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Purpose: To investigate spectral distortion caused by bowtie (BT) scatter in photon-counting CT (PCCT) and its impact of the accuracy of material decomposition.

Methods: GPU-accelerated Monte Carlo (MC) simulations of a PCCT scanner with 1 mm detector pixels, a 1- dimensional anti-scatter grid (ASG) with 30:1 grid ratio, and five energy channels per pixel (25/35/50/65/80keV) were performed. Beam collimation was varied 80 – 160 mm (at isocenter). X-ray tube voltage was 120 kVp. An ~80 mm thick Aluminum BT was placed 48 mm from the x-ray focal spot. Spectral and spatial distributions of scatter and Scatter-to-Primary ratio (SPR) of water cylinders (150 mm – 300 mm diameter) were compared for two MC simulation settings: (a) scatter occurs both in the BT and in the object (ground-truth); and (b) BT acts only as a beam shaper but does not cause scatter. Water cylinders with Calcium wedge inserts (5 mm …