Mr. Steven Freestone Profile

Steven Freestone

at Varex Imaging Corp

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Publications (2)

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Proceedings Article | 16 March 2020 Paper

Analysis of a new indium gallium zinc oxide (IGZO) detector

Steven Freestone, Richard Weisfield, Carlo Tognina, Isaias Job, Richard Colbeth

Proceedings Volume 11312, 113123W (2020) https://doi.org/10.1117/12.2549469

KEYWORDS: Sensors, Amorphous silicon, Imaging systems, Field emission displays, Modulation transfer functions, Spatial frequencies, CMOS sensors, Signal to noise ratio, Sensor performance, X-rays

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Many improvements have been made to amorphous silicon (a-Si) flat panel detectors (FPDs) to meet the market needs for different x-ray imaging applications. With the current generation of a-Si FPDs the performance is limited by the a-Si thin film transistors (TFTs). The low electron mobility of a-Si necessitates large TFT’s with large parasitic dataline capacitance, which increases electronic noise and reduces the pixel fill factor (FF). In other words, large TFT’s negatively impact Signal-to-Noise Ratio (SNR). CMOS FPDs were introduced to provide improved low dose imaging performance and faster readout times, but the increase in cost can be prohibitive. IGZO TFTs have an electron mobility that is <10x higher than a-Si, which facilitates a reduction in the size of the TFT while also reducing the pixel discharge time, resulting in an increase to both the detector readout rate and the SNR. Reducing the TFT size is particularly important in achieving adequate low dose performance in dynamic detectors with pixels approaching 100μm. A 31cm x 31cm (100μm) FPD using IGZO TFTs was evaluated at 25 frames/second (fps) in 1x1, 2x2, 3x3, and 4x4 binning. In the 1x1 standard noise configuration, the noise equivalent dose (NED) was 24nGy with a max linear dose (MLD) of 10uGy. The NED was reduced to 6.6nGy in the 2x2, 3.4nGy in the 3x3, and 2.4nGy in the 4x4 mode. The linearity of the IGZO imager was comparable to a-Si imager. The 1x1 MTF was 57.5% at 1 lp/mm and 28.5% at 2lp/mm. The quantum limited DQE in the 1x1 binning mode was 79% at 0 lp/mm and 47% at 1 lp/mm. The 1x1 DQE measured at NED was 71% at 0 lp/mm, 29% at 1 lp/mm. This paper will explore how to optimally employ IGZO and present data from a first IGZO imager, showing that IGZO is an excellent technology for the future of FPDs.

Proceedings Article | 9 March 2018 Presentation + Paper

X-ray performance of new high dynamic range CMOS detector

Arundhuti Ganguly, P. Gerhard Roos, Tom Simak, J. Michael Yu, Steven Freestone, Donald Hondongwa, Richard Colbeth, Ivan Mollov

Proceedings Volume 10573, 105730N (2018) https://doi.org/10.1117/12.2293771

KEYWORDS: Sensors, X-rays, CMOS sensors, Imaging systems, X-ray imaging, X-ray detectors, Modulation transfer functions, Scintillators, Image resolution, Semiconducting wafers

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A new high dynamic range CMOS x-ray detector is described. This sensor was designed specifically for x-ray imaging as opposed to the common approach of modifying a 3T optical sensor design. This allowed for a highly linear, wide dynamic range operation that has otherwise been a major drawback of CMOS x-ray detectors. The design is scalable from small tiles to large wafer-scale imagers fabricated on 300mm wafers. The performance of such a detector built using a 9.4cm x 9.4cm tile is reported. The pixel size of this detector is 76 μm and it can be operated in the native resolution or 2x2 binned mode. Measurements were performed with a thallium-doped cesium iodide (CsI(Tl)) scintillator deposited on a reflective aluminum substrate. The imager was operated at 30 frames/second. The linearity, dynamic range, sensitivity, MTF, NPS and DQE at RQA5 were measured using the standard protocols. Linearity was measured to be better than 0.2%. Using 600 μm CsI(Tl) scintillator, the maximum linear dose was 9 μGy with high gain and 56 μGy with low gain settings. This is comparable to conventional amorphous silicon flat panel detectors. The MTF is dominated by the scintillator and is 58% at 1 lp/mm and 28% at 2 lp/mm. The DQE is 70% at 0 lp/mm and 12% at the Nyquist frequency of 6.6 lp/mm. The high resolution combined with the large dynamic range and excellent DQE makes this CMOS detector particularly suitable for dynamic imaging including fluoroscopy, angiography and conebeam CT.

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