CR Mammography: Image Quality Measurement and Model Calculation for Needle vs. Powder Imaging Plate

Abstract

Computed radiography (CR) is a digital radiography technology in which a storage phosphor plate is used to store a latent X-ray image. The plate is exposed in a light-tight cassette and then read out in a digitizer to create the digital image. Traditionally, CR powder imaging plates (PIP) are used based on BaFBr_1 − xI_x:Eu^2 + phosphor. The active layer consists of phosphor micro-crystals in a polymer binder. A needle imaging plate (NIP), created by vapor deposition of needle-shaped phosphor crystals, is expected to lead to better image quality. A first reason is that lateral light spread is less in NIP. Further, the system gain is higher, because more storage centers are created per unit of absorbed X-ray energy, because read-out depth can be higher and because the stimulated light escape efficiency is higher. The more transparent NIP guarantees a more constant image contribution over the thickness of the plate. Finally, the NIP layer is more homogeneous than the PIP layer, which leads to a lower degree of screen-structure noise. Measurements confirm that CsBr:Eu^2 + NIP’s in CR mammography have significantly better image quality (DQE), especially in the high frequency range. A linear-systems approach is used to model signal and noise transfer in a CR system using PIP or NIP. The transfers are described by cascading transfer relationships for each process. The calculated image quality (DQE) is in good agreement with measurement for both the NIP and the PIP systems.