Sci—Thur PM: Imaging — 03: A novel Čerenkov detector based on air‐spaced light guiding taper for megavoltage x‐ray imaging

Abstract

Electronic Portal Imaging Devices (EPIDs) have been used in radiation therapy and are still needed on linear accelerators (Linacs) equipped with kilovoltage cone beam CT (kV‐CBCT) or MRI systems. Recently a new concept of a high quantum efficiency (QE) Čerenkov Portal Imaging Device (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 fibres 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 this work we 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, a much superior light collection efficiency is achieved. Our Monte Carlo studies have shown that the modified new CPID has a QE more than an order of magnitude greater than that of current clinical systems and yet a spatial resolution similar to that of current flat‐panel based EPIDs. Furthermore it has been 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.