New methods of radiation dosimetry in medicine by using optical fibers

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The purpose of the present study is the development of novel radiation dosimeters and new methods of radiation dosimetry in medicine by using optical fiber as detector material of x-ray dosimeters. The study was focused on the following three developments.1. A long dosimeter on the order of one meter was devised by using a plastic scintillating fiber as detector material for measuring dose integral by x-ray CT exposure on a human phantom along lines parallel to the axis of rotation of the x-ray source. Uniform sensitivity of the dosimeter was attained over a range of about one meter by processing light signals derived from both ends of the fiber. Measurements of CT doses with the long dosimeter indicated the underestimation of saturated dose integral that had been obtained with conventional CT chambers with a short detection length.2. A small sized and tissue equivalent detector consisting of a plastic scintillating fiber chip coupled with an optical fiber light guide was devised for the precise dosimetry of small field therapeutic x-ray beams. Light output from the detector was converted to electrical signal with unity gain photodiodes without using photomultipliers, and a novel method to estimate absorbed dose from pulse height spectra was developed to obtain dose values with a large signal-to-noise ratio and excellent reproducibility.3. A small sized and highly sensitive x-ray detector with directional independence of response was devised for the dosimetry of CT exposure by using a plastic scintillation fiber chip and a pair of optical fiber light guides. Dose profiles in a human chest phantom along lines parallel to the axis of rotation of the x-ray source and point doses at various positions in the phantom were measured for helical CT examinations to estimate tissue or organ doses.