THE USE OF THE EXTENDED FILM-FOCUS DISTANCE TECHNIQUE FOR DOSE REDUCTION: EXAMINATION OF THE THORACIC (RIB) CAGE

Abstract

Aim: The technique of extended Film-Focal Distance (FFD) on chest radiographs was used to determine the optimum FFD and dose parameters to ensure safe radiological practices according to the ALARA principle, without compromising the image quality which is required by the Radiologists to accurately interpret and conclude the diagnosis from Chest X-Ray (CXR) lms. Materials and methods: PHILIPS MCD-105 mobile portable X-Ray machine with a maximum voltage of 105 kVp was used in this study. AGFA X-ray lms were used to obtain all the chest radiographs. A perspex phantom was constructed to house the rib cage of a human adult obtained from the anatomy laboratory. The whole phantom arrangement was to simulate the chest part of an adult human. The phantom was exposed ve times maintaining a constant voltage of 70 kV and tube load of 10 mAs by varying the FFDs at 110, 120, 130, 140, and 150 cm. A RaySafe Thin-X RAD dosemeter was used to determine the input and output radiation doses during each exposure at varying FFD values. Results and Discussion:The maximum value of the absorbed dose recorded in this study was 335 μGy at FFD 110 cm and this value kept decreasing with increasing FFDs with its lowest value at FFD at 150 cm, which is 222 μGy. This conrms that FFD has a signicant effect on the dose delivered according to inverse square law on chest X-ray examinations. Conclusion: Even though the radiograph at FFD 150 cm had the best contrast of them all due to low exposure but the radiograph at FFD 140 cm is found to give the optimum image quality with a dose even lower than the internationally accepted maximum value.