Perceptibility of defects in an aluminum test object: a comparison of the RVG-S and first generation VIXA systems with and without added niobium filtration

Comparison of psychophysical properties of two intraoral digital sensors on low-contrast perceptibility

OBJECTIVES

The psychophysical properties of a new complementary metal oxide semi-conductor-based detector, ProSensor(®) (Planmeca Oy, Helsinki, Finland), in terms of dose response function and perceptibility curve test were performed and compared with those of a charged couple device-based sensor, Dixi(®) (Planmeca Oy).

METHODS

Dose response functions at 66 kVp for a Dixi and a ProSensor were determined by means of multiple exposures to a homogeneous X-ray field covering the whole exposure range. The entry dose of each exposure was measured and registered. The mean grey level in each image was plotted as function of the corresponding exposures for both digital systems. Radiographs of a test object containing ten holes of increasing depth were obtained throughout the exposure range of the two digital sensors at 66 kVp. 12 observers were asked to register the number of perceptible holes in each radiograph. Based on the mean value of the observers' evaluation, the perceptibility curves were constructed, and the integral value under the perceptibility curves were compared between two intraoral sensors.

RESULTS

The results based on dose response function showed that the ProSensor was more sensitive than the Dixi sensor. Paired t-test showed that the minimal perceptible low-contrast details were significantly higher for the ProSensor than for the Dixi sensor (p < 0.001). The integrals below the two perceptibility curves were 33.4 and 69.2 for the Dixi and ProSensor, respectively.

CONCLUSION

Applying the new ProSensor may be beneficial to patients owing to its reduced radiation dose and increased perception for low-contrast details in dentistry.

Perceptibility curve test for conventional and colour-coded radiographs

OBJECTIVE

A colour scale has been developed as a complement to the grey scale in radiographs. The aim of the present study was to compare colour-coded with conventional black-and-white radiographs in terms of the perceptibility curve test.

MATERIALS AND METHODS

Twelve digital radiographs were exposed on a test object from low to high exposures and recorded using the DIXI system (Planmeca Oy, Helsinki, Finland). An aluminium test object was used containing ten object details in the form of holes with depths ranging from 0.03 mm to 0.30 mm in steps of 0.03+/-0.01 mm. The new colour scale was used to transform the conventional black-and-white radiographs into colour radiographs by specially designed software. Ten observers were asked to analyse four sets of radiographs, i.e. colour-coded and black-and-white radiographs, and reversed colour-coded and reversed black-and-white radiographs. The object detail having the lowest perceptible contrast in each radiograph was registered for each observer. Perceptibility curves were plotted based on the mean value of observer data.

RESULTS

Results from the perceptibility curve test showed that the information in colour-coded radiographs was at least as good as that in black-and-white radiographs. In fact, in the lower exposure range, colour-coded radiographs exhibited better perception than conventional black-and-white radiographs.

CONCLUSION

Radiographs that are colour-coded with the applied colour scale may be used as an alternative to conventional black-and-white radiographs.

Comparison of the psychophysical properties of various intraoral film and digital systems by means of the perceptibility curve test

OBJECTIVES

To compare psychophysical properties of two intraoral films and three digital systems using the perceptibility curve (PC) test.

MATERIALS AND METHODS

A 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. The PCs for the two intraoral films, UltraSpeed and EktaSpeed Plus, were constructed employing exposure and exposure differences from dose response functions. Integrals of the PCs were calculated to obtain the psychophysical properties of the two films. Psychophysical properties of the two films were compared with those of the three digital systems published previously (CDR, Dixel and Digora).

RESULTS

The PC for the EktaSpeed Plus showed a slightly higher peak than that for the UltraSpeed. Available exposure ranges were comparable. The PC for the EktaSpeed Plus was shifted to the left of the exposure axis indicating its higher sensitivity as compared with UltraSpeed. All three digital systems had narrower but higher peaks compared with the films. The integrals for the digital systems were considerably larger than those for the two film types.

CONCLUSIONS

All the three digital systems have superior psychophysical properties compared with the two tested films.

Conventional and predicted perceptibility curves for contrast-enhanced direct digital intraoral radiographs

OBJECTIVES

To construct Perceptibility Curves (PCs) for contrast-enhanced digital intraoral radiographs.

METHODS

Radiographs of a test object having holes of increasing depths were exposed using three digital systems, the CDR (Schick Technologies, Long Island, NY, USA), the Dixel (J Morita MFG, Kyoto, Japan) and the Sens-A-Ray (Regam Medical Systems, Sundsvall, Sweden). The radiographs were contrast-enhanced and PCs constructed in the conventional way using 10 observers. Predicted PCs were calculated and compared with observer data.

RESULTS

The PCs showed that contrast enhancement is effective for the perception of small contrast details, especially in the low exposure range. Predicted PCs demonstrated excellent agreement with observer data.

CONCLUSIONS

Contrast enhancement should be advantageous in digital radiography. The effects of contrast enhancement on PCs may be predicted without previous knowledge of observer performance.

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, 28 mm in both height and length and 10 mm in thickness. The test object had ten contrast details in the form of round holes with diameters of 1.5 mm. The depths of the holes ranged from 0.05 mm to 0.5 mm in steps of 0.05 +/- 0.01 mm. An X-ray unit was used that operated in the range of 60 to 90 kVp. 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, delta logE as a function of the number of object details was calculated. Finally the reciprocal of all values of (delta logE)min were calculated. The number of details which the observers perceived decreased linearly with decreasing delta logE 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.

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.