A piecewise-focused high DQE detector for MV imaging.
OBJECTIVE
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 0 1 0 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.


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/cm(2) 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.
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Josh
J
Star-Lack
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Daniel
D
Shedlock
Agile Technologies, Inc., 10337 Yellow Pine Lane, Knoxville, Tennessee 37932.
Dennis
D
Swahn
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Dave
D
Humber
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Adam
A
Wang
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Hayley
H
Hirsh
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
George
G
Zentai
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Daren
D
Sawkey
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Isaac
I
Kruger
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Mingshan
M
Sun
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Eric
E
Abel
Varian Medical Systems, 3120 Hansen Way, Palo Alto, California 94304.
Gary
G
Virshup
Department of Radiology, Stanford University, Stanford, California 94305.
Mihye
M
Shin
Department of Radiology, Stanford University, Stanford, California 94305.
Rebecca
R
Fahrig
D054893:Cone-Beam Computed Tomography / Q000295:instrumentation*; D055615:Electrical Equipment and Supplies; D061089:Radiotherapy, Image-Guided; D050397:Radiotherapy, Intensity-Modulated; D012588:Scintillation Counting