X-ray imaging performance of structured cesium iodide scintillators

• Published in: Medical physics (Lancaster) Vol. 31; no. 9; pp. 2594 - 2605
• Main Authors: Zhao, Wei, Ristic, Goran, Rowlands, J. A.
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01-09-2004
• Subjects: Algorithms; Ancillary equipment; caesium compounds; Cesium - radiation effects; cesium iodide; CMOS image sensors; diagnostic radiography; digital radiography; Doping; Dose-Response Relationship, Radiation; Equipment Design; Equipment Failure Analysis - methods; flat‐panel detectors; Gamma Cameras; Image quality; image resolution; indirect detection; Iodides - radiation effects; Medical imaging; Metal insulator semiconductor structures; Modulation transfer functions; Nuclear medicine imaging; optical sensors; optical transfer function; Quantum measurement theory; Radiographic Image Enhancement - instrumentation; Radiometry; Reproducibility of Results; scintillation; scintillation counters; Scintillation detectors; Sensitivity and Specificity; Swank factor; thallium; X-Rays; X‐ and γ‐ray sources, mirrors, gratings, and detectors; X‐ray detection; X‐ray detectors; X‐ray imaging; X‐ray optics; digital radiography; indirect detection; flat-panel detectors; Swank factor; cesium iodide
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.1782676

Columnar structured cesium iodide (CsI) scintillators doped with Thallium (Tl) have been used extensively for indirect x-ray imaging detectors. The purpose of this paper is to develop a methodology for systematic investigation of the inherent imaging performance of CsI as a function of thickness and design type. The results will facilitate the optimization of CsI layer design for different x-ray imaging applications, and allow validation of physical models developed for the light channeling process in columnar CsI layers. CsI samples of different types and thicknesses were obtained from the same manufacturer. They were optimized either for light output (HL) or image resolution (HR), and the thickness ranged between 150 and 600 microns. During experimental measurements, the CsI samples were placed in direct contact with a high resolution CMOS optical sensor with a pixel pitch of 48 microns. The modulation transfer function (MTF), noise power spectrum (NPS), and detective quantum efficiency (DQE) of the detector with different CsI configurations were measured experimentally. The aperture function of the CMOS sensor was determined separately in order to estimate the MTF of CsI alone. We also measured the pulse height distribution of the light output from both the HL and HR CsI at different x-ray energies, from which the x-ray quantum efficiency, Swank factor and x-ray conversion gain were determined. Our results showed that the MTF at 5 cycles/mm for the HR type was 50% higher than for the HL. However, the HR layer produces ∼36% less light output. The Swank factor below K-edge was 0.91 and 0.93 for the HR and HL types, respectively, thus their DQE(0) were essentially identical. The presampling MTF decreased as a function of thickness L. The universal MTF, i.e., MTF plotted as a function of the product of spatial frequency f and CsI thickness L, increased as a function of L. This indicates that the light channeling process in CsI improved the MTF of thicker layers more significantly than for the thinner ones.