Analysis of performance of Active Pixel Sensor (APS) as sensing element for a Real-time Active Pixel Dosimeter for Interventional Radiology

Purpose: we present the results obtained considering a CMOS Active Pixel Sensor as an X-Ray radiation detector for the individual dosimetry of medical personnel involved in interventional radiology procedures. The RAPID project was approved and funded by the National Institute of Nuclear Physics (INFN). Methods and materials: A phantom made of 20x20x3 cm3 PMMA slabs was used to diffuse the X-ray photons from an interventional angiography system. The sensor, five TLDs and a commercial active pixel dosimeter were mounted in a plastic holder that was moved along the z axis at a distance of 0 to 100 cm from the phantom (a typical range between medical staff and a patient during IR procedures). A two-threshold clustering algorithm has been implemented with the goal to obtain the number of detected photons in a frame and integrated signal due to photons over a frame. A photon is defined as a cluster of topologically connected pixels where at least one pixel signal is over a given threshold (10 times the single pixel noise) and all of the rest is over a second threshold (3 times the single pixel noise). The reconstructed photon signal is the sum of all the pixel signals belonging to the cluster. Results: Data were recorded varying the X-ray tube settings (continuous/pulsed mode, kV, current, pulse parameters), the sensor parameters (gain, integration time) and the relative positions between sensor and phantom (distance, orientation). A strong correlation has been observed among the two dosimetric observables, with relative uncertainties less than 5%. The sensor response has been compared with measurements performed using both TLD and the commercial active pixel dosimeter for the observables. A reasonable linearity could be observed in all cases and for the dosimetric variables, the correlation holds for both pulsed and continuous mode. Finally changing the tube voltage did not change the linearity of the sensor response. In order to obtain a relative calibration, the sensor has also been exposed to a certified X-ray beam, the results show a linear correlation between the observables and the certified dose rate. Conclusion: The operation of an Active Pixel Sensor as sensing element for monitoring individual dosimetry during interventional radiology procedures has been verified, obtaining a precision in the measurement of dose and dose-rate better than 10%, even for the most demanding protocols.