A Digital Fluoroscopic Imaging System for Radiotherapy Localization

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Abstract
We have been developing a digital fluoroscopic imaging device to replace the portal films that are currently used to verify patient positioning during radiotherapy treatments. Our system differs in two ways from previously reported devices. Our x-ray detector, which consists of a 1.0-mm copper plate with a 400-mg/cm2 Gd2O2S:Tb phosphor layer bonded to the plate, has a much thicker phosphor layer than used previously. Thus, the light output from the detector is also much larger. The increased light output reduces the contribution by the secondary (light) quanta to noise in the images. In addition, the operation of the T.V. camera has been modified so that the light from the phosphor screen is integrated on the target of the T.V. camera for periods of 0.2-2.0 seconds. Integration of the light increases the video signal relative to the fixed noise current generated by the camera, and thus minimizes the camera's contribution to noise in the images. Image quality improves if either a thicker phosphor detector or target integration is used to form the image, and the best images result if these two are used in combination. The images obtained from the imaging system are comparable to film, and show that our fluoroscopic imaging system represents a definite alternative to film as a method of verifying patient positioning in radiotherapy.

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