Highlights
- •Test method to assess how CT parameters affect image quality and Hounsfield units.
- •CNR and HCSR varied most with reconstruction algorithm, reconstruction FOV and mAs.
- •Hounsfield units changed with acquisition FOV and reconstruction algorithm selected.
- •The results will enable optimisation of CT protocols used for radiotherapy planning.
Abstract
Purpose
To define a method and investigate how the adjustment of scan parameters affected
the image quality and Hounsfield units (HUs) on a CT scanner used for radiotherapy
treatment planning. A lack of similar investigations in the literature may be a contributing
factor in the apparent reluctance to optimise radiotherapy CT protocols.
Method
A Catphan phantom was used to assess how image quality on a Toshiba Aquilion LB scanner
changed with scan parameters. Acquisition and reconstruction field-of-view (FOV),
collimation, image slice thickness, effective mAs per rotation and reconstruction
algorithm were varied. Changes were assessed for HUs of different materials, high
contrast spatial resolution (HCSR), contrast-noise ratio (CNR), HU uniformity, scan
direction low contrast and CT dose-index.
Results
CNR and HCSR varied most with reconstruction algorithm, reconstruction FOV and effective
mAs. Collimation, but not image slice width, had a significant effect on CT dose-index
with narrower collimation giving higher doses. Dose increased with effective mAs.
Highest HU differences were seen when changing reconstruction algorithm: 56 HU for
densities close to water and 117 HU for bone-like materials. Acquisition FOV affected
the HUs but reconstruction FOV and effective mAs did not.
Conclusions
All the scan parameters investigated affected the image quality metrics. Reconstruction
algorithm, reconstruction FOV, collimation and effective mAs were most important.
Reconstruction algorithm and acquisition FOV had significant effect on HU. The methodology
is applicable to radiotherapy CT scanners when investigating image quality optimisation,
prior to assessing the impact of scan protocol changes on clinical CT images and treatment
plans.
Keywords
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Article info
Publication history
Accepted:
November 30,
2017
Received in revised form:
October 8,
2017
Received:
May 30,
2017
Identification
Copyright
Crown Copyright © 2017 Published by Elsevier Ltd on behalf of Associazione Italiana di Fisica Medica. All rights reserved.
