Daniel Shedlock

Synthesis and characterization of a BaGdF5:Tb glass ceramic as a nanocomposite scintillator for x-ray imaging.

Transparent glass ceramics with embedded light-emitting nanocrystals show great potential as low-cost nanocomposite scintillators in comparison to single crystal and transparent ceramic scintillators. In this study, cubic structure BaGdF5:Tb nanocrystals embedded in an aluminosilicate glass matrix are reported for potential high performance MeV imaging applications. Scintillator samples with systematically varied compositions were prepared by a simple conventional melt-quenching method followed by annealing. Optical, structural and scintillation properties were characterized to guide the design and optimization of selected material systems, aiming at the development of a system with higher crystal volume and larger crystal size for improved luminosity. It is observed that enhanced scintillation performance was achieved by tuning the glass matrix composition and using GdF3 in the raw materials, which served as a nucleation agent. A 26% improvement in light output was observed from a BaGdF5:Tb glass ceramic with addition of GdF3.

Prototype 1.75 MV X-band linear accelerator testing for medical CT and industrial nondestructive testing applications

Flat panel imagers based on amorphous silicon technology (a-Si) for digital radiography are accepted by the medical and industrial community as having several advantages over radiographic film-based systems. Use of Mega-voltage x-rays with these flat panel systems is applicable to both portal imaging for radiotherapy and for nondestructive testing (NDT) and security applications. In the medical field, one potential application that has not been greatly explored is to radiotherapy treatment planning. Currently, such conventional computed tomographic (CT) data acquired at kV energies is used to help delineate tumor targets and normal structures that are to be spared during treatment. CT number accuracy is crucial for radiotherapy dose calculations. Conventional CT scanners operating at kV X-ray energies typically exhibit significant image reconstruction artifacts in the presence of metal implants in human body. Using the X-ray treatment beams, having energies typically ≥6MV, to acquire the CT data may not be practical if it is desired to maintain contrast sensitivity at a sufficiently low dose. Nondestructive testing imaging systems can expand their application space with the development of the higher energy accelerator for use in pipeline, and casting inspection as well as certain cargo screening applications that require more penetration. A new prototype x-band BCL designed to operate up to 1.75 MV has been designed built and tested. The BCL was tested with a prototype portal imager and medical phantoms to determine artifact reductions and a PaxScan 2530HE industrial imager to demonstrate resolution is maintained and penetration is improved.

TH‐A‐141‐08: Instrument Design to Measure the Optical Properties of Reflectance and Transmittance

PURPOSE To estimate performance of crystaline scintillator-based pixelated detectors depending on the optical properties of the crystal surface (cut, etched, polished) and the septum material between each crystal. We propose a new device design to measure the optical reflectance and transmittance properties of candidate crystal-septum structures. METHODS Using a measurement device including a laser and an arc of photodiode detectors, the reflectance of two sandwich samples that each mimic a pixel was measured: CdWCO4 -glue-ESR-glue-CdWCO4 and CdWO4 -glue-(Al-sputtered-ESR)-glue-CdWO4 . Reflectance was normalized to that of ESR (a multi-layer optical film, highly specular reflector). To provide the required range of incident light angles at the interface of interest, a BGO hemisphere with a high index of refraction (n=2.2) was glued (Meltmount, n=1.7) to the top surface of the sandwich. RESULTS The sandwich structure demonstrated constant reflectivity over all laser angles with reflectances of 95% and 60% for ESR and Al-sputtered-ESR sandwiches respectively. This is because the highest incident angle achieved at the crystal-glue interface was only 25.8 degrees due to the large difference in refractive indices between air (n=1) and CdWO4 (n=2.2), which is below the critical angle for total internal reflection. With the BGO hemisphere added, there are two boundaries where total internal reflection can occur, and three levels of reflectivity were detected with the steps corresponding to the critical angles of 45 degrees and 54 degrees. Normalization is required to remove the influence of the Meltmount and BGO crystal from the measured reflectivities. CONCLUSION The optical reflectance due to surface conditions and septum materials can be accurately measured via the addition of a high-refractive-index hemisphere to a sandwich structure that has equivalent characteristics to the finished pixel matrix. Reflectance and transmittance are both important to develop an efficient pixelated detector, therefore the instrument has been modified and being assembled to measure them. This work is supported by National Institutes of Health (NIH R01 CA138426) and the Richard M. Lucas Foundation. There is no conflict of interest to disclose.

