University of Oulu

S.I. Inkinen et al 2020, JINST, 15, T08005,

Calibration method and photon flux influences tiled flat-panel photon counting detector image uniformity in computed tomography

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Author: Inkinen, S. I.1; Juntunen, M. A. K.1,2,3; Kotiaho, A.1,2,3;
Organizations: 1Research Unit of Medical Imaging, Physics and Technology, University of Oulu, Aapistie 5A, FI-90220, Oulu, Finland
2Medical Research Center, University of Oulu and Oulu University Hospital, Oulu, Aapistie 5A, FI-90220, Oulu, Finland
3Department of Diagnostic Radiology, Oulu University Hospital, POB 50, FI-90029, Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 8.8 MB)
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Language: English
Published: IOP Publishing, 2020
Publish Date: 2021-01-25


The aim of this study is to compare how different calibration methods influence the image quality of photon-counting detector computed tomography (PCD-CT) at high and low photon fluxes. We investigate the performance of flat-field correction, signal-to-equivalent thickness calibration (STC), and polynomial correction (PC) methods using polymethyl methacrylate (PMMA) and iron as calibration materials. Two different cylindrical imaging phantoms containing contrast targets were scanned: an agar phantom and a phantom consisting of titanium hip implant embedded in agar. The scans were acquired using 120 kVp, and the energy thresholds of the PCD were set at 10 keV and 60 keV to obtain low energy (10–60 keV), high energy (60–120 keV) and total energy images (10–120 keV). Additionally, virtual monochromatic images (VMIs) with energies between 60–180 keV with 20 keV increments were generated from PC data. The reconstructions were made using filtered back projection, and image quality was assessed by evaluating image noise, contrast-to-noise ratio (CNR), and image uniformity. Overall, STC with PMMA as calibration material yielded the best image quality in terms of CNR and uniformity. Flat-field correction produced uniform reconstruction at low photon flux, but the performance degraded substantially at high flux. STC with iron as calibration material did not improve the reconstructions of the titanium hip implant. The beam hardening effects arising from metal were reduced when the VMI energy was increased while the CNR evaluated from agar phantom decreased with increasing energy of the VMI. Over the methods investigated, STC with PMMA was the most optimal calibration method for PCD-CT, yielding excellent image uniformity with both photon flux conditions.

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Series: Journal of instrumentation
ISSN: 1748-0221
ISSN-E: 1748-0221
ISSN-L: 1748-0221
Volume: 15
Issue: 8
Article number: T08005
DOI: 10.1088/1748-0221/15/08/T08005
Type of Publication: A1 Journal article – refereed
Field of Science: 114 Physical sciences
217 Medical engineering
Copyright information: © 2020 CERN. Published by IOP Publishing Ltd on behalf of Sissa Medialab. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.