**4. Discussion**

The present study has demonstrated that the volumetric tomography images have the potential to be the practical extraoral imaging modality for proximal caries detection.

In the most of studies (Velders et al., 1996; Svanæs et al., 2000; Wenzel, 2001; Khan et al., 2004; Young & Featherstone, 2005), the proximal carious lesion was evaluated by using the visual criterion. Although the visual criterion used was somewhat subjective, it represented the best clinical representation of a proximal carious lesion. In previous similar in vitro studies (Jesse, 1999; Kooistra et al., 2005), the gold standard for comparisons was histological section of the extracted teeth. Caries depth was evaluated in these sections based solely on the microscopic evaluation of a color change between involved and uninvolved dentin.

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rather than a source area. Clinical radiation generators emit X-rays from a source area and not from a singular point. This means that radiographic images are subject to loss of image detail and geometric unsharpness. The loss of image detail and sharpness is a function of the dimensions of the focal spot. The greater the focal spot size, the greater the loss of detail. Since X-ray photons cannot be focused into a sharp image as light can be focused through a camera lens, the image captured by conventional film or DDR sensors will never have a crisp, in focus appearance like a photograph focused through a lens. Radiographic images will always be subject to a certain degree of geometric unsharpness that will limit the resolution of the image that can be captured. The photon dispersion pattern cannot be improved by the sensor and will continue to limit image quality and sharpness for both conventional and DDR systems (Langland & Sippy, 1973; Hedrick et al., 1994).

The CBCT scanners utilize a two-dimensional, or panel, detector, which allows for a single rotation of the gantry to generate a scan of the entire head, as compared with conventional CT scanners whose multiple "slices" must be stacked to obtain a complete image. Cone beam technology utilizes X-rays much more efficiently, requires far less electrical energy, and allows for the use of smaller and less expensive X-ray components than fan-beam technology. In addition, the fan-beam technology used in conventional CT scanners does not lend itself to miniaturization because it requires significant space to spiral around the entire body (Sukovic, 2003; Marmulla et al., 2005).

Jaffray & Siewerdsen (2000) noted that the CBCT approach offers two important features that dramatically reduce its cost in comparison to a conventional scanner. First, the cone beam nature of the acquisition does not require an additional mechanism to move the patient during the acquisition. Second, the use of a cone beam, as opposed to a fan beam, significantly increases the X-ray utilization, lowering the X-ray tube heat capacity required for volumetric scanning. For the same source and detector geometry, the efficiency roughly scales with slice thickness. For example, the X-ray utilization increases by a factor of 30 in going from a 3 mm slice in a conventional scanner to a cone angle corresponding to a 100 mm slice with a cone beam system. This would reduce heat load capacity dramatically.

In summary, cone beam CT is a versatile emerging technology whose high and isotropic spatial resolution, undistorted images, compact size and relatively low cost, make it a perfect candidate for a dedicated dentomaxillofacial imaging modality. When combined with dedicated software packages, it can provide practitioners with a complete solution for demanding tasks (Mozzo et al., 1998; Sukovic, 2003; Schulze et al., 2005).

For approximal caries detection, the sensitivity of the volumetric dental tomography images was found to be slightly less than conventional radiographs and digital images in this study. Overall, the three methods were not statistically significantly different for the determination depth of the approximal carious lesions in vitro.
