Novel dual screen-dual film combination for mammography

Abstract

A new dual screen-dual film mammography combination was constructed which made of two phosphor screens and two films loaded into a single x-ray cassette. The screens and films were combined so that a single emulsion film (Film #1, Kodak Min-R E film) was placed in direct contact with the phosphor side of the first screen (Screen #1). Screen #1 was made of the Kodak Min-R phosphor (34.0 mg/cm 2 Gd 2 O 2 S:Tb) coated on a 4 mil transparent Mylar backing. A double emulsion film (Film #2, Kodak T-Mat G film) was sandwiched between Screen #1 backing and the phosphor side of the second screen (Screen #2). Screen #2 was a Kodak Insight ME screen that has a Gd 2 O 2 S:Tb coating weight of 43.1 mg/cm 2 and a reflective coating between its phosphor layer and support layer. The relative sensitometric responses and spatial resolution properties of the two films were measured as a function of x-ray tube potential (kVp). A series of a contrast-detail phantom images was obtained by varying the x- ray exposure level at 28 kVp. An observer performance study was subsequently carried out to investigate the low contrast performance using the dual screen-dual film combination. The effective speeds of the two films differed by a factor of 2.12 to 2.67 between 25 to 30 kVp. Film #2 contrast was a factor of two or greater than Film #1 in the range where Film #1 optical density values were under 0.7. Measured MTF curves from digitized edge images showed that Film #1 MTF performance was comparable to a standard Kodak Min-R screen-Min-R E film combination. The limiting spatial resolution was found to be 19.5 lp/mm for Film #1 and approximately 11 lp/mm for Film #2. Observer performance studies showed that the threshold contrast in detecting small (less than 10 mm) breast lesions could be up to a factor of two lower on Film #2 images when regions of interest are underexposed on Film #1 images.