Development and performance measurements of a micro-dose calibrator

Objectives: A prototype of a new first in class micro-dose calibrator has been designed and built. The micro-dose calibrator is designed to measure, using a 60 second integration time, a dose with an accuracy of 1% within an activity range of 1 KBq up to 3.5 MBq. Methods: Using the design concept of a segmented well counter, 8 detector modules made up of an isosceles trapezoid shaped NaI crystal coupled to a photomultiplier tube (PMT) for readout were built and assembled to form the body of the micro-dose calibrator. The NaI crystal portion of the detector module is 112.9 mm long, has an inner width of 21.04 mm, outer width of 46.08 mm and a depth of 30.23 mm. The acute angle is 67.5 degrees. With the 8 detector modules assembled into an octagon, the inner bore is 50.08 mm in diameter with a depth of 112.9mm. The NaI/PMT detector modules are each independently read out using 250MS/s digitizers. The digitizers generate a data packet for each pulse, containing the integrated charge of the pulse and a corresponding time stamp. Using a pipeline coincidence processing algorithm, N fold coincidence events are tagged and recorded as the data is passed through the acquisition system. Event rates are measured by summing singles rates, 2 fold up to 8 fold coincidence rates. This data processing enables the 8 individually operated and readout detector modules to act as one NaI crystal but with 8 times the bandwidth allowing the device to measure higher activity rates than a conventional single crystal NaI well counter. To extend the operational range of the device, a three-function pulse pileup live time model corrects for non-linear response at higher data rates consisting of a unitary, quadratic and linear functions. Results: The range of the micro-dose calibrator for an ¹⁸F radioactive source is between 1 KBq and 3.5 MBq maintaining less than 1% deviation from a linear response. At the top end of the activity range, the micro-dose calibrator records over 1.5 M correlated events/sec. The low end of the activity is limited by background rate which can range between 50 and 1K correlated events/sec depending on the surrounding ambient radiation typical of a hot lab. The activity of a 30 KBq ¹³⁷Cs point source was measured once every 24 hours over a period of 10 days using a 60 second integration period. The background subtracted rate measurement recorded a one sigma deviation from the mean of 0.168%. Various radioisotopes were placed at 5mm spacing intervals along the center axis of the micro-dose calibrator. The recorded activity of these sources was with a +/- 1% range in the center 4 cm of the FOV. Conclusions: The micro-dose calibrator is designed to provide accurate measurement of activity levels below the sensitivity of a standard dose calibrator which is limited down to 370 KBq and above the operational range of a standard well counter which is limited up to 10-20 KBq. The physical design of the device allows one to measure object sizes as large as 50mm diameter x 40 mm length which cannot be accomplished with a standard well counter, this includes the critical direct measurement of a dose of activity in a syringe before it’s injected into the animal being studied. This instrument is designed for experiments and research involving bio-distributions, cell labeling and other pre-clinical research activities in which doses below the sensitivity of a dose calibrator are being used. Funded by the NCI Contract No. HHSN261200800001E.

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