POSITION ESTIMATION AND ERROR CORRECTION IN A 2D POSITION-SENSITIVE NAI(TL) DETECTOR

Removal of spatial distortion for a two-dimensional position-sensitive NaI(Tl) detector used in a positron-emission tomography (PET) scanner is investigated. The Anger-type detector has an active area of 45 x 9 cm2 and is 2.5 cm thick and is optically coupled to a 3 x 10 array of 5 cm square photomultiplier tubes. Due to the nature of the scintillation light spread and collection there is a substantial difference between the true and calculated scintillation position as calculated from a biased centroid algorithm. To generate an error-correction lookup table the detector is illuminated with a tightly collimated gamma-ray beam at regular intervals and the measured positions at these locations are compared with the known true positions. Since it is impractical to collect this data for all the possible position channels, a table of 1024 x 200 distortion correction factors is interpolated from a sparse grid of measured points that covers the detector surface. The effects of different sampling distances for the measured points and two different interpolation schemes are compared. We conclude that the optimal combination is a sampling distance of 7.5 mm and a two-dimensional bilinear interpolation algorithm, which results in an average rms error in the corrected position of 0.2 mm. The bilinear interpolation algorithm is modified to work on the distorted rectangular grid that results from the measured locations of the data points.