:Experiments using an RMI 156 Phantom
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For phantom image quality evaluation, the recommended optical density was previously 1.2, but has recently been revised to 1.4. This is probably because the optical density of maximum latitude has shifted upwards with newly developed mammography dedicated films. Contrasts of disk, three groups of calcification specks (C1, C2 and C3) and the largest two masses (Ml and M2) were measured on RMI-156 phantom images with the optical density in the range 0.7 to 2.0, using image processing software after digitization of the films. The pixel value was converted to optical density using the calibration data. The exposure conditions were 2 X-ray units with an anti-scatter grid, Mo/ Mo and Mo/Rh, 28 kV and MinR 2000/MinR 2000. Seven to 10 images were analyzed per one series to cover the optical density range of 0.7 to 2.0. Contrasts at various densities were compared with those at 1.2 for each object ; an increase in contrasts was large for a disk, and Cl and C2, but quite small for masses, especially for M2, on Mo/Mo images. On Mo/Rh images, an increase in contrasts was smaller for all test objects than with Mo/Mo images at an optical density greater than 1.2. To increase the optical density from 1.2 to 1.4, the radiation dose had to be raised by 13% to 15%. Although there appears to be a difference between mammography X-ray units in the contrast-density relationship, the contrasts of disk and calcification specks, having high object contrast, increase with an increase in optical density. However, this increase in optical density seems to have little benefit with respect to masses.
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