Method of and device for determining the distribution of radiation absorption in a slice of a body

Computed tomography apparatus for determining the distribution of radiation absorption properties in a planar slice of a body comprising:

• radiation source means which generate a planar beam of penetrating radiation and radiate a plane in the body therewith from a plurality of directions;
• detector means which measure radiation from the radiation source means which has passed through the body and which determine therefrom absorption values which are a measure of the radiation attenuation along measuring paths in the plane which pass through the body from the source;
• a positioning zone for the body situated between the source means and the detector means;
• central processing means which determine absorption coefficients of the radiation absorption distribution from the absorption values;
• a memory for storing the absorption values and the absorption coefficients;
• and
• display means which display the radiation absorption distribution;
• wherein an examination zone in the plane lies within and is wholly surrounded by the positioning zone and, as an improvement, the apparatus further comprises:
• means which operate the radiation source means to produce two different intensities of radiation so that a first, higher intensity of radiation is used to radiate the body along paths which pass through the examination zone and a second, lower intensity of radiation is used to radiate the body along all paths which pass through the entire positioning zone, the higher intensity radiation being used to determine a first set of absorption values and the lower intensity of radiation being used to determine a second set of absorption values;
• and
• arithmetic means which determine the number k of measuring paths which extend through the examination zone in each direction, which determine and sum quotients of the first absorption values Q.sub.1 (P.sub.1, .theta.) and second absorption values Q.sub.2 (P.sub.2, .theta.) associated with the measuring paths, which divide the sum thus obtained by the number of measuring paths, thereby determining a correction factor C(.theta.) in accordance with the formula
• C(.theta.)=((.upsilon.(Q.sub.1 (P.sub.1,.theta.)/Q.sub.2 (P.sub.1,.theta.)k
• and which multiply those second absorption values which were measured along measuring paths in that direction which pass through the positioning zone and do not pass through the examination zone by the correction factor.