CT scanograms are routinely performed at the start of CT procedures for the purposes of selecting cross-sectional imaging planes. Their use as a low-dose alternative to film/screen radiography has also been demonstrated for measurement of leg lengths and angles prior to prosthetic surgery, although patient dose is limited by machine selectable factors. We set out to determine whether further dose reductions are possible without compromising the necessary information content of the image, simply by placing additional filtration between the x-ray source and the patient.
Materials/Methods An ionisation chamber was used to measure CT scanner output as the x-ray beam of a Philips Tomoscan operating at its minimum available parameters (100kVp and 50mA) was filtered using various thicknesses of aluminium and copper. A Rando anthropomorphic phantom was then scanned under the same set of conditions. The quality of the image was assessed in terms of suitability for its clinical purpose. Patient risk was quantified by loading the phantom with thermoluminescent dosemeters and scanning it at the filtration which gave the lowest exposure consistent with acceptable image quality. This model enabled us to calculate effective dose.
Results and Conclusions Images of the phantom acquired under various beam qualities are presented, together with the associated estimates of effective dose. The images obtained demonstrate that heavy filtration of the x-ray beam allows for substantial dose reduction in CT scanography, while still permitting satisfactory visualisation of bony landmarks.
O'Neil K, Lecomber AR, Harman T, Tennant D. Oral presentation: Patient dose optimisation in CT scanography. IOS (UK Radiological Congress), Birmingham, 22-24 May 2000.
Dr. Andy Lecomber, NGH Unit, Newcastle upon Tyne. Tel: +44 (0)191-233-6161
Regional Medical Physics Department, Newcastle General Hospital, Newcastle upon Tyne, NE4 6BE, UK.
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Regional Medical Physics Department, a Clinical Directorate of the Newcastle upon Tyne Hospitals NHS Foundation Trust.
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