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Title: Thick, segmented CdWO{sub 4}-photodiode detector for cone beam megavoltage CT: A Monte Carlo study of system design parameters

Abstract

Megavoltage (MV) imaging detectors have been the focus of research by many groups in recent years. We have been working with segmented CdWO{sub 4} crystals in contact with photodiodes in our lab. The present study uses both x-ray and optical photon transport Monte Carlo simulations to analyze the effects of scintillation crystal height, septa material, beam divergence, and beam spectrum on the modulation transfer function, MTF(f) and zero frequency detective quantum efficiency, DQE(0), of a theoretical area detector. The theoretical detector is comprised of tall, segmented CdWO{sub 4} crystals and two dimensional photodiode arrays with a pitch of 1 mm and a fill factor of 72%. Increasing the crystal height above 10 mm does not result in an improvement in the DQE(0) if the reflection coefficient of the septa is less than 0.8. For a reflection coefficient of 0.975 for the septa, there is a continual gain in the DQE(0) up to 30 mm tall crystals. Similar calculations show that employing a 3.5 MV beam without a flattening filter increases the DQE(0) for 20 mm tall crystals by 9% compared to a typical 6 MV beam with a flattening filter. The severe degradations due to beam divergence on MTF(f) aremore » quantified and suggest the use of focused detectors in MV imaging. It is found that when the effect of optical photons is considered, the presence of divergence can appear as a shift in the location of the input signal as well as loss of spatial resolution.« less

Authors:
; ;  [1];  [2]
  1. Department of Medical Physics, Cross Cancer Institute, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2 (Canada) and Departments of Oncology and Physics, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2 (Canada)
  2. (Canada) and Department of Oncology, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2 (Canada)
Publication Date:
OSTI Identifier:
20853824
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 33; Journal Issue: 12; Other Information: DOI: 10.1118/1.2370503; (c) 2006 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
62 RADIOLOGY AND NUCLEAR MEDICINE; BEAMS; CADMIUM TUNGSTATES; COMPUTERIZED SIMULATION; COMPUTERIZED TOMOGRAPHY; CRYSTALS; MONTE CARLO METHOD; PHOTODIODES; PHOTON TRANSPORT; QUANTUM EFFICIENCY; SPATIAL RESOLUTION; TRANSFER FUNCTIONS

