ESTABLISHMENT OF RQT BEAM QUALITY (100 TO 150 kV) FOR COMPUTED TOMOGRAPHY APPLICATIONS

Abstract

In Malaysia, the radiation beam qualities for calibration of dosimeters in computed tomography (RQT series) were established at the Secondary Standard Dosimetry Laboratory (SSDL) of the Malaysian Nuclear Agency by using a constant potential industrial X-ray machine and a 0.6 cc PTW UNIDOS ionization chamber calibrated at the International Atomic Energy Agency (IAEA). Through the experimental method of additional filtration determination, the results demonstrated that all measured first half-value layer (HVL) filtrations (mm Al) for each RQT 8 (100 kV), RQT 9 (120 kV) and RQT 10 (150 kV) comply with tolerance limits of ± 3% as recommended by the IEC-61676. For RQT 8, a repeated experiment to determine accurate additional filtration has to be done with different added filtration RQT (mm Cu) thickness as the value initially goes beyond the 3% difference. Compared to prior radiation quality series RQT determinations, the added filtration for RQT 8 changed from 0.1 mm Cu in 2019 to 0.2 mm Cu in 2020. When compared against values established over the previous three years, all three RQTs for 2020 exhibit differences in measured first HVL filtrations (mm Al), albeit remaining within the 3% difference standard recommended in the TRS 457 standard of the IAEA. This change arises from x-ray tube aging, anode roughening and inherent filtration alteration, leading to a perceived need for RQT trial and error re-evaluation to reduce the percentage decrement. Yearly monitoring of the beams should be performed to determine possible radiation quality changes, taking corrective action where necessary to remain within the prescribed tolerance limit. The standard radiation qualities should be maintained, allowing for calibration accuracy to confirm dosimeter readings.