Analysis of the imaging performance in indirect digital mammography detectors by linear systems and signal detection models P. Liaparinos n , N. Kalyvas, I. Kandarakis, D. Cavouras Department of Medical Instruments Technology, Technological Educational Institute, 122 10 Athens, Greece article info Article history: Received 8 May 2012 Received in revised form 17 July 2012 Accepted 1 August 2012 Available online 16 September 2012 Keywords: Modelling Digital mammography X-ray converters abstract Purpose: The purpose of this study was to provide an analysis of imaging performance in digital mammography, using indirect detector instrumentation, by combining the Linear Cascaded Systems (LCS) theory and the Signal Detection Theory (SDT). Observer performance was assessed, by examining frequently employed detectors, consisting of phosphor-based X-ray converters (granular Gd 2 O 2 S:Tb and structural CsI:Tl), coupled with the recently introduced complementary metal-oxide- semiconductor (CMOS) sensor. By applying combinations of various irradiation conditions (filter-target and exposure levels at 28 kV) on imaging detectors, our study aimed to find the optimum system set-up for digital mammography. For this purpose, the signal to noise transfer properties of the medical imaging detectors were examined for breast carcinoma detectability. Methods: An analytical model was applied to calculate X-ray interactions within software breast phantoms and detective media. Modeling involved: (a) three X-ray spectra used in digital mammo- graphy: 28 kV Mo/Mo (Mo: 0.030 mm), 28 kV Rh/Rh (Rh: 0.025 mm) and 28 kV W/Rh (Rh: 0.060 mm) at different entrance surface air kerma (ESAK) of 3 mGy and 5 mGy, (b) a 5 cm thick Perspex software phantom incorporating a small Ca lesion of varying size (0.1–1 cm), and (c) two 200 mm thick phosphor-based X-ray converters (Gd2O2S:Tb, CsI:Tl), coupled to a CMOS based detector of 22.5 mm pixel size. Results: Best (lowest) contrast threshold (CT) values were obtained with the combination: (i) W/Rh target-filter, (ii) 5 mGy (ESAK), and (iii) CsI:Tl-CMOS detector. For lesion diameter 0.5 cm the CT was found improved, in comparison to other anode/filter combinations, approximately 42% than Rh/Rh and 55% than Mo/Mo, for small sized carcinoma (0.1 cm) and approximately 50% than Rh/Rh and 125% than Mo/Mo, for big sized carcinoma (1 cm), considering 5 mGy X-ray beam. By decreasing lesion diameter and thickness, a limiting CT (100%) was occurred for size values less than 0.2 cm. Conclusion: CT was found to be affected by the selection of target/filter and exposure combination. It was found that the optimum thickness of CsI:Tl was approximately 190 mm and for Gd 2 O 2 S:Tb 120 mm for the studied energy and ESAK range. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Image quality for medical purposes is related to the useful diagnostic information that can be extracted from an image. This quality can be evaluated by the performance of ‘‘some type of observer’’ on a particular diagnostic classification task [1,2]. The methodology used for human performance assessment can pro- vide a relationship between objective image quality level and subjective diagnostic performance. In diagnostic medical imaging applications (i.e. X-ray mammography, chest radiography and X-ray computed tomography), where X-ray detectors are employed for image formation [3], an example of a classification task is the lesion detection. X-ray detectors can play a critical role in improv- ing image quality, since they can accurately acquire and depict spatial information carried by the X-ray beam transmitted through the patient. In X-ray digital mammography, a very particular application of X-ray projection imaging, an attenuation profile of a human breast is projected onto a radiation detector often consisting of a phosphor-based X-ray converter, to convert X-ray energy to visible light photons. These photons are then captured by an electronic optical sensor. These converters are classified into two general categories: (a) indirect imaging detectors, using a gran- ular phosphor screen (e.g., Gd 2 O 2 S:Tb) and (b) indirect imaging detectors, using a columnar phosphor screen (e.g., CsI:Tl) and exhibit advantages in several medical and industrial imaging applications [4]. Direct imaging detectors based on a-Se converting material are also currently employed in commercially available Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.08.014 n Corresponding author. E-mail address: liapkin@teiath.gr (P. Liaparinos). Nuclear Instruments and Methods in Physics Research A 697 (2013) 87–98