A PRELIMINARY ATTEMPT TOWARD THE DEVELOPMENT OF A BREAST PHANTOM FOR EVALUATION OF MAMMOGRAPHY TECHNIQUES

Abstract

A preliminary approach was made for the development of satisfactory breast phantoms for evaluating mammography techniques. The initial effort was made to select the test object materials which simulate the breart tissues. A physical comparison of the test objects with breast tissues was made by obtaining the image contrast theoretically and experimentally. A good agreement of the results obtained by the two methods made it possible to expect roentgenographic detail at a visual threshold where the image contrast would not be measured. The results obtained by this study are summarized as follows : 1) Lucite absorber was more absorbent than the average breast by 25%, which corresponds to a 5 cm thick water-polystyrene composite absorber in which polystyrene comprised 36% in thickness (nearly equal to that by weight) at 30kVp, and the percentage varies from 35% to 40% in the range of 40〜20kVp. This implies the possibility of making a solid absorber by combining plastic plates, i. e., 4cm Lucite plus 1 cm polystyrene as a substitute for an average breast containing 50% fatty tissue, and 2cm Lucite plus 3cm polystyrene for an atrophic breast containing 70% fatty tissue, polystyrene being very nearly fat equivalent. 2) A high Z material like aluminum offers a negligible scattering medium compared with plastic absorbers. An aluminum test piece placed in direct contact with the film yields a higher image contrast than that placed further from the film so that it should be positioned 5 mm above the film when minimizing a geometrical effect. 3) Mammary carcinoma and blood vessels were theoretically simulated by a solution containing 1% cupric sulfate by weight in distilled water. Roentgenographic evaluation proved that the solution filled in cylinders is a satisfactory substitute for blood vessels. Water columns were less opaque. X-ray absorption of microcalcifications in the paraffin tissue block was the same as the one of quartz glass because they can be assumed to contain 50% tricalcium phosphate. 4) A reduction in the percentage of polystyrene from the composite absorber markedly decreased the image contrast of low Z materials. It is believed that fat involution in the breast is a very important factor for detecting a small carcinoma or fine blood vessels but not so much for calcifications. 5) Minimum perceptible contrast of cylindrical objects was presumed to be 0.008 to 0.01,the corresponding diameteer of which was 1 mm for water and 0.6mm for 1% cupric sulfate solution. Calcification required a rather high contrast of 0.06 for its detection and this implies that visibility of such minute flecks would be limited by the grain of the film. A tentative drawing of the phantom consisting of two parts was made. One represents the contour of the breast by a sloped Lucite block, and the other, positioned at the bottom, is made of 1 cm thick Lucite slab of 8×12cm in size. A number of tapped holes filled with a diluted solution of cupric sulfate, and microbeads of quartz glass enbedded in areas of 5×5 cm each in the slab are used to nullify the heel effect. This slab can be detached and placed at any level of the plastic blocks of uniform thickness for evaluation of changes in the object-to-film distance.