Sensitometric response of the Sens-A-Ray, a charge-coupled imaging device, to changes in beam energy

Evaluation of image quality parameters of representative intraoral digital radiographic systems

OBJECTIVE

The aim of this study was to compare imaging properties of 20 intraoral digital systems objectively.

STUDY DESIGN

Using a direct current x-ray source and a radiographic phantom, a series of radiographs was made from the lowest exposure time until the sensor saturated. Images were captured and stored. Incident exposures were measured using a radiation meter. Gray scale, spatial resolution, and contrast/detail detectability were evaluated. Presence of 7 distinct steps spanning the gray levels from 0 to 255 was used to define the exposure latitude. An "optimal" exposure, the lowest exposure where maximum spatial resolution and contrast/detail detectability were achieved, was determined.

RESULTS

The systems varied greatly in latitude, "optimal" exposure, and image quality. This may not be readily apparent to the naked eye or when clinical images are compared.

CONCLUSIONS

Objective assessment of image quality with a quality assurance tool makes it possible to evaluate and compare the various intraoral digital systems.

The influence of tube potential on periodontal bone level measurements and subjective image quality using a digital photostimulable storage phosphor sensor

Objectives

The purpose of the present study was to determine the measurement accuracy and subjective image quality for periodontal disease diagnosis when using two X-ray tube voltages with a digital photostimulable storage phosphor sensor.

Material and Methods

A digital photostimulable storage phosphor (PSP) sensor (Vistascan) and a multipulse X-ray generator (Prostyle Intra) with two tube voltages were used in this study. The front, premolar and molar region of two adult human cadaver skulls jaws were imaged using the X-ray tube at 63 kV and 70 kV, both at 8 mA and decreasing exposure times (160 ms, 120 ms and 80 ms). A standardized exposure protocol containing waxed occlusal keys and an aiming device ensured proper and reproducible beam alignment. Three observers assessed the digital radiographs for 31 selected periodontal bone loss sites. Radiographic measurements were compared to physical measurements (Standard). Subjective ratings of lamina dura, crater defect and furcation involvement visibility, contrast perception and bone quality were also performed.

Results

Multiple regression equation of the variables kV and exposure time demonstrated no significant difference for the periodontal bone level measurements (P > 0.05). In 90.3% and 96.7% of the measurements for 70 kV and 63 kV respectively, deviation was within 1 mm. The subjective ratings produced similar findings in terms of image quality for both tube voltages and the three exposure times.

Conclusions

The results of the present study revealed that tube voltages of 63 kV and 70 kV provided similar accuracy and image quality for periodontal disease diagnosis.

Effect of bit depth and kVp of digital radiography for detection of subtle differences

OBJECTIVES

The purpose of this study is to investigate the effects of different bit depths and kilovoltage peak (kVp) values used in intraoral digital radiography on observer performance in detecting subtle radiographic density differences.

STUDY DESIGN

Using an intraoral CCD sensor set at 8- or 12-bit depth, kVp set at 60 or 70, and 14 different exposure times, digital radiographs were acquired of a specially designed aluminum step-wedge phantom with small holes of different depths in each step. Ten observers examined all images.

RESULTS

The observers counted more holes with the 12-bit images compared with the 8-bit images, particularly at 60 kVp. Significantly more holes were also counted with images taken at 70 kVp compared with those taken at 60 kVp.

CONCLUSION

Regarding the detection of subtle radiographic density differences, 12-bit images and 70 kVp were superior to 8-bit images and 60 kVp.

Influence of tube potential setting and dose on the visibility of lesions in intraoral radiography

OBJECTIVES

The objectives were to determine the diagnostic accuracy of different image receptors when first, the tube potential setting was increased from 60 kV to 90 kV and second, when the milliampere setting was reduced at unchanged tube potential for storage phosphor plates.

METHODS

Intraoral radiographs (films of speed class F, storage phosphor plates) were taken of 12 dried human skulls prepared with lesions in both teeth and peri-implant bone, in ascending size. Five observers assessed the presence (response=1) or absence (response=0) of lesions. The digital intraoral radiograph was exposed at 60 kV and 90 kV with half and a quarter of the film exposure. Some of the radiographs were assessed a second time. In total, 6000 assessments were made. Intrarater agreement was expressed by Cohen's kappa coefficient.

RESULTS

The digital combinations showed the lowest diagnostic accuracy in all combinations, but the results demonstrated an equivalence of all combinations of films and storage phosphor plates. The differences in diagnostic accuracy were low (94-96.5%). The kappa coefficient for intrarater agreement was high (0.85).

