Volume 34, Issue 6, June 2007
Index of content:
- Imaging Scientific Session: L100J
- X‐ray Imaging‐New Detectors, Techniques
MO‐E‐L100J‐01: Enhancement of Signal Performance Through Innovative Pixel Design for Indirect Detection Active Matrix Flat‐Panel Arrays34(2007); http://dx.doi.org/10.1118/1.2761257View Description Hide Description
Purpose: Active matrix flat‐panel imager technology is being introduced to an ever‐increasing variety of imaging applications. This has been greatly facilitated by improvements in optical signal collection achieved through innovations in array pixel design. Such improvements are particularly important for applications involving low exposures and high spatial frequencies, where DQE can be strongly attenuated due to relatively high additive electronic noise. In this presentation, recent innovations to pixel design are described and the effect on various signal properties, as determined through measurements and calculations related to novel prototypes, is reported. Method and Materials: Optical and radiation signal measurements were performed on prototype imagers incorporating a series of six increasingly sophisticated array designs, with pixel pitches ranging from 75 to 127 μm. The most recent discrete photodiode array employs aggressive design rules that significantly increase optical fill factor while also incorporating a clamp circuit. Two other array designs incorporate a continuous photodiode structure with the goal of providing an optical fill factor of 100%. Results: The new 127 μm pitch discrete photodiode design achieves a sensitivity consistent with its design goal of an ∼85% optical fill factor while the 75 and 90 μm pitch continuous photodiode designs demonstrate sensitivity corresponding to an ∼95% fill factor. These designs demonstrated no degradation in MTF due to charge sharing and the pixel clamp reduces memory effects at high signal levels. The effects of these enhanced sensitivities on DQE are illustrated and compared to the performance of arrays based on earlier generations of design. Conclusion: Aggressive application of design rules can achieve very high levels of optical fill factor and sensitivity resulting in improvements in DQE. Continuous photodiode designs extend optical fill factors almost to the theoretical limit, even for very small pixels, and are well suited to designs containing more complex pixel circuits.
MO‐E‐L100J‐02: Limits On Achievable Performance Levels for Active Matrix Flat Panel Imagers Incorporating Active Pixel Sensor Architectures34(2007); http://dx.doi.org/10.1118/1.2761258View Description Hide Description
Purpose: The relatively high level of additive noise in active matrix flat‐panel imagers (AMFPIs) leads to significant loss of DQE under conditions of low exposures per frame and/or high spatial frequencies. One promising method for dealing with such performance loss involves in‐pixel signal amplification through incorporation of additional circuitry to each pixel — a concept generally referred to as an active pixel sensor. In this presentation, an examination of performance levels that can be achieved with active pixel sensors based on thin‐film electronics will be presented. Method and Materials: Calculations of the DQE of various active pixel sensor designs were performed using the cascaded systems formalism. Inputs to the calculations come from empirical measurements, published data, analytic calculations, and results derived from detailed circuit simulations. The calculations were performed for a series of designs, under fluoroscopic irradiation conditions, and using various readout protocols. Results: Our model calculations indicate that, under conditions of low exposure and/or high spatial frequency, substantial DQE restoration can be achieved via a combination of in‐pixel amplification circuitry and correlated double sampling protocols — each specifically selected to eliminate specific noise contributions. In particular, it appears feasible to reduce thermal noise associated with the pixel reset TFT and preamplifier noise to negligible levels and thereby increase DQE. However, the degree of performance improvement that can be practically realized will also depend upon the quality and properties of the thin film transistors used in the pixel circuit as well as selection of circuits that will yield well in manufacture. Conclusion: The degree of performance improvement made possible through array architectures comprising active pixel sensors is critically dependent on many factors including the properties of the transistors, the engineering rules governing pixel and array design, the choice of pixel circuit, and the method of array readout.