TH-A-141-10: A Piecewise-Focused Pixelated Detector for MV Imaging

PURPOSE For portal imaging, high DQE detectors can be constructed from thick pixelated scintillator arrays that absorb MV x-rays. However, due to beam divergence, MTF and DQE losses can be significant for off-axis elements not focused towards the source. We present a novel focusing approach based on situating a shaped fiber optic plate (FOP) between rectilinear scintillator arrays and an amorphous silicon flat panel imager (AMFPI). METHODS The entire FOP comprises seven wedge-shaped sections that are fused together so that the center of each section points towards the source focal spot which is located 1500 mm away. The arc-shaped FOP directs light from the scintillator assembly to the AMFPI. The scintillator assembly consists of seven identical rectilinear sub-arrays, each with a 1 degree bevel, that are close-packed end-to-end. Each 15mm thick CdWO4 sub-array comprises 66×66 elements with a pixel pitch of 0.784 mm resulting in a piecewise-focused area detector having dimensions 365 mm × 52 mm. RESULTS Monte Carlo simulations of radiative and optical transport with a 6MV source predict a DQE(0) of 23%. With no beam divergence, MTF and DQE values at a spatial frequency of 0.4mm-1 are 0.5 and 15% respectively. Maximum off-axis MTF and DQE losses occur at the subarray edges (divergence angle =1 degree) and are only 6% and 12% respectively at 0.4mm-1 . If the detector were not focused, MTF and DQE losses at 0.4mm-1 would be 30% and 85% respectively. These losses would occur at edge of the detector where the beam divergence is 7 degrees. CONCLUSION A novel approach to creating a focused detector for MV portal, cone-beam or helical CT imaging is presented. Compared to previously proposed designs, all the block arrays are identical and rectilinear scintillator thus reducing costs and simplifying manufacturing processes. Assembly and experimental measurements are underway. NIH Academic-Industrial Partnership NIH RO1 CA138426; Varian Medical Systems.

1D pixelated MV portal imager with structured privacy film: a feasibility study

Modern amorphous silicon flat panel-based electronic portal imaging devices that utilize thin gadolinium oxysulfide scintillators suffer from low quantum efficiencies (QEs). Thick two dimensionally (2D) pixelated scintillator arrays offer an effective but expensive option for increasing QE. To reduce costs, we have investigated the possibility of combining a thick one dimensional (1D) pixelated scintillator (PS) with an orthogonally placed 1D structured optical filter to provide for overall good 2D spatial resolution. In this work, we studied the potential for using a 1D video screen privacy film (PF) to serve as a directional optical attenuator and filter. A Geant4 model of the PF was built based on reflection and transmission measurements taken with a laser-based optical reflectometer. This information was incorporated into a Geant4-based x-ray detector simulator to generate modulation transfer functions (MTFs), noise power spectra (NPS), and detective quantum efficiencies (DQEs) for various 1D and 2D configurations. It was found that the 1D array with PF can provide the MTFs and DQEs of 2D arrays. Although the PF significantly reduced the amount of optical photons detected by the flat panel, we anticipate using a scintillator with an inherently high optical yield (e.g. cesium iodide) for MV imaging, where fluence rates are inherently high, will still provide adequate signal intensities for the imaging tasks associated with radiotherapy.

A novel method for quantification of beam's‐eye‐view tumor tracking performance

Purpose: In‐treatment imaging using an electronic portal imaging device (EPID) can be used to confirm patient and tumor positioning. Real‐time tumor tracking performance using current digital megavolt (MV) imagers is hindered by poor image quality. Novel EPID designs may help to improve quantum noise response, while also preserving the high spatial resolution of the current clinical detector. Recently investigated EPID design improvements include but are not limited to multi‐layer imager (MLI) architecture, thick crystalline and amorphous scintillators, and phosphor pixilation and focusing. The goal of the present study was to provide a method of quantitating improvement in tracking performance as well as to reveal the physical underpinnings of detector design that impact tracking quality. The study employs a generalizable ideal observer methodology for the quantification of tumor tracking performance. The analysis is applied to study both the effect of increasing scintillator thickness on a standard, single‐layer imager (SLI) design as well as the effect of MLI architecture on tracking performance. Methods: The present study uses the ideal observer signal‐to‐noise ratio (d′) as a surrogate for tracking performance. We employ functions which model clinically relevant tasks and generalized frequency‐domain imaging metrics to connect image quality with tumor tracking. A detection task for relevant Cartesian shapes (i.e., spheres and cylinders) was used to quantitate trackability of cases employing fiducial markers. Automated lung tumor tracking algorithms often leverage the differences in benign and malignant lung tissue textures. These types of algorithms (e.g., soft‐tissue localization – STiL) were simulated by designing a discrimination task, which quantifies the differentiation of tissue textures, measured experimentally and fit as a power‐law in trend (with exponent β) using a cohort of MV images of patient lungs. The modeled MTF and NPS were used to investigate the effect of scintillator thickness and MLI architecture on tumor tracking performance. Results: Quantification of MV images of lung tissue as an inverse power‐law with respect to frequency yields exponent values of β = 3.11 and 3.29 for benign and malignant tissues, respectively. Tracking performance with and without fiducials was found to be generally limited by quantum noise, a factor dominated by quantum detective efficiency (QDE). For generic SLI construction, increasing the scintillator thickness (gadolinium oxysulfide – GOS) from a standard 290 μm to 1720 μm reduces noise to about 10%. However, 81% of this reduction is appreciated between 290 and 1000 μm. In comparing MLI and SLI detectors of equivalent individual GOS layer thickness, the improvement in noise is equal to the number of layers in the detector (i.e., 4) with almost no difference in MTF. Further, improvement in tracking performance was slightly less than the square‐root of the reduction in noise, approximately 84–90%. In comparing an MLI detector with an SLI with a GOS scintillator of equivalent total thickness, improvement in object detectability is approximately 34–39%. Conclusions: We have presented a novel method for quantification of tumor tracking quality and have applied this model to evaluate the performance of SLI and MLI EPID designs. We showed that improved tracking quality is primarily limited by improvements in NPS. When compared to very thick scintillator SLI, employing MLI architecture exhibits the same gains in QDE, but by mitigating the effect of optical Swank noise, results in more dramatic improvements in tracking performance.

