Impact of x-ray tube voltage, field size and object thickness on scattered radiation distribution in diagnostic radiology: A Monte Carlo investigation

The contamination of diagnostic radiology images with scattered photons effectively degrades image quality and decreases low contrast detectability. The magnitude of scattered radiation strongly depends on the object under study and exposure settings. Thorough knowledge of scatter distribution as function of X-ray tube settings and scanning parameters is essential for optimization of system design geometry, selection of imaging protocols and development of optimum anti-scatter grids. The general practice of developing theoretical scatter models involves experimental measurements or deterministic mathematical modeling, whereas most recent publications focus on the use of Monte Carlo simulations. In this study, the general- purpose MCNP4C Monte Carlo computer code was used for calculation of scatter distribution and scatter-to-primary ratio (SPR) as function of tube voltage, field size and object thickness. Scattered radiation was separated from the primary component using the surface source method implemented in the MCNP4C code. The measured spectra published in the Handbook of Computed Tomography X-ray Spectra were used for validation of simulated spectra in the exit window of the X-ray tube and after the X-ray tube and after thephantoms using different materials and thicknesses. Following validation of simulation results, the standard conditions proposed in the IEC 60627 standard were used for scatter distribution characterization. The SPR varied between 0.17 and 13.06 for water phantom thicknesses varying between 3 and 40 cm, whereas it varied between 0.11 and 1.79 for field size varying between 5><5 and 30x30 cm (80 kVp and 20 cm water phantom), and between 2.38 and 2.8 for tube voltages varying between 40 and 150 kVp (30x30 cm field size and 20 cm water phantom), respectively. The results indicate that the MCNP4C code is a useful tool for modelling of scattered radiation in diagnostic radiology provided careful adjustment of the appropriate MCNP cards is- carried out by experienced users.