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

Abstract

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.

Accurate MTF measurement in digital radiography using noise response

“…It follows that consecutive binomial processes can be combined into a single binomial process with a probability equal to that of the product of the probabilities of each individual stage. 37 Blurring processes may be either stochastic ͑caused by the spatial spreading or scattering of stochastic quanta͒ or deterministic ͑e.g., pixelization resulting from digital sampling͒. The signal at the output of stage i of a blurring process is given by…”

“…Additional details can be seen in a number of works. [33][34][35][36][37][38][39][40] Amplification processes may involve binary selection or conversion of quanta from one form to another. The signal at the output of stage i of an amplification process is described by…”

“…[37][38][39][40] Careful inspection of the resultant noise response of such systems from a theoretical framework, as described by the two-dimensional noise power spectrum ͑NPS͒, indicates that the resolution response ͑i.e., the MTF͒ is inherently included in such information. 41,42 In this work, we present an exact relationship using a generalized cascaded linear systems analysis.…”

Evaluation of the imaging performance of LSO powder scintillator for use in X-ray mammography

“…Uniform irradiation of the LSO screen-film combination was performed employing the same exposure conditions as those for MTF measurements [12]. The irradiated films were developed and digitized with the same parameters as in the MTF measurements.…”

Integrated model for estimating phosphor signal and noise transfer characteristics on medical images: application to CdPO3Cl:Mn phosphor screens

“…The input signal-to-noise ratio SNR 2 has been previously considered to be equal to the incident X-ray fluence q)x (SHAW and VAN METTER, 1984; However, it must be pointed out that X-ray imaging detectors (SANDBORG and CARLSON, 1992;WILLIAMS et al, 1999b) yield an output signal proportional to the total energy absorbed within the detector. Thus, the incident X-ray energy fluence q)x E, rather than X-ray photon fluence, should be used for input signal calculation.…”