Kinya Kuroyanagi

Intraoral radiographic storage phosphor image mean pixel values and signal-to-noise ratio: Effects of calibration

OBJECTIVE The DIGORA intraoral radiographic storage phosphor system needs to be calibrated before images are made. Calibration involves inputting of the maximum exposure to be used. This investigation studied the effects of different maximum exposure calibration settings on the mean pixel value for selected regions of interest and the signal-to-noise ratio for images of a test phantom. STUDY DESIGN A dental phantom containing a step wedge made of different thicknesses of homogeneously radiopaque bone-equivalent material was imaged at 70 kVp with exposures ranging from 12.8 to 105.2 microC.kg-1. Images were displayed through use of imaging software, and a region of interest was set for each bone-step. The mean pixel values and their standard deviations were measured. RESULTS Except for very low exposures (< 10% of the calibrated maximum), there was a linear relationship between exposure and the pixel values within the regions of interest irrespective of the calibrated maximum exposure. Low exposures resulted in underexposed low-contrast images. Low calibrated maximum exposures (< 54.5 microC.kg-1) resulted in low-density images with poor signal-to-noise ratios. CONCLUSIONS Because of the very wide image latitude of the DIGORA system, loss of image quality was not observed as a result of adjustments in the calibration setting over the range of exposure commonly used in dental practice. The highest accepted exposure was limited by prior calibration; hence, if diligence is applied, patient exposure can be minimized without detriment to image quality.

Effects of projection geometry and number of projections on accuracy of depth discrimination with tuned-aperture computed tomography in dentistry

OBJECTIVE The purpose of this study was to determine the degree to which the number and angular disparity of component projections influence depth discrimination with tuned-aperture computed tomography. STUDY DESIGN Groups of three tiny steel spheres served as fiducial references on and in four partially edentulous mandibles. Two spheres were attached to the facial and lingual surfaces of each mandible, and the third was fixed in the apical region of an open tooth socket. Errors in estimates of the depth of the apically positioned sphere relative to the other two spheres were determined from three-dimensional tuned-aperture computed tomography reconstructions. These data were compared with actual measurements produced independently with an optical micrometer. Multiple projections required by the tuned-aperture computed tomography reconstruction algorithm were produced from radially symmetric exposures bearing angular disparities of 5, 15, 30, and 45 degrees. The number of symmetrically dispersed projections per tuned-aperture computed tomography reconstruction likewise was varied systematically (2, 4, 8, 12, and 16 projections). These variables were manipulated through the use of a balanced factorial design. Depth estimates were performed by trained observers; the estimates were based on the determination of tuned-aperture computed tomography slices perceived as imaging the respective apical spheres in sharpest focus. Specimen and observer effects were also considered as independent variables. Resulting data were normalized by logarithmic transformation and analyzed statistically by analysis of variance. RESULTS Significant differences (p < 0.005) were demonstrated for angular disparity and specimen effects, but the number of projections and the effect of the observer were not found to be statistically significant. CONCLUSIONS In dentistry, angular disparities of 15 degrees or greater should be used when tuned-aperture computed tomography is being applied to diagnostic tasks requiring maximal depth discrimination accuracy.

Sens-A-Ray characteristics with variations in beam quality***

This study examined the relationship between x-ray beam quality and image characteristics with the use of the Sens-A-Ray (Regam Medical Systems, Sundsvall, Sweden). To evaluate the x-ray response, properties of the image sensor, dark current and statistical noise, sensitivity, and contrast resolution were measured. Tube voltage, tube current, and exposure time were varied. Exposures were determined with the use of an ionization chamber. Pixel values beneath a lead strip were used to calculate dark current effects. Contrast was measured with pixel values from the images of a step-wedge. Dark current noise increased in proportion to exposure time. Although the sensitivity at each tube voltage was linear and proportional to the exposure, the gradient was inversely proportional to increased tube voltage. After subtraction of the dark current and statistical noise, the gradient of the sensitivity at low tube voltage remained steeper than at high tube voltages. Hence, sensitivity decreased with increased tube voltage. Sensitivity gradients at each different tube current were similar. Irrespective of exposure, contrast resolution between adjoining step-wedge steps > or = 9 mm were too small to distinguish at 50 kVp. The gray levels for each step below 12 mm in thickness were distinguishable with 90 kVp. Increased dark current noise consequent to long exposure times narrowed the available gray scale range. Lower tube voltage resulted in higher sensitivity with larger gradients than higher tube voltages.

