Power-law analysis of nonlinear active-pixel detector responses as a function of mammographic energy

Abstract

Abstract This study investigates the imaging performance of a complementary metal-oxide-semiconductor (CMOS) active pixel sensor (APS) array coupled with a gadolinium oxysulfide phosphor. A wide range of mammographic energy was considered, which corresponds to an aluminum-equivalent thickness in the range of 0.515 to 0.689 mm in terms of half-value layers. The performance metrics include the modulation-transfer function (MTF), Wiener noise-power spectrum (NPS), and detective quantum efficiency (DQE). The detector showed a nonlinear response at low exposure ranges, so it was described using a power-law model to linearize image data before evaluating the MTF, NPS, and DQE. We show that the NPS, which is measured from images without linearization but corrected by the exponent of the power function (viz., nonlinearity correction), can be consistent with that obtained using the linearization procedure. Negligible difference in MTFs measured with and without linearization showed good agreement between the nonlinearity-corrected and linearized DQEs. The DQE performance of the detector was comparable to published data for conventional digital mammography detectors. However, the detector design with a CMOS APS array in conjunction with a thin phosphor was energy-sensitive. The overall spatial-resolution performance (the size of effective aperture) improved by about 13%, the noise performance was degraded by about 22%, and the zero-frequency DQE was reduced by about 26% when measured with a Mo-target beam quality of 25 kV to 31 kV and a similar exposure of about 6 mR. The detector design may be appropriate for operation in a narrow range of energy. Further design considerations of the phosphor with output that tolerates variations in energy could lead to applications in convertible mammography and tomosynthesis.