An interface has been developed to capture frames taken by X-ray array imagers up to 64×64 pixels. An application
specific integrated circuit (ASIC) designed solely for X-ray flat panel imaging readout circuitry, manufactured by
FLIR® called Indigo (also known as ISC9717), was used as part of charge-amplifier block. An Altera Cyclone II FPGA
is used to serve three purposes: Create pulses required for gate-driver block, Receive fast-stream data coming from the
Indigo chip, and Send data through RS-232 protocol over a serial cable to a personal computer.
Initial results for a 32×32 passive pixel sensor (PPS) with lateral amorphous Selenium metal-semiconductor-metal
(MSM) photodetector were presented in [1]. This work focuses more on methods used to improve the images obtained
from the array. Sharper images produced in sync with the light source are presented. In addition, insight into array
readout circuitry and capturing a frame from an array is discussed.
An array of ring voltage controlled oscillators (RVCO) aiming for photon quantum shot noise limited applications such
as protein crystallography is presented. The pixilated array consists of 24 by 21 RVCO pixels. RVCO pixel converts x-ray
generated input charge into an output oscillating frequency signal. This architecture can be used in both direct and
indirect detection schemes. In this paper the direct detection using a layer of amorphous selenium (a-Se) coupled with
the RVCO array is proposed. Theoretical and Experimental results for an in-house fabricated array of RVCOs in
amorphous silicon (a-Si) technology are presented. All different requirements for protein crystallography application are
listed in this paper and also the way this array addresses each of these requirements is discussed in details in this paper.
The off-panel readout circuitry, designed and implemented in-house, is given in this paper. The off-panel readout circuits
play an important role in the imaging applications using frequency based pixels. They have to be optimized in order to
reduce the fixed pattern noise and fringing effects in an imaging array containing many such RVCO pixels. Since the
frequency of oscillation of each of these pixels is in the range of 100 KHz, there is no antenna effect in the array.
Antenna effect becomes an important issue in other technologies such as poly silicon (poly-Si) and CMOS technologies
due to higher frequency of oscillation ranges (more than 100 MHz). Noise estimations, stability simulations and
measurements for some randomly selected pixels in the array for the fabricated RVCO array are presented. The reported
architecture is particularly promising for large area photon quantum shot noise applications, specifically protein
crystallography. However, this architecture can be used for low dose fluoroscopy, dental computed tomography (CT) and
other large area imaging applications limited by input referred electronic noise due to its very low input referred
electronic noise, high sensitivity and ease of fabrication in low cost a-Si technology.
Previously, we reported on a single-pixel detector based on a lateral a-Se metal-semiconductor-metal structure, intended
for indirect conversion X-ray imaging. This work is the continuous effort leading to the first prototype of an indirect
conversion X-ray imaging sensor array utilizing lateral amorphous selenium. To replace a structurally-sophisticated
vertical multilayer amorphous silicon photodiode, a lateral a-Se MSM photodetector is employed which can be easily
integrated with an amorphous silicon thin film transistor passive pixel sensor array. In this work, both 2×2 macro-pixel
and 32×32 micro-pixel arrays were fabricated and tested along with discussion of the results.
KEYWORDS: X-rays, Sensors, Imaging systems, Amorphous silicon, Signal to noise ratio, Prototyping, X-ray imaging, Amplifiers, Modulation transfer functions, Active sensors
Active pixel sensor (APS) circuits are an alternate to passive pixel sensor (PPS) technology which, when integrated with
a direct detection amorphous selenium (a-Se) photoconductor, can enable high performance, digital x-ray imaging
applications such as real-time fluoroscopy due to their better
signal-to-noise ratios at low dose. This paper presents
experimental imaging results from a prototype 64×64 APS pixel array fabricated in a-Si technology. The prototype APS
array is coated with a one millimeter thick layer of a-Se and the experimental results are evaluated using a standard
radiography x-ray beam quality RQA5. The APS experimental results are compared with a standard real-time detector
(FPD14) imaging array under the same x-ray beam conditions. In addition, we will theoretically examine the best
achievable performance for our APS array fabricated in
state-of-the-art a-Si technology and compare the results to
state-of-the-art PPS panels for real-time fluoroscopy.
A detailed experimental and theoretical investigation of noise in both current mode and voltage mode amorphous silicon
(a-Si) active pixel sensors (APS) has been performed. Both flicker (1/f) and thermal are considered in this study. The
experimental result in this paper emphasizes the computation of the output noise variance. The theoretical analysis shows
that the voltage mode APS has an advantage over the current mode APS in terms of the flicker noise due to the operation
of the readout process. The experimental data are compared to the theoretical analysis and are in good agreement.
A 2-TFT current-programmed, current-output active pixel sensor in amorphous silicon (a-Si:H) technology is introduced
for digital X-ray imaging, and in particular, for mammography tomosynthesis and fluoroscopy. Pixel structure, operation
and characteristics are presented. The proposed APS circuit was fabricated and assembled using an in-house bottom gate inverted staggered a-Si:H thin film transistor (TFT) process. Lifetime, transient performance as well as sensitivity to temperature measurements were carried out. An off-panel current amplifier with double sampling capability required for 1/f noise reduction is proposed and implemented in CMOS 0.18 micron technology. The results are promising and demonstrate that the proposed APS compensates for electrical and thermal stress causing shift in the threshold voltage of a-Si TFTs.
KEYWORDS: Signal to noise ratio, Sensors, Imaging arrays, Active sensors, Californium, Prototyping, Computer simulations, Switches, Amplifiers, Medical imaging
We present an analysis of output referred pixel electronic noise as a function of position in the active matrix array for
both active and passive pixel architectures. Three different noise sources for Active Pixel Sensor (APS) arrays are
considered: readout period noise, reset period noise and leakage current noise of the reset TFT during readout. For the
state-of-the-art Passive Pixel Sensor (PPS) array, the readout noise of the TFT switch is considered. Measured noise
results are obtained by modeling the array connections with RC ladders on a small in-house fabricated prototype. The
results indicate that the pixels in the rows located in the middle part of the array have less random electronic noise at the
output of the off-panel charge amplifier compared to the ones in rows at the two edges of the array. These results can
help optimize for clearer images as well as help define the
region-of-interest with the best signal-to-noise ratio in an
active matrix digital flat panel imaging array.
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.
KEYWORDS: Computer programming, X-rays, Sensors, Amorphous silicon, Imaging systems, Capacitors, Capacitance, Active sensors, Transistors, Signal to noise ratio
A dual mode current-programmed, current-output active pixel sensor (DCAPS) in amorphous silicon (a-Si:H) technology
is introduced for digital X-ray imaging, and in particular, for hybrid fluoroscopic and radiographic imagers. Here, each
pixel includes an extra capacitor that selectively is coupled to the pixel capacitance to realize the dual mode behavior.
Pixel structure, operation and characteristics are presented. The proposed DCAPS circuit was fabricated and assembled
using an in-house bottom gate inverted staggered a-Si:H thin film transistor (TFT) process. Gain, lifetime, transient
performance as well as noise analysis were carried out. The results are promising and demonstrate that the DCAPS
enables dual mode X-ray imaging while compensating for the long term electrical and thermal stress related a-Si TFT
threshold voltage (Vt) shift.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.