Radiation exposure with the RVG-S and conventional intraoral X-ray film

The exposure required to obtain optimal image quality was determined using conventional intraoral x-ray film (Eastman Kodak, Rochester, NY) and the RVG-S Radio VisioGraphy-S: Trophy Radiology, Vincennes, France) CCD-based intraoral radiographic imaging system. The RVG-S permitted dose reductions of 50 to 65% for individual exposures in comparison with Ektaspeed film, and 73 to 76% when compared to Ultraspeed film. The dose dynamic for the RVG-S was 8.6 times narrower than that for conventional film. Perception of low contrast details was not significantly different between either type of x-ray film and the RVG-S.The exposure required to obtain optimal image quality was determined using conventional intraoral x-ray film (Eastman Kodak, Rochester, NY) and the RVG-S Radio VisioGraphy-S: Trophy Radiology, Vincennes, France) CCD-based intraoral radiographic imaging system. The RVG-S permitted dose reductions of 50 to 65% for individual exposures in comparison with Ektaspeed film, and 73 to 76% when compared to Ultraspeed film. The dose dynamic for the RVG-S was 8.6 times narrower than that for conventional film. Perception of low contrast details was not significantly different between either type of x-ray film and the RVG-S.

Effect of number of projections on accuracy of depth discrimination using tuned-aperture computed tomography for 3-dimensional dentoalveolar imaging of low-contrast details

OBJECTIVE The purpose of this study was to test the hypothesis that the number of projections influences the accuracy of a simple depth discrimination task when tuned-aperture computed tomography is used. STUDY DESIGN In each of 4 partially edentulous mandibles, 2 radiopaque steel spheres were attached to the facial and lingual surfaces and 1 ceramic sphere was place in the apical region of an open tooth socket. Errors in estimates of the depth of the apically positioned ceramic sphere relative to the 2 steel spheres were determined from 3-dimensional tuned-aperture computed tomography reconstructions. These data were compared with actual measurements produced independently by means of an optical micrometer. Multiple projections were produced from radially symmetric exposures bearing an angular disparity of 15 degrees. The number of symmetrically dispersed projections per tuned-aperture computed tomography reconstruction was varied systematically (2, 4, 8, 12, and 16 projections). The consequences of this variable, as well as specimen and observer effects, were evaluated in a balanced factorial experimental design. Depth estimates were performed by 10 trained observers. The depth reported was that corresponding to the tuned-aperture computed tomography slice perceived to yield the image of the ceramic sphere in sharpest focus. Resulting data were normalized by logarithmic transformation and analyzed statistically by analysis of variance. RESULTS No statistically meaningful effects were found for the number of projections (P = .607) or for different observers (P = .093), but a significant specimen effect was demonstrated (P = .006). CONCLUSIONS Factors other than high contrast limit the perception of image sharpness under these conditions. Depth may be estimated accurately from relatively small numbers of projections.

Beam quality and image contrast with VIXA-2

PurposeCurrent CCD-based intraoral radiographic systems permit the use of any dental X-ray generator. As a consequence, beam quality can be altered. This study was carried out to investigate studied the effects of varying beam quality on the VIXA-2 image contrast (Gendex Dental Systems srl Milan, Italy).MethodsImages were made of a standard aluminum stepwedge and the pixel value of each step was measured. An optical bench was used to standardize geometric projection. Soft-tissue equivalent attenuation was effected using 1.75 cm plexiglass. Exposures were made at 2–48 impulses using 50–90 kVp settings at 10 kVp intervals. Exposures (μC/kg) were determined using a beryllium-windowed ionization chamber.ResultsThe pixel values for each step decreased both with increased exposure (μC/kg) and with increased kVp. The relationship between exposure and pixel value was not linear. The longest scale of contrast was obtained at 17.3, 15.2, 13.5, 11.7, and 11.3 μC/kg respectively at 50, 60, 70, 80, and 90 kVp. The gradient for pixel values along the steps was steeper at lower kVp settings than at higher kVp settings.ConclusionsThe VIXA-2 can be operated at a wide range of kVp settings. Gamma conversion inherent in the VIXA-2 creates wide variations in the pixel values for different stepwedge thicknesses.

