The aim of the present study was to determine the allowable range of X-ray projection angle for the second and thereafter roentgenographies in relation to the direction of X-ray beam at the initial one in order to obtain the identical view when several roentgenograms taken with lapse of days should be compared in intraoral roentgenography.
Experimental procedures used were as follows:
The unit was geometrically arranged so as to obtain Object Focus Distance of 510mm and Object Film Distance of 67mm and prepared to permit changing of projection angle freely with precision of 0.5°.
The methods used were bisecting angle method (with film-dental axis of 10°, 20°, and 30°C) and paralleling method.
At each roentgenography, the film which produced an isometric image of the tooth length was regarded as the standard view, and roentgenography was performed with projection angles adjusted towards the direction which gives shorter image of the tooth length (1° to 5°).
The X-ray site was 45 for the maxilla and 56 for the mandibulla.
For collection of data, the dental film of the original data was magnified 3.1 times and printed on gravure film with transparent base.
Using a microphotometer, the septa interalveolaria between 4 and 5 and that between 5 and 6 were scanned from the processus alveolaris to the body and the data obtained were input a mamory of 2KW through A/D converter of 8 bits.
The aperture of the microphotometer was 0.25×2.5mm
2.
From the mircophotometer, the function of density level of trabeculation was subjected to Fourier transformation and then further to inverse Fourier transformation through a low cut filter with a cut off frequency of 0.2 1p/cm to obtain a function of traf eculation only.
Using the data processed as described above, cross correlation coefficient betvc een the standard view and those views obtained with various angles.
For gross assessment of the image of trabeculation, the standard view and the views taken at various angles were shown to seven dentists with clinical experiences of 4 to 5 years to let then choose the views differing from the standard view.
Following results were obtained.
1) At each roentgenography, the views with cross correlation coefficient of 75% or higher for the correlation with the standard view was regarded as identical view. It was found the maxilla should be projected with the angle within 2° irrespective of the methods used. On the other hand, the mandibula should be projected with the angle of less than 5° when paralleling method is used and less than 45° for the film-dental axis angle of 10° and less than 3° for those of 20° and 30° when the bisecting method is used.
2) When the film is set with changing of the film-dental axis angle with lapse of time on the assumption that requirements for the bisecting method are always met, the film should be set at angles within 5° from the initial film-dental axis angle.
3) When a film holder for standardized intraoral roentgenography was used, little or on reduction in coefficient for correlations between views taken with lapse of time.
4) When difference was found grossly between two films taken on different days with film setting by free hand, the X-ray projection angle differed by more than 3°.
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