Development of a Multichannel Dosimeter Based on Scintillating Fibers for Medical Applications

Dosimetry with plastic scintillators is an interesting alternative for the measurement of the absorbed dose. A scintillator does barely disturb the radiation field due to its mass absorption coefficients which is water-equivalent in a wide range of energies. Furthermore, plastic scintillation dosimeters provide a fast and direct reading of the measured value combined with a high spatial resolution. In the set-up described so far, the light produced by the scintillators was transported via light-guides to single-channel or multi-channel photomultipliers to be transformed into an electric current read out by pico-amperemeters. The use of photomultipliers becomes expensive and complicated for a dosimeter system with many parallel channels. For such applications, an image intensifier coupled to a CCD is a simpler approach which can read out some 80 fibers in parallel.

The described dosimeter will consist altogether of 72 channels, 36 of these channels with a scintillator 1mm³ and an optical fiber. The remaining 36 channels are without scintillator. They will be used for the suppression of the Cerenkov background produced in the light guiding fibers. The read-out system is set up in such a way that the incoming light guides are kept at fixed positions by a mask to ensure a good optical contact between fibers and the image intensifier. The image intensifier transforms the light from all 72 fibers into electrons, amplifies them in parallel, transforms them back into photons and re-emits an amplified image of the incoming light signals.
The image emitted by the image intensifier is transported via an optical fiber taper to a CCD. This fiber taper is a conical optical light guide consisting of a large number of thin glass fibers. The diameter of these fibers all decrease towards one end of the taper, so an image transmitted by the taper will be reduced in size, but its relative spatial resolution will be maintained. The CCD integrates the light signal of each of the 72 fibers in parallel over 20 milliseconds. The image is transformed into standard TV format and read out by a personal computer equipped with a frame grabber, an extension card for a PC converting a video signal into digital information.

First measurements were made at the Clinic for Radiation Therapy and Radio-Oncology in the Klinikum Wuppertal GmbH.

M. Vaupel

Last Modification: Juli 16, 2004