Beams-eye-view imaging applications such as real-time soft-tissue motion estimation are hindered by the inherently low image contrast of electronic portal imaging devices (EPID) currently available for clinical use. We introduce and characterize a novel EPID design that provides substantially increased detective quantum efficiency (DQE), contrast-to-noise ratio (CNR) and sensitivity without degradation in spatial resolution. The prototype design features a stack of four conventional EPID layers combined with low noise integrated readout electronics. Each layer consists of a copper plate, a scintillator ([Formula: see text]) and a photodiode/TFT-switch (aSi:H). We characterize the prototype's signal response to a 6 MV photon beam in terms of modulation transfer function (MTF), DQE and CNR. The presampled MTF is estimated using a slanted slit technique, the DQE is calculated from measured normalized noise power spectra (nNPS) and the MTF and CNR is estimated using a Las Vegas contrast phantom. The prototype has been designed and built to be interchangeable with the current clinical EPID on the Varian TrueBeam platform (AS-1200) in terms of size and data output specifications. Performance evaluation is conducted in absolute values as well as in relative terms using the Varian AS-1200 EPID as a reference detector. A fivefold increase of DQE(0) to about 6.7% was observed by using the four-layered design versus the AS-1200 reference detector. No substantial differences are observed between each layer's individual MTF and the one for all four layers operating combined indicating that defocusing due to beam divergence is negligible. Also, using four layers instead of one increases the signal to noise ratio by a factor of 1.7.

/pdf/a-novel-epid-design-for-enhanced-contrast-and-detective-3bogxuszmx.pdf

A novel EPID design for enhanced contrast and detective quantum efficiency.

Purpose:
Electronic portal imagers
(EPIDs)
with high detective quantum efficiencies (DQEs) are sought to facilitate
the use of the megavoltage (MV) radiotherapy treatment beam for image guidance.
Potential advantages include high quality (treatment) beam’s eye view
imaging, and improved cone-beam computed
tomography
(CBCT)
generating images with more accurate electron density maps with immunity
to metal artifacts. One approach to increasing detector sensitivity
is to couple a thick pixelated scintillator array to an active matrix flat panel
imager (AMFPI) incorporating amorphous silicon thin film
electronics. Cadmium tungstate (CWO) has many desirable scintillation properties
including good light output, a high index of refraction, high optical
transparency, and reasonable cost. However, due to the 
cleave plane inherent in its crystalline
structure, the difficulty of cutting and polishing CWO has, in part, limited its
study relative to other scintillators such as cesium iodide and bismuth germanate
(BGO). The goal of this work was to build and test a focused large-area pixelated
“strip” CWO detector.
0 1 0
Methods:
A 361  ×  52 mm scintillator assembly that contained a total of 28 072 pixels was
constructed. The assembly comprised seven subarrays, each 15 mm thick. Six of the
subarrays were fabricated from CWO with a pixel pitch of 0.784 mm, while one array
was constructed from BGO for comparison. Focusing was achieved by coupling the
arrays to the Varian AS1000 AMFPI through a piecewise linear arc-shaped fiber
optic plate. Simulation and experimental studies of modulation transfer
function
(MTF) and
DQE were undertaken using a 6 MV beam, and comparisons were made between the
performance of the pixelated strip assembly and the most common EPID configuration
comprising a 1 mm-thick copper build-up plate attached to a 133 mg/cm2
gadolinium oxysulfide scintillator screen (Cu-GOS). Projection radiographs and
CBCT
images of phantoms were acquired. The work also introduces the
use of a lightweight edge phantom to generate MTF measurements at MV
energies and shows its functional equivalence to the more cumbersome slit-based
method.
Results:
Measured and simulated DQE(0)’s of the pixelated CWO detector were 22% and
26%, respectively. The average measured and simulated ratios of CWO
DQE(f) to Cu-GOS DQE(f) across the frequency
range of 0.0–0.62 mm−1 were 23 and 29, respectively. 2D and 3D
imaging studies confirmed the large dose efficiency
improvement and that focus was maintained across the field of view. In the CWO
CBCT
images, the measured spatial resolution was
7 lp/cm. The contrast-to-noise ratio was dramatically improved reflecting a
22 × sensitivity increase relative to Cu-GOS. The CWO scintillator material showed
significantly higher stability and light yield than the BGO material.
Conclusions:
An efficient piecewise-focused pixelated strip scintillator for MV imaging is
described that offers more than a 20-fold dose efficiency improvement over
Cu-GOS.

