Emi, LLC

U.S. Philips Corporation

Thorn Emi Plc

Richard M Waltham

Godfrey N Hounsfield

Daniel J Pisano, Jr

Erlvada A Olson

A digital phase-locked loop using a binary overflowing accumulator as a numerically controlled oscillator has external compensation to eliminate frequency-dependent phase offset problems while not requiring any second-order filtering. Operation of the NCO adder initiates with value set by the center frequency control and under the latching action of the latch clocked by the reference oscillator produces a linearly stepped output. Detection of the most-significant bit values produces a square wave at the output. The phase detector latch is used for phase correction, while comparator eliminates steady-state phase error.

Phase-locked loops

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).

Computed tomography apparatus with detector sensitivity correction

Computerized tomographic X-ray apparatuses direct X-rays, usually in a fan-shaped distribution, through the body of a patient. In some examples the fan distribution may subtend angles of 50.degree. or more at the source. It has been found that such tubes, particularly of the rotating anode type, tend to have X-ray emission from regions surrounding the focal spot and it is not convenient to use anode shields to eliminate it. This off-focus radiation can present problems, especially when attenuators are used to equalize absorbing paths through the patient, since the off-focus radiation can sometimes be less attenuated than the main beam. It is proposed to correct the signals produced by the X-ray detectors, for off-focus radiation. A profile of off-focus or halo radiation is produced and the attenuation imposed on it is estimated. The estimates are used to suitably modify the signals produced by the X-ray detectors (or signals derived from the detector outputs) so as to correct for the presence of off-focus radiation.

Medical radiographic apparatus and method

This application describes a rotation-only CT scanner with beam deflections; the deflection and the rotation being so related as to permit the derivation of absorption data in groups focussed on respective 'pivot points' outside the detector locus. Data derived from different detectors, but focussed on the same pivot point, relate to groups of paths, some of which overlap, and the overlap is used to compare the performances of different detectors. To reduce data handling, data provided by adjacent detectors can be combined.

Radiography

In a computerized tomographic apparatus in which the source and detectors are moved angularly around the body, the angular movement of the source is, in effect, periodically arrested so that the detectors, which are of greater angular extent than the distribution of radiation produced by the source, slide through the distribution. In this way successive detectors can be made to view the same beam in the distribution.

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London, United Kingdom

Richard M Waltham