Despite improved DQEs of recently commercialized flat- panel detectors, two problems intrinsic to projection x-ray imaging with area detectors remain and degrade the low contrast performance: (1) low photon fluences at the detector in heavily attenuated regions, and (2) lack of efficient methods to reject scattered radiation. The expectation that these problems would be resolved or become less important through improved DQEs is somewhat illusive. One major benefit with digital detectors is the prospect of digital image processing and analysis. However, this benefit is hard to realize due to the low signal-to-noise ratios in heavily attenuated regions where image processing often finds most use. To resolve the above mentioned problems, we propose to construct and investigate a scanning equalization digital radiography (SEDR) system using an aSi:H and CsI:Tl based flat-panel detector. Slit scanning imaging with regionally adjustable beam width will be used to achieve effective scatter rejection and exposure equalization. A pre-exposure image will be used to compute the necessary beam width modulations for the scan, eliminating the need for using a radiation detector array. A novel image readout scheme will be used to provide electronic aft-collimation to achieve scatter rejection without attenuating primary x-rays and without using a heavy, bulky aft-collimator. A re-scaling technique will be implemented to convert equalized digital chest images into images with the appearance of conventional chest images but with enhanced contrast and equalized SNRs. To evaluate the proposed technique, we will measure and characterize the performance of the proposed SEDR system. We will study and demonstrate image quality improvements with phantom images. We will also conduct a comprehensive 350 patient ROC study to compare the scanning equalization DR chest images with regular DR images with and without digital image enhancement processing. The proposed system is expected to improve the low contrast performance of digital chest images by: (1) more effectively rejecting scattered radiation without attenuating primary x-rays, (2) increasing x-ray fluences in heavily attenuated regions to improve SNRs and reduce impact of scattered radiation there, (3) providing high quality image data with improved contrast-to-noise ratios and equalized SNRs for further digital image processing and analysis without suffering excessive noise limitations in heavily attenuated regions.
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