A RECORDING SYSTEM FOR MAMMOGRAPHY

Abstract

Visibility and perceptibility of the imaging systems were assessed visually and theoretically in combination with X-ray tubes having a molybdenum or a tungsten target. Visibility, being here to mean the ability to detect minute objects, was quantitized with the diameter in μm of a just perceptible object. Concept of perceptibility proposed by Belder and others was developed to conform the abstract value to the visibility defined as above, and this proscess was used to discuss perceptible information amount(PIA)throughout a certin thickness range of the breast. Visibility was confirmed with predicted value by the ratio of the object contrast to minimum perceptible contrast on the asumption that both were roughly a linear function of MTF correspon ding to the object diameter. The results obtained by this study are summarized as follows : I. The visibility afforded by a tungsten tube with varied tube voltages corresponding to the system speed became better with increasing image contrast but with increasing MTF by a molybdenum tube. This implies that sharpness should be emphsized the most for a molybdenum tube as the contrast benefiting from reduction of the tube voltage is less than that by a tungsten tube by one-half at the most. In addition, a molybdenum tube is potentially capable of producing nearly adequate contrast even at a relatively higher tube voltage, When ultra-high speed screen systems are employed, an orthochromatic film back by a screen containing rare earth phosphors, their high speed benefit should be used for improvement of the total system resulution by reducing geometric unsharpness, Prevalence of this type of film, which tends to be used with excessively low kilovoltage, prompts therefore a need of amolybdenum tube having a small focal spot size of 0.3mm as the tube voltage should be kept 25kVp or greater, or a need of a much more smaller focus for magnification technique as the contrast loss accompanied by higher tube voltage may be less than offset by an improved geometry against a tungsten tube, in which sharpness was of second importance compared to the need for adquate contrast to exceed visual threshold. II. Increase in density beyond 1.2 adversely affected the visibility for Lo-dose system due to decrease in film contrast Actual mammograms of 1.0 density, even with a non-screen film, gave an impression so extremely dark compared to that given in ordinary radiograms which include bones or opaque medium, that the density of the midpoint of the breast or whole breast except its perihery remained within 0.6 density. The mammograms of 1.0 density therefore required 10 times greater intensity of illumination than that required for the practical density of 0.6,which differed substantially from the values of phantom works, but will exert a higher contrast potential if extraneous light was effectively masked out all but the relevant area of the film exposed to uniform density by compressing the whole breast.