Tomography

Claim 1

A method of operating on electrical signals, each indicative of a line integral of absorption suffered by penetrating radiation, on traversing a respective beam path through a slice disposed cross-sectionally of a body under examination so as to produce a representation of the absorption coefficient, with respect to said radiation, at each of a plurality of locations distributed over said slice; the beam paths comprising a set of substantially parallel paths at each of a plurality of angular orientations relative to the body; the method comprising the steps of: (a) convolving the signals relating to a first set of said paths each with a plurality of values of Walsh functions of different sequency to produce a plurality of convolved signals of respective sequencies, each containing contributions from the signals relating to each beam path in said first set. (b) forming the scalar product of each convolved signal with a respective series of coefficients each determined by a first sequency factor which is constant throughout the series and is of the same sequency as the relevant convolved signal and a second sequency factor which is different for each coefficient of the series, (c) storing said scalar products in a coefficient store, (d) repeating the method steps (a), (b) and (c) for the signals relating to the other sets of beam paths, and (e) inverting the products stored in said coefficient store in dependence upon respective values of Walsh transform functions to evaluate said absorption coefficients.

Claim 2

A method of operating on electrical signals, each indicative of a line integral of absorption suffered by penetrating radiation, on traversing a respective beam path through a slice disposed cross-sectionally of a body under examination so as to produce a representation of the absorption coefficient, with respect to said radiation, at each of a plurality of locations distributed over said slice; the beam paths comprising a set of substantially parallel paths at each of a plurality of angular orientations relative to the body; the method comprising the steps of: (a) convolving the signals relating to a first set of said paths each with a plurality of values of Walsh functions of different sequency to produce a plurality of convolved signals of respective sequencies, each containing contributions from the signals relating to each beam path in said first set. (b) forming the scalar product of each convolved signal with a respective series of coefficients each determined by a first sequency factor which is constant throughout the series and is of the same sequency as the relevant convolved signal and a second sequency factor which is different for each coefficient of the series, (c) storing said scalar products in a coefficient store, (d) repeating the method steps (a), (b) and (c) for the signals relating to the other sets of beam paths, and (e) inverting the products stored in said coefficient store in dependence upon respective values of Walsh transform functions to evaluate said absorption coefficients.