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Determination of Calibration Cycle for Computed Tomography at Cancer Institute of Guyana

Nirvanie Amisha Sukdeo, Petal P Surujpaul and Sayan Chakraborty

Objective: The research aims to evaluate the optimum time for calibration cycle for the 4 slice GE lightspeed QXi CT unit. The workload and X-ray tube output were assessed in order to evaluate the performance of the X-ray tube. The Computed Tomography Dose Index (CTDI) and Dose Length Product (DLP) were the scan parameters assessed to evaluate X-ray tube output and was compared to the International Atomic Energy Agency (IAEA) standards.

Method: The CTDI phantom and a Raysafe X2 CT Calibration detector were used to obtain measured CTDI and DLP values for the common CT protocols (head, neck, sinus and chest). Peripheral CTDI phantom measurements were taken and compared to the displayed CTDI values for the scan protocols. CTDI_air measurements were taken as a control and to confirm output consistency. Patient measurements were also done for comparative purposes. Exposure/electro-technical parameters were also recorded and compared. The workload of the institution was calculated for a time period of three months (January, February and March) in the year 2018. These measurements were compared to the National Council on Radiation Protection (NCRP 147) standards. A Geometric Distribution was conducted where the Raysafe X2 pencil ionization chamber was placed at varying distances from isocenter along the width of the scanner couch. Air measurements for Head and Abdomen protocols were measured and compared.

Results: The results obtained showed significant variations in CTDI readings for the head, neck, sinus and chest protocols. The variation of displayed and measured CTDI and DLP readings were due to the exposure time, pitch factor, fluence rate and X-ray tube heating. There was larger variation of the pitch factor in Air measurements as compared to patient and phantom readings. X-ray tube heating was prevalent for air measurements done for the protocols. The fluence rate was the major factor that varies the patient measurements. Variations of the Geometric Distribution were attributed to the influence of the anode heel effect.

Conclusion: The calibration cycle was determined by evaluating percentage variation between preliminary and final readings for a period of one year. The variation in DLP and CTDI values was found to be 6% which was within the IAEA standards. Therefore, the time for the calibration cycle was determined to be at least twice per year.