In a block detector system, detector elements vary in efficiency because of the position of the element in the block, physical variations in the crystal and light guides and variations in the gains of the photomulti-plier tubes. These variations result in substantial high-frequency non-uniformities in the raw data. In particular there is a systematic variation in detector efficiency with the crystal position within the block (the "block profile") which results in significant variations in the sensitivity of the tomograph in the axial direction. Radially the effect is not so great, because any one pixel in the image is viewed by many detectors and there is a tendency for these effects to cancel out during reconstruction. Nevertheless, failure to correct for them leads to radial streaking in the image, and the systematic block profile effects can reinforce during reconstruction, resulting in circular "saw-tooth" artefacts. Detector efficiency, and in particular the block profile, can be affected by count rate. One result of pulse pileup within a block detector is the shifting of detected events towards the centre of the block . This is not really a normalisation effect in the conventional sense, but since it results in a systematic change in the apparent efficiency of the lines of response with position in the block it manifests itself in a very similar way. The
effect can be reduced by measuring normalisation coefficients at a similar count-rate to that used during data acquisition, or by creating a rate-dependent lookup table of normalization coefficients .
If this is not possible, any resulting image artefacts may be reduced by extracting systematic effects from the raw data after normalisation but prior to reconstruction.
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