34(2007); http://dx.doi.org/10.1118/1.2761259View Description Hide Description
Purpose: Most electronic portal imaging devices (EPIDs) developed so far use a Cu plate/phosphor screen to absorb x rays. The main problem with this approach is that the Cu plate/phosphor screen must be thin (∼ 2 mm) in order to obtain a high spatial resolution, resulting in a low quantum efficiency (QE) for megavoltage (MV) x rays (typically 2–4%). In addition, the phosphor screen contains high atomic number (high‐Z) materials, resulting in an over‐response of the detector to low energy x rays in dosimetric verification. Our goal is to develop a new high QE MV x‐ray detector made of a low‐Z material for both geometrical and dosimetric verification in radiotherapy. Method and Materials: Our approach uses radiation‐induced light (Cherenkov radiation) in optical fibers that are made of low‐Z materials. With our approach, a thick (∼ 10–30 cm) fiber‐optic taper consisting of a matrix of optical fibers aligned with the incident x rays is used to replace the thin Cu plate/phosphor screen to dramatically increase the QE. The feasibility of this approach has been investigated using a single optical fiber embedded in a solid material. The spatial resolution expressed by the modulation transfer function(MTF) and the signal‐to‐noise ratio of the proposed detector at low doses (∼ one Linac pulse) have been measured. Results: It is predicted that, using this approach, a detective quantum efficiency (DQE) of an order of magnitude higher at zero frequency can be obtained while maintaining a reasonable MTF, as compared to current EPIDs. Conclusion: This work demonstrated the feasibility of using Cherenkov radiation for portal imaging applications [Work supported by the Individual Discovery Grant Program awarded by National Sciences and Engineering Research Council of Canada (NSERC)].
MO‐E‐L100J‐04: Scatter Rejection and Low‐Contrast Performance of a Slot‐Scan Digital Chest Radiography System with Electronic Aft‐Collimation: A Phantom Study34(2007); http://dx.doi.org/10.1118/1.2761260View Description Hide Description
Purpose: To investigate scatter rejection and low‐contrast performance of an electronic aft‐collimation based slot‐scan imaging technique for chest imaging.Method and Materials: The slot‐scan imaging technique was implemented with a 1.6 cm wide fan‐beam and a modified flat‐panel (FP) detector for electronic aft‐collimation. During the scan, the leading edge line of the scanning fan‐beam is reset to erase the scatter accumulated while the trailing edge line is read out to acquire the image signals following the fan‐beam exposure. Two images acquired with the same techniques were subtracted from each other for measuring the noise levels. A 2‐mm thick lead plate with a 2‐D array of holes was used to measure the primary signals which were then subtracted from those obtained without the lead plate to determine the scatter components. A 2‐D array of aluminum beads (3mm in diameter) is used as the low‐contrast objects to measure the contrast ratios (CRs) and contrast‐to‐noise ratios (CNRs) for evaluation of the low‐contrast performance in chest images.Results: The slot‐scan imaging method resulted in lower average scatter‐to‐primary ratios (SPRs) and improved CRs, primary signal‐to‐noise ratios (PSNRs) and CNRs than the anti‐scatter grid method. Slot‐scan imaging used in conjunction with grid resulted in further reduction of the SPRs and improvement of CRs. However, in most cases, the PSNRs were degraded and CNRs were at the same level or even degraded. Conclusion: Slot‐scan imaging with electronic aft‐collimation can effectively reject scatter without having to attenuate the primary x‐rays. It resulted in almost doubled CNR improvement as compare to the anti‐scatter grid method. Further improvement by using slot‐scan imaging with an anti‐scatter grid was found to be limited. Conflict of Interest: This work was supported in part by a research grant EB00117 from the NIH‐NIBIB and the research grants CA51248 and CA104759 from the NIH‐NCI.
MO‐E‐L100J‐05: High Quantum Efficiency Portal Imaging Using a Structured Fiber‐Optic Scintillation Glass Array (FOSGA)34(2007); http://dx.doi.org/10.1118/1.2761261View Description Hide Description
Purpose: Preliminary studies of a high quantum efficiency (QE) portal imager based on a fiber‐optic scintillation glass array (FOSGA) within a focused polymer‐tungsten grid and coupled to a‐Si photodiodes are presented. These include characterization of intrinsic imaging performance using Monte Carlo simulations and validation of mechanical fabrication accuracy and automated glass fiber insertion. Method and Materials: The scintillator array comprises of high‐density Tb‐activated scintillation glass (proprietary material composition) fibers drawn into uniform fiber‐optic bundles. The polymer‐tungsten grid is constructed using tomo‐lithographic molding (TLM), a patent‐pending manufacturing technology that can create precisely focused grids using cast‐molding and stack‐lamination. Glass fibers are inserted using an automated loading jig based on electrostatic alignment and vaccum gradients for precise pixel loading. Monte Carlo simulations were used to model the effects of scintillator geometry (thickness and fill factor) on intrinsic detector performance indicated by the modulation transfer function(MTF), and detective quantum efficiency (DQE). Results: Results indicated that DQE(0) increased and MTF decreased with scintillator thickness and fill factor. However, upon calculating DQE(f) ≈ DQE(0)MTF(f)2, it was found that the effect of thickness and fill factor on DQE(0) was more significant than that on the MTF. Intrinsic QE = 0.43 and DQE(0) = 0.25 could be obtained for a 5 cm thick scintillator array with a fill factor ⩾ 70%. Conclusions: FOSGA provides high QE due to good x‐ray absorption, scintillation, and fiber‐optic coupling characteristics of Tb‐glass fibers, while the polymer‐tungsten grid improves spatial resolution by limiting the spread of secondary electrons leading to greater MTF and DQE. Based on high QE, high DQE megavoltage imaging in conjunction with cost‐effective methodologies for accurate (±3%) mass‐production of large‐field uniform detector arrays, FOSGA is well suited for 2D localization and verification imaging as well as megavoltage computed tomography (MVCT) for image guided radiation therapy(IGRT).
