Scanned-slot digital mammography

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Abstract
A scanned-slot digital radiography system is being developed for use in mammography. The system consists of a novel fiber-optic "reformatter" which couples a strip of phosphor material to a light amplifier and a specially designed CCD camera. The reformatter provides an excellent means of converting a slot-shaped image to a format more suitable for digitization. A mammogram is produced by scanning the system in steps across the breast from the chest wall to the nipple. The pixels are of dimension 56 μm by 49 μm at the detector input. Images are currently acquired using a 40 kV tungsten anode spectrum with a measured half-value layer of 0.82 mm Al. The limiting resolution [MTF(f)=O.05] in the scanning direction is 9.2 cycles/mm. The quantum interaction efficiency of the phosphor is 64%, and the low-frequency detective quantum efficiency (DQE) was measured to be 0.60 ± 0.07. The high-frequency DQE is superior to that of mammographic film-screen systems because of the ability to remove fixed pattern noise from images. Phantom images produced with the scanned-slot system, and with a state-of-the-art film screen mammography system were compared. The scanned-slot system demonstrated better contrast sensitivity using a lower mean glandular dose than the film-screen mammography system.

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