Won Y. Choi

Ultrahigh-resolution plastic graded-index fused image plates

Fiber-optic imaging systems such as the medical endoscope, the boroscope, the fused-image faceplate, and the image conduit are now made from glass step-index (SI) fibers, and the image resolution of the SI fiber-optic imaging systems is limited to ~5 microm. Ultrahigh-resolution fiber-optic fused-image plates with fiber diameter sizes of 5 and 2.8 microm were fabricated with plastic graded-index (GRIN) fibers. The measured image resolutions of the 5-microm SI and GRIN-based guides were comparable, and the resolution of the plastic GRIN image guides improved as the fiber diameter was reduced from 5 to 2.8 microm.

Imaging characteristics of plastic scintillating fiber screens for mammography

A scanning slot digital mammography system using a plastic scintillating fiber screen (SFS) is currently being developed. To improve the x-ray interaction efficiency and absorption efficiency of an SFS, high Z elements can be added into the scintillating fiber core. In this paper, we investigate theoretically the zero spatial frequency detective quantum efficiency, DQE(0), and modulation transfer function, MTF(f), of three 2 cm thick SFSs made of polystyrene, polystyrene loaded with 5% by weight of lead, and polystyrene loaded with 10% by weight of tin scintillating fibers. X-ray interaction efficiency, scintillating light intensity distributions and line spread functions were generated using Monte Carlo simulation. DQE(0) and MTF(f) were computed for x-ray energies ranging from 15 to 50 keg. Loading high Z elements into the SFS markedly increased the DQE(0). For x-ray energies used for mammography, DQE(0) values of both high Z element loaded SFSs are about a factor of three higher than the DQE(0) of an Min-R screen. At mammographic x-ray energies, MTF(f) values of all three SFSs are greater than 50% at 25 lp/mm spatial frequency, and were found to be dominated by the 20 micrometer individual scintillating fiber diameter used. The results show that both high DQE(0) and spatial resolution can be achieved with the high Z element loaded SFSs, which make these SFS attractive for use in a scanning slot detector for digital mammography.

Detective quantum efficiency of a CsI:Tl scintallator-based scanning slot x-ray detector for digital mammography

An investigation was made of the key determinants of the detective quantum efficiency at zero spatial frequency [DQE(0)] of a CsI:Tl scintillator based scanning slot x- ray detector for digital mammography. The slot x-ray detector was made of a prismatic type thallium activated CsI scintillator (150 micrometer thick) optically coupled to CCDs by fiber optical image guides. Monte Carlo calculations were performed to generate the CsI:Tl scintillator Swank factor on the basis of the energy deposition from pencil beam x-ray sources and light transmission within the CsI:Tl scintillator. A theoretical expression for the detector DQE(0) was obtained which was used to investigate the detector imaging performance as a function of x-ray energy, x-ray exposure, CCD electronic noise level, and optical coupling efficiency of the fiber optic image guide. The Swank factor of the CsI:Tl scintillator was close to unity at x-ray energies below Iodine K-edge (33.2 keV), but decreased to approximately 0.8 at higher x-ray energies up to 40 keV. DQE(0) of the slot x-ray detector was approximately 75% at 15 keV but decreased to approximately 40% at 30 keV. Optimum DQE(0) performance of the slot x-ray detector was generally obtained at a detector x-ray exposure level above approximately 5 to 10 mR and an electronic noise level below approximately 50 electrons rms. A drop in the optical coupling efficiency of the image guide from 1.0 to 0.3 reduced the detector DQE(0) from approximately 75% to approximately 55% in the mammography x-ray energy range. The key finding in this study is that the choice of the x-ray energy has a major impact on the DQE(0) of a CsI:Tl scintillator based slot x-ray detector. Since the x-ray photon energy also affects x-ray tube loading, mean glandular dose and subject contrast, the choices of optimal x-ray spectra from current mammography x-ray tubes require further investigation.

