Monte Carlo performance on the x-ray converter thickness in digital mammography using software breast models P. Liaparinos a) Department of Medical Instruments Technology, Technological Educational Institute, 12210 Athens, Greece K. Bliznakova Department of Medical Physics, School of Medicine, University of Patras, 26500 Rio, Patras, Greece (Received 7 February 2012; revised 18 September 2012; accepted for publication 18 September 2012; published 12 October 2012) Purpose: In x-ray mammography, some of the components that play significant role to early diag- nosis are the x-ray source, the breast composition as well as the composition of the x-ray converter. Various studies have previously investigated separately the influence of breast characteristics and detector configuration on the optimization of mammographic imaging systems. However, it is impor- tant to examine the combined effect of both components in improving the signal transfer properties in mammography systems of the mammograms. In the present study, the authors compared and eval- uated x-ray converters using software breast models and realistic mammographic spectra in terms of: (a) zero-frequency detective quantum efficiency (DQE) and (b) sensitivity. The impact of x-ray converter thickness on contrast threshold (C TH ) for observer assessment, based on the Rose model, was demonstrated as well. Methods: Monte Carlo techniques were applied to simulate the x-ray interactions within the soft- ware breast phantoms and thereafter within the detective medium. Simulations involved: (a) two mammographic x-ray spectra: 28 kV Mo, 0.030 mm Mo, and 32 kV W, 0.050 mm Rh of differ- ent entrance surface air kerma (ESAK: 3–7 mGy), (b) realistic breast models (dense and fatty) and (c) x-ray converter materials most frequently considered in investigations on energy integrating digi- tal mammography detectors: the Gd 2 O 2 S:Tb granular phosphor, the CsI:Tl structured phosphor, and the a-Se photoconductive layer. Detector material thickness was considered to vary in the range from 50 mg/cm 2 up to 150 mg/cm 2 . Results: The Monte Carlo study showed that: (a) the x-ray beam becomes less penetrating after passing through dense breasts leading to higher values of zero-frequency DQE of the x-ray imaging converters and improved C TH values in all cases considered, (b) W/Rh target/filter combination results in improved C TH values at higher ESAK values, and (c) a-Se shows higher zero-frequency DQE values than the phosphor-based converters, Gd 2 O 2 S:Tb and CsI:Tl. However, thicker layers of CsI:Tl could be comparable to a-Se layers achieving approximately 27.6% C TH improvement at a thickness of 150 mg/cm 2 . Conclusions: The present Monte Carlo investigation indicates that in the energy range employed in mammography, an upper limit, approximately 100 mg/cm 2 , should be considered in the devel- opment of thicker a-Se converters. On the other hand, above this thickness value, CsI:Tl converter could improve its imaging performance. © 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4757919] Key words: Monte Carlo, x-ray converters, mammography, breast models, contrast threshold I. INTRODUCTION Imaging of the breast is indicated for the early detection and diagnosis of cancer. X-ray mammography, that is, the radiog- raphy of the breast, is essentially the only widely used imag- ing modality for breast cancer screening due to its increased diagnostic accuracy. 1 In particular, the benefits of mammo- graphic imaging systems are attributed to the differences in tissue x-ray attenuation and to the capability of the system to transform the x-rays to visible signals. Today, an x-ray mammography imaging system can be op- timized through suitable selection of exposure conditions, de- tector type, dedicated post processing, viewing conditions, and computer-aided detection. 2 Observer models can also be used to predict the performance of an imaging system for a particular task. 3 The development of observer mod- els overcomes some of the limitations in using the over- all signal-to-noise properties of the detector in the sense that they are applied in clinical x-ray irradiation and im- age observation conditions. Two significant studies 3, 4 have been published recently investigating the performance of dig- ital mammographic systems. According to these studies, one of the major factors affecting the accuracy of the diagno- sis is the detector performance and, in particular, the com- position of the converter material. Different x-ray converter materials have been considered in digital imagers for diag- nostic x-ray applications: 3, 5, 6 (i) Indirect based on powder phosphor (granular gadolinium oxysulphide: Gd 2 O 2 S:Tb), 6638 Med. Phys. 39 (11), November 2012 © 2012 Am. Assoc. Phys. Med. 6638 0094-2405/2012/39(11)/6638/14/$30.00