Comparison of contrast-to-noise ratios for Bayesian processing and grids in digital chest radiography

Previously, we have demonstrated the ability of Bayesian image estimation (BIE) to reduce scatter and improve image contrast to noise ratio (CNR) in chest radiography without degradation of resolution. Here, we compare the effectiveness of BIE to a standard 12:1 grid. Images of a geometric phantom with two inches of added polystyrene were obtained both with and without a 12:1, 150 Ip/mm grid. Images were acquired with standard protocols: 120 kVp, 72 inch source to image distance, and PA positioning. Images were acquired on a calibrated photostimulable phosphor system. An image exposure was used corresponding to the same patient dose as when acquiring film/screen chest images using a phototimer. The image acquired without the grid was processed by BIE for 6 iterations. Contrast, noise, and CNR were calculated and compared for the image acquired with the grid and the BIE processed image in different regions. BIE processing improved image CNR by 200 to 350% over that provided by the anti-scatter grid for the different regions. BIE provides higher CNR than that of a 12:1 grid. Because of this increase in CNR, Bayesian processed images will show an increase in detectability of low contrast objects, such as subtle lung nodules.