The Use of a Novel Radiation Detector on Quantifying PET/Nuclear Medicine Occupational and Non-occupational Doses and Calibration of MOSFET Radiation Detectors against Effective Energy

Project 1: Dose reduction for PET technologists by the automatic dose draw/injection system

* Purpose: To evaluate the dose reduction by the installed automatic dose draw/injection machine.

* Materials & Methods: Six RadEye detectors were given to six PET technologists. A RadEye detector recorded data every 25 seconds throughout the day. Technologists logged their activities as follows: dose draw/injection, patient positioning, patient transport, patient care and non-specific. One technologist performed dose drawing/injection manually while others used the Trasis system. The Trasis machine was monitored with a Radeye detector during the period as well.

* Results: The average dose reduction brought by Trasis is 75% for dose draw and 70% for dose injection. Qualitatively, instead of dose draw/injection, patient positioning has become the most significant contributing factor to overall PET technologist dose. In addition, the current average daily dose for a PET technologist is about 0.03 mSv, which on average is 36% less than before [12]. PET technologists typically received a dose of 0.007 mSv from dose draw/injection, 0.005 mSv from patient transport, 0.013 mSv from patient positioning, 0.001 mSv from patient care, and 0.003 mSv from non-specific per working day. This would result in an annual dose of 8 mSv which is approximately 16% of occupational dose limit (50 mSv).

* Conclusions: The installation of automatic dose draw/injection machine has clear benefits to the PET technologists. The radiation doses for PET technologists are well within the annual limit of doses to occupational radiation workers.

Project 2: Validation of ceiling shielding in CT/PET room with RADEYE

* Purpose: To measure and the magnitude of scattered radiation levels in CT/PET suite and to evaluate the effectiveness of shielding of the ceiling.

* Materials & Methods: Six RadEye detectors were placed in the CT/PET room, four in the ceiling, and two at one meter above floor. A RadEye detector recorded data every 25 seconds throughout the day. The detector was turned on at the beginning of the day (6 am) and the doses were transferred to a laptop for analysis at the end of the day (5 pm). The dose to a non-radiation worker above the CT/PET room was estimated based on the ceiling data. The magnitude of transmitted CT radiation in the room above was measured separately with RadEye.

* Results: The CT dose contributed about 80% of the total dose while the PET contributed 20% within the scanning room. No dose contribution was measured above the floor from CT scanning. The combined dose from both PET and CT scan in a room above at 2.2 meter was 2.4x10-6 mSv per week, assuming an occupancy factor of 1.

* Conclusions: This study quantified the CT and PET doses contributions separately in the clinical CT/PET room. An analytical model was developed to calculate the non-occupational personnel dose above the CT/PET room and the calculated results were confirmed by physical measurements. The actual physical dose was much smaller than the NCRP design goals of 0.02 mSv/wk.

Project 3: Evaluating MOSFET dependency on effective energy over diagnostic energy range

* Purpose: To characterize MOSFET calibration factors (CF) as a function of effective energy over diagnostic energy range.

* Materials & Methods: Five new MOSFETs were used in the study. The calibration factors were measured in two ways: 1) fixed kVp, fixed SSD, fixed FOV, and varying filtration; and 2) fixed filtration, fixed SSD, fixed FOV, and varying kVp. Effective energy was computed as a function of kVp and filtration by SpekCalc.

* Results: CF was independent with HVL in the range of HVL = 5 to HVL = 9mm Al at a fixed 120 kVp. CF depended linearly with kilo-voltage (kVp) from 80 to 140 kVp at a fixed filtration. In addition, a strong non-linear correlation of average CF versus effective energy was generated for effective energies in the diagnostic range (Goodness of fit of 0.98).

* Conclusions: A correlation of second degree polynomial was seen between calibration factor and effective energy over diagnostic range. Hence, we created a calibration curve so that under a given fixed kVp or filtration, the calibration factor is automatically generated. A high correlation between CF versus effective energy was found over the diagnostic energy from 45 keV to 65 keV. This suggests that we could estimate the calibration factor with in-house generated MOSFET aging data, which would have a direct impacted CF linearly.