EXPERIMENTAL CORRELATION-BASED IDENTIFICATION OF X-RAY CT POINT-SPREAD FUNCTION .2. SIMULATION AND DESIGN OF INPUT SIGNAL

The preferred signals for non-parametric correlation-based point sprea d function identification are white noise or pseudo-random binary sequ ences (PRBSs). Given the difficulty of building a phantom based on eit her of these signals, a new input is devised that corresponds to pseud o-randomly located holes. The positions of the holes correspond to zer os in a 2-D PRES. To optimise the design of the phantom and to ensure proper imaging procedure, a number of simulations are conducted. The e ffects of the following parameters on identification quality are inves tigated: the size of the holes and their minimum separation, the perio d of the PRBS, input-output translational and rotational mis-registrat ion, pixel size and the presence of cupping. The factors affecting ide ntification quality the most are rotational alignment, hole size and s eparation, as well as sequence length. During simulations, a point spr ead function offering characteristics similar to the Philips Tomoscan CX is identified. Optimal results are obtained when the signal consist s of 0.6 mm holes, separated by 0.9 mm, whose position is based on a 3 2 x 32 PRBS generated with a ten-stage shift-register. When adequate r otational alignment is provided, it is shown that the pseudo-randomly located holes signal is a good substitute for a purely white signal wh en identifying the PSF of a CT scanner.