A low-noise CCD coupled to a structured CsI(Tl) scintillator via a fiber optic plate was operated as X-ray detector for digital mammography. The imaging capabilities of the device were measured in terms both of spatial resolution (MTF) and of noise properties (DQE, NPS). The detector was characterized using a standard mammographic tube with and without the coupling to a pair of anti-scatter projective grids. Breast phantom images were collected and compared to a MonteCarlo simulation of the apparatus. Contrast enhancement was achieved by using the anti-scatter grids.
A 4096 pixel Photon Counting Chip (PCC) has been developed and tested. It is aimed primarily at medical imaging although it can be used for other applications involving particle counting. The readout chip consists of a matrix of 64 by 64 identical square pixels, whose side measures 170 micrometers and is bump-bonded to a similar matrix of GaAs or Si pixel diodes covering a sensitive area of 1.18 cm2. The electronics in each cell comprises a preamplifier, a discriminator with variable threshold and a 3-bit threshold tune as well as a 15-bit counter. Each pixel can be individually addressed for electrical test or masked during acquisition. A shutter allows for switching between the counting and readout modes and the use of static logic in the counter enables long data taking periods. Electrical test of the chip have shown a maximum counting and readout modes and the use of static logic in the counter enables long data taking periods. Electrical test of the chip have shown a maximum counting rate of up to 2 MHz in each pixel. The minimum reachable threshold is 1400 e with a variation of 350 e rms that can be reduced to 80 e rms after tuning with the 3-bit adjustment. Electrical noise at the input is 170 e rms. Several read-out chips have been bump bonded to 200 micrometers thick GaAs pixel detectors. Test with (gamma) -ray and (beta) sources have been carried out. A number of objects have been imaged and a 260 micrometers thick aluminum foil which represents a contrast to the surrounding air of only 1.9 percent has been correctly imaged.
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