A Generalized Monte Carlo Source Model for Estimating Radiation Dose from Spiral CT Scan Protocols

This study is designed to accurately simulate a helical CT-scanner for the purpose of estimating radiation dose and to perform validation experiments involving several dose profile and ionization chamber measurements. To accomplish this we have modified a general purpose Monte Carlo transport code (MCNP4B) as the framework for simulating the x-ray source and its movement, photon transport and radiation dose calculation. The movement of the source in either axial or spiral modes (with user selected pitch) was modeled explicitly. CT system components were modeled using published information about x-ray spectra as well as information provided by the manufacturer (e.g. geometry, internal filtration and the beam shaping filters for head and body scans). For the purposes of validation, dosimetry phantoms and measurement devices (ion chamber and thermoluminsecent dosimeters (TLDs)) were also modeled. Simulations for single axial dose profiles and spiral scans with pitch=1 and 2 were performed using head and body Computed Tomography Dose Index (CTDI-acrylic) phantoms on both central and peripheral positions within the phantoms. Comparisons were made using profile measurements (made with TLDs) as well as integrated measurements using a pencil ionization chamber. For the single axial exposure, the TLD simulation and measurements agree to within 9% for both the head and body phantoms. For a single axial exposure to an ion chamber, the simulation and measurement data agreed within 0.5% for the body phantom and to within 11% for the head phantom. For the spiral pitch 1 and pitch 2 scans in both the head and body phantoms, the TLD measured and simulated results showed good agreement in terms of amplitude and shape. The ability to model the spiral source movement along with the possibility of importing a patient's voxelized image data may allow the calculation of organ dose and effective dose using patient specific models.