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Beam Profile Assessment in Spectral CT Scanners

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Received: 23 July 2017 | Revised: 23 November 2017 | Accepted: 7 December 2017 DOI: 10.1002/acm2.12260 MEDICAL IMAGING Beam profile assessment in spectral CT scanners Marzieh Anjomrouz1,2 | Muhammad Shamshad1,2 | Raj K. Panta1,2 | Lieza Vanden Broeke3 | Nanette Schleich4 | Ali Atharifard1,2 | Raja Aamir1,2 | Srinidhi Bheesette1,5 | Michael F. Walsh2 | Brian P. Goulter2 | Stephen T. Bell2,5 | Christopher J. Bateman1,2,3 | Anthony P. H. Butler1,2,3,5 | Philip H. Butler2,3,5 1Department of Radiology, University of Otago, Christchurch, New Zealand Abstract 2MARS Bioimaging Ltd, Christchurch, New In this paper, we present a method that uses a combination of experimental and Zealand modeled data to assess properties of x-ray beam measured using a small-animal 3Department of Physics and Astronomy, fi University of Canterbury, Christchurch, spectral scanner. The spatial properties of the beam pro le are characterized by New Zealand beam profile shape, the angular offset along the rotational axis, and the photon 4 Department of Radiation Therapy, count difference between experimental and modeled data at the central beam axis. University of Otago, Wellington, New Zealand Temporal stability of the beam profile is assessed by measuring intra- and interscan 5European Organization for Nuclear count variations. The beam profile assessment method was evaluated on several Research (CERN), Geneva, Switzerland spectral CT scanners equipped with Medipix3RX-based detectors. On a well-cali- Author to whom correspondence should be brated spectral CT scanner, we measured an integral count error of 0.5%, intrascan addressed. Marzieh Anjomrouz E-mail: [email protected] count variation of 0.1%, and an interscan count variation of less than 1%. The angu- lar offset of the beam center ranged from 0.8° to 1.6° for the studied spectral CT Funding Information scanners. We also demonstrate the capability of this method to identify poor per- University of Otago; University of Canterbury; MARS Bioimaging Ltd.; CERN; formance of the system through analyzing the deviation of the experimental beam National Heart Foundation; Maurice and profile from the model. This technique can, therefore, aid in monitoring the system Phyllis Paykel Trust performance to obtain a robust spectral CT; providing the reliable quantitative images. Furthermore, the accurate offset parameters of a spectral scanner provided by this method allow us to incorporate a more realistic form of the photon distribu- tion in the polychromatic-based image reconstruction models. Both improvements of the reliability of the system and accuracy of the volume reconstruction result in a better discrimination and quantification of the imaged materials. PACS 87.57.-s, 87.57.cf, 87.57.cm, 87.57.cp, 87.57.N-, 87.57.Q-, 87.59.-e KEY WORDS beam profile, geometric calibration, Medipix detector, spectral CT, x-ray source model 1 | INTRODUCTION power, and propagation. The characterization of a beam is specific to the type of beam, which could be monochromatic 1–3 or polychro- A broad definition of the beam profile analysis encompasses all beam matic.4 For a laser beam, parameters such as alignment, focus spot properties, such as spatial, temporal and spectral characteristics, size, and beam uniformity are typically analyzed to optimize laser ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2018 MARS Bioimaging Ltd. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine. | J Appl Clin Med Phys 2018; 19:2:287–297 wileyonlinelibrary.com/journal/jacmp 287 288 | ANJOMROUZ ET AL. performance.1,2 Characterizing the beamlines is also essential in par- Reconstruction problems can also arise when random fluctuations ticle accelerators for them to be operated with optimal output.3 The occur in the beam profile due to instability of CT scanner components, polychromaticity of the x-ray beams used in the computed tomogra- such as the x-ray tube and detector. Fluctuations in the beam profile phy (CT) scanners necessitates the accurate modeling of beam pro- are more likely when spectral data are acquired over a long exposure file in these machines. CT scanners are used for diagnostic imaging time. Relatively long exposure time is required because photon count- (kilovoltage range) and image-guided radiotherapy (megavoltage ing detectors can optimally operate at low photon flux.14 The use of range). Many methods have been published for beam profile mea- low photon flux ensures maintaining spatial and spectral fidelity of the surements of such systems. Among them, the work published