The effects of structural misalignment are not easy to predict. They can often be determined by examining data used for calculating the transaxial geometric factors after normalisation for all other known components. On the GE Advance (GE Medical Systems, Milwaukee,WI), this process reveals high-frequency non-uniformities which are consistent in every sinogram, regardless of ring difference. These non-uniformities are correlated with rotational misalignments of the block modules, which extend for the entire length of the tomograph. However, examination of data from the Siemens/CTI ECAT 962 tomograph (CTI Inc., Knoxville, TN), which also has block modules that extend for the entire length of the tomograph, does not reveal these consistent non-uniformities. On the Advance, the consistency of the non-uniformities can be exploited in a simple manner to yield the required correction factors. Data from a rotating line source is
corrected for all known normalisation effects and then summed over all ring differences, yielding a matrix with the same dimensions as a single sino-gram. Each element in the matrix is then divided by the mean over all the matrix elements and inverted to yield the muivj.
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