Digital Processing Of Minimal Radiographs For Exposure Reductions

AbstractUntil now the application of computerized digital imaging process has been concen-trated in the area of imaging production. A digital imaging process technique has beendeveloped in our experimental work. Based on our preliminary experimental resultsradiographs of a minimal exposure (about 1/4 of the standard exposure) is digitized andcan be regenerated with the digital imaging process reported here to yield a digitalimage with good resolution similar to the radiograph produced by 100% of the standardexposure. This digital imaging process can reduce exposure by a factor of 4 for a radio -logic image and hopefully it could be developed for further reduction of radiation ex-posure per a given radiological study.IntroductionIn the past decade great effort has been made toward the development for digitalencoding fluoroscopic images in the digital angiography and for digitization of the out-put images of x -ray detectors in the CT.1 -15 Until now the application of computerizeddigital imaging process has been concentrated in the area of image production, and verylittle has ever been mentioned about the potential reduction_of radiation exposure usingthe computerized digital imaging process in the literature. 2,5,6,7 We have worked on thedirect digitizing radiographs reported previously 15,16 and one of us has had involvementwith the development of a digital radiography system which has been reasonably successfulwith good resolution and definition. Based on our preliminary experimental results,radiographs of a minimal optimal exposure (about 1/4 of the standard exposure) are digit-ized and can be regenerated with the proposed digital image with good resolution similarto the radiographs produced by the standard exposure. Experimentally, the proposedcomputer manipulation procedure can reduce radiation exposure for a diagnostic imageto 25% of the standard level or decrease the exposure time by a factor of ten or greater.The reported digital imaging process using point by point compensation is possible onlyon digitized radiologic images, and this explains the reason why the idea has never beenexplored in the image intensification of radiography. 1 -5Methods and resultsAn A -P projection of a hand was radiographed using 100 %, 50% and 25% of the standardexposure respectively and the three differently exposed radiographs were obtained usingthe conventional radiographic development method. Each of the three radiographs wasseparately digitized by a drum scanner (Optionics International's C4100) to give a cor-responding digitized radiograph of 100 %, 50% and 25% of the standard exposure. Theradiographic digitizing technique which was ulilized in this study was developed anddescribed in the previous reports.1$,16 Figure I is the digitized radiograph of 100%exposure. Figure 2 is the digitized radiograph of 50% exposure and Figure 3 is thedigitized radiograph of 25% exposure. The spatial resolution is 0.35mm between twoneighboring pixels.The gray level at each pixel may take on one of the 256 possible value. The grayscale G was calibrated against the optical density scale D by the use of a KodakCalibrated Neutral Density Attenuator; and the Figure 4 illustrates the result of therelationship between the gray value G and optical density scale D.By identifying the corresponding pixels on the two differently exposed radiographsof 100% and 25% respectively and observing their respective gray values G100 for radio-graph of 100% and G25 for radiograph of 25% a gray scale mapping was experimentallyestablished. This was done in two corresponding windows of 32x100 pixels, one in eachradiograph is illustrated in Figure I and Figure 3. The result produced by the mappingis shown by the curve A in Figure 5 where G25 denotes the gray value at a pixel in thedigitized radiograph with 25% exposure and G100 denotes the gray value at the correspond-ing pixel in the digitized radiograph with 100% exposure.