High Resolution Active Pixel Sensor X-Ray Detectors for Digital Breast Tomosynthesis
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Current large area x-ray detectors for digital breast tomosynthesis (DBT) are based on the amorphous silicon (a-Si:H) passive pixel sensor (PPS) technology. However, PPS detectors suffer from a limited resolution and high electronic noise. The limited imaging performance of PPS detectors will lead to blurred and noisy DBT images. To address these issues, we propose high-resolution large-area active pixel sensor (APS) x-ray detectors based on the complementary metal-oxide-semiconductor (CMOS) technology to improve the imager resolution and noise properties.
We evaluated the two-dimensional (2D) x-ray imaging performance as measured by the modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) for a 50 µm pixel pitch CMOS APS x-ray detector named DynAMITe. The DynAMITe detector has very low electronic noise of 150 e-. Excellent imaging performance (DQE as high as 0.5) has been achieved over a wide spatial frequency range (0 "“10 mm-1) at very low air kerma below 10 µGy. Also, a 2D cascaded system analysis model has been developed to describe the signal and noise transfer for the CMOS APS x-ray imaging systems. The developed 2D cascaded system analysis model has been extended to the three-dimensional (3D) spatial frequency space to simulated the 3D MTF, NPS and DQE characteristics using DBT radiation conditions and acquisition geometries. Next, the 3D cascaded system analysis for the DynAMITe detector was integrated with an object task function, a medical imaging display model, and the human eye contrast sensitivity function to calculate the detectability index (d') and area under the ROC curve (AUC). It has been demonstrated that the display pixel pitch and zoom factor should be optimized to improve the AUC for detecting small microcalcifications. Detector electronic noise of smaller than 300 e- and a high display maximum luminance (>1000 cd/cm2) are desirable to distinguish microcalcifications of 150 µm or smaller in size. For low contrast mass detection, a medical imaging display with a minimum of 12 bits gray levels is needed to realize accurate luminance levels. A wide projection angle range of greater than Â±30Â °~ in combination with the image gray level magnification could improve the mass detectability especially when the anatomical background noise is high.