| 摘要 |
In a rotary fan beam computed tomography apparatus using recurrent relative displacement between the source and detectors (e.g. a deflected spot X-ray tube 9) for the recalibration (22) of detectors in chain-like sequences across the detector array 10 by successive pair-wise common-path sensitivity comparisons starting from a terminal detector 25, 26, each sequence normally involves 30 or more successive comparisons, and consistent but unpredictable errors are found to occur, leading to incorrect Houndsfield values in the computed image matrix (24). The improvement comprises locating at least one radiation transparent detector 27 of high stability in front of the array 10 at an intermediate point and using the output to further correct the chain-corrected detector sensitivity values. A detector 27 comprising a plastics scintillator optically coupled to a photomultiplier is described, whose output pulses are counted (36) during a rotational scan and compared with the mean corresponding measurement from detectors 28 lying behind the detector 27, to form a sensitivity ratio (43). From a corresponding ratio and data derived during calibration, a measured sensitivity value for detectors 28, is determined for each scan and is compared with the corresponding chain-corrected sensitivity value to generate a further sensitivity correction value which is then distributed among the detectors of the comparison sequence (44).
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