A novel method for contrast-to-noise ratio (CNR) evaluation of digital mammography detectors.
ABSTRACT 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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ABSTRACT: Iterative reconstruction of CT images is characterized by reduced image noise and may allow reduction in radiation exposure. We investigated the influence of an IRT technique on image quality and radiation dose savings when applied to temporal bone CT. Based on the typical image quality level of adult subjects using routine radiation dose and FBP, an exsomatized cadaveric head with CNR characteristics closest to the level of clinical subjects was identified. Cadaver acquisitions were performed at multiple levels of tube current exposure. Reconstructions were performed using FBP and IRT (iDose), with multiple iDose levels applied for each acquisition. Transverse and coronal reformations of all reconstructions were evaluated subjectively and objectively. Phantom tests were performed to validate the protocol optimizations with iDose, specifically the spatial resolution relative to routine dose acquisitions. Finally, the results of protocol optimization with iDose were clinically validated in 50 patients. At the same radiation dose, the image CNR of iDose reconstructions was higher than that of FBP and progressively increased with higher iDose levels. The combination of 100 mAs/section and iDoseL5 was the lowest dose that met the requirements for diagnostic acceptability, with CNR slightly higher than our routine institution protocol of 200 mAs/section with FBP reconstruction. Spatial resolution characteristics were similar between FBP and iDose at all different strengths. The findings were consistent among the cadaver, phantom, and clinical acquisitions. The iDose IRT can help reduce radiation dose of temporal bone CT by 50% relative to routine institution protocols with FBP, while maintaining diagnostic image quality.American Journal of Neuroradiology 02/2012; 33(6):1020-6. · 3.17 Impact Factor
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ABSTRACT: This work examines the use of a detectability index to control an Automatic Exposure Control (AEC) system for an amorphous-Selenium digital mammography detector. The default AEC mode for the system was evaluated using homogeneous poly(methyl methacrylate) (PMMA) plates of thickness 20, 40, 60 and 70 mm to find the tube potential and anode/filter settings selected by the system. Detectability index (d') using a non-prewhitened model observer with eye filter (NPWE) was calculated for these beam qualities as a function of air kerma at the detector. AEC settings were calculated that gave constant d' as a function of beam quality for a homogeneous background; a target d' was used that ensured the system passed the achievable image quality criterion for the 0.1 mm diameter disc in the European Guidelines. Threshold gold thickness was measured using the CDMAM test object as a function of beam quality for the AEC mode, which held pixel value (PV) constant, and for the constant d' mode. Threshold gold thickness for the 0.1 mm disc increased by a factor of 2.18 for the constant PV mode, while constant d' mode held threshold gold thickness constant to within 7% and signal-difference-to-noise-ratio (SdNR) constant to within 5%. The constant d' settings derived for homogeneous images were then applied to a phantom with a structured background. Threshold gold thickness for the 0.13 mm disc increased by a factor of 1.90 for the constant PV mode, while constant d' mode held threshold gold thickness constant within 38% for 0.13 mm disk.Medical Imaging 2011: Physics of Medical Imaging; 02/2011
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ABSTRACT: To compare the diagnostic performance of new and established full-field digital mammography (FFDM) systems. During a 15-month period, 1038 asymptomatic women who visited for mammography were prospectively included from two institutions. For women with routine two-view mammograms from established FFDM systems, bilateral mediolateral oblique (MLO) mammograms were repeated using the new FFDM system. One of the four reviewers evaluated two-sets of bilateral MLO mammograms at 4-week intervals by using a five-point score for the probability of malignancy according to a Breast Imaging Reporting and Data System. The lesion type and breast density were determined by the consensus of two readers at each institution. The dichotomized mammographic results correlated with a final pathologic outcome and follow-up data. Receiver operating characteristic (ROC) curves, sensitivity, and specificity were compared in general and according to the lesion type and breast density. Of the 1038 cases, 193 (18.6%) had cancer. The areas under the ROC curve (AUC), sensitivity, and specificity of the established system were 0.815, 65.3%, and 90.2%, respectively. Those of the new system were 0.839, 68.4%, and 91.7%, respectively. There were no significant differences in the AUCs, sensitivities or the specificities in general between new and established systems (Ps = 0.194, 0.590, 0.322, respectively). We found no significant difference in these parameters according to lesion type or breast density. The new FFDM system has a comparable diagnostic performance with established systems.Korean journal of radiology: official journal of the Korean Radiological Society 01/2013; 14(2):164-170. · 1.32 Impact Factor