DEVELOPMENT OF A 2ND-GENERATION FIBEROPTIC ONLINE IMAGE VERIFICATION SYSTEM

Purpose: We have previously reported the development of a fiber-optic fluoroscopic system for on-line imaging on radiation therapy machines with beam-stops because of space limitation. While the images were ade quate for clinical purposes in most cases, an undesirable grid artifac t existed and distracted visualization. The resolving power of the sys tem, limited by the 1.6 mm X 1.6 mm dimension of the input fibers, app eared insufficient in some cases. This work identifies solutions to re duce grid artifact and to improve the resolution of the system. Method s and Materials: In the clinical system, it was found that the scannin g mechanism of the newvicon camera was deflected differently at variou s gantry positions because of the different orientation of the earth's magnetic field. The small image misregistration produced grid artifac t during image normalization, particularly near boundaries of the fibe r bundles. One approach taken to reduce magnetic field effects was to shield the camera with mu-metal. Alternatively, a charged-coupled-devi ce camera was used instead of the newvicon camera. As for improving sp atial resolution, fibers with smaller input dimension were used. A 20 cm X 20 cm high resolution fiber-optic prototype consisting of 250 X 2 50 fibers, each with an input dimension of 0.8 mm X 0.8 mm was constru cted. Its performance was tested using several phantoms studies. Resul ts: Both shielding the newvicon camera with mu-metal or replacing it w ith a charge-coupled-device camera reduced grid artifact. However, opt imal shielding could not be made for our clinical system because of th e space limitation of its housing. High contrast resolution was improv ed, the 30% value of the modulation transfer function occurred at 0.3 linepairs per mm for the clinical system and at 0.7 linepairs per mm f or the high-resolution prototype. However, because of the larger degre e of transmission non-uniformity of the prototype, it was less effecti ve using the current setup in detecting low contrast objects. Conclusi ons: The results are encouraging and demonstrate successful reduction of grid artifact and improvement of high contrast spatial resolution u sing the proposed methods. The less effective low contrast detection w as related to reduced light collection efficiency due to use of protot ype fibers whose productions were not closely monitored. The findings are being considered in our construction of a second generation clinic al fiber-optic on-line image verification system.