Better X-ray diagnostics with lower doses: development of new image capture and reconstruction methods for three-dimensional medical imaging to optimise the relationship between medical diagnosis and radiation exposure

Medical imaging is the major source of radiation exposure in the population of the Federal Republic of Germany. In 2001, the average mean effective dose per inhabitant from medical investigations was estimated to be 2.0 mSv per year, with a total average exposure of about 4.7 mSv. Computed tomography (CT) investigations represent only five per cent of the medical imaging investigations performed, but contribute just under 40 per cent of the exposure due to medical imaging. Recently, a new CT technology - multi-slice CT - has become available which allows simultaneous visualisation of multiple connected layers in the body. This technique enables investigations to be performed much faster as well as offering new diagnostic possibilities; it also means that the number of investigations with CT scanners will increase, contributing to an even higher average radiation exposure in the population. Technical improvements in modern CT systems have so far only managed to reduce the dose per CT investigation by about 30 per cent. It seems that in terms of CT technology the only effective way to reduce the dose further would be to accept a lower image quality, which mainly means an increased noise level. For the last one and a half years, the research group 'Medical Physics' in the Institute of Radiation Protection has been developing methods for reducing the dose from CT investigations without reduction in image quality. Part of this work is being done in collaboration with the University of Oregon. The basis of these methods is a newly developed image reconstruction algorithm for calculating final images from CT raw data. This algorithm uses the information derived from the scan more efficiently than the filtered back projection commonly in use today; an image quality at least as good can be obtained with only half of the raw data - and only half the dose - needed at present in a similar calculation time. A variety of new image capture geometries have also been developed based on this algorithm, which could lead to further dose savings while maintaining image quality. (orig.)