Charles David Greskovich

Development of the HiLightTM scintillator for computed tomography medical imaging

Abstract The image quality of computed tomography (CT) medical scanners is extremely sensitive to afterglow, radiation damage and optical non-uniformities of scintillators used in X-ray detectors. This represents a tough challenge in the design of scintillator materials with optimum properties. Discussion will center on the development and properties of the first commercialized transparent ceramic scintillator, the (Y,Gd)2O3:Eu-based HiLightTM scintillator used in GE Medical Systems CT products. The flexibility of the ceramic scintillator platform has enabled it to be engineered to satisfy the changing needs of CT imaging, which is demonstrated by its successful incorporation into over 8000 CT systems worldwide since 1988. The ceramic process makes possible uniform co-doping at ppm levels to control electronic defects responsible for afterglow, reducing it to levels below detectibility in CT images. Annealing of the material in controlled oxygen atmospheres, combined with rapid oxygen diffusion along grain boundaries in the ceramic, reduces radiation damage to negligible values. Transient thermoluminescence of these materials will be discussed as a diagnostic of electronic trap levels responsible for both afterglow and radiation damage. Finally, with the increased scan speed requirements of modern CT systems, energy transfer between the Eu activator and other rare-earth ions can be used to speed the radiative decay of the scintillator, ensuring the materials continued viability in future CT systems.

Processing and application of solid state converted high-strain materials

The property enhancement offered by single crystal relaxor ferroelectrics combined with the manufacturability advantages offered by injection molding has the potential of producing single crystal 1-3 piezocomposites at an affordable production-viable rate. Two methods of texturization/recrystallization are being evaluated: an integrated multi-seed process and epitaxial growth. The integrated seed approach involves incorporation of oriented single crystal PMN-PT seeds into injection molding feedstock prior to fabrication of 1-3 ceramic preforms. After sintering, an additional texturization and growth step is carried out. This step is intended to drive recrystallization at multiple sites within the ceramic body extending the oriented texture throughout the matrix. The epitaxial growth approach involves nucleation and growth in the dense ceramic body initiated from a compatible external seed crystal. Recrystallization is achieved through direct contact between a ceramic preform and a seed substrate coupled with appropriate thermal and atmospheric growth conditions.

High Resolution X-Ray Computed Tomography for Composites and Electronics Inspection

A useful computed tomography (CT) or digital radiography (DR) system must simultaneously resolve both the smallest meaningful features and least relevant differences in density in the objects it is designed to inspect. The advent of structural composites and high-density electronic assemblies requires digital x-ray systems that are tailored to their properties. The critical flaw size in graphite composites is thought to be 25 to 50 μm. Similarly, prototype military electronics are now being assembled on circuit boards with conductive patterns only 100 μm in width.

Single‐phase inhomogeneity in ceramic garnets

Sintered ceramic garnets of composition (Y,Gd)3 (Fe,Al)5012 are of technological importance for the construction of microwave phase shifters. Mossbauer spectroscopy near Tc provides a unique monitor of the effect of ceramic processing variables, as well as sintering time at temperature, on chemical homogeneity. Near Tc we observe the coexistence of ferrimagnetic and paramagnetic phases, with the width of the coexistence region, ΔTc, varying sharply between samples. The ΔTc values are a quantitative measure of local chemical fluctuations, presumably in the Fe/Al ratio, which remain after the particular preparation. Roughly similar fluctuations are also inferred from x‐ray line broadening, but which much less precision. Concurrent with the decrease in ΔTc from 9°C to 3°C, the X‐band resonance line width decreases from 67 to 45 Oe. This change is more than twice the maximum estimated for the observed slight decrease in porosity, and illustrates the influence of single‐phase inhomogeneity on the microwave properties of substituted garnets.Sintered ceramic garnets of composition (Y,Gd)3 (Fe,Al)5012 are of technological importance for the construction of microwave phase shifters. Mossbauer spectroscopy near Tc provides a unique monitor of the effect of ceramic processing variables, as well as sintering time at temperature, on chemical homogeneity. Near Tc we observe the coexistence of ferrimagnetic and paramagnetic phases, with the width of the coexistence region, ΔTc, varying sharply between samples. The ΔTc values are a quantitative measure of local chemical fluctuations, presumably in the Fe/Al ratio, which remain after the particular preparation. Roughly similar fluctuations are also inferred from x‐ray line broadening, but which much less precision. Concurrent with the decrease in ΔTc from 9°C to 3°C, the X‐band resonance line width decreases from 67 to 45 Oe. This change is more than twice the maximum estimated for the observed slight decrease in porosity, and illustrates the influence of single‐phase inhomogeneity on the microwave pro...