by images in two aspects.5,15 Firstly, the small pixel size of the photon Malts et al. can be referred to. They presented a method of charac- counting detectors such as Medipix3RX favors the use of x-ray tubes terizing the spatial variation in the intensity and energy of the inci- with small focal spot sizes (e.g., 50 lm) to maximize spatial resolu- dent beam in diagnostic and treatment cone beam CT.4 tion.5,14,16 Striking the smaller area of the anode target by electrons, in Beam profile characterization is also a prerequisite to optimize turn, generates a lower photon flux.14,17 Secondly, due to limited performance of spectral CT scanners operating on the basis of pho- pulse resolution time of such detectors, the energy information of a ton-counting detectors. The optimal performance of the spectral CT high flux beam cannot be resolved correctly. The energy of coincident scanner is achievable when the energy and position of the incident photons is accumulated and registered at a higher energy of each ini- photon are measured accurately.5 Beam profile assessment methods tial photon. This pulse pile-up effect results in the loss of spectral examining various properties of the beam profile can be used to iden- information.16,18 To minimize the occurrence of this effect, incident tify the parameters that prevent accurate measurement of photon photon flux needs to be reduced. energy and position. The properties of the beam such as photon inten- Acquiring data with longer exposure time, while using the low sity and its angular distribution not only needs to be characterized at photon flux, provides sufficient counts; resulting in a higher signal to initial installation, but beam profiles also need to be regularly assessed noise ratio in reconstructed images. However, the detector perfor- for identifying the distortion caused by either deterioration of the mance may degrade due to increasing ASIC temperature and as a x-ray tube performance during its lifetime or instabilities of other scan- consequence, charge loss occurs due to detector polarization during ner components. In this study, aforementioned properties are charac- long acquisition time. Therefore, the beam profile stability needs to terized for the beam profiles measured using MARS small-animal be monitored in such a system to ensure that there is no count drift spectral CT manufactured by MARS Bioimaging Ltd., New Zealand. during imaging. In response to this need, we have developed a beam The MARS spectral CT scanners use Medipix photon counting profile assessment and characterization method. The method enables detectors to provide 3D tomographic images with both high spatial quantification of the temporal and spatial properties of beam profiles and high spectral resolution. The energy resolved information enables and assessment through comparison with modeled beam profiles. simultaneous discrimination and quantification of different materials In this paper, we introduce a parametrized semi-analytic source based on their spectral signatures.6,7 The spectral imaging allows the model and the experimental requirements. We then explain the pro- extraction of functional and anatomical features of the tissues via trac- cedure of developing the beam profile assessment method, and pre- ing biomarkers and pharmaceuticals in a low dose and noninvasive sent method evaluation results obtained from one well-calibrated way.8,9 MARS imaging has been used in various preclinical applications and two poorly calibrated MARS spectral CTs. such as characterizing the composition of excised vulnerable atherosclerosis plaques in arteries,10 functional imaging of arthritic cartilage,11 and targeting cancerous cells using nanoparticles.12 2 | MATERIALS AND METHODS The x-ray tube used in the spectral CT provides a cone shape photon distribution which typically varies over the imaging field.4 A workflow diagram of the beam profile assessment method is Furthermore, x-ray tube manufacturing and alignment variation of depicted in Fig. 1. An experimental beam profile is provided to the the beam direction with detector plane in a spectral CT scanner also method. Then a modeled beam profile is prepared from a semi-analytic makes the photon beam profile specific to that system. For instance, source model based on the equivalent spatial parameters of the mea- relative geometric offsets due to tube anode orientation may spa- surement. Measured and modeled data are then preprocessed to reach tially shift the recorded beam profile. To identify such a variation, the same level of conformity to be comparable with each other. In the the beam profile of each spectral
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