  The image quality of X-ray imaging relies heavily on the X-ray output number
which is dependent on the X-ray tube current and the exposure time. Hybrid
X-ray imaging modalities like X-ray luminescence CT (XLCT) and X-ray
fluorescence CT (XFCT) rely on the intensity of the X-ray tube to provide an
accurate image reconstruction of the nanoprobe distribution in the imaging
sample. A limiting factor of good image quality is the radiation dose that will
be delivered to the imaging object. To accurately estimate the absorbed dose in
an imaging protocol, it is better to simulate the X-ray imaging with a Monte
Carlo platform such as GATE (Geant4 Application for Tomographic Emission).
However, the input of GATE is a photon number of the simulated X-ray tube. So
far, there is no good way to setup the photon number for a desired X-ray tube
current. In this work, the accumulated radiation dose of a micro-CT X-ray tube
at different current exposure times was recorded with a general-purpose ion
chamber. GATE was used to model the total absorbed dose (cGy) in the sensitive
volume of the ion chamber with different X-ray output numbers. A linear
regression model was generated between the X-ray photon number in the GATE
simulations and the tube current exposure time (mAs). The findings of this work
provide an approach to correlate the X-ray tube current exposure time (mAs) to
the X-ray photon number in GATE simulation of the X-ray tube.
