Patient dose reduction in CT

Abstract

CT is undoubtedly a powerful diagnostic tool, but data from surveys of CT practice show that, over the 30 years of clinical use, CT has contributed an ever-increasing percentage to the population dose from medical X-ray examinations [1, 2]. Although some regard the radiation dose levels from CT as beneficial [3], the generally held view is one of alarm at the increasing dose contribution, and the consensus amongst the radiology professions is that steps should be taken to reverse, or at least arrest, this trend. However, the reasons for this increasing dose contribution should be examined. Is it only as a result of increased numbers of examinations, or also as a result of an increase in dose per examination? Increased numbers of examinations, if correctly justified, must be viewed as resulting in a net benefit to patient management. However, if the doses to individual patients are increasing, the reasons must be carefully examined. Monitoring of trends in CT patient doses is currently particularly important as the technology is evolving rapidly. Multislice CT scanners, capable of simultaneously acquiring four slices in a single rotation, were introduced into clinical practice in 1999 and, as a result of targeted government funding to replace older scanner models, their adoption in the UK has been rapid. After only 6 years since their launch, multislice systems constitute around 70% of the CT scanners operating in England [4]. Multislice scanning has led to a major revolution in CT practice. Four slice scanners have been followed by 8-slice, 16-slice and up to 64-slice scanners. The greatly reduced examination times on these systems and their capacity to scan long lengths with narrow slices have further increased the scope of CT as a diagnostic tool. The use of submillimetre slice widths increases the spatial resolution in the scan axis direction, allowing a high spatial resolution in all planes. This isotropic resolution capability results in high quality three-dimensional (3D) and multiplanar reconstructions which are of particular benefit in angiography and endoscopy studies. The 16and 64-slice scanners are also making an impact in the field of cardiology, and CT cardiac angiography in particular is now rivalling conventional diagnostic techniques, and has the advantage of being non-invasive. The trend in technology is generally towards more dose efficient equipment, with solid-state detectors, an emphasis on high geometric dose efficiency and the introduction of dose-saving and dose-awareness features. However, the ever increasing capabilities of modern CT scanners, in terms of fast examination time and more powerful X-ray tubes, allow for the scanning of longer patient lengths and the use of higher tube currents. Unfortunately, there is still little published data available on trends in doses following the introduction of multislice CT. Two recent studies suggest average increases in effective dose per patient of 10% and 34% for multislice compared with single slice scanning [5, 6]. The recently published results of the 2003 UK CT dose survey [7] show that there has been a reduction in average patient doses from CT examinations since the last national UK CT dose survey published in 1991 [8]. However, they also show that doses from multislice are generally slightly higher than current dose levels from single slice CT scanners.