Paul Leblans

Storage Phosphors for Medical Imaging

Computed radiography (CR) uses storage phosphor imaging plates for digital imaging. Absorbed X-ray energy is stored in crystal defects. In read-out the energy is set free as blue photons upon optical stimulation. In the 35 years of CR history, several storage phosphor families were investigated and developed. An explanation is given as to why some materials made it to the commercial stage, while others did not. The photo stimulated luminescence mechanism of the current commercial storage phosphors, BaFBr:Eu2+ and CsBr:Eu2+ is discussed. The relation between storage phosphor plate physical characteristics and image quality is explained. It is demonstrated that the morphology of the phosphor crystals in the CR imaging plate has a very significant impact on its performance.

A new needle-crystalline computed radiography detector.

The most successful digital radiography detectors to date have been storage phosphor plates used in computed radiography (CR). The detector is cheap, has good producibility, and is robust. Direct radiography (DR) systems are being developed based on flat-panel technology. Better image quality is claimed for some DR systems. On the other hand, DR detectors have low producibility and robustness, and a high price. A new CR detector is being developed at Agfa that combines the advantages of CR and DR. It is a storage phosphor plate made up of needle-shaped crystals. The phosphor efficiently converts absorbed x-ray quanta into photostimulable centers for efficient read out. It has a large dynamic range and its emision is efficiently detected with both photomultiplier tube (PMT) and charge coupled device (CCD). It is shown that CR systems based on the new detector offer image quality that matches that of the best DR systems.

New needle-crystalline CR detector

The storage phosphor RbBr:Tl+ can be grown in needles via vacuum deposition. Thanks to reduced lateral light diffusion thick needle screens still offer acceptable resolution. Due to its low intrinsic X-ray absorption, however, a RbBr:Tl+ needle screen does not lead to a better absorption/resolution compromise than a BaFBr1-xIx:Eu2+ powder screen. CsBr:Eu2+ does combine high specific X-ray absorption and the possibility of needle growth. Its blue emission, peaking at 440 nm and near IR stimulation band, with maximum at 685 nm, make it well suited for use in CR systems. Sensitivity and sharpness of a 500 (mu) thick CsBr:Eu2+ needle screen were measured in a flying-spot scanner. The number of photostimulated light quanta per absorbed X-ray quantum is higher than for BaFBr1-xIx:Eu2+. At 70 kVp and 0.5 mm Cu filtration, equal sharpness is obtained for 85% vs. 46% X-ray absorption in BaFBr1-xIx:Eu2+ screens. DQE was measured at 2.5 (mu) Gy, 70 kVp, and 0.5 mm Cu filtration for a CsBr:Eu2+ needle screen in a flying-spot scanner. Up to 3 lp/mm, DQE was 2 times higher than for state-of-the-art CR systems and equal to the DQE claimed for flat panel DR systems, based on a-Si photodiodes combined with a CsI:Tl scintillator layer.

CR mammography: image quality measurement and model calculation for needle vs powder imaging plate

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 BaFBr1−xIx:Eu2+ 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:Eu2+ NIPs 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.

Paramagnetic Eu2+ center as a probe for the sensitivity of CsBr x-ray needle image plates

Needlelike CsBr:Eu x-ray storage phosphor plates exhibit an excellent sensitivity as expressed by their photostimulated luminescence (PSL) intensity and have ever growing applications in medical radiography. An earlier electron paramagnetic resonance (EPR) study revealed spectra due to a Eu2+ center with axial ⟨100⟩ symmetry. In this work, its EPR intensity is shown to exhibit a linear correlation with the PSL intensity and the sensitivity of the plates. It can thus be expected that the structural information retrieved from EPR and related techniques may substantially contribute to the understanding of the physics behind the writing and reading processes in the phosphor.