WE-DE-BRA-05: Monte Carlo Simulation of a Novel Multi-Layer MV Imager

PURPOSE To develop and validate a Monte Carlo (MC) model of a novel multi-layer imager (MLI) for megavolt (MV) energy beams. The MC model will enable performance optimization of the MLI design for clinical applications including patient setup verification, tumor tracking and MVCBCT. METHODS The MLI is composed of four layers of converter, scintillator and light detector, each layer similar to the current clinical AS1200 detector (Varian Medical Systems, Inc). The MLI model was constructed using the Geant4 Application for Tomographic Emission (GATE v7.1) and includes interactions for x-ray photons, charged particles and optical photons. Computational experiments were performed to assess Modulation Transfer Function (MTF), Detective Quantum Efficiency (DQE) and Noise Power Spectrum normalized by photon fluence and average detector signal (qNNPS). Results were compared with experimental measurements. The current work incorporates, in one model, the complete chain of events occurring in the imager; i.e. starting from x-ray interaction to charged particle transport and energy deposition to subsequent generation, interactions and detection of optical photons. RESULTS There is good agreement between measured and simulated MTF, qNNPS and DQE values. Normalized root mean squared error (NRMSE) between measured and simulated values over all four layers was 2.18%, 2.43% and 6.05% for MTF, qNNPS and DQE respectively. The relative difference between simulated and measured values for qNNPS(0) was 1.68% and 1.57% for DQE(0). Current results were obtained using a 6MV Varian Truebeam™ spectrum. CONCLUSION A comprehensive Monte Carlo model of the MLI prototype was developed to allow optimization of detector components. The model was assessed in terms of imaging performance using standard metrics (i.e. MTF, qNNPS, DQE). Good agreement was found between simulated and measured values. The model will be used to assess alternative detector constructions to facilitate advanced clinical imaging applications including MV-CBCT and tumor tracking. The project was supported, partially, by a grant from Varian Medical Systems, Inc., and Award No. R01CA188446-01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

SU-F-J-41: Experimental Validation of a Cascaded Linear System Model for MVCBCT with a Multi-Layer EPID

PURPOSE The purpose of this study was to validate the use of a cascaded linear system model for MV cone-beam CT (CBCT) using a multi-layer (MLI) electronic portal imaging device (EPID) and provide experimental insight into image formation. A validated 3D model provides insight into salient factors affecting reconstructed image quality, allowing potential for optimizing detector design for CBCT applications. METHODS A cascaded linear system model was developed to investigate the potential improvement in reconstructed image quality for MV CBCT using an MLI EPID. Inputs to the three-dimensional (3D) model include projection space MTF and NPS. Experimental validation was performed on a prototype MLI detector installed on the portal imaging arm of a Varian TrueBeam radiotherapy system. CBCT scans of up to 898 projections over 360 degrees were acquired at exposures of 16 and 64 MU. Image volumes were reconstructed using a Feldkamp-type (FDK) filtered backprojection (FBP) algorithm. Flat field images and scans of a Catphan model 604 phantom were acquired. The effect of 2×2 and 4×4 detector binning was also examined. RESULTS Using projection flat fields as an input, examination of the modeled and measured NPS in the axial plane exhibits good agreement. Binning projection images was shown to improve axial slice SDNR by a factor of approximately 1.4. This improvement is largely driven by a decrease in image noise of roughly 20%. However, this effect is accompanied by a subsequent loss in image resolution. CONCLUSION The measured axial NPS shows good agreement with the theoretical calculation using a linear system model. Binning of projection images improves SNR of large objects on the Catphan phantom by decreasing noise. Specific imaging tasks will dictate the implementation image binning to two-dimensional projection images. The project was partially supported by a grant from Varian Medical Systems, Inc. and grant No. R01CA188446-01 from the National Cancer Institute.

