Comparison of CsI:Tl and Gd2 O2 S:Tb indirect flat panel detector x-ray imaging performance in front- and back-irradiation geometries.

PURPOSE: The detective quantum efficiency (DQE) of indirect flat panel detectors (I-FPDs) is limited at higher x-ray energies (e.g., 100-140 kVp) by low absorption in their scintillating x-ray conversion layer. While increasing the thickness of the scintillator can improve its x-ray absorption efficiency, this approach is potentially limited by reduced spatial resolution and increased noise due to depth dependence in the scintillator's response to x rays. One strategy proposed to mitigate these deleterious effects is to irradiate the scintillator through the pixel sensor in a "back-irradiation" geometry. This work directly evaluates the impact of irradiation geometry on the inherent imaging performance of I-FPDs composed with columnar CsI:Tl and powder Gd2 O2 S:Tb (GOS) scintillators. METHODS: A "bidirectional" FPD was constructed which allows scintillator samples to be interchangeably coupled with the detector's active matrix to compose an I-FPD. Radio-translucent windows in the detector's housing permit imaging in both "front-irradiation" (FI) and "back-irradiation" (BI) geometries. This test device was used to evaluate the impact of irradiation geometry on the x-ray sensitivity, modulation transfer function (MTF), noise power spectrum (NPS), and DQE of four I-FPDs composed using columnar CsI:Tl scintillators of varying thickness (600-1000 µm) and optical backing, and a Fast Back GOS screen. All experiments used an RQA9 x-ray beam. RESULTS: Each I-FPD's x-ray sensitivity, MTF, and DQE was greater or equal in BI geometry than in FI. The I-FPD composed with CsI:Tl (1 mm) and an optically absorptive backing had the largest variation in sensitivity (17%) between FI and BI geometries. The detector composed with GOS had the largest improvement in limiting resolution (31%). Irradiation geometry had little impact on MTF(f) and DQE(f) measurements near zero frequency, however, the difference between FI and BI measurements generally increased with spatial frequency. The CsI:Tl scintillator with optically absorptive backing (1 mm) in BI geometry had the highest spatial resolution and DQE over all frequencies. CONCLUSIONS: Back irradiation may improve the inherent x-ray imaging performance of I-FPDs composed with CsI:Tl and GOS scintillators. This approach can be leveraged to improve tradeoffs between detector dose efficiency, spatial resolution and noise for higher energy x-ray imaging.

An apparatus and method for directly measuring the depth-dependent gain and spatial resolution of turbid scintillators.

PURPOSE: Turbid (powder or columnar-structured) scintillators are widely used in indirect flat panel detectors (I-FPDs) for scientific, industrial, and medical radiography. Light diffusion and absorption within these scintillators is expected to cause depth-dependent variations in their x ray conversion gain and spatial blur. These variations degrade the detective quantum efficiency of I-FPDs at all spatial frequencies. Despite their importance, there are currently no established methods for directly measuring scintillator depth effects. This work develops the instrumentation and methods to achieve this capability. METHODS: An ultra-high-sensitivity camera was assembled for imaging single x ray interactions in two commercial Gd2 O2 S:Tb (GOS) screens (Lanex Regular and Fast Back, Eastman Kodak Company). X ray interactions were localized to known depths in the screens using a slit beam of parallel synchrotron radiation (32 keV), with beam width (~20 µm) much narrower than the screen thickness. Depth-localized x ray interaction images were acquired in 30 µm depth-intervals, and analyzed to measure each scintillator's depth-dependent average gain g ¯ ( z ) and modulation transfer function MTF(z,f). These measurements were used to calculate each screen's expected MTF(f) in an energy-integrating detector (e.g., I-FPD). Calculations were compared to presampling MTF measurements made by coupling each screen to a high-resolution CMOS image sensor (48 µm pixel) and using the slanted-edge method. RESULTS: Both g ¯ ( z ) and MTF(z,f) continuously increased as interactions occurred closer to each screen's sensor-coupled surface. The Regular yielded 1351 ± 66 and 2117 ± 54 photons per absorbed x ray (42-66 keV-1 ) in interactions occurring furthest from and nearest to the image sensor, while the Fast Back yielded 833 ± 22 and 1910 ± 39 photons (26-60 keV-1 ). At f = 1 mm-1 , MTF(z,f) varied between 0.63 and 0.78 in the Regular and 0.30-0.76 in the Fast Back. Calculations of presampling MTF(f) using g ¯ ( z ) and MTF(z,f) showed excellent agreement with slanted-edge measurements. CONCLUSIONS: The developed instrument and method enable direct measurements of the depth-dependent gain and spatial resolution of turbid scintillators. This knowledge can be used to predict, understand, and potentially improve I-FPD imaging performance.

Opportunities for Fluorochlorozirconate and Other Glass-Ceramic Detectors in Medical Imaging Devices.

This article gives an overview of fluorochlorozirconate glass-ceramic scintillators and storage phosphor materials: how they are synthesized, what their properties are, and how they can be used in medical imaging. Such materials can enhance imaging in x-ray radiography, especially mammography and dental imaging, computed tomography, and positron emission tomography. Although focusing on fluorochlorozirconate materials, the reader will find the discussion is relevant to other luminescent glass and glass-ceramic systems.

Scanning translucent glass-ceramic x-ray storage phosphors.

A simple benchtop apparatus has been built, to measure the x-ray imaging properties of fluorozirconate-based glass-ceramic x-ray storage phosphor materials. The MTF degradation due to stimulating light spreading in the plate is lower in comparison to optically turbid screens resulting in higher image MTF. In addition, the degree of transparency, or the amount of light scattering at the wavelength of the stimulating (laser) light is adjustable by means of the glass preparation process. The amount of stimulating exposure required for plate readout is generally higher than in previous systems, but well within the range of commercially available laser systems, for practical readout times. The effects of flare or unwanted readout due to back-reflection from the imaging plate is also less than in previous systems.A novel telecentric scanning system has been developed that is able to rapidly read out the latent image stored in the translucent imaging plates. This system features a reflective primary scan mirror to achieve telecentricity, optical correction for scan line bow, and the design should enable the construction of a relatively inexpensive scanner system for the translucent x-ray storage plates.

Screen optics effects on detective quantum efficiency in digital radiography: zero-frequency effects.

Indirect flat panel imagers have been developed for digital radiography, fluoroscopy and mammography, and are now in clinical use. Screens made from columnar structured cesium iodide (CsI) scintillators doped with thallium have been used extensively in these detectors. The purpose of this article is to investigate the effect of screen optics, e.g., light escape efficiency versus depth, on gain fluctuation noise, expressed as the Swank factor. Our goal is to obtain results useful in optimizing screens for digital radiography systems. Experimental measurements from structured CsI samples were used to derive their screen optics properties, and the same methods can also be applied to powder screens. CsI screens, all of the same thickness but with different optical designs and manufacturing techniques, were obtained from Hamamatsu Photonics Corporation. The pulse height spectra (PHS) of the screens were measured at different x-ray energies. A theoretical model was developed for the light escape efficiency and a method for deriving light escape efficiency versus depth from experimental PHS measurements was implemented and applied to the CsI screens. The results showed that the light escape efficiency varies essentially linearly as a function of depth in the CsI samples, and that the magnitude of variation is relatively small, leading to a high Swank factor.