Effects of bowtie scatter on material decomposition in photon-counting CT

W. Yang Tai, Donghyeon Lee, Xiaohui Zhan, Katsuyuki Taguchi, Wojciech Zbijewski

Research output: Chapter in Book/Report/Conference proceedingConference contribution


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. Conclusions: Algorithmic scatter corrections will likely be necessary for precise PCCT material decomposition for body-sized objects and wide collimations. While adequate results can be obtained using simplified models that ignore BT scatter, achieving <10% decomposition accuracy might require incorporating BT scatter.

Original languageEnglish (US)
Title of host publicationMedical Imaging 2023
Subtitle of host publicationPhysics of Medical Imaging
EditorsLifeng Yu, Rebecca Fahrig, John M. Sabol
ISBN (Electronic)9781510660311
StatePublished - 2023
EventMedical Imaging 2023: Physics of Medical Imaging - San Diego, United States
Duration: Feb 19 2023Feb 23 2023

Publication series

NameProgress in Biomedical Optics and Imaging - Proceedings of SPIE
ISSN (Print)1605-7422


ConferenceMedical Imaging 2023: Physics of Medical Imaging
Country/TerritoryUnited States
CitySan Diego


  • multi-material decomposition
  • photon-counting CT
  • x-ray scatter correction

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Atomic and Molecular Physics, and Optics
  • Biomaterials
  • Radiology Nuclear Medicine and imaging


Dive into the research topics of 'Effects of bowtie scatter on material decomposition in photon-counting CT'. Together they form a unique fingerprint.

Cite this