Discussion - References.
A number of fundamental issues need to be resolved before digital radiology can be fully incorporated into clinical practice (Van Der Stelt 1995). The radiological diagnostic techniques used in the present study were chosen to reflect current interest in comparing the performances of digital and conventional radiology and assessing their applicability to root-canal measurements (Shearer et al.1990, Shearer et al.19 91, Hedrick et al.1994, Ellingsen et al.1995, Ong & Pitt Ford1995).
When obtaining radiographic images of any kind, the purpose of the radiographic sensing component (either digital or conventional) is to capture X-ray photon density emerging from the target tissues. This is, in turn, determined by the tissues involved and the radiation source used. I n this sense, the image will be more precise if a point-source is employed. Another important consideration in this context is the size of the focal point of the X-ray generator, because it exerts a decisive influence upon the capacity of the system to discriminate between two points in close proximity to each other (Sprawls 1977). In practice, however, the X-ray generators emit from an extensive zone rather than from a point-source; consequently, the X-ray images obtained suffer a degree of distortion and loss of detail. As the photon dispersion pattern cannot be improved upon by the sensor, image quality and precision remains limited in both conventional and digital radiographic systems.
In the case of the PSP (Digora) technique, endodontic measurements were found to be possible at exposure levels considerably lower than those required by the conventional film techniques (Borg & Grondahl 1996). In this context, although the exposure time may be shortened, the images do not lose quality as a result (Hayakawa et al.1998). On comparing the Digora system with the conventional radiography for determining the root-canal dimensions, no significant differences were observed, although the file-tip visualization was, nevertheless more exact with PSP system, particularly when using small size files (Cederberg et al.19 98).
However, it is possible to improve precision in the capture of the incident X-ray photons by reducing the size of the corresponding sensor units. In the case of conventional X-ray systems, this size basically corresponds to film grain size. The current tendency is to use increasingly faster fil ms in order to limit patient exposure; however, faster fil ms are associated with the larger grain sizes, which result in poorer image detail; so, as our study was carried out in vitro, we decided to use D-speed film (Ellingsen et al.19 95).
The sensor unit used in digital radiological systems is the pixel. Consequently, the image quality increases with the number of pixels per unit surface. This increase in detail can be achieved by either reducing pixel size or increasing the surface of the sensor. In this sense, RVG has a smaller pixel size than the Digora system. In the present study, a single X-ray source was used with controlled exposure time, focal distance and position. The characteristics of the conventional film used, the RVG sensor and the Digora imaging plate were also kept constant throughout the study.
It is important to mention the exposure times used to obtain the digital images (Ellingsen et al.19 95), because, if the same times are applied to all extracted teeth equally, some regions in the digital images may disappear upon adjusting brightness and contrast. A parallel beam technique with two angles (08 and 208 mesial) was used in the horizontal plane in order to extrapolate the situation to in vivo practice, differentiating certain structures (i.e. distinguishing roots) but without creating excessive superpositioning or distortion.
Radiographic visualization is influenced by a number of physical and technical factors. In order to avoid one of the main sources of error in the production of radiographic images, an automatic developer was used to secure similar conditions for all the radiographic fil ms used. The conventional X-ray viewing was made in the most uniform and simple way possible. Magnification was also used in an attempt to eliminate variables capable of altering image visualization (Ellingsen et al. 1995,Ong&Pitt Ford1995).Although these authors used 2x and 4x magnifications, 6x magnification was used in the present study, because it improved visualization of the images.
The digital techniques make it possible to modify and analyze images in an attempt to improve the diagnostic capacity of these systems. Although such technical possibilities may seem obvious, a number of experimental studies have been made to more precisely defifine their true of feet (Kullendorf et al.1996). Magnification of the original image does significantly increase diagnostic accuracy (Svanaes et al.1996). On the other hand, if the observer has a certain experience with these techniques, imaging exerts no influence upon diagnostic accuracy (Powell-Cullingford & Pitt Ford1993).
The file sizes used in our study were similar to those employed previously (Cox et al.1991, Powell-Cullingford & Pitt Ford 1993).O n comparing the reliability of RVG with conventional radiography in evaluating the locations of size 08 and10 file tips with respect to the radiographic apex, conventional X-ray (D-speed) were clearly superior (Ellingsen et al.1995). However, when using the size15 files, similar results were obtained with both techniques (Sanderink et al.19 94).
The magnification of digital images provides a better image, although, this does not mean that more details are visible than in the original image. In the present study, we used no digital image modifications other than changes in the brightness and contrast, and magnification, because no advantage would be afforded by doing so (Leddy et al.1994). On the other hand, on the screen images obtained with both digital techniques were used, as they offered better quality than the paper-printed images (Griffiths et al.19 92).
Observation of the canal-measurement images with the two digital systems revealed difficulties in precisely locating the tip of the smallest size file, although the performance improved with the increasing file size. It was also seen that this problem could be solved by magnifying the images, because the zoom function improves visualization. On the other hand, when the images were examined by several observers, with and without magnification, greater discrepancy existed amongst them than when the comparisons were made by the same observers with different types of images and techniques (Antrim 1983).
When comparing RVG and Regam digital radiographic images with conventional E-speed images in the determination of root-canal length, no significant differences were recorded between the images obtained via direct digital thermal printout and conventional X rays (Hedrick et al.1994). On contrasting conventional radiography with RVG, the latter has been seen to offer canal measurements of a quality similar to those afforded by conventional film (Shearer et al.19 90, Ellingsen et al.19 95).
Imaging with either the conventional or digital techniques was based on the reproducible and precise positioning of each tooth, thus allowing the comparison of images of the same tooth without varying the projection geometry, or doing so in a pre-established manner.
For all three file sizes used, the conventional film was seen to yield results close to 0. Similar fifindings were reported in another study involving size10 files (Griffiths et al.1992). However, as the file size was increased, the negative values of the Digora system were seen to decrease gradually in greater proportion than in the case of the RVG values. Likewise, all three techniques were seen to produce few measurements in excess of the mean value (Versteeg et al.1997). Positive measurements with conventional radiography only occurred when magnification was used, at 08 and 208.T his could be owing to the problems in identifying the foramen with 6_magnification. The positive measurements were ones 0.01mm in magnitude which is of no clinical relevance. Thus, it can be confirmed that conventional radiography with any file size and regardless of the projection angle or magnification involved, offered the most accurate results. The RVG withfifile sizes10and15alsoyieldedgood performance, and could replace conventional film for obtaining root-canal dimensions (Shearer et al.19 91). In turn, the reliability of Digora in obtaining working length measurements was similar to that of conventional film (Borg & Grondahl1996).
On the other hand, the three techniques showed considerable discrepancy in terms of the actual measurements obtained. However, this does not contradict the above because it referred to mean values. As an example, conventional film and RVG both gave values close to 0; however, there is little to determine that when one technique normally measures in excess, the other must also be expected to do so, or vice versa.
One study reported that in 82% of cases the file tip coincided with the radiographic apex, and that magnification did not influence the conventional radiographic technique (Olson et al.1991).
For fifile sizes of10 or smaller, digital radiography does not afford sufficient diagnostic accuracy for determining working length. In contrast, for file sizes 15 or larger, the technique approaches the performance of conventional film (Sanderink et al.19 94). A number of authors have reported no significant differences between RVG and conventional radiography (Leddy et al.19 94) in performing root-canal measurements with size 10 files, or between the Digora system and conventional film (Cederberg et al.19 98).
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