Analysis of the detective quantum efficiency of a developmental detector for digital mammography

We are developing a modular detector for applications in full field digital mammography and for diagnostic breast imaging. The detector is based on a design that has been refined over the past decade for applications in x-ray crystallography [Kalata et al., Proc. SPIE 1345, 270-279 (1990); Phillips et al. ibid. 2009, 133-138 (1993), Phillips et al., Nucl. Instrum. Methods Phys. Rev. A 334, 621-630 (1993)]. The full field mammographic detector, currently undergoing clinical evaluation, is formed from a 19 cm x 28 cm phosphor screen, read out by a 2 x 3 array of butted charge-coupled device (CCD) modules. Each 2k x 2k CCD is optically coupled to the phosphor via a fiber optic taper with dimensions of 9.4 cm x 9.4cm at the phosphor. This paper describes the imaging performance of a two-module prototype, built using a similar design. In this paper we use cascaded linear systems analysis to develop a model for calculating the spatial frequency dependent noise power spectrum (NPS) and detective quantum efficiency (DQE) of the detector using the measured modulation transfer function (MTF). We compare results of the calculation with the measured NPS and DQE of the prototype. Calculated and measured DQEs are compared over a range of clinically relevant x-ray exposures and kVps. We find that for x-ray photon energies between 10 and 28 keV, the detector gain ranges between 2.5 and 3.7 CCD electrons per incident x-ray, or approximately 5-8 electrons per absorbed x ray. Using a Mo/Mo beam and acrylic phantom, over a detector entrance exposure range of approximately 10 to 80 mR, the volume under the measured 2-d NPS of the prototype detector is proportional to the x-ray exposure, indicating quantum limited performance. Substantial agreement between the calculated and measured values was obtained for the frequency and exposure dependent NPS and DQE over a range of tube voltage from 25 to 30 kVp.