A fast, GPU accelerated Monte Carlo engine for simulating relevant photon interaction processes over the diagnostic
energy range in third-generation CT systems was developed to study the relative contributions of bowtie and object
scatter to the total scatter reaching an imaging detector.
Primary and scattered projections for an elliptical water phantom (major axis set to 300mm) with muscle and fat inserts
were simulated for a typical diagnostic CT system as a function of anti-scatter grid (ASG) configurations. The ASG
design space explored grid orientation, i.e. septa either a) parallel or b) parallel and perpendicular to the axis of rotation,
as well as septa height. The septa material was Tungsten. The resulting projections were reconstructed and the scatter
induced image degradation was quantified using common CT image metrics (such as Hounsfield Unit (HU) inaccuracy
and loss in contrast), along with a qualitative review of image artifacts.
Results indicate object scatter dominates total scatter in the detector channels under the shadow of the imaged object
with the bowtie scatter fraction progressively increasing towards the edges of the object projection. Object scatter was
shown to be the driving factor behind HU inaccuracy and contrast reduction in the simulated images while shading
artifacts and elevated loss in HU accuracy at the object boundary were largely attributed to bowtie scatter. Because the
impact of bowtie scatter could not be sufficiently mitigated with a large grid ratio ASG, algorithmic correction may be
necessary to further mitigate these artifacts.
Scatter presents as a significant source of image artifacts in cone beam CT (CBCT) and considerable effort has been devoted to measuring the magnitude and influence of scatter. Scatter management includes both rejection and correction approaches, with anti-scatter grids (ASGs) commonly employed as a scatter rejection strategy. This work employs a Geant41,2 driven Monte Carlo model to investigate the impact of different ASG designs on scatter rejection performance across a range of scanner coverage along the patient axis. Scatter rejection is quantified in terms of scatter to primary ratio (SPR). One-dimensional (1D) ASGs (grid septa running parallel to patient axis) are compared across a range of septa height, septa width and septa material. Results indicate for a given septa width and patient coverage, SPR decreases with septa height but demonstrates diminishing returns for larger height values. For shorter septa heights, higher Z materials (e.g., Tungsten) exhibit superior scatter rejection to relatively lower Z materials (e.g., Molybdenum). For taller septa heights, the material difference is not as significant. SPR has a relatively weak dependence on septa width, with thicker septa giving lower SPR values at a given scanner coverage. The results are intended to serve as guide for designing post patient collimation for whole body CT scanners. Since taller grids with high Z materials pose a significant manufacturing cost, it is necessary to evaluate optimal ASG designs to minimize material and machining costs and to meet scatter rejection specifications at given patient coverage.