CONCLUSIONS

Regarding peri-implant and decayed lesions, intraoral films and storage phosphor plates demonstrated equal quality in this in vitro study regardless of exposure at 60 kV or 90 kV.

Performance evaluation and testing of digital intra-oral radiographic systems

Four digital intra-oral radiographic systems were tested and evaluated; three charge-coupled device (CCD) based systems from RVG, Visualix and Sidexis and a photostimulable phosphor (PSP) system from DenOptix. Image quality was assessed using three purpose-built phantoms to measure uniformity, low contrast detail detectability and signal-to-noise ratio (SNR). Limiting resolution was measured using a 20 lp mm(-1) bar pattern. Radiation output inaccuracies caused difficulties in achieving optimum doses for CCD systems. However, the accuracy was improved by using K-edge filters. SNR measurements proved to be a useful tool in assessing system performance. Each system has specific attributes: resolution was highest for the RVG system, the Visualix system measured the highest SNR and the lowest exposure settings were on the Sidexis system. Test methods and phantoms developed are suitable for acceptance testing and commissioning digital dental X-ray systems and for programming each system to produce an optimum level of image quality.

Effect of beam energy and filtration on the signal-to-noise ratio of the Dexis intraoral X-ray detector

OBJECTIVE

To evaluate the signal-to-noise ratio (SNR) of the Dexis charge-coupled device (CCD) intraoral X-ray detector, with special reference to the influence of beam energy and filtration.

MATERIAL AND METHODS

Digital radiographic images were made of a nine-step dental aluminium step wedge using a Dexis intraoral detector. The X-ray generator was a GE 1000 operating at 10 mA with 2.7 mm aluminium filtration. The peak tube voltage settings applied were 50 kVp, 60 kVp, 70 kVp, 80 kVp and 90 kVp. Various exposure times were used at each beam energy and the exposures (in micro C kg(-1)) were determined in each case using a 3 cl beryllium-windowed ionization chamber. SNR was defined using the surrogate measure of mean pixel value (surrogate for signal) divided by the standard deviation of the pixel value (surrogate for noise). SNRs were measured at separate regions of interest (ROIs) at the centre and at both sides of the detector for each aluminium step as well as without the step.

RESULTS

For beam energies of 50 kVp and 60 kVp, the estimated SNR improved both with increased exposure and with increased filtration (thickness of the aluminium step), and image saturation did not occur within the exposure time range permitted by the Dexis software. At 70 kVp and above, the SNR was optimized in the middle of the exposure range: 4 micro C kg(-1) with 290:1 at the ninth step (13.5 mm Al) for 70 kVp; 3.5 micro C kg(-1) with 240:1 at the ninth step (13.5 mm Al) for 80 kVp; 3.5 micro C kg(-1) with 160:1 at the ninth step (13.5 mm Al) for 90 kVp. Saturation did occur at these beam energies within the exposure time range permitted by the Dexis software.

CONCLUSION

For the Dexis intraoral radiographic imaging system, estimated SNR improved both with higher filtration and with lower kVp. The Dexis detector was capable of generating acceptable images of the step wedge at a wide range of kVp settings.

Optimum exposure ranges for computed dental radiography

OBJECTIVE

Computed dental radiography (CDR; Schick Technologies Inc, Lond Island City, NY, USA) can be used with any dental X-ray generator. The optimum exposure at various tube voltage settings was studied.

METHODS

Images were made of a dental QA jaw phantom and a standard aluminium stepwedge. Exposures were made between 50 and 90 kVp at 10 kVp intervals. Two contrast indices were calculated from the bone stepwedge pixel values: CI(1), the ratio between the highest and lowest pixel values, and CI(2), the difference between them.

RESULTS

Contrast indices were greatest at low kVp. The gradual decrease in CI(1) with increased exposure demonstrated that the optimum exposure range was always relatively wide. Maximum CI(2) values were found at exposures of 27, 17, 15, 11 and 9microC kg-1 at 50, 60, 70, 80 and 90 kVp, respectively, at the centre of the optimum exposure range. Pixel values for each aluminium step increased both with increased exposure and with increased kVp. The longest contrast scale was obtained at 11.0, 9.6, 8.7, 7.2 and 7.0 microC kg-1 at 50, 60, 70, 80 and 90 kVp. The steepest slopes were obtained either with thin aluminium steps or at low kVp.

CONCLUSIONS

CDR is a fast CCD-based system and is capable of operating at a wide range of kVp settings.