A CODED-APERTURE DEVICE FOR ONLINE IMAGING WITH MEGAVOLTAGE PHOTON BEAMS

A prototype coded aperture device based upon a liquid-filled strip ion chamber has been investigated as an on-line imaging unit for portal i maging of megavoltage photon beams. The strip ion chamber was used to collect integrated data for a series of pixels along a line across the field of view. By translating the detector, in equal steps in a direc tion perpendicular to its length, the data for a complete projection o f the image were collected. Multiple-projection data, corresponding to different angular views, were collected by sequentially rotating the detector in equal angular steps and then translating through the field . The portal image was reconstructed from the multiple-projection data by a convolution-filtered back-projection algorithm. The phantom imag es taken with a cobalt-60 radiation source were found to be of similar contrast to film images. The signal-to-noise ratio, including the noi se propagation of the reconstruction process for an image of 101 x 101 pixels, was 207:1. For a translational step of 2 mm the spatial resol ution of the system was found to be 4.2 mm, as determined from the ful l width at half maximum of the point spread function.