Room: AAPM ePoster Library
Purpose: To determine the stability of a scanning tank electrometer for operating a novel calibration ion chamber design in nuclear medicine.
Methods: An electrometer from a scanning tank (Standard Imaging DoseView) was tested first with a high-ohmic load (1TO resistor) and then with a novel design ion chamber in order to test its constancy over time. The electrometer was controlled via its RS-232 serial interface. A Matlab routine was written to implement the electrometer’s binary command set and command frames were sent and received from a desktop PC. After first zeroing the unit, constancy tests were carried out with a bias of -300V for up to 25 hours.
Results: When connected to the 1TO resistor, the ion chamber’s current first rose by up to 1.5 pA from its baseline value within the first 5 minutes. Within the first hour current did not vary more than 1.5 pA (<0.5%); after 25 hours the current had dropped by 3pA from its baseline value. The measured resistance was 0.96TO, i.e. within the 10% tolerance specification of the resistor. When connected to the ion chamber, an intial current spike of up to 20 pA was observed; after 10 minutes the current returned to within 0.5 pA of its baseline value.
Conclusion: The electrometer’s constancy is sufficient to provide reproducible measurements within 0.5% at 300pA within more than one hour from zeroing. At least 10 minutes are required after connecting an ion chamber until the electrometer’s readings are reproducible within 0.5 pA, owing to the initial current spike caused by the relatively large ion chamber (sensitive volume 89 ccm). Future tests with radioisotopes will investigate the system under realistic operating conditions.
Funding Support, Disclosures, and Conflict of Interest: Standard Imaging, Madison, WI, provided the DoseView electrometer.