Needle vs. Powder Imaging Plate for Computer Radiography: Image Quality Measurement and Model Calculation

摘要

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 scanner to create the digital image. Conventionally, CR powder imaging plates (PIP) are used based on BaFBr_(1-x)I_x:Eu~(2+) phosphor. The active layer consists of phosphor microcrystals in a polymer binder. A needle imaging plate (NIP), created by of vapor deposition of needle-shaped phosphor crystals, leads to better image quality because thicker phosphor layers, having higher X-ray absorption can be used. BaFBr_(1-x)I_x:Eu~(2+) is an excellent storage phosphor. It decomposes upon vaporization, however. For that reason, it is impossible to vapor deposit BaFBr_(1-x)I_x:Eu~(2+) needle crystals. At Agfa an excellent new storage phosphor, CsBr:Eu~(2+), was discovered. Since CsBr melts congruently, it allows thermal vapor deposition and production of NIP's. Measurements demonstrate that CsBr:Eu~(2+) NIP's allow to double CR image quality (DQE). 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 the PIP system. The model overestimates the NIP system DQE at high spatial frequencies. An overestimation of the system gain may be the reason.