Citation Formats

Monajemi, T. T., Fallone, B. G., Rathee, S., and Department of Medical Physics, Cross Cancer Institute, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2. Thick, segmented CdWO{sub 4}-photodiode detector for cone beam megavoltage CT: A Monte Carlo study of system design parameters. United States: N. p., 2006. Web. doi:10.1118/1.2370503.
Monajemi, T. T., Fallone, B. G., Rathee, S., & Department of Medical Physics, Cross Cancer Institute, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2. Thick, segmented CdWO{sub 4}-photodiode detector for cone beam megavoltage CT: A Monte Carlo study of system design parameters. United States. doi:10.1118/1.2370503.
Monajemi, T. T., Fallone, B. G., Rathee, S., and Department of Medical Physics, Cross Cancer Institute, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2. Fri . "Thick, segmented CdWO{sub 4}-photodiode detector for cone beam megavoltage CT: A Monte Carlo study of system design parameters". United States. doi:10.1118/1.2370503.
@article{osti_20853824,
title = {Thick, segmented CdWO{sub 4}-photodiode detector for cone beam megavoltage CT: A Monte Carlo study of system design parameters},
author = {Monajemi, T. T. and Fallone, B. G. and Rathee, S. and Department of Medical Physics, Cross Cancer Institute, University of Alberta, 11560 University Avenue, Edmonton, Alberta T6G 1Z2},
abstractNote = {Megavoltage (MV) imaging detectors have been the focus of research by many groups in recent years. We have been working with segmented CdWO{sub 4} crystals in contact with photodiodes in our lab. The present study uses both x-ray and optical photon transport Monte Carlo simulations to analyze the effects of scintillation crystal height, septa material, beam divergence, and beam spectrum on the modulation transfer function, MTF(f) and zero frequency detective quantum efficiency, DQE(0), of a theoretical area detector. The theoretical detector is comprised of tall, segmented CdWO{sub 4} crystals and two dimensional photodiode arrays with a pitch of 1 mm and a fill factor of 72%. Increasing the crystal height above 10 mm does not result in an improvement in the DQE(0) if the reflection coefficient of the septa is less than 0.8. For a reflection coefficient of 0.975 for the septa, there is a continual gain in the DQE(0) up to 30 mm tall crystals. Similar calculations show that employing a 3.5 MV beam without a flattening filter increases the DQE(0) for 20 mm tall crystals by 9% compared to a typical 6 MV beam with a flattening filter. The severe degradations due to beam divergence on MTF(f) are quantified and suggest the use of focused detectors in MV imaging. It is found that when the effect of optical photons is considered, the presence of divergence can appear as a shift in the location of the input signal as well as loss of spatial resolution.},
doi = {10.1118/1.2370503},
journal = {Medical Physics},
number = 12,
volume = 33,
place = {United States},
year = {Fri Dec 15 00:00:00 EST 2006},
month = {Fri Dec 15 00:00:00 EST 2006}
}
  • We describe the components of a bench-top megavoltage computed tomography (MVCT) scanner that uses an 80-element detector array consisting of CdWO{sub 4} scintillators coupled to photodiodes. Each CdWO{sub 4} crystal is 2.75x8x10 mm{sup 3}. The detailed design of the detector array, timing control, and multiplexer are presented. The detectors show a linear response to dose (dose rate was varied by changing the source to detector distance) with a correlation coefficient (R{sup 2}) nearly unity with the standard deviation of signal at each dose being less than 0.25%. The attenuation of a 6 MV beam by solid water measured by thismore » detector array indicates a small, yet significant spectral hardening that needs to be corrected before image reconstruction. The presampled modulation transfer function is strongly affected by the detector's large pitch and a large improvement can be obtained by reducing the detector pitch. The measured detective quantum efficiency at zero spatial frequency is 18.8% for 6 MV photons which will reduce the dose to the patient in MVCT applications. The detector shows a less than a 2% reduction in response for a dose of 24.5 Gy accumulated in 2 h; however, the lost response is recovered on the following day. A complete recovery can be assumed within the experimental uncertainty (standard deviation <0.5%); however, any smaller permanent damage could not be assessed.« less
  • Megavoltage cone-beam computed tomography (MV CBCT) is a highly promising technique for providing volumetric patient position information in the radiation treatment room. Such information has the potential to greatly assist in registering the patient to the planned treatment position, helping to ensure accurate delivery of the high energy therapy beam to the tumor volume while sparing the surrounding normal tissues. Presently, CBCT systems using conventional MV active matrix flat-panel imagers (AMFPIs), which are commonly used in portal imaging, require a relatively large amount of dose to create images that are clinically useful. This is due to the fact that themore » phosphor screen detector employed in conventional MV AMFPIs utilizes only {approx}2% of the incident radiation (for a 6 MV x-ray spectrum). Fortunately, thick segmented scintillating detectors can overcome this limitation, and the first prototype imager has demonstrated highly promising performance for projection imaging at low doses. It is therefore of definite interest to examine the potential performance of such thick, segmented scintillating detectors for MV CBCT. In this study, Monte Carlo simulations of radiation energy deposition were used to examine reconstructed images of cylindrical CT contrast phantoms, embedded with tissue-equivalent objects. The phantoms were scanned at 6 MV using segmented detectors having various design parameters (i.e., detector thickness as well as scintillator and septal wall materials). Due to constraints imposed by the nature of this study, the size of the phantoms was limited to {approx}6 cm. For such phantoms, the simulation results suggest that a 40 mm thick, segmented CsI detector with low density septal walls can delineate electron density differences of {approx}2.3% and 1.3% at doses of 1.54 and 3.08 cGy, respectively. In addition, it was found that segmented detectors with greater thickness, higher density scintillator material, or lower density septal walls exhibit higher contrast-to-noise performance. Finally, the performance of various segmented detectors obtained at a relatively low dose (1.54 cGy) was compared with that of a phosphor screen similar to that employed in conventional MV AMFPIs. This comparison indicates that for a phosphor screen to achieve the same contrast-to-noise performance as the segmented detectors {approx}18 to 59 times more dose is required, depending on the configuration of the segmented detectors.« less
