A piecewise-focused high DQE detector for MV imaging.
2015
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.
Keywords:
- Correction
- Source
- Cite
- Save
- Machine Reading By IdeaReader
52
References
24
Citations
NaN
KQI