Calculation of OTF, NPS, and DQE for Oblique X-Ray Incidence on Turbid Granular Phosphors

Digital breast tomosynthesis (DBT) is an imaging modality in which tomographic sections of the breast are generated from a limited range of x-ray tube angles. One drawback of DBT is resolution loss in the oblique projection images. The purpose of this work is to extend Swank’s formulation of the transfer functions of turbid granular phosphors to oblique x-ray incidence, using the diffusion approximation to the Boltzmann equation to model the spread of light in the phosphor. As expected, the modulation transfer function (MTF) and noise power spectra (NPS) are found to decrease with projection angle regardless of frequency. By contrast, the dependence of detective quantum efficiency (DQE) on projection angle is frequency dependent. DQE increases with projection angle at low frequencies, and only decreases with projection angle at high frequencies. Importantly, the x-ray quantum detection efficiency (AQ) and the Swank information factor (AS) are also found to be angularly dependent.