A novel method for contrast-to-noise ratio (CNR) evaluation of digital mammography detectors

BreastCheck, The National Cancer Screening Service, 36 Eccles Street, Dublin 7, Ireland.
European Radiology (Impact Factor: 4.01). 06/2009; 19(9):2275-85. DOI: 10.1007/s00330-009-1409-3
Source: PubMed


The purpose of this study was to test a new, simple method of evaluating the contrast-to-noise ratio (CNR) over the entire image field of a digital detector and to compare different mammography systems. Images were taken under clinical exposure conditions for a range of simulated breast thicknesses using poly(methyl methacrylate) (PMMA). At each PMMA thickness, a second image which included an additional 0.2-mm Al sheet was also acquired. Image processing software was used to calculate the CNR in multiple regions of interest (ROI) covering the entire area of the detector in order to obtain a 'CNR image'. Five detector types were evaluated, two CsI-alphaSi (GE Healthcare) flat panel systems, one alphaSe (Hologic) flat panel system and a two generations of scanning photon counting digital detectors (Sectra). Flat panel detectors exhibit better CNR uniformity compared with the first-generation scanning photon counting detector in terms of mean pixel value variation. However, significant improvement in CNR uniformity was observed for the next-generation scanning detector. The method proposed produces a map of the CNR and a measurement of uniformity throughout the entire image field of the detector. The application of this method enables quality control measurement of individual detectors and a comparison of detectors using different technologies.

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    • "The PMMA and the Al attenuation is equivalent to a 10 mm thickness plate of PMMA (Young et al 2006b). Some authors suggested that the CNR is the parameter indicated to assess the image quality in digital mammography (Young et al 2006c, Samei et al 2005, Baldelli et al 2009). The CNR for a particular set of clinical exposure parameters was determined using a 20 mm side square of Al foil, 99% of purity and 0.2 mm thickness, placed over a 20 mm thickness PMMA plate of 24 cm × 30 cm (Al-PMMA phantom). "
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