A phantom for testing Cone Beam CTs
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TECHNICAL NOTE
A phantom for testing Cone Beam CTs Steven Muir1 · Johnny Laban2 Received: 11 August 2020 / Accepted: 30 October 2020 © Australasian College of Physical Scientists and Engineers in Medicine 2020
Abstract Cone Beam Computed Tomography (CBCT) scanners are becoming more common for dental and maxillofacial/head scanning, but performing image quality tests on these systems is difficult. There are quality assurance (QA) phantoms commercially available but they can be expensive, bulky and not optimised for CBCT imaging limits. Smaller phantoms often lack features that are recommended for testing CBCT systems. A custom made phantom can provide more useful test objects in a more convenient size and at a lower cost. The proposed phantom is called the “Karu” Cone Beam CT Phantom and is constructed with a 3D printed poly lactic acid (PLA) shell, with 3D printed inserts for holding the test details in place. Tests included are geometric accuracy (in three dimensions), Hounsfield Unit (HU) accuracy, low contrast detectability, spatial resolution (using line pairs), and uniformity/artifacts/noise. The phantom was scanned on a number of scanners and was clearly able to differentiate scanners producing poorer quality images from better quality ones. The phantom could be produced for under NZ $2000. Keywords Cone Beam CT · Phantom · Quality control test
Introduction Cone Beam CT scanners utilise a flat panel digital X-Ray detector and an X-Ray tube that rotates either a partial or full revolution around a patient and obtains a three dimensional dataset that can be reconstructed in any plane or as a 3D surface. For scanners used in dental/maxillofacial work, the field of view and voxel size are smaller than a typical multi-slice volume CT, giving very good spatial resolution. Cone beam artefacts and increased scatter radiation tends to result in poorer low contrast resolution and Hounsfield Unit accuracy and the small voxel size results in increased noise compared to a multi-slice volume CT. The X-Ray doses are higher than traditional dental X-Ray machines, and so the importance of performing quality control tests to ensure they are delivering acceptable images, is more important.
* Steven Muir [email protected] 1
Medical Physicist, Medical Physics and Bioengineering Department, Christchurch Hospital, Private Bag, Christchurch 4710, New Zealand
Medical Physicist, Photon Physics Ltd., Christchurch, New Zealand
2
The European Commission [1] recommends that six image quality tests are performed on CBCT scanners, which are: uniformity & artefacts, noise, geometrical precision, voxel density values, low-contrast (contrast detail) resolution, and spatial resolution. A joint publication by EFOMP, ESTRO and IAEA [2] also recommends these six tests. Phantoms provided by CBCT scanner manufacturers seldom have image quality test objects, which limits the number of tests that can be performed. Third party phantoms such as the PhantomLabs Catphan [3], ACR 464 CT phantom [4], Leeds SedentexCT [5] and Proden
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