A piecewise-focused high DQE detector for MV imaging

Purpose
A novel Megavoltage (MV) multi-layer imager (MLI) design featuring higher detective quantum efficiency and lower noise than current conventional MV imagers in clinical use has been recently reported Optimization of the MLI design for multiple applications including tumor tracking, MV-CBCT and portal dosimetry requires a computational model that will provide insight into the physics processes that affect the overall and individual components’ performance The purpose of the current work was to develop and validate a comprehensive computational model that can be used for MLI optimization

Methods
The MLI model was built using the Geant4 Application for Tomographic Emission (GATE) application The model includes x-ray and charged particle interactions as well as the optical transfer within the phosphor A first prototype MLI device featuring a stack of four detection layers was used for model validation Each layer of the prototype contains (copper buildup sheet, phosphor screen, photodiode array) The model was validated against measured data of Modulation Transfer Function (MTF), Noise Power Spectrum (NPS) and Detective Quantum Efficiency (DQE) MTF was computed using a slanted slit with 23° angle and 01 mm width NPS was obtained using the autocorrelation function technique DQE was calculated from MTF and NPS data The comparison metrics between simulated and measured data were the Pearson's correlation coefficient (r) and the normalized root-mean-square error (NRMSE)

Results
Good agreement between measured and simulated MTF and NPS values was observed Pearson's correlation coefficient for the combined signal from all layers of the MLI was equal to 09991 for MTF and 09992 for NPS; NRMSE was 00121 for MTF and 00194 for NPS Similarly, the DQE correlation coefficient for the combined signal was 09888 and the NRMSE was 00686

Conclusions
A comprehensive model of the novel MLI design was developed using the GATE toolkit and validated against measured MTF, NPS and DQE data acquired with a prototype device featuring four layers This model will be used for further optimization of the imager components and configuration for clinical radiotherapy applications

This article is protected by copyright All rights reserved

/pdf/a-novel-multilayer-mv-imager-computational-model-for-31xmih909d.pdf

A novel multilayer MV imager computational model for component optimization

We have developed a Monte Carlo computational model of a clinically employed electronic portal imaging device (EPID), and demonstrated the impact of phosphor optical properties on the imaging performance. The EPID model was built with Geant4 application for tomographic emission. Both radiative and optical transport were included in the model. Modulation transfer function (MTF), normalized noise-power spectrum times the incident x-ray fluence (qNNPS), and detective quantum efficiency (DQE) were calculated for simulated and measured data, and their agreement was quantified by the normalized root-mean-square error (NRMSE). MTF was computed using a 100 µm wide slit tilted by 1.5° and qNNPS was estimated using the Fujita-Lubberts-Swank method. DQE was calculated from MTF and qNNPS data. The NRMSE value was 0.0467 for MTF, 0.0217 for qNNPS, and 0.0885 for DQE, showing good agreement between measurement and simulation. Five major optical properties, phosphor grain size, phosphor thickness, phosphor refractive index, binder refractive index, and packing ratio were tested for their influence on the qNNPS, MTF, and DQE(0) of the model. Generally, the effect on the qNNPS is greater than MTF, and no impact on DQE(0), except from phosphor thickness, was observed. Multiple applications, such as imager design optimization and investigations of the dosimetric performance, are expected to benefit from the validated model.

/pdf/a-monte-carlo-study-of-the-impact-of-phosphor-optical-3w7xx15hox.pdf

A Monte Carlo study of the impact of phosphor optical properties on EPID imaging performance

The performances of the pixelated CsI(Tl) scintillation screens based on oxidized silicon micro-pore array templates with different CsI(Tl) micro-column shapes and array structures in X-ray imaging were simulated using the Geant4 Monte Carlo simulation code. The shapes of the micro-columns include square, hexagonal and circular, and the array structures include square and hexagonal arrangements. The pitch size of the pixelated CsI(Tl) scintillation screens was set to 4 µm, and the incident X-ray energy was set to 20 keV. The ratios of the number of scintillation photons that propagate along the CsI(Tl) micro-columns to the total number of scintillation photons of the micro-columns gradually decrease with the increase in total reflection time on the lateral surfaces of the micro-columns. However, these ratios are closely related to the shapes of the micro-columns and the incident positions of X-ray on the cross-sections of the micro-columns, especially for the circular micro-column. The sequence of bottom light outputs stimulated by a uniform flood field of X-ray from high to low corresponds to the circular, square and hexagonal CsI(Tl) micro-columns with the same cross-section areas. In addition, all spatial resolutions in terms of modulation transfer functions (MTFs) for the pixelated CsI(Tl) scintillation screens with square and hexagonal array structures are over 100 lp/mm. However, the resolution for the pixelated screen with the hexagonal array structure is approximately 8.5% higher than that for the screen with the square array structure. Moreover, the former screen has a higher detective quantum efficiency (DQE) than the latter screen at the same thickness. The pixelated CsI(Tl) scintillation screen with circular micro-column and hexagonal array structure in X-ray imaging has superior performance compared to other pixelated screens in this work.

