Two-Step Image Registration for Dual-Layer Flat-Panel Detectors

For a single exposure in radiography, a dual-layer flat-panel detector (DFD) can provide spectral images based on copper filtering or beam hardening, and efficiently utilize the transmitted x-ray photons to improve the detective quantum efficiency (DQE) performance. In this paper, we present a registration method for x-ray images acquired from DFD considering only spatial translations and scale factors. Note that the spatial translation difference between the upper and lower detector layers arises when attaching the layers, and the scale difference arises from the x-ray projection relative to the source and detector distances. The conventional registration methods have inconsistent estimate accuracies depending on the captured object scene even when using entire pixels, and have deteriorated frequency performance due to the interpolation method employed. The proposed method consists of two steps; the first step is conducting a spatial translation according to the Fourier shift theorem with a subpixel registration, and the second step is conducting a scale transformation using cubic interpolation to process the x-ray projections. To estimate the subpixel spatial translation, a maximum-amplitude method using a small portion of the slant-edge phantom is used. The performance of the proposed two-step method is first theoretically analyzed, and then observed by conducting extensive experiments and measuring the noise power spectrum and DQE. An example for registering chest images is also shown. For DFD, it is shown that the proposed method shows a better registration result than the conventional one-step registration.