歯科 X 線写真の画質についての基礎的研究

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It may appear that the absolute criteria for the evaluation of the image of radiograph do not exist, because the evaluation is greatly swayed by the final subjective judgement through the vision of the observer (the dentist). But as the formation of radiograph is based on physical reactions between the x-ray source, the objects, the x-ray film and so on, the physical factors on the radiograph image can be chosen as important objective criteria for the evaluation of the image quality. The major physical factors on the radiograph image are usually classified into three categories, namely, density, contrast and sharpness. The effects that the mutual relations between physical elements, such as x-ray source, objects and x-ray film, have to each of the major physical factors were examined experimentally, using dental x-ray source, several kinds of phantoms of oral tissues and dental films, The results obtained were summrized as follows : 1) The higher is the γ (density gradient) of dental x-ray film having the density (D=1.0∿1.2) optimum for the best diagnosis on the details of the object, the better becomes the image contrast. Nevertheless, as for the wider density range (D=0.3∿1.8) optimum for the universal diagnosis throughout the whole image, the higher the γ value the less the obtainable information. The optimum density range for dental radiograph image should be limited to about 0.45∿1.8, because the most radiopaque tissue like a tooth is contained in the object (the oral tissues). 2) The effective value of γ differs from one another even when the film density is identical. The thicker the layer and the higher the value μ (absorption coefficient) of the object, the lower the effective value of γ. 3) In general, the higher the value μ of the object the better the contrast of radiograph image, particularly where the object layer is thin. But when the value μ is quite high as a tooth, the contrast is extremely bad where the object layer is thick (more than ca. 8mm), and when the value μ is quite low as muscles the contrast is very bad where the object layer is thin (less than ca. 10mm). As for the object having a medium value of μ as bones the contrast is nearly proportional to the difference of the object thickness. 4) In general, the lower the value KVp (tube voltage) the better the contrast of radiograph image, particularly where the object layer is thin. But when the value μ is quite high as a tooth and that layer is thicker than ca. 8mm. the lower the value KVp the worse the contrast. 5) The contrast of radiograph image of decayed bones tissues in the soft tissue differs in accordance with the range and degree of decay. The larger the range and the harder the degree of decay, the better the contrast. 6) Density difference (contrast) is hardly obtained on the dental x-ray film image by the soft tissue of less than 10mm thickness. The thickness contrast (density difference obtained by every unit thickness) of the bone tissue is ca. (0.02∿0.05)/mm. and that of tooth tissue is ca. (0.04∿0.07)/mm. 7) The contrast of radiograph image is decreased by the effect of scattering x-ray, The value of decreasing rate was ca. 12% on the dental phantom image, but the value was augumented to ca. 15∿20% on the clinical dental radiograph image because of the additional scattering x-ray from the surrounding tissues. 8) The image unsharpness on dental radiograph film results mainly from two geometrical factors. One is the well known penumbra effect caused by the extension of tube focus, and the other is the geometrical absorption distance effect which has been hitherto payed no attention. For most of the dental image unsharpness the absorption distance effect (absorption unsharpness) is responsible. Only by the restricted cases where the object edge is located to the very proximity of the x-ray center axis, appears the penumbra effect. 9) The width of absorption unsharpness is proportional to the product of object thickness and deflection of object edge from x-ray center axis, and inversely proportional to FMD (focus-object distance). 10) The width of the genuine image of minute (comparing to the focus extention) object becomes narrower than that of the object itself by the width of image unsharpness, and is reduced to zero when the latter becomes to the same value as the former, that is, when the deflection angle reaches nearly a/b (a and b are the width and the thickness of the minute object, respectively). In other words, the image sharpness of minute object is inversely proportional to the deflection angle of object thickness axis from the direction of radiation.
九州歯科学会の論文
1977-03-31

著者

犬塚孝
九州歯科大学歯科放射線学教室

歯科 X 線写真の画質についての基礎的研究

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