Method of determining the body contour for the reconstruction of the absorption of radiation in a flat zone of a body

In computed tomography, a method of determining the contour of a body in a plane, said body being positioned on an examination table and further being contained within a positioning zone in the plane, comprising the steps of;

• completely irradiating an examination zone, which is situated within the positioning zone, from a plurality of different directions in the plane of examination with primary radiation which passes through the body along a plurality of beam paths and measuring the radiation transmitted through the body to determine primary radiation absorption values along each of said beam paths;
• irradiating the positioning zone in a region which adjoins the examination zone and is tangent to the perimeter of the positioning zone;
• from a plurality of directions in the plane with auxiliary radiation which is substantially absorbed by the body;
• measuring the auxiliary radiation with auxiliary detectors to determine the position of auxiliary radiation beams which are tangent to the body;
• determining the position of contour points of the body from the position of said auxiliary radiation beams which are tangent to the body;
• wherein, as an improvement, the step of determining the position of contour points comprises:
• determining the position of a plurality of first contour points from the position of auxiliary radiation beams which are tangent to the body and do not pass through the examination table;
• determining the position of further contour points by calculating the distance between a further contour point and a first contour point as the quotient of the absorption value determined from the measurement of primary radiation along a beam path which passes through said first point and said further point divided by an assumed value of a mean absorption coefficient in the body.