A megavoltage MTF measurement technique for metal screen-film detectors.

A technique to measure the MTF of metal screen-film detectors at megavoltage x-ray energies has been devised. It employs an exposure through a slit 25 micron wide and a method to average numerous line spread functions from a single film image. That the slit is wide compared to those used at diagnostic x-ray energies serves two purposes: it facilitates the use of a laser alignment procedure designed to ensure correct orientation of the slit-forming tungsten blocks in the x-ray beam, and it also increases the contrast in the slit image, thereby reducing the data analysis problems caused by film noise. Further reduction in the effects of film noise is accomplished by the averaging method. A correction is made to account for the effect of the wide slit on the MTF. The validity of this correction, the laser alignment, and the averaging method have been confirmed by simulations.