/pdf/simulated-performances-of-pixelated-csi-tl-scintillation-2b2n2g2q1g.pdf

Simulated performances of pixelated CsI(Tl) scintillation screens with different micro-column shapes and array structures in X-ray imaging.

Purpose: Performance optimization of indirect x-ray detectors requires proper characterization of both ionizing (gamma) and optical photon transport in a heterogeneous medium. As the tool of choice for modeling detector physics, Monte Carlo methods have failed to gain traction as a design utility, due mostly to excessive simulation times and a lack of convenient simulation packages. The most important figure-of-merit in assessing detector performance is the detective quantum efficiency (DQE), for which most of the computational burden has traditionally been associated with the determination of the noise power spectrum (NPS) from an ensemble of flood images, each conventionally having 107 − 109 detected gamma photons. In this work, the authors show that the idealized conditions inherent in a numerical simulation allow for a dramatic reduction in the number of gamma and optical photons required to accurately predict the NPS. Methods: The authors derived an expression for the mean squared error (MSE) of a simulated NPS when computed using the International Electrotechnical Commission-recommended technique based on taking the 2D Fourier transform of flood images. It is shown that the MSE is inversely proportional to the number of flood images, and is independent of the input fluence provided that the input fluence is above a minimal value that avoids biasing the estimate. The authors then propose to further lower the input fluence so that each event creates a point-spread function rather than a flood field. The authors use this finding as the foundation for a novel algorithm in which the characteristic MTF(f), NPS(f), and DQE(f) curves are simultaneously generated from the results of a single run. The authors also investigate lowering the number of optical photons used in a scintillator simulation to further increase efficiency. Simulation results are compared with measurements performed on a Varian AS1000 portal imager, and with a previously published simulation performed using clinical fluence levels. Results: On the order of only 10–100 gamma photons per flood image were required to be detected to avoid biasing the NPS estimate. This allowed for a factor of 107 reduction in fluence compared to clinical levels with no loss of accuracy. An optimal signal-to-noise ratio (SNR) was achieved by increasing the number of flood images from a typical value of 100 up to 500, thereby illustrating the importance of flood image quantity over the number of gammas per flood. For the point-spread ensemble technique, an additional 2× reduction in the number of incident gammas was realized. As a result, when modeling gamma transport in a thick pixelated array, the simulation time was reduced from 2.5 × 106 CPU min if using clinical fluence levels to 3.1 CPU min if using optimized fluence levels while also producing a higher SNR. The AS1000 DQE(f) simulation entailing both optical and radiative transport matched experimental results to within 11%, and required 14.5 min to complete on a single CPU. Conclusions: The authors demonstrate the feasibility of accurately modeling x-ray detector DQE(f) with completion times on the order of several minutes using a single CPU. Convenience of simulation can be achieved using GEANT4 which offers both gamma and optical photon transport capabilities.

/pdf/rapid-monte-carlo-simulation-of-detector-dqe-f-2jolz06lv4.pdf

Rapid Monte Carlo simulation of detector DQE(f).

An imaging device includes: a scintillator layer; and an array of photodiode elements; wherein the scintillator layer is configured to receive radiation that has passed through the array of photodiode elements. An imaging device includes: a scintillator layer having a plurality of scintillator elements configured to convert radiation into photons; and an array of photodiode elements configured to receive photons from the scintillator layer, and generate electrical signals in response to the received photons; wherein at least two of the scintillator elements are separated by an air gap. An imaging device includes: a first scintillator layer having a plurality of scintillator elements arranged in a first plane; and a second scintillator layer having a plurality of scintillator elements arranged in a second plane; wherein the first scintillator layer and the second scintillator layer are arranged next to each other and form a non-zero angle relative to each other.

High dqe imaging device

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Rapid Monte Carlo simulation of detector DQE(f).

High dqe imaging device