CALCULATION OF SINGLE DETECTOR EFFICIENCIES AND EXTENSION OF THE NORMALIZATION SINOGRAM IN PET

A fast iterative method is presented for calculating single detector e fficiencies for positron emission tomographs. These efficiencies can b e used to extend the normalization scan to areas outside that covered by the normalization source. The root mean square (rms) error of the c alculated single detector efficiencies decreases exponentially with th e number of iterations. Thirty iterations per normalization image are sufficient and take about 1 s on a SUN Classic. The geometry factors a re composed of factors which only depend on the distance from the cent re of the field-of-view (FOV) and of factors which show a more complex pattern over the normalization sinogram. The geometry factors are spe cific to each scanner. On the ECAT 931 scanner the complex part of the geometry factors showed a diamond shaped pattern (caused by the varyi ng sensitivity of single detectors in a detector block with varying an gle of incidence) and S-shaped curves (representing attenuation caused by supporting rods for the ring source). The coefficient of variation of the diamond-shaped pattern was 4% for detectors farthest from the centre of the FOV. Extensions of the normalization scan may, therefore , contain a relative rms error of about 4% if the applied geometry fac tors only take the distance from the centre of the FOV into account.