# A Novel and Reliable Pixel Response Correction Method (DAC-Shifting) for Spectral Photon-Counting CT Imaging

## TL;DR

A new method called DAC-shifting improves image quality in spectral photon-counting CT by correcting pixel response issues, leading to fewer artefacts and more accurate results.

## Contribution

DAC-shifting is a novel post-processing method that explicitly measures and corrects pixel response in spectral photon-counting CT imaging.

## Key findings

- DAC-shifting reduced CT-number variation by 47.4% in homogeneous materials.
- DAC-shifting outperformed PMMA-based STC in correcting pixel responses and handling detector instability.
- DAC-shifting produced sharper attenuation peaks and more uniform CT values in K-edge imaging.

## Abstract

Spectral photon-counting cone-beam computed tomography (CT) imaging is challenged by individual pixel response behaviours, which lead to noisy projection images and subsequent image artefacts like rings. Existing methods to correct for this either use calibration measurements, like signal-to-thickness calibration (STC), or perform a post-processing ring artefact correction of sinogram data or scan reconstructions without taking the pixel response explicitly into account. Here, we present a novel post-processing method (digital-to-analogue converter (DAC)-shifting) which explicitly measures the current pixel response using flat-field images and subsequently corrects the projection data. The DAC-shifting method was evaluated using a repeat series of the spectral photon-counting imaging (Medipix3) of a phantom with different density inserts and iodine K-edge imaging. The method was also compared against polymethyl methacrylate (PMMA)-based STC. The DAC-shifting method was shown to be effective in correcting individual pixel responses and was robust against detector instability; it led to a 47.4% average reduction in CT-number variation in homogeneous materials, with a range of 40.7–55.6%. On the contrary, the STC correction showed varying results; a 13.7% average reduction in CT-number variation, ranging from a 43.7% increase to a 45.5% reduction. In K-edge imaging, DAC-shifting provides a sharper attenuation peak and more uniform CT values, which are expected to benefit iodine concentration quantifications.