|[Frontiers in Bioscience 1, b5-7, August 1, 1996]|
COMPARISON OF CONTRAST DETAIL CURVES OF FULL FIELD DIGITAL
WITH SCREEN FILM BREAST PHANTOM IMAGES
Aparna Visweswaran, M.D., Hong Liu, Ph.D., Laurie L. Fajardo, M.D., Gia A. DeAngelis, M.D.
Department of Radiology, University of Virginia Health Sciences Center, Charlottesville, VA 22908, USA
Received 05/15/96; Accepted 07/08/96; On-line 08/01/96
The CDMAM Phantom Type 3.2 (Nuclear Associates, Carle Place, NY), specifically designed to evaluate mammographic systems, consists of an aluminum base with gold disks of various thicknesses (depth) and diameters, attached to a plexiglass cover (10). The diameter of the phantom objects (gold disks) ranges from 3.2 to 0.10 mm and the thickness of the objects ranges from 0.05 to 1.60 µm. The disks are arranged in a matrix of squares comprising 16 rows and 16 columns. Within each row, the diameter of the gold disk is constant with logarithmically increasing thickness. Within each column, the thickness of the gold disk is constant and its diameter increases logarithmically. In each square, there are two identical disks, one in the center and the other at a randomly chosen corner. For the imaging experiments, the phantom was positioned on each system with the smallest disk diameters at the chest wall side of the mammographic device. Four plexiglass plates were placed on top of the CDMAM phantom to simulate a 5 cm thick breast. The phantom and plexiglass plates were imaged at equivalent radiation doses on both the full breast digital and the screen film mammographic systems. Screen film breast phantom images were obtained at 25 kVp and 194 mRad mean glandular dose (Figure 2a); digital breast phantom images were obtained at 33 kVp and 194 mRad (Figure 2b). Using the method devised by the CDMAM phantom developer (11), the resultant images were evaluated to determine the threshold contrast (the minimal perceptible disk thickness) detectable for each diameter. For each square in the analog and digital images, a single observer identified the location of the eccentric disk. At least three squares were observed in each column and each row. The indicated positions for the eccentric disks were then compared to the true locations. For each row, where the diameter of the disks is constant, the thinnest disk correctly localized is taken as the minimal detectable thickness for the given diameter.
Fig. 2 (a) Analog screen-film CDMAM phantom image. This image was obtained at 25 kVp and 194 mRad mean glandular dose, using a clinical screen-film mammography system, (Model: M-III, LORAD Corporation, Danbury, CT)
Fig. 2 (b) Digital CDMAM phantom image. This image was obtained at 33 kVp and 194 mRad using the prototype full breast digital mammography unit (LORAD/Thermotrex, San Diego, CA).