Phototimer setup for CR imaging

A study was performed to investigate the feasibility of using the standard deviation (sigma) of the pixel values in a computed radiography (CR) image and a measure of the median incident exposure on the imaging plate (IP) as parameters for setting up phototimers in a CR system. Slabs of Lucite(TM) 4-, 6-, and 8-in.-thick were imaged with a CR system at 70, 90, and 125 kVp at various mA s values both with grid and without grid. Incident TP exposures were measured with an ionization chamber. Images were analyzed on a workstation. The a's in the "flat field" images were found to be approximately related to the mean incident exposure E by the relationship: sigma proportional toE(-1/2), indicating the quantum-noise-limited operation of the system. Derived relationships between the reading sensitivity of the (IP) reader (S number) and sigma can be used to obtain images with a specific noise level. At our institution, where a 400 speed screen-film system is used for general radiography and 200 speed for chest radiography, radiologists generally find CR image quality acceptable when sigma less than or equal to 11 (S less than or equal to 400) for general radiography (50-90 kVp), and sigma less than or equal to8 (S less than or equal to 200) for chest radiography (125 kVp). However, factors other than the amount of x-ray quanta that form the useful image, such as the image processing mode and the amount of collimation, may affect both the sensitivity value and the image quality. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)01811-3].