Purpose: Many radiology departments perform conventional film/screen radiography to assess leg lengths and angles prior to prosthetic surgery. CT scanography is an alternative technique which can provide similar or superior image quality easily and more reliably. To compare techniques, a full body phantom loaded with thermoluminescent dosemeters was used to simulate a patient.
Materials/Methods: The phantom was radiographed at typical patient factors (85kV, 80mAs, 200cm focus-skin distance), and was also scanned using 100kV, 50mA and 500cm scan length. Using this model, we were able to calculate and compare effective doses for the two techniques.
Results: In male patients, CT gave effective doses approximately ten times lower than those for plain radiography; in female patients, fifteen times lower. The reduction is due principally to the higher tube potential, higher beam filtration and postero-anterior projection used in CT scanograms.
Discussion Under the 1997 Medical Exposures Directive (97/43/EURATOM), diagnostic reference levels (DRLs) should be established for "medical radiodiagnostic practices" in "groups of standard-sized patients" using "broadly defined types of equipment". CT scanners and plain radiography units clearly form distinct types of equipment on which leg lengths can be measured. Given the large apparent dose differences between the two techniques, we would recommend that individual departments should determine DRLs for each technique and, where possible, should recommend CT as the lower dose method.
O'Neil K, Lecomber AR, Laidlow D, Asaad S, Tennant D. Poster presentation. Evaluation of CT scanography as a simple and reliable low-dose technique. 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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