ASSESSING THE IMAGE QUALITY OF A CCD-BASED DIGITAL INTRAORAL RADIOGRAPHY SYSTEM: APPLICATION OF PERCEPTIBILITY CURVE TEST

The perceptibility curve test is a method for evaluating the psychophysical properties of radiographic systems. The concept of the perceptibility curve is based on the minimum perceptible exposure differences that are recorded by a particular imaging system. The perceptibility curve test was applied to a Charge-Coupled Device (CCD)-based digital intraoral radiography system in this study. A test object was made of a square aluminum block, 28mm in both height and length and 10mm in thickness. The test object had ten contrast details in the form of round holes with diameters of 1.5mm. The depths of the holes ranged from 0.05mm to 0.5mm in steps of 0.05±0.01mm. An X-ray unit was used that operated in the range of 60 to 90kVp. Test radiographs were shown to observers in random order. Six observers were asked to rate them regarding the number of spots representing holes that they could perceive. The smallest perceptible difference in gray level on the computer monitor, a 15-inch cathode-ray tube (CRT), was then found. The minimum difference in gray level between a perceptible detail and the background was determined. Dose response functions were employed to determine exposures corresponding to these gray levels. For each case, ΔlogE as a function of the number of object details was calculated. Finally the reciprocal of all values of (ΔlogE) min were calculated. The number of details which the observers perceived decreased linearly with decreasing ΔlogE values. The maximum number of details that observers perceived was eight or nine. The perceptibility curves showed that their peaks shifted slightly with a change of the kVp but that their shapes were similar at different kVp settings.