Beam quality and image contrast with VIXA-2

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.

Signal-to-noise ratios of 6 intraoral digital sensors

OBJECTIVE

The objective of this study was to compare the signal-to-noise ratios (SNRs) of 6 intraoral direct digital sensors. The sensors were Visualix 1, Visualix 2, DenOptix, Computed Dental Radiography (CDR), CDR APS, and Digora.

STUDY DESIGN

The sensors were exposed by using a dental x-ray machine operating at 50 kV, 8 mA and various exposure times. The phantom used was a hole measuring 1 mm in diameter and 1.25 mm in depth, drilled in a 10-mm-thick aluminum block. Digital images were transferred as 8-bit tagged image file format files and analyzed by using a personal computer.

RESULTS AND CONCLUSIONS

SNR values increased up to a maximum value at a certain exposure level and then, depending on the system, either decreased or maintained a high level with increased exposure. The Digora system produced the highest SNR values over a large exposure range, whereas the Visualix 2 and DenOptix systems produced the lowest SNR values. The 2 CDR sensors produced rather high SNRs but over a very small exposure range.

Assessments of the physical performance of 2 generations of 2 direct digital intraoral sensors

OBJECTIVE

To evaluate the performance of 4 intraoral direct digital sensors regarding their fundamental physical characteristics. The sensors are made by Schick Technologies Inc (Long Island City, NY) and Gendex Dental Systems (Milan, Italy).

STUDY DESIGN

The sensors were exposed by using a Prostyle dental x-ray machine (Planmeca Oy, Helsinki, Finland) operating at 50 kV, 8 mA and various exposure times. Three test phantoms were used: a homogeneous 10-mm thick aluminum block, an aluminum block with a pattern of holes of varying sizes, and a resolving power target. Digital images were transferred as 8 bit TIFF files and analyzed by using a personal computer.

RESULTS AND CONCLUSIONS

Some improvements could be observed in the physical performance of the new generation of direct digital radiographic sensors when compared with the earlier generation. Smaller pixels and higher quantum efficiency have improved sensor performance.

Technical report. Processing to achieve high-contrast images with computed dental radiography

OBJECTIVES

To determine the effects of equalization on image contrast and signal-to-noise ratio (SNR) with Computed Dental Radiography (CDR).

METHODS

Pixel value distributions were measured for various tissue thicknesses of a dental quality assurance jaw phantom and the average pixel values and SNR calculated.

RESULTS

Equalization resulted in enhanced image contrast of underexposed images by effecting pixel value adjustment with little change in SNR. Overexposed images demonstrated minimal difference in contrast or SNR when equalization was applied.

CONCLUSION

The equalize function of CDR is a convenient method to provide high-contrast images from underexposed images without detriment to the SNR. As it does not compensate for overexposure, the function will not lead to inappropriate overdosage of the patient.

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.