INFLUENCE OF SPURIOUS COMPONENTS OF MODULATION TRANSFER FUNCTION (MTF) ON THE IMAGE IN INTRAORAL ROENTGENOGRAPHY

Abstract

Spurious components exist in Modulation Transfer Function (MTF) in intraoral roentgenography. We examined the influence of these spurious components on diagnosis in the present study. Materials and Methods
Spurious components may be caused by the bisecting angle technique and the focal spot size. The focal spot size was determined by pinhole roentgnography. In the case of the bisecting angle technique, a slit roentgonogram was taken by 10μm pb slit. From these roentgenogram, the MTF was determined. The influence of spurious components on diagnosis was examined as follows: Roentgenograms were taken by varying the object film distance. These roentgenograms were reduced to dental size and duplicated. These duplicate films were then examined by 11 dentists as to whether or not the diagnosis of periodontal space and laminadura could be made. As the object, the lower incisor region of dry skull was used.
Next, roentgenograms were taken by artificially changing the spurious gain. Referring to these roentgenograms, the relationship of spurious gain to inages of the periodontal space and laminadura was evaluated. Roentgenograms for evaluating were taken as below: A routine roentgenogram was first taken. Then, the object was laterally shifted about 100μm or 200μm with fixed film, and a roentgenogram was taken again. In this way, it is a type of double exposure technique. Results
It was revealed from this experiment that the spurious component exists between 5lp/mm to 10lp/mm in the MTF and that the influence on diagnosis was grestest when the gain was 0.5 or more. In these roentgenograms, the periodontal space appeared as double black lines and the laminadura as double white lines. In particular, the laminadura was found to be easily misjudged as hyperplasis in diagnosis.