Image Reconstruction (II): Computerized Scanner Explosion

Computerized x-ray scanners have had a dramatic impact on the practice of neuroradiology since their introduction 3 years ago. Efforts are now under way to extend the range of applicability of image reconstruction from projections (the principle on which x-ray scanners are based) in medicine still further. In particular, researchers are working on fast x-ray scanners, on the combination of image reconstruction with nuclear medicine, and on low dose methods of imaging. As now performed, the scanning procedure is necessarily slow. The basic scanning action consists of the motion of a narrow x-ray beam and a single detector across the head or body of a patient; this linear scan is followed by a rotation of the frame holding the x-ray source and detector about the patient and another linear scanning motion. A two-dimensional image of the cross section of the head or body in the plane that is traced out by the x-ray beam during the scanning is typically reconstructed from 180 or more one-dimensional projections, one of which is produced by each linear scan (Science, 7 November, p. 542). The time to read the detector 40,000 or more times and the time to move the x-ray source and detector repeatedly through the linear and rotational motions together add up to about 4.5 minutes. Ways to reduce this time include the use of many detectors in the plane of the cross section, so that many data points are taken simultaneously, and the reduction or elimination of all mechanical motion. One reason for needing faster scanners is that any motion of the patient during the scanning introduces artifacts into the reconstructed image, and it is often difficult for sick patients to remain sufficiently immobile for 5 minutes. Even chest motion during respiration could be overcome with 5to 20-second scanning times because the breath could be held that. long. And more patients could be accommodated, if the examination time were reduced. Manufacturers and academic investigators who are now in the process of building computerized x-ray scanners that are equipped with multiple detector arrays are also using an x-ray source geometry called a fan beam. In conventional x-radiography, the x-ray tube emits a diverging cone of radiation that may be 40 centimeters in diameter by the time it reaches the film. In the first generation of computerized x-ray scanners, the x-rays are collimated into a thin beam which, in conjunction with a