| Images with reduced rib shadows can be obtained on chest radiography by using a higher voltage or applying an additional filter, owing to an increase in effective energy. Although images with poor discriminative ability result from radiography at a high effective energy level when screen/film systems are used, a result of a general decrease in the contrast of the subject, shadows of soft tissues are apparently emphasized when the CR system is used. Because tone is adjusted according to the width of the histogram by the exposure-data-recognizer(EDR)mechanism, a higher effective energy level is expected to be advantageous in chest radiography using the CR system. As long as the incident dose to the imaging plate(IP)is maintained at a nearly fixed level using a phototimer, EDR-enhanced noise, soft tissue signals, and the SNR in the lung field are reduced, with an increase in effective energy. All in all, the high effective energy technique has reduced the ability to diagnose low contrast masses. Increasing the dose is extremely efficient in the high SNR of the lung field and maintains the advantage of high-voltage radiography in the CR system using the EDR mechanism. The increase ratio requisite for exposure is about 1.4 in Cu 0.1 mm+Al 2 mm, about 1.6 in Cu 0.3 mm using 141 kV, with the proviso that the increase ratio is 1.0 when the sensitivity of the phototimer is normal. |
