口内法撮影における被曝線量軽減

第4報フィルム観察における拡散光画像の応用

抄録

According to Oldendorf, the underexposed film produces a very faint image when viewed in conventional transillumination, that is, with the light passing through the film perpendicular to its surface. In dark-field illumination, the light passes obliquely through the film. With the underexposed film, each of the deposited emulsion grains becomes a point of scintillation in the oblique light against dark background. Thus, a useful “positive” image is produced. He mentioned that this will lead to reduction of x-ray exposure.
In view of this fact, we have carried out a fundamental experiment in order to reduce x-ray exposure to the patients using this technique in intraoral roentgenography. When this technique is employed, the use of a low fog film is required. With an ordinary periapical film the experiment is not feasible. Therefore, Eastman Kodak subtraction masking film was selected. The calculation of the standard exposure for this film was madefrom the characteristic curves of Fuji periapical film and subtraction masking film. When roent-genography is taken by Oldendorf's method, photographic contrast may appear in the under-exposed area and even the image which cannot be demonstrated on the ordinary illuminator may be observed with good contrast. However, in high density area white and black parts are reversed, showing the same image as that obtained on the ordinary illuminator.The density where this conversion occurs is approximately 0.5.
It was found that by use of this method x-ray exposure can be reduced to about 80 % of that in the convensional method.