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 – 25 mm Ca thickness) were used to evaluate material decomposition errors due to object and BT scatter using least-squares projection-domain decomposition.

Results: Even with an 1D ASG, the low-energy channel SPRs for large objects (300 mm diameter) might be as high as 5 (at 80 mm collimation) – 10 (160 mm collimation). When BT scatter is ignored in scatter modeling, the SPRs are underestimated by 10% - 30%, with the relative error increasing with channel energy. Scatter introduces substantial biases in material decomposition: without any correction, the relative errors of Ca path length estimates range from 6%-10% for a 150 mm object and 80 mm collimation to 50%-100% for a 300 mm object at 160 mm collimation. Approximate correction using only object scatter reduces these biases to generally <5%, except for the largest phantom and collimation, where ignoring BT scatter leads to 6% - 12% underestimation of Ca thickness.