Suitability of the general-purpose graphic printer as an image output device for digital dental x-ray images.

OBJECTIVES The purpose of this study was to evaluate the suitability of the general-purpose graphic printer as an image output device for digital dental x-ray images. METHODS The image quality as obtained by a thermal printer and by a dye sublimation printer was investigated. A grid pattern image was used to check parallelism and verticality of lines in each hard copy. A step-wedge image was printed with each printer, and the optical density, gradient, and root mean square granularity were compared. Depiction ability was also compared by using test images, including small signals. RESULTS All of the lines were parallel and vertical on hard copies of both printers. The dye sublimation printer showed better results on optical density, gradient, root mean square granularity, and depiction ability. CONCLUSION The dye sublimation printer produces images of the depiction ability comparable to the cathode ray tube display and seems suitable as an image output device for digital dental x-ray images.

Pixel Value Modification Using RVG-4 Automatic Exposure Compensation for Instant High-Contrast Images

The RVG-4 permits automatic exposure compensation (AEC). The purpose of this investigation was to determine the effects of AEC on image contrast. Images were made either with or without a dental QA jaw phantom using a fixed image projection geometry. Exposures were 6.3 through 27.3 μC/kg using an X-ray generator operated at 70 kVp. Region of interest pixel value distributions were measured at tissue thicknesses in this phantom, and the average pixel values and signal-to-noise ratios (SNR) were calculated. The use of AEC without an object in place resulted in a disproportionate relationship between pixel value and exposure with a marked reduction in SNR. The use of AEC on under- and over-exposed images of the phantom simultaneously enhanced image contrast and reduced SNR. Thus, AEC provides a convenient and quick method for achieving high-contrast images with sub-optimal exposures, however, this could lead to inappropriate patient dosages if the function is used for over-exposed images. AEC reduces the SNR and produces disproportionate pixel values relative to exposure.

Computed Dental Radiography System versus Conventional Dental X-ray Films for Detection of Simulated Proximal Caries

The diagnostic accuracy of intraoral images obtained with a first generation Computed Dental Radiography (CDR) digital dental imaging system is compared with the diagnostic accuracy of conventional dental x-ray film images using a receiver operating characteristics (ROC) analysis. Fifteen dentition phantoms were constructed. Each contained 4 teeth, consisting of premolar and molar teeth with 6 proximal surfaces. Mechanical defects were created randomly on the proximal surfaces and plugged with dentine powder to simulate caries. Fifteen images of each modality were rated by nine dentists in separate sessions according to their confidence in their ability to detect the simulated caries. The images examined were as follows: original CDR images, CDR images magnified 2×, CDR images magnified 4 ×, Ektaspeed Plus dental x-ray film images, and Ultraspeed dental x-ray film images, both unmagnified and magnified 2 ×. A series of ROC curves indicated the limited usefulness of magnification for the CDR system, and consistently similar success in detecting simulated proximal caries for original CDR images and original film images. The magnification of original film images did not always promote greater success than that obtained with the original film images.

Signal-to-noise ratio: Computed Dental Radiography versus Sens-A-Ray

The signal-to-noise ratio (SNR) describes the ability of a detector to differentiate a signal from random fluctuations in signal intensity or noise in an image. The dose-response curves and the SNRs were measured and compared between Computed Dental Radiography (CDR) and Sens-A-Ray. The dose-response curves at 60, 70 and 80 kVp of both systems indicated that the pixel values increased in proportion to the radiation exposure. The pixel value gradient was slightly steeper for the CDR than for the Sens-A-Ray. Sensitivity increased slightly at the higher kVp setting with the CDR only. All SNRs increased with increased exposure for both systems. The SNR for the CDR was superior to that for the Sens-A-Ray, even with low exposures. The SNR for CDR increased steeply as exposure and average pixel values increased. On the other hand, the SNR for the Sens-A-Ray showed only a relatively slight increase with exposure. In conclusion, due to the high SNR in the lower exposure range, the CDR has the capability of substantially reducing the level of exposure in comparison with the Sens-A-Ray.