  • Megavoltage computed tomography (MVCT) is a potential imaging tool for positioning and dose delivery verification during image guided radiotherapy. The problem with many MVCT detectors, however, is their low detective quantum efficiency (DQE) which leads to poor low contrast resolution (LCR) and high image noise. This makes separating the tumors from the soft tissue background difficult. This manuscript describes the imaging performance of our bench-top MVCT scanner that uses an 80-element detector array consisting of CdWO{sub 4}-photodiode elements with a DQE of 19% in 6 MV and 26% in Co{sup 60} beams [T. T. Monajemi, S. Steciw, B. G. Fallone,more » and S. Rathee, 'Modelling scintillator-photodiodes as detectors for megavoltage CT', Med. Phys. 31, 1225-1234 (2004)] at zero frequency. The imaging experiments presented were carried out mainly in a Co{sup 60} teletherapy unit, while the beam hardening characteristics of the system were also presented for a 6 MV beam. During image evaluation, persistent ring artifacts, caused by air gaps at the ends of the eight-element detector blocks, were removed by using a calibration procedure. The measured contrast of a low contrast target with a 20 mm diameter was determined to be independent of dose, between 2.1 and 17 cGy. The measured LCR of a target with a nominal contrast of 2.8% was reduced from 2.3% to 1.2% when the contrast target diameter was reduced from 15 to 5 mm, using 17 cGy for imaging. The signal to noise ratio of this system is shown to be proportional to the square root of dose. Most importantly, a low contrast target with a diameter of 6 mm and a nominal contrast level of 1.5% is resolved with a radiation dose of 2.1 cGy in the Co{sup 60} beam. The spatial resolution in the Co{sup 60} beam is limited to one line pair per centimeter mainly due to the size of the Co{sup 60} source.« less
  • Purpose: Active matrix flat-panel imagers (AMFPIs) incorporating thick, segmented scintillators have demonstrated order-of-magnitude improvements in detective quantum efficiency (DQE) at radiotherapy energies compared to systems based on conventional phosphor screens. Such improved DQE values facilitate megavoltage cone-beam CT (MV CBCT) imaging at clinically practical doses. However, the MV CBCT performance of such AMFPIs is highly dependent on the design parameters of the scintillators. In this paper, optimization of the design of segmented scintillators was explored using a hybrid modeling technique which encompasses both radiation and optical effects. Methods: Imaging performance in terms of the contrast-to-noise ratio (CNR) and spatial resolutionmore » of various hypothetical scintillator designs was examined through a hybrid technique involving Monte Carlo simulation of radiation transport in combination with simulation of optical gain distributions and optical point spread functions. The optical simulations employed optical parameters extracted from a best fit to measurement results reported in a previous investigation of a 1.13 cm thick, 1016μm pitch prototype BGO segmented scintillator. All hypothetical designs employed BGO material with a thickness and element-to-element pitch ranging from 0.5 to 6 cm and from 0.508 to 1.524 mm, respectively. In the CNR study, for each design, full tomographic scans of a contrast phantom incorporating various soft-tissue inserts were simulated at a total dose of 4 cGy. Results: Theoretical values for contrast, noise, and CNR were found to be in close agreement with empirical results from the BGO prototype, strongly supporting the validity of the modeling technique. CNR and spatial resolution for the various scintillator designs demonstrate complex behavior as scintillator thickness and element pitch are varied—with a clear trade-off between these two imaging metrics up to a thickness of ∼3 cm. Based on these results, an optimization map indicating the regions of design that provide a balance between these metrics was obtained. The map shows that, for a given set of optical parameters, scintillator thickness and pixel pitch can be judiciously chosen to maximize performance without resorting to thicker, more costly scintillators. Conclusions: Modeling radiation and optical effects in thick, segmented scintillators through use of a hybrid technique can provide a practical way to gain insight as to how to optimize the performance of such devices in radiotherapy imaging. Assisted by such modeling, the development of practical designs should greatly facilitate low-dose, soft tissue visualization employing MV CBCT imaging in external beam radiotherapy.« less
  • Purpose: Active matrix flat-panel imagers incorporating thick, segmented scintillators for megavoltage cone-beam CT (MV CBCT) imaging have demonstrated strong potential for facilitating soft-tissue visualization at low, clinically practical doses. In order to identify scintillator design parameters that optimize performance for this purpose, a modeling technique which includes both radiation and optical effects and which lends itself to computationally practical implementation has been developed and explored. Methods: A hybrid modeling technique, based on Monte Carlo event-by-event simulation of radiation transport and separate determination of optical effects, was devised as an alternative to computationally prohibitive event-by- event simulations of both radiation andmore » optical transport. The technique was validated against empirical results from a previously reported 1.13 cm thick, 1.016 mm element-to-element pitch BGO scintillator prototype. Using this technique, the contrast-to-noise ratio (CNR) and spatial resolution performance of numerous scintillator designs, with thicknesses ranging from 0.5 to 6 cm and pitches ranging from 0.508 to 1.524 mm, were examined. Results: CNR and spatial resolution performance for the various scintillator designs demonstrate complex behavior as scintillator thickness and pitch are varied - exhibiting a clear trade-off between these two imaging metrics up to a thickness of ~3 cm. Based on these results, an optimization map highlighting those regions of design that provide a balance between these metrics was created. The map indicates that, for a given set of optical parameters, scintillator thickness and pitch can be judiciously chosen to maximize performance without resorting to thicker, more costly scintillators. Conclusion: Modeling radiation and optical effects in thick, segmented scintillators through use of a hybrid modeling technique provides a practical way to gain insight as to how to optimize the performance of such devices for radiotherapy imaging. Assisted by such modeling, the development of practical designs should greatly facilitate low-dose, soft tissue visualization through MV CBCT imaging. This project was supported in part by NIH grant R01 CA051397.« less