A NONINVASIVE METHOD TO CONTROL THE TUBE CURRENT CALIBRATION OF DIAGNOSTIC-RADIOLOGY EQUIPMENT

The correct calibration of the tube current of diagnostic X ray equipm ent is important to ensure optimal image quality. This decreases the n umber of retakes which will reduce the radiation dose to the patient a nd the radiation worker. The direct measurement of the tube current is time-consuming and is not part of the non-invasive quality control pr ogramme. A technique was therefore developed where readings from a NER O system were used to control the tube current calibration of diagnost ic X ray equipment. If the tube potential and timer are accurately cal ibrated, it is possible to derive the consistency of the tube current calibration from the exposure measurement. The exposure of 27 X ray ma chines was measured using a 65 cm focus-detector distance at 80 kV. Pr evious tests on the different machines showed that the calibration of the timer and tube potential was correct within 5%. The half value lay er (HVL) for each machine was determined at 80 kV. The range of HVL va lues was from 2.44 to 3.62 mm Al at 80 kV and the corresponding exposu re from 0.06 to 0.19 mGy (mA.s)(-1) (95% confidence level). If the exp osure is not within these limits with a correct tube potential and tim er calibration, it will be indicative of a faulty tube current value. A non-invasive method was developed to control the tube current calibr ation of diagnostic X ray machines and this study showed that it could be implemented successfully.