| Detectors of at least several centimeters long are required for accurate dosimetry of patient exposure from x-ray computed tomography(CT)devices, since scattered x-rays are largely distributed in the human body and their contribution to total absorbed dose is significant. The present paper describes the development of a novel dosimeter for x-ray CT devices that uses a tissue-equivalent thin plastic scintillating fiber as a detector. In usual scintillating fiber detectors, the lightemitted in the fibers is transformed to an electrical signal by a photomultiplier tube(PMT)attached to the end of the fiber. In this case, uniformity of sensitivity along the fiber axis cannot be obtained because of the attenuation of light intensity in the fiber. To obtain uniform sensitivity of the scintillating fiber detector, we developed a new method using two PMTs attached to both ends of the fiber and processed coincident signals from both PMTs. Using this method we were able to construct a long scintillation detector with an effective detection length of 880 mm for x-ray CT dosimetry. The measurement of absorbed dose in a human phantom for x-rays from CT devices indicated that the absorbed dose obtained with CT ion chambers was approximately 30% less than that measured with our new dosimeter. |
