Monte Carlo simulation of a quantum noise limited Čerenkov detector based on air-spaced light guiding taper for megavoltage x-ray imaging.
OBJECTIVE
Electronic Portal Imaging Devices (EPIDs) have been widely used in radiation therapy and are still needed on linear accelerators (Linacs) equipped with kilovoltage cone beam CT (kV-CBCT) or MRI systems. Our aim is to develop a new high quantum efficiency (QE) Čerenkov Portal Imaging Device (CPID) that is quantum noise limited at dose levels corresponding to a single Linac pulse.


METHODS
Recently a new concept of CPID for MV x-ray imaging in radiation therapy was introduced. It relies on Čerenkov effect for x-ray detection. The proposed design consisted of a matrix of optical fibers aligned with the incident x-rays and coupled to an active matrix flat panel imager (AMFPI) for image readout. A weakness of such design is that too few Čerenkov light photons reach the AMFPI for each incident x-ray and an AMFPI with an avalanche gain is required in order to overcome the readout noise for portal imaging application. In this work the authors propose to replace the optical fibers in the CPID with light guides without a cladding layer that are suspended in air. The air between the light guides takes on the role of the cladding layer found in a regular optical fiber. Since air has a significantly lower refractive index (∼ 1 versus 1.38 in a typical cladding layer), a much superior light collection efficiency is achieved.


RESULTS
A Monte Carlo simulation of the new design has been conducted to investigate its feasibility. Detector quantities such as quantum efficiency (QE), spatial resolution (MTF), and frequency dependent detective quantum efficiency (DQE) have been evaluated. The detector signal and the quantum noise have been compared to the readout noise.


CONCLUSIONS
Our studies show that the modified new CPID has a QE and DQE more than an order of magnitude greater than that of current clinical systems and yet a spatial resolution similar to that of current low-QE flat-panel based EPIDs. Furthermore it was demonstrated that the new CPID does not require an avalanche gain in the AMFPI and is quantum noise limited at dose levels corresponding to a single Linac pulse.
Imaging Research, Sunnybrook Health Sciences Centre, Department of Medical Biophysics, University of Toronto, Toronto M4N 3M5, Canada.
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Teymurazyan
Imaging Research, Sunnybrook Health Sciences Centre, Department of Medical Biophysics, University of Toronto, Toronto M4N 3M5, Canada; Thunder Bay Regional Research Institute (TBRRI), Thunder Bay P7A 7T1, Canada; and Department of Radiation Oncology, University of Toronto, Toronto M5S 3E2, Canada.
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Rowlands
Imaging Research, Sunnybrook Health Sciences Centre, Department of Medical Biophysics, University of Toronto, Toronto M4N 3M5, Canada; Department of Radiation Oncology, University of Toronto, Toronto M5S 3E2, Canada; Odette Cancer Centre, Toronto M4N 3M5, Canada; and Department of Physics, Ryerson University, Toronto M5B 2K3, Canada.
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Pang
D000388:Air; D054893:Cone-Beam Computed Tomography / Q000295:instrumentation*; D008027:Light; D009010:Monte Carlo Method; D010315:Particle Accelerators; D011789:Quantum Theory; D011829:Radiation Dosage; D035501:Uncertainty