Ring filter to locate external boundaries in SPECT using scattered radiation.

A novel form of filter for SPECT is described, in which, after back projection and summation, the reconstructed signal is a measure of the total activity within a ring of specified radius, centre and width. The filter is applied to the problem of using Compton scattered radiation to locate external boundaries. In the simple case of the determination of the radius of a circular scattering body of known centre, the filter output would identify a transition region and define an appropriate threshold as the boundary was crossed. However it can also be applied to locate the boundaries seen in individual SPECT projections and hence trace out the envelope of the scattering body. Monte Carlo simulation based on 99mTc is used to test the performance of the filter in a range of situations, with encouraging results.