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Landry, Guillaume; Gaudreault, Mathieu; Elmpt, Wouter van; Wildberger, Joachim E. and Verhaegen, Frank (2016): Improved dose calculation accuracy for low energy brachytherapy by optimizing dual energy CT imaging protocols for noise reduction using sinogram affirmed iterative reconstruction. In: Zeitschrift für Medizinische Physik, Vol. 26, No. 1: pp. 75-87

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The goal of this study was to evaluate the noise reduction achievable from dual energy computed tomography (CT) imaging (DECT) using filtered backprojection (FBP) and iterative image reconstruction algorithms combined with increased imaging exposure. We evaluated the data in the context of imaging for brachytherapy dose calculation, where accurate quantification of electron density rho(e) and effective atomic number Z(eff) is beneficial. A dual source CT scanner was used to scan a phantom containing tissue mimicking inserts. DECT scans were acquired at 80 kVp/140Sn kVp (where Sn stands for tin filtration) and 100 kVp/140Sn kVp, using the same values of the CT dose index CTDIvol for both settings as a measure for the radiation imaging exposure. Four CTDIvol levels were investigated. Images were reconstructed using FBP and sinogram affirmed iterative reconstruction (SAFIRE) with strength 1,3 and 5. From DECT scans two material quantities were derived, Z(eff) and rho(e). DECT images were used to assign material types and the amount of improperly assigned voxels was quantified for each protocol. The dosimetric impact of improperly assigned voxels was evaluated with Geant4 Monte Carlo (MC) dose calculations for an I-125 source in numerical phantoms. Standard deviations for Z(eff) and rho(e) were reduced up to a factor similar to 2 when using SAFIRE with strength 5 compared to FBP. Standard deviations on Z(eff) and rho(e) as low as 0.15 and 0.006 were achieved for the muscle insert representing typical soft tissue using a CTDIvol of 40 mGy and 3 mm slice thickness. Dose calculation accuracy was generally improved when using SAFIRE. Mean (maximum absolute) dose errors of up to 1.3% (21%) with FBP were reduced to less than 1% (6%) with SAFIRE at a CTDIvol of 10 mGy. Using a CTDIvol of 40mGy and SAFIRE yielded mean dose calculation errors of the order of 0.6% which was the MC dose calculation precision in this study and no error was larger than +/- 2.5% as opposed to errors of up to -4% with FPB. This phantom study showed that the SAFIRE image reconstruction algorithm provided reduced standard deviations of Z(eff) and rho(e) in uniform regions of interest while preserving mean Z(eff) and rho(e) values. This resulted in improved material type assignment. The use of SAFIRE improved brachytherapy dose calculations for the materials from the phantom investigated in this study using I-125.

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