Fifty-eighth annual meeting of the american association of physicists in medicine
WE-AB-207A-10: Transmission Characteristics of a Two Dimensional Antiscatter Grid Prototype for CBCT
Scattered radiation remains to be a major contributor to image quality degradation in CBCT. To address the scatter problem, a focused, 2D antiscatter grid (2DASG) prototype was designed, and fabricated using additive manufacturing processes. Its scatter and primary transmission properties were characterized using a linac mounted CBCT system.
The prototype 2DASG was composed of rectangular grid holes separated by tungsten septa, and has a grid pitch of 2.91 mm, grid ratio of 8, and a septal thickness of 0.1 mm. Each grid hole was aligned or focused towards the x-ray source in half-fan (i.e. offset detector) geometry of the Varian TrueBeam CBCT system. Scatter and primary transmission experiments were performed by using acrylic blocks and the beam-stop method. Transmission properties of a radiographic ASG (1DASG) (grid ratio of 10) was also performed by using the identical setup.
At 30 cm phantom thickness, scatter to primary ratio (SPR) was 4.51 without any ASG device. SPR was reduced to 1.28 with 1DASG, and it was further reduced to 0.28 with 2DASG. Scatter transmission fraction (Ts) of 1DASG was 21%, and Ts was reduced to 5.8% with 2DASG. The average primary transmission fraction (Tp) of 1DASG was 70.6%, whereas Tp increased to 85.1% with 2DASG. Variation of Tp across 40 cm length (the long axis of flat panel detector) was 2.6%.
When compared to conventional ASGs, the focused 2DASG can vastly improve scatter suppression and primary transmission performance. Due to precise alignment of 2DASG's grid holes with respect to beam divergence, high degree of primary transmission through the 2DASG was maintained across the full length of the prototype. We strongly believe that robust scatter rejection and primary transmission characteristics of our 2DASG can translate into both improved quantitative accuracy and soft tissue resolution in linac mounted CBCT systems.