Monte Carlo simulations for therapy imaging

In therapy imaging there is a need to optimize imaging protocols and different correction methods for e.g. photon attenuation, contribution from scatter and other physics-related effects that limits the possibility to achieve accurate quantitative results. Physical phantom studies are important but lack flexibility and are often difficult to make patient realistic. A complement is to simulate the study using a Monte Carlo-based model of the imaging process including accurate description of the interaction processes and a realistic model of the patient and the imaging device. With the Monte Carlo method it is also possible to simulate and calculate parameters that otherwise would be impossible to measure from a physical experiment. An example of such is the simulation of a scatter and an attenuation-free situation. Results from such simulations can be very important in a validation procedure serving as reference data for which results obtained by different correction methods can be compared to. The Monte Carlo method has also been used to calculate absorbed doses from reference phantom or patient-specific phantom obtained from a CT study. This paper give an overview of the method and show examples of how Monte Carlo simulation can be used to evaluate therapy imaging with different radionuclides but also how simulations can be used for calculation of necessary parameters for correction of scatter, attenuation and collimator response.