This paper describes and demonstrates an electronic collimation method, referred to as the alternate line erasure and readout (ALER) technique, for implementing slot-scan digital radiography technique with an amorphous silicon (a-Si) thin-film transistor (TFT) array based flat-panel detector. An amorphus selenium (a-Se) flat-panel detector was modified to implement the ALER technique for slot-scan imaging. A stepping-motor driven fore-collimator was mounted in front of an X-ray tube to generate a scanning X-ray fan beam. The scanning speed and magnification were adjusted to synchronize the fan beam motion with the image line readout rate. The image lines on the leading and trailing edges of the fan beam were tracked and alternately reset and read out, respectively. The former operation resulted in the erasure of the scatter signals accumulated in the leading edge image line prior to the arrival of the fan beam. The latter operation resulted in the acquisition of fan beam exposure data integrated in the trailing edge image line right after the fan beam passed. To demonstrate the scatter rejection capability of this technique, an anthropomorphic chest phantom was placed in PA position and scanned at a speed of 576 lines (8.0 cm)/s at 117 kVp and 32 mA. A tungsten bar is placed at the entrance side of the chest phantom to measure the scatter-to-primary ratio (SPR), scatter reduction factor (SRF), and contrast-to-noise ratio degradation factor (CNRDF) in the slot-scan images to evaluate the effectiveness of scatter rejection and the resultant improvement of image quality. SPR and CNRDF in the open-field images were also measured and used as the reference for comparison. A scatter reduction by 86.4 to 95.4% across lower lung and heart regions has been observed with slot-scan imaging. The CNRs have been found to be improved by a factor of 2 in the mediastinum areas over the open-field image as well.

The computed radiography (CR) technique, also known as the storage phosphor imaging technique, has evolved to be a major candidate for large-scale implementation of digital radiography during the past decade. In order to obtain a reasonable spatial resolution, the storage phosphor plate used is generally limited in thickness. This leads to X rays being only partially absorbed by the detector. Useful information may be contained in the X rays transmitted through the detector. Multiple-plate imaging techniques may be used to capture and utilize the X rays more efficiently. In this paper, an image fusion method, based on the Rayleigh principle and the Karhunen-Loeve (K-L) transform, is presented for optimizing the signal-to-noise ratio (SNR) of the fused image on a pixel-by-pixel basis. Because the multiple-plate images contain the same structural information, the signal components of the images are highly correlated with one another. Thus, the K-L transform is applied to decompose each of the multiple-plate images into an eigen image (the estimated signal) and a residual image (the estimated noise). An average representation entropy measure is maximized for selecting the number of eigen components to be included in the signal estimation. An experimental study, using an anthropomorphic chest phantom, is presented to illustrate pixelwise fusion of multiple-plate images. Experimental results show that the SNR of the fused image was improved by 12-48%, depending upon the anatomical regions of interest in the image.