A Non-Invasive Method to Control the Tube Current Calibration of Diagnostic Radiology Equipment

The correct calibration of the tube current of diagnostic X ray equipment is important to ensure optimal image quality. This decreases the number of retakes which will reduce the radiation dose to the patient and the radiation worker. The direct measurement of the tube current is time-consuming and is not part of the non-invasive quality control programme. A technique was therefore developed where readings from a NERO system were used to control the tube current calibration of diagnostic X ray equipment. If the tube potential and timer are accurately calibrated, it is possible to derive the consistency of the tube current calibration from the exposure measurement. The exposure of 27 X ray machines was measured using a 65 cm focus-detector distance at 80 kV. Previous tests on the different machines showed that the calibration of the timer and tube potential was correct within 5%. The half value layer (HVL) for each machine was determined at 80 kV. The range of HVL values was from 2.44 to 3.62 mm Al at 80 kV and the corresponding exposure from 0.06 to 0.19 mGy (mA.s)-1 (95% confidence level). If the exposure is not within these limits with a correct tube potential and timer calibration, it will be indicative of a faulty tube current value. A non-invasive method was developed to control the tube current calibration of diagnostic X ray machines and this study showed that it could be implemented successfully.