Computerised tomography scanner with two processing cycles - has two scan cycles using adjustable collimator and variable beam-current

摘要

X-ray CT-scanners having two processing cycles first calculate absorption values within the region of interest and second over the entire scanned area. The region of interest receives the unattenuated beam, the remaining area receives filtered rays. This suffers from the drawback of increased noise and a shift of the X-ray spectrum requiring elaborate correction procedures and resulting in reduced accuracy of measured absorption values. This is avoided by using two scan cycles and an adjustable exit collimator. During the first cycle, only the region of interest (11) is scanned by setting the collimator (12) i.e. its two sections (13,14) accordingly, using full X-ray intensity. For the second cycle the collimator sections (13,14) are adjusted so as to scan the full slice area (4) with reduced X-ray intensity by reducing the tube beam current. Correction factors are determined from these two sets of measurements.