Abstract During computer tomography examinations, ring-like artefacts occasionally occur because the X-ray source becomes hot, so that the X-ray focus changes its position in space. Because the distance between the detector and the diaphragm which stops the radiation beam is several times larger than the distance between the X-ray focus and the diaphragm, the shifting of the radiation beam on the detector is even greater. If the detector consists of detection elements which are not exactly identical, it may occur that the sensetivities of the detection elements apparently change with respect to each other due to such a shift, the reconstruction then giving rise to said artefacts. In accordance with the invention, the position where the radiation beam is incident on the detector is measured and the diaphragm for the stopping of the radiation beam is readjusted so that the radiation is each time incident in the same area.
A device for determining local absorption differences in an object, comprising an X-ray source for generating a flat, fan-shaped X-ray beam for irradiating the object during an examination, and an X-ray detector which comprises a large number of detection elements for measuring radiation transmitted by the object, the X-ray source comprising a diaphragm having a slit-shaped aperture and an X-ray tube which is arranged behind the diaphragm, characterized in that the device further comprises adjusting means for moving the diaphragm and the X-ray tube with respect to each other and an auxiliary detector for controlling the adjusting means so that the fan-shaped beam is always incident on the X-ray detector in a fixed, predetermined area.
Claim 2
Independent Claim
115 words
A device for determining local absorption differences in an object, comprising an X-ray source for generating a flat, fan-shaped X-ray beam for irradiating the object during an examination, and an X-ray detector which comprises a large number of detection elements for measuring radiation transmitted by the object, the X-ray source comprising a diaphragm having a slit-shaped aperture and an X-ray tube which is arranged behind the diaphragm, characterized in that the device further comprises adjusting means for moving the diaphragm and the X-ray tube with respect to each other and an auxiliary detector for controlling the adjusting means so that the fan-shaped beam is always incident on the X-ray detector in a fixed, predetermined area.
Claim 3
Dependent Claim
85 words
A device as claimed in claim 1, further comprising a controller comprising two inputs and a pair of auxiliary detectors which are adjacently arranged in the direction transverse to the flat X-ray beam, the output signals of said auxiliary detectors being equal if the radiation beam is incident on the detector in the predetermined area and being unequal if this is not the case, the auxiliary detectors being connected to the inputs of the controller and the controller being connected to operate the adjusting means.