Resolution of positron emission tomography (PET) systems benefits from information about depth of interaction (DOI)
within scintillation crystals, particularly in small bore scanners or parallel plate detectors. In this investigation, the ability
of the dual-ended readout detector module configuration to resolve DOI and crystal index was evaluated for a variety of
detector pitches and light guide thicknesses to validate the dual-ended readout method. Experimental results with oneto-
one coupling between saw-cut 2mm pitch LYSO scintillation crystals and silicon photomultipliers (SiPMs) achieved
2.1 mm DOI resolution. Monte Carlo simulations were used to investigate the effect of larger detector pitches and
varied light guide thickness on the crystal index identification accuracy and DOI resolution for a pixilated crystal array
in dual-ended readout configuration. It is reported that the accuracy in identifying a 2 mm scintillation crystal was >80%
for detector pitches < 6 mm and that DOI resolution was < 2 mm for all detector pitches and light guide thicknesses.
A single photon counting Voltage Controlled Oscillator (VCO) based pixel architecture in amorphous silicon (a-Si)
technology is reported for large area digital medical imaging. The VCO converts X-ray generated input charge into an
output oscillating frequency signal. Experimental results for an in-house fabricated VCO circuit in a-Si technology are
presented and external readout circuits to extract the image information from the VCO's frequency output are discussed.
These readout circuits can be optimized to reduce the fixed pattern noise and fringing effects in an imaging array
containing many such VCO pixels. Noise estimations, stability simulations and measurements for the fabricated VCO
are presented. The reported architecture is particularly promising for large area photon counting applications (e.g. low
dose fluoroscopy, dental computed tomography (CT)) due to its very low input referred electronic noise, high sensitivity
and ease of fabrication in low cost a-Si technology.
We detail the integration of amorphous silicon (a-Si) active pixel sensor (APS) test arrays with an overlying amorphous
selenium (a-Se) x-ray photoconductor, and report on results of their x-ray response and imaging properties. The
a-Se/a-Si APS arrays incorporate a two-transistor (2T) gate-switched pixel amplifier architecture designed to provide high
detector array resolution, as well as a controllable on-pixel gain. The direct x-ray detectors consist of in-house
fabricated, dual mode active and passive sensor arrays with detector element (del) pitches of 100 μm and 200 μm, coated
with 80 μm thick stabilized amorphous selenium. These selenium layers were selected for preliminary work and
represent a quantum efficiency (QE) of 69% for x-ray spectra (tungsten target, 2 mm Al filtration) of 30 kVp. Detector
response was evaluated for a-Se biasing electric fields of both 5 V/μm and 10 V/μm.
A detector dark current of 110 pA/cm2 (0.01 pA/100 μm del) at 10V/μm electric field, a controllable detector conversion
gain up to 15.3 nA/mR at 30 kVp were measured. Active pixel gains of 6.7 and 9.6 were measured for 100μm and
200μm pitch detectors respectively. The amplified readout exhibits a better detection limit (by one order of magnitude)
compared to the passive readout implemented on the same pixel. Capabilities of amplified pixels such as nondestructive
readout, as well as programmable pixel conversion gain, and dynamic range control are demonstrated. In light of their
adaptable gain and dynamic range, these detectors represent a promising technology for high-resolution high gain x-ray
digital imaging, particularly in mammography tomosynthesis.
Amplified Pixel Sensor (APS) architectures using two transistors per pixel are introduced in this research for digital
mammography tomosynthesis that requires high resolution and low noise imaging capability. The fewer number of on-pixel
elements and reduced pixel complexity result in a smaller pixel pitch and higher gain, which makes the two-transistor
(2T) APS architectures promising for high resolution, low noise and high speed digital imaging including
medical imaging modalities such as tomosynthesis and cone beam computed tomography. Measured results from in-house
fabricated test arrays using amorphous silicon (a-Si) thin film transistor (TFTs) are presented as well as driving
schemes for minimizing the threshold voltage metastability problem and increasing frame rate. The results indicate that a
pixel input referred noise value of down to 220 electrons is achievable with a 50μm pixel pitch a-Si 2T APS.
Cone beam computed tomography (CBCT) has been recently reported using flat panel imagers (FPI). Here, detector
technology capable of high speed imaging, high spatial resolution, large volume coverage, better contrast resolution and,
in particular, lowered patient dose is required. Employing active matrix flat panel imagers (AMFPIs) as cone beam CT
detectors has been proposed as a solution for improving volume coverage, contrast and resolution; however, clinical
evaluations have shown that they suffer from low speed read out.
Unlike passive pixel architecture which is currently the state-of-the-art technology for AMFPIs, our preliminary studies
have shown that novel amplified pixel sensor (APS) architectures can overcome the low readout speed, and moreover,
they provide gain which can be traded for higher frame rate and lower X-ray doses. Although APS architectures can
meet the high dynamic range and low noise requirements of CT imaging, linearity and variations between pixel
characteristics are major issues. In this study we will investigate novel APS architectures to address these concerns.
Amorphous silicon photodiodes are increasingly being used as fundamental components in digital diagnostic medical imaging system including large area chest radiography, mammography and real time fluoroscopy. The intrinsic a-Si:H material (i-a-Si:H), commonly deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), is well known to suffer from both light and bias stress induced instabilities over time that can result in an increase in dark current and a decrease in photoconductivity. In contrast, research in Hot-Wire Chemical Vapor Deposition (HWCVD) indicates that a-Si:H films grown by HWCVD can have superior physical and electronic properties to those grown by PECVD.
In this research, we report on the material properties and stability of i-a-Si:H material by comparing the photoconductivity degradation of the HWCVD and PECVD films over time. Then, we discuss the p-i-n diode fabrication process and examine the leakage and photo-current degradation in the HWCVD and PECVD photodiode structures over time via bias and time stress measurements. Also, we investigate the quantum efficiency degradation over time in a-Si:H p-i-n detectors grown by PECVD.
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