MO‐E‐L100J‐06: Slot Scan Imaging with a High Frame Rate Flat Panel Detector‐Measurement and Correction for In‐Slot Scatter34(2007); http://dx.doi.org/10.1118/1.2761262View Description Hide Description
Purpose: To report on the use of a new scatter measurement technique to characterize the scatter rejection properties and to further correct for the in‐slot scatter in slot scan imaging with a high frame rate flat panel detector.Method and Materials: A new slot scan imaging technique was developed with a high frame rate (7.5 fps) flat panel detector. 300 images were acquired as an x‐ray fan beam, defined by a slot collimator placed in front of the tube, was scanned across an anthropomorphic chest phantom. Time‐intensity curves were formed from the image sequence for all pixels. They were integrated over the fan beam passing frames to form the scatter rejected image signals and over all frames to form the open field image signals. By assuming the primary component to be constant during the fan beam exposure, the scatter components were extracted from the time‐intensity curves to estimate the in‐slot scatter using an iterative background estimation algorithm. The results were used to measure the scatter signals and to correct for in‐slot scatter. Results: Slot scan images of an anthropomorphic chest phantom were acquired and formed for various projected fan beam widths. 2D maps of scatter‐to‐primary ratios and contrast‐to‐noise ratio (CNR) improvement factors were generated and plotted for the slot‐scan images and compared with those for the full field, non‐grided images . In the lungs, CNR was improved by 30 to 50%, and in the mediastinum, CNR was improved by 60 to 150%. Conclusion: We have described and demonstrated the use of a new scatter measurement technique to estimate and correct for the in‐slot scatter on a pixel‐by‐pixel basis in slot scan imaging with a high frame rate flat panel detector.
This work was supported in part by research grants EB00117 from the NIH‐NIBIB and CA51248 and CA104759 from the NIH‐NCI.
34(2007); http://dx.doi.org/10.1118/1.2761263View Description Hide Description
Purpose: Current CT scanners collect the projection images by a step‐and‐shoot process using a single x‐ray source. The inefficient serial data collection scheme severely limits the data collection speed. Multiplexing technique, which has been widely adopted in communication devices, holds the promise to significantly increase the data throughput. It however, has not been applied to x‐ray radiography, mainly due to limitations of the current x‐ray source technology. Method and Materials: We demonstrated the feasibility of multiplexingradiography technique based on the frequency division multiplexing (FDM) principle and the multi‐pixel x‐ray technology. The carbon nanotube based field emission multi‐pixel x‐ray source can generate spatially and temporally modulated x‐ray radiation. During the multiplexingimaging process, all the x‐ray pixels were turned on simultaneously with each beam modulated at a different frequency. The superimposed x‐ray signals were captured by an x‐ray detector and then demultiplexed to recover the original nine projection images from different view angles. Results: In general a factor of N/2 (N= total number of images) increase in the speed can be achieved using the multiplexing scheme. This becomes significant when N is large, for example for clinical CT scanners which use ∼1000 views per gantry rotation. On the other hand, if the total imaging time and x‐ray dose are kept the same as used in the sequential process, then the x‐ray power, i.e. the tube current, can be reduced by a factor of N/2 by multiplexing because the exposure time per image is now longer. Conclusion: In summary, we show the feasibility of multiplexingradiography that enables simultaneous collection of multiple projection images. Overall the experiment has sufficiently demonstrated the efficiency of multiplexing for data collection compared to the current serial approach. It has the potential to significantly increase the imaging speed for CT scanning without compromising the imaging quality.