Masato Tatsumi

Assessment of image quality in dental radiography, part 2 Optimum exposure conditions for detection of small mass changes in 6 intraoral radiography systems

OBJECTIVE The purpose of this study was to compare 2 film systems and several digital intraoral systems with regard to visual image quality through use of a test phantom developed for this purpose. STUDY DESIGN The detectors used for digital imaging were as follows: Computed Dental Radiography (CDR), Digora, Dixel, and Sens-A-Ray without scintillator layer. Two types of digital images were prepared for the observer performance test: one with original gray scales and another with contrast enhancement. Images with and without enhancement from the 4 systems were displayed to 7 observers. The change in the average number of perceptible holes was plotted against exposure, and modified perceptibility curves were created and compared with curves for the film systems. The exposure level at which the maximum number of holes was perceived was defined to be optimum. The optimum exposure levels were determined for each digital system and compared with that of the film systems. At the optimum exposure, the average maximum numbers of perceptible holes in each digital system with and without contrast enhancement were compared with the maximum numbers for the film systems. The minimum exposure levels were determined to be those at which the number of perceptible holes exceeded the number for film, and the possibility of exposure reduction was evaluated. RESULTS All digital systems except the Digora system showed lower optimum exposures than E-speed film. In all digital images without enhancement, however, the maximum number of perceptible holes was significantly lower than that for the film systems at that exposure. With contrast enhancement, all digital systems except the Sens-A-Ray system showed visibility superior to that of the film systems. With the CDR, Digora, and Dixel systems, exposures could be further reduced by a considerable amount, with greater retention of information than was associated with film. CONCLUSIONS Our results strongly suggest that digital systems, if properly used, can exceed film systems in the detection of small mass changes.

Assessment of image quality in dental radiography, part 1 ☆ ☆☆ ★ ★★: Phantom validity

OBJECTIVE The purpose of this study was to describe and validate an image-quality phantom to be used in dental radiography for comparison of film and digitally acquired images. STUDY DESIGN An aluminum block of 12 steps, with 7 holes in each step, was covered by acrylic blocks. This phantom was radiographed with Kodak Ultra-speed and Ektaspeed Plus films at 70, 65, and 60 kVp with the whole exposure range available. All together, 50 dental films were randomly sequenced and presented to 7 observers. The average number of perceptible holes from all steps was plotted against exposure for each tube voltage and film type, generating a modified perceptibility curve. The tentative optimum exposure level was determined from perceptibility curves in each experimental condition and compared with that determined by means of the standard aluminum stepwedge and the preset time of the x-ray machine. The density range of this phantom at the optimum exposure was compared with that of clinical dental radiographs. Validity of the phantom was evaluated according to the optimum exposure level from the modified perceptibility curves and the overall density range. Finally, the average maximum numbers of perceptible holes at the tentative optimum exposure level were compared for each tube voltage and film type. The statistical test used was a 2-way factorial analysis of variance. RESULTS The exposure at the perceptibility curve peak approximated that obtained by means of the standard aluminum step-wedge and the time preset by the manufacturer. The overall density range at the perceptibility curve peak covered the clinical density range for each tube voltage and film type. There were no statistically significant differences between film types or among tube voltages. CONCLUSIONS The x-ray attenuation range for this phantom seemed to approximate clinical conditions. In addition, differences in image quality could be quantitatively evaluated by means of the number of the holes seen in the phantom.

Analysis of different approaches to constructing perceptibility curves

ObjectivesTo construct and analyze perceptibility curves (PCs) according to two different approaches.Material and methodsA test object was used to determine the exposures and exposure differences between the total thickness of the test object and details consisting of holes of increasing depth. Two digital systems were employed to predict PCs according to the two different approaches. One approach defined exposures and exposure differences from dose-response functions, including secondary and scattered radiation. The other defined exposure and exposure differences as calculated transmitted radiation flux from the primary beam behind the test object, excluding secondary and scattered radiation. Integrals of the PCs and of the minuimum perceptible gray-level differences as functions of background gray levels were calculated. The validity of the different types of PCs was analyzed. Another test object was used to predict observer performance for the two systems.ResultsThe integrals of PCs obtained according to the above first-mentioned approach and integrals of gray-level differences as functions of background gray level were equal. The same integrals using the second approach were different. The second approach, however, successfully predicted observer performance for the two systems.ConclusionsOnly the first-mentioned approach gives PCs that are true representations of psychophysical properties. The second approach may, however, be employed to predict observer performance when different radiographic systems are employed.