A microCT X-ray head model for spectra generation with Monte Carlo simulations

The X-ray spectrum from a small-animal computed tomography (CT) device directly influences the dose delivered to the animal as well as the image quality. For any given imaging task, the spectrum needs to be optimized. The aim of this study is to present a model of a small animal microCT X-ray source and to calculate practical spectra for different anode and filter combinations by using the Monte Carlo method. The GATE simulation package was used for this work. The X-ray source of the MicroCAT II scanner was modeled as a tungsten enclosure containing the X-ray tube with dielectric oil and a thin carbon exit window. The anode was a tungsten or molybdenum cylinder block tilted with respect to the electron-beam axis. Beam filtration was achieved through a disk of variable thickness and composition positioned at the exit window. Photon detection was performed with a thick (1 cm) cylindrical tungsten plate (20 cm diameter). Simulated spectra were compared with spectra measured with a CdTe detector for three beams: 50, 70 and 80 kVp with 2 mm of aluminum filtration. There was a reasonable agreement between measurements and calculations. Identified features of the measured spectra were: K absorption edges of Cd and Te (from the detector itself) and characteristic X-rays from Te (Kα and Kβ) and possibly Sb (Kα). Discrepancies between simulations and measurements can be attributed to elements that have been left out of simulations, imperfect detector alignment, and uncertainty in elemental composition of simulated materials. The Monte Carlo method is a convenient and fast tool allowing the calculation of spectra possessing the essential features of real spectra. These and other spectra will be used in future work to study the influence of X-ray spectrum on dose and image quality for various X-ray CT system configurations.