WE‐DE‐BRA‐07: Megavoltage Spectral Imaging with a Layered Detector

PURPOSE The aim of the current work is to investigate the feasibility of megavoltage spectral imaging using a multiple layered detector for enhancement of low contrast detectability through material segmentation and discrimination (such as bone, markers and metal implants). Potentially the technique can be applied to improve detection and reduce dose in Megavoltage Cone Beam Computed Tomography (MV-CBCT). METHODS Experiments were performed with a prototype multi-layer imager (MLI) which has higher detective efficiency and lower noise characteristics than conventional Electronic Portal Imaging Devices (EPIDs). Images of a solid water phantom were acquired at 2.5 MV, 6MV and 6MV without flattening filter (FFF). The following materials were placed within a stack of solid water: aluminum, copper and gold. Material separation was assessed based on Contrast-to-Noise Ratio (CNR) of the weighted image, formed by a weighted subtraction of the images from two layers of the MLI. A range of weighting factors were investigated for material separation. RESULTS CNR can be minimized for each material by appropriate selection of the subtraction weighting factor. This is equivalent to a selective subtraction of specific materials from the image. Using multiple layers simultaneously also decreases the dose requirement and removes any registration errors. The minimum CNR for aluminum, copper and gold at the weighted image formed with 2.5MV was obtained at weighting factors equal to 0.92, 0.76 and 0.64 respectively. The corresponding values at 6MVFFF were 0.99, 0.92 and 0.78 respectively. CONCLUSION In the current work, an MV spectral imaging feasibility study was attempted using a novel multi-layer prototype EPID imager. Initial results suggest that material separation based on spectral differences between different layers is possible. This spectral imaging technique has potential advantages in MV-CBCT for real-time target tracking, patient set-up imaging and adaptive radiotherapy. The project was supported, partially, by a grant from Varian Medical Systems, Inc., and Award No. R01CA188446-01 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Cancer Institute or the National Institutes of Health.

WE-DE-BRA-04: A Cost-Effective Pixelated EPID Scintillator for Enhanced Contrast and DQE.

PURPOSE Beams-eye-view imaging applications such as real-time soft-tissue motion estimation and MV-CBCT are hindered by the inherently low image contrast of electronic portal imaging devices (EPID) currently in clinical use. We investigate a cost effective scintillating glass that provides substantially increased detective quantum efficiency (DQE) and contrast to noise ratio (CNR). METHODS A pixelated scintillator prototype was built from LKH-5 glass. The array is 12mm thick; 42.4×42.4cm2 wide and features 1.51mm pixel pitch with 20µm separation (glue+septa). The LKH-5 array was mounted on the active matrix flat panel imager (AMPFI) of an AS-1200 (Varian) with the GdO2S2:Tb removed. A second AS-1200 was utilized as reference detector. The prototype EPID was characterized in terms of CNR, modulation transfer function (MTF) and DQE. Additionally, the visibility of various fiducial markers typically used in the clinic as well as a realistic 3D-printed lung tumor model was assessed. All items were placed in a 12cm thick solid water phantom. CNR is estimated using a Las Vegas contrast phantom, presampled MTF is estimated using a slanted slit technique and the DQE is calculated from measured normalized noise power spectra (NPS) and the MTF. RESULTS DQE(0) for the LKH-5 prototype increased by a factor of 8× to about 10%, compared to the AS-1200 equipped with its standard GdO2S2:Tb scintillator. CNR increased by a factor of 5.3×. Due to the pixel size the MTF50 decreased by about 55% to 0.23lp/mm. The visibility of all fiducial markers as well as the tumor model were however markedly improved in comparison to an acquisition with the same parameters using the GdO2S2:Tb scintillator. CONCLUSION LKH-5 scintillating glasses allow the cost effective construction of thick pixelated scintillators for portal imaging which can yield a substantial increase in DQE and CNR. Soft tissue and fiducial marker visibility was found to be markedly improved. The project was supported in part by NIH grant R01CA188446-01 and a grant from Varian Medical Systems.