was reported in the past and
is based on a 500 μm thick columnar CsI scintilator. The resulting depth-dependent optical photon responses
were fit to a parametrized Gaussian mixture model. The model parameters were the depth-dependent radial
shift of the response peak, the depth dependent widths of the Gaussians, and the depth-dependent magnitude
of the Gaussians in the mixture. The depth-dependent optical spread has a maximum spatial shift of 53 μm.
The optical collection efficiency at the photo-diode layer followed a power law varying from 90% for interactions
at the scintillator exit surface to 20% for interactions at the detector entrance. The responses were consequently
incorporated into penMesh, a PENELOPE based Monte Carlo x-ray, electron/positron transport simulation
package for generating clinically realistic images of triangular mesh phantoms. The resulting detector responses
from this empirical model were compared against the full x-ray/electron/optical photon simulation using the
package MANTIS, showing good agreement. The simulation speed, using the optical transport model in penMesh,
increases by two orders of magnitude compared to MANTIS.