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 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 IP exposures were measured with an ionization chamber. Images were analyzed on a workstation. The sigma's in the "flat field" images were found to be approximately related to the mean incident exposure E by the relationship: sigma is proportional to E(-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 < or = 11 (S< or =400) for general radiography (50-90 kVp), and sigma < or =8 (S< or =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.
Christodoulou
E G
EG
Department of Radiology, University of Michigan Medical Center, Ann Arbor 48109-0030, USA. manos@umich.edu
Goodsit
M M
MM
Chan
H P
HP
Hepburn
T W
TW
D006801:Humans; D015233:Models, Statistical; D019047:Phantoms, Imaging; D011859:Radiography / Q000295:instrumentation* / Q000379:methods*; D013902:Radiography, Thoracic / Q000295:instrumentation* / Q000379:methods*; D012680:Sensitivity and Specificity; D013997:Time Factors; D014965:X-Rays