High-resolution digital x-ray imaging system based on the scintillating plastic microfiber technology

The design and construction of a prototype high resolution digital x-ray imaging system, employing a novel scintillating microfiber detector as the x-ray imaging sensor is described. The fiber detector has an active imaging area of 5 cm X 5 cm and thickness of 1 cm. The optical image formed in the fiber detector is read out by a high resolution image intensifier- CCD camera system. Investigations on the image quality of the prototype x-ray system are reported. MTF measurement shows >= 10 line pairs/mm spatial resolution of the scintillating fiber detector over the diagnostic x-ray energy range. The feasibility of the medical diagnostic application of the scintillating fiber detector is analyzed, and the new sensor is shown to be adaptable to imaging tasks where both high resolution and high detector sensitivity are required.

Performance of a CsI:TL-screen-based scanning slot detector for digital mammography

Thallium activated CsI scintillation screen (CsI:Tl) has advantages over rare-earth phosphor screens when used in a scanning slot x-ray detector for digital mammography. The scintillation decay time for CsI:Tl is only approximately 1 microsecond(s) which eliminates the afterglow effect associated with the use of rare-earth phosphor. The CsI needles serve to limit the spread of the scintillation light which permits the use of a relatively thick CsI:Tl screen to improve detector x-ray interaction efficiency without sacrificing resolution performance. A prototype scanning slot detector was made of a CsI:Tl screen and was optically coupled to two CCDs by plastic optical fiber image guides. The screen was composed of prismatic CsI crystals with a needle size of approximately 5 micrometers . Image guides used in the detector had an input to output ratio of 1:1, and were made of 10 micrometers diameter plastic optical fibers. Each CCD, operated in the time-delayed integration mode, was an 1100 X 330 pixel array with pixel size of 24 micrometers . A full scale scanning slot detector will contain eight rather than two such modules. The x-ray interaction efficiency of the slot x-ray detector was calculated to be approximately 84 percent at 20 keV for the CsI:Tl screen, and was approximately 20 percent greater than that of a 31.7 mg/cm2 thick Gd2O2S:Tb phosphor screen. Limiting spatial resolution was investigated by taking the images of a 1 degree star resolution test pattern as a function of detector scan speed. Limiting spatial resolution was approximately 15 1p/mm at a scan speed of 4 cm/s and improved only slightly to approximately 16 1p/mm as the scanning speed decreased to 1 cm/s.

Spatial-frequency-dependent DQE performance of a CsI:Tl-based x-ray detector for digital mammography

Monte Carlo calculations were performed to generate the point spread functions of x-ray photons absorbed in a CsI:Tl x-ray detector at the x-ray energies normally used in mammography (i.e., 20 keV to 50 keV). The corresponding modulation transfer functions [MTF(f)] for the CsI:Tl screen were also computed, taking into account the optical spread of light within the CsI:Tl crystals. The computed MTF(f)s were dominated by scintillation light lateral dispersion within the CsI:Tl screen. For the photon energy range encountered in digital mammography, the MTF(f) was a minimum at an x-ray photon energy just above the k-edge of Iodine (33 keV). Noise propagation theory for a cascaded imaging system was subsequently used to derive a theoretical expression for the detector DQE(f), including the dependence of DQE(f) on the spatial distribution of x-ray photon energy deposition. Detector performance was investigated as a function of x-ray exposure, CCD electronic noise, coupling efficiency of the fiber optical coupler, and the CCD quantum efficiency. Although most of the x-rays are absorbed via the photoelectric effect, the deposited x-ray energy spread within the CsI:Tl screen from the emission of characteristic x-rays can have a marked effect on detector performance, and the DQE(f) was found to decrease rapidly with photon energy just above the Iodine K-edge. X-ray exposure levels to the detector should be greater than or equal to 5 mR with a CCD electronic noise of approximately 20 electrons rms to ensure that DQE(f) performance is not significantly degraded at the spatial frequencies important in digital mammography (i.e., 0 to 10 lp/mm). Light dispersion within the CsI:Tl crystals was the major factor degrading imaging system DQE(f) at higher spatial frequencies. Optical coupling efficiency and CCD quantum efficiency are important system design parameters, which need to be maintained at a relatively high value. An optical coupling efficiency of approximately 0.7, and a CCD quantum efficiency of approximately 0.4, would still permit system DQE values greater than 60% at a spatial frequency of 5 lp/mm.