Dr. Winston Guangda Ji Profile

Dr. Winston Guangda Ji

at Sunnybrook Health Sciences Ctr

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Author

Publications (5)

Proceedings Article | 20 April 2005 Paper

Effect of flare on computed radiography systems

John Rowlands, Stephen Kengyelics, Andrew Davies, Winston Ji, Kuo Yan

Proceedings Volume 5745, (2005) https://doi.org/10.1117/12.597382

KEYWORDS: Chromium, X-rays, Radiography, Modulation transfer functions, Light scattering, Computing systems, Waveguides, Scanners, X-ray imaging, Photons

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Proceedings Article | 28 June 2001 Paper

Effect of depth-dependent modulation transfer function and K-fluorescence reabsorption on the detective quantum efficiency of indirect-conversion flat-panel x-ray imaging systems using CsI

John Rowlands, Winston Ji, Wei Zhao

Proceedings Volume 4320, (2001) https://doi.org/10.1117/12.430931

KEYWORDS: X-ray detectors, Light scattering, Quantum efficiency, Data modeling, Scattering, X-ray imaging, Imaging systems, X-rays, Sensors, Modulation transfer functions

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Proceedings Article | 25 April 2000 Paper

Direct-conversion flat-panel x-ray imaging: reduction of noise by presampling filtration

John Rowlands, Winston Ji, Wei Zhao, Denny Lee

Proceedings Volume 3977, (2000) https://doi.org/10.1117/12.384519

KEYWORDS: Interference (communication), Photoresistors, Prototyping, Electrodes, Spatial frequencies, X-ray imaging, Imaging systems, X-rays, Modulation transfer functions, Sensors

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Proceedings Article | 28 May 1999 Paper

Optimization of the presampling modulation transfer function of flat-panel detectors for digital radiology

John Rowlands, Winston Ji

Proceedings Volume 3659, (1999) https://doi.org/10.1117/12.349536

KEYWORDS: X-ray detectors, Radiography, Electrodes, Imaging systems, Photoresistors, Spatial frequencies, X-rays, X-ray imaging, Sensors, Modulation transfer functions

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Large area, flat panel solid state detectors are being investigated for digital radiography and fluoroscopy. These detectors employ an x-ray imaging layer of either photoconductor ('direct' conversion method) or phosphor ('indirect' conversion method) to detect x-rays. In both cases the image formed at the surface of the layer is read out in situ using an active matrix array. Depending upon the resolution of the layer compared to the pixel size, undersampling of the image and hence aliasing may occur. Aliasing is always present regardless of the pixel size in direct detectors based on amorphous selenium because of its high intrinsic resolution. Aliasing gives rise to increased noise which results in reduction of detective quantum efficiency DQE at high spatial frequencies. The aliasing can be reduced or even eliminated by blurring prior to pixel sampling (e.g., by scattering in a phosphor layer). However, blurring, which may be quantified by the spatial frequency f dependent modulation transfer function MTF(f), also has a deleterious effect: the imaging system becomes much more susceptible to noise for example that arising in the charge amplifiers or secondary quantum statistics. Note that in principle, the system MTF can be corrected to any desired values in a digital system thus MTF has no predictive value for the quality of an imaging system, rather it is the DQE(f) which determines the overall signal to noise ratio independently of the MTF enhancement chosen. Nevertheless, determining the ideal level of presampling blurring (i.e., the Presampling Modulation Transfer function) is not straightforward. A problem caused by blurring is that the degree of blurring often depends on the depth of absorption of the x-ray in the imaging layer. In such cases (as pointed out by Lubberts) additional noise is transferred to the image. The predictions of a Lubberts model will be compared with published measurements of DQE for both direct and indirect detectors. A preliminary conclusion, is that blurring by CsI phosphor layers is non-ideal and leads to a significant loss of DQE at high spatial frequencies while no such loss is occurring in (alpha) -Se layers due to the equal MTF (in this case MTF(f) approximately equals 1) at all depths. Thus the only method which appears practical to cause blurring so as to avoid noise aliasing while avoiding the Lubberts depth dependent effect is to have a perfect MTF in the imaging layer and then blur before sampling. Such an approach has been proposed for the direct method based on the use of a partially conducting layer. Theoretical estimates of the final DQE(f) to be expected using this variation of the direct conversion method are produced.

Proceedings Article | 2 May 1997 Paper

Flat panel detector for digital radiology using active matrix readout of amorphous selenium

John Rowlands, Wei Zhao, Ira Blevis, Gendi Pang, Winston Ji, Stephen Germann, Safa Kasap, David Waechter, Zhong Shou Huang

Proceedings Volume 3032, (1997) https://doi.org/10.1117/12.273974

KEYWORDS: Sensors, X-rays, X-ray imaging, Electrodes, Direct methods, Imaging systems, Radiography, Selenium, Photoresistors, Fluoroscopy

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