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

Health Sciences Center, The University of Louisville, Kentucky, USA

*Corresponding to: Dr Allan G Farman, Division of Radiology and Imaging Sciences, School of Dentistry, The University of Louisville, Louisville, KY 40292, USA; Email: agfarm01{at}louisville.edu

Received 27 August 2003; revised 23 January 2004; accepted 3 February 2004

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 µC 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 µC kg^–1 with 290:1 at the ninth step (13.5 mm Al) for 70 kVp; 3.5 µC kg^–1 with 240:1 at the ninth step (13.5 mm Al) for 80 kVp; 3.5 µC 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.

Keywords: charge-coupled device; digital imaging; filtration; radiography, dental; X-ray beam energy