TU-FG-209-01: A Model of Noise Anisotropy in Amorphous Selenium Digital Detectors




One of the main advantages of amorphous selenium (a-Se) photoconductors is high spatial resolution. It has also been shown that the noise power spectra (NPS) is effectively white (frequency-independent) at various angles of x-ray incidence. One limitation of previous studies on NPS is that the effect of digitization has not been modeled. This work demonstrates that the NPS becomes colored (frequency-dependent) once digitization is taken into account.


A mammography system is modeled by tracing rays between the focal spot and the detector. At each point on the entrance surface of a-Se, the path length of the ray through the thickness of a-Se is determined. The PSF is calculated from first principles based on the existence of ionizations at each point along the path length. To model the effect of digitization, the PSF is convolved with a product of rectangle functions. The NPS is then calculated from the discrete Fourier Transform of the autocovariance of the digitized PSF.


At the midpoint of the chest wall, the angle of x-ray incidence is normal to the detector. It is demonstrated that the NPS is white at this position, as expected. The angle of incidence becomes oblique with increasing distance from the midpoint of the chest wall. At these positions, the NPS decreases up to the alias frequency, and becomes increasingly colored as the angle of incidence deviates more considerably from the normal. In addition, the NPS is found to be orientation-dependent. Specifically, the NPS is colored along the direction of the incident ray, but is white along the direction perpendicular to the incident ray.


It is demonstrated that oblique x-ray incidence yields cross talk between detector elements. Hence, although the NPS of the a-Se photoconductor is white, the NPS of the digital image is colored.

Support for Raymond Joseph Acciavatti was provided by the Postdoctoral Fellowship Grant PDF14302589 from Susan G. Komen. The content is solely the responsibility of the authors and does not necessarily represent the official views of the funding agency.