1. Field of the Invention
The present invention is directed to a method for operating a medical imaging apparatus, and in particular to an improvement in the filtered back-projection method for producing such an image.
2. Description of the Prior Art
A typical computer tomography apparatus includes a measuring unit having an x-ray source and a radiation receiver. The receiver is composed of a row in individual detector elements, and the x-ray source emits a fan-shaped x-ray beam which, after passing through an examination subject, is incident on the radiation receiver. Each individual detector of the radiation receiver generates an electrical signal corresponding to the intensity of the radiation incident thereon. Means are provided for rotating the x-ray beam around the examination subject for transirradiation of a slice of the examination subject from different directions in the fan plane. A measured value processing unit is supplied with the signals from the individual detectors, obtained at the various transirradiation directions, identifies the attenuation values of predetermined points in the transirradiated plane of the examination subject from those signals. An image reproduction system generates a visual image from the calculated attenuation values. A processing unit undertakes a convolution or filtering of the measured data obtained from the detector signals, and generates a back-projection.
In a medical imaging apparatus of this type, for generating an image of a slice of the examination subject, a defined characteristic .circle.(r) of the examination subject is approximated and visually displayed for each location r in the slice plane through the subject. For that purpose, measured data are acquired corresponding to the integrals of .circle.(r) along a series of lines in the slice plane. The lines L(.gamma., .delta.) are characterized by two parameters, as are the corresponding measured values f(.gamma., .delta.), according to the relationship: ##EQU1## wherein ds is an element of the line L(.gamma., .delta.).
In parallel ray tomography devices, .delta. is the distance of the line L(.gamma., .delta.) from the center of the device and .gamma. is the angle between the normal proceeding through the center on the line L(.gamma.,.delta.) and a fixed reference direction. In fan beam machines, .gamma. is the angle which the straight line between the tip of the fan and the center of the measuring system (i.e., the central ray) describes relative to a reference line, and .delta. is the angle between the central ray of the x-ray beam and the ray under consideration.
In an medical imaging apparatus of the type described above, operating according to known back-projection techniques, the image contains artifacts which are based on statistical unreliabilities of the measured values, and on deterministic measured value errors, such as calibration errors and linearity errors.