The ImagingRing System (medPhoton, Salzburg, Austria) is a novel X-ray planar and cone beam computed tomography system for in room imaging in particle therapy. The aim of this study was to establish a Monte Carlo model of the ImagingRing System for future research on scatter effects. The X-ray head was modeled using the Monte Carlo toolkit GATE (v8.0, GEANT4 Application for Tomographic Emission) and GEANT4 (GEometry ANd Tracking v.10.3).In a first step, the tungsten anode and the electron beam emerging from the cathode were modeled using GATE. Its surrounding glass and oil, as well as the polycarbonate exit cone were modeled directly while the primary collimator, collimator jaws and flattening filter were imported from vendor supplied CAD-files. Next, experimental characterization was performed. Half-Value-Layers (HVL) in aluminum were determined with a selection of filtration levels and types using a NOMEX multimeter (PTW, Freiburg, Germany). In the simulated model, the energy spectrum was tuned by approximating the energy of the electron beam by a linear combination of discrete energies. The resulting MC based HVLs in aluminum were compared to experimental data. The best approximation for the energy spectrum was determined by minimizing a cost function. The physical dimensions of the electron focal spot on the anode were measured using a dedicated slit camera (PTW, Freiburg, Germany). The slit camera restricted the beam to a narrow slit. The projection of this slit was then registered by the detector of the ImagingRing System. Subsequently, findings on position and shape of the electron focal spot were implemented in the simulation. In addition, two-dimensional dose distributions in abscence of a flattening filter were first measured, using the scintillation based Lynx detector (IBA, Schwarzenbruck, Germany), and then simulated. The average deviation between measured and simulated HVLAl was within 3% for the whole clinical energy range between 80 keV to 120 keV. This agreement was X Iwithin measurement uncertainty. The size and shape of the simulated electron focal spot agreed closely with the experimentally measured data. The heel e_ect stemming from the anode was clearly visible in both the experimental data and the MC simulation and the intensity profiles matched. In conclusion, a GATE based X-ray head model was established, that accurately resembles experimental measurements. The presented method was shown to provide a realistic X-ray distribution, enabling the estimation of imaging doses when implementing new clinical protocols or predict the impact of technical changes of the X-ray source. The developed method can be transferred to model other commercial X-ray units. The established model also allows detailed MC based investigations of the head scatter to improve imaging quality.