Skin sparing in interventional radiology: the effect of copper filtration

Complex and lengthy interventional radiological techniques have resulted in a number of patients developing skin reactions in recent years. To safegua rd against these side effects, we have investigated the degree to which ent rance skin dose can be reduced by inserting 0.18 mm and 0.35 mm copper filt ration in the incident beam. The potential reduction was measured on a 22 c m water phantom for each of eight models of a fluoroscopy unit. Using the c atheter laboratory fluoroscopy unit on which radiofrequency ablations are r outinely performed, we assessed the relative effectiveness of adding filtra tion and increasing the kV:mA ratio. Image quality was subjectively assesse d for diagnostic and therapeutic acceptability in two groups of 10 patients undergoing radiofrequency ablations, pacemaker insertions or electrophysio logy studies. One of the groups was screened with 0.35 mm copper filtration in place and the other group acted as the control. Maximum patient skin do se proved difficult to measure directly because of the unpredictable dose p attern. This pattern was studied in four patients using a film method in co njunction with thermoluminescent dosemeters. Copper filtration 0.35 mm thic k inserted in the beams of the eight fluoroscopy units produced a mean redu ction in entrance dose to the phantom of 58% with a mean increase in tube l oading of 29%. At 100 kV the increased loading on the X-ray tube was equiva lent to increasing the anteroposterior separation of the patient by 2 cm. M easurements on the catheter laboratory unit showed that the tube voltage wo uld need to be raised above the normal diagnostic range to obtain an equiva lent entrance dose reduction without the filter. The blackening of films un der the patients showed complex patterns, but the estimated skin doses were consistent with those predicted by the phantom experiments. All six cardio logists considered there to be insignificant detriment to image quality in the procedures investigated.