# Can Planning Images Reduce Scatter in Follow-Up Cone-Beam CT?

## TL;DR

This paper evaluates different scatter estimation methods for cone-beam CT, demonstrating that incorporating accurate scatter estimates into statistical reconstruction improves image quality, with Monte-Carlo based estimates offering marginal benefits over simpler measurement-based methods.

## Contribution

It introduces a method to match offline Monte-Carlo scatter estimates with new measurements and compares their effectiveness in statistical reconstruction for CBCT.

## Key findings

- Statistical reconstruction with scatter estimates outperforms analytic and iterative pre-correction methods.
- Monte-Carlo scatter estimates provide only slight improvements over measurement-based methods.
- Incorporating scatter estimates significantly enhances image accuracy in CBCT reconstruction.

## Abstract

Due to its wide field of view, cone-beam computed tomography (CBCT) is plagued by large amounts of scatter, where attenuated photons hit the detector, and corrupt the linear models used for reconstruction. Given that one can generate a good estimate of scatter however, then image accuracy can be retained. In the context of adaptive radiotherapy, one usually has a low-scatter planning CT image of the same patient at an earlier time. Correcting for scatter in the subsequent CBCT scan can either be self consistent with the new measurements or exploit the prior image, and there are several recent methods that report high accuracy with the latter. In this study, we will look at the accuracy of various scatter estimation methods, how they can be effectively incorporated into a statistical reconstruction algorithm, along with introducing a method for matching off-line Monte-Carlo (MC) prior estimates to the new measurements. Conclusions we draw from testing on a neck cancer patient are: statistical reconstruction that incorporates the scatter estimate significantly outperforms analytic and iterative methods with pre-correction; and although the most accurate scatter estimates can be made from the MC on planning image, they only offer a slight advantage over the measurement based scatter kernel superposition (SKS) in reconstruction error.