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personnel was definitely lower and the risk of contamination during phantom manipulation was eliminated. The acquisition was performed exactly in the same way and with the same time needed for a patient. At the end of CTQ the 68Ge phantom was packed and shipped to the next PET/CT site. Using the 68Ge phantom we observed a variability in BAC (ISV=24.4%) 1/3 lower than in 18F phantom (ISV=67.8%). The variability in SBR (ISV=19.8%) was 1/3 of what was found with 18F phantoms (ISV=50.6%). The drastic drop in variability was only due to phantom preparation. Initially, at the start of the CTQ program, a re-calibration of the PET/CT scanners was required when BAC variability was higher than 10%. The large reduction in inter-scanner variability was confirmed also when comparing only the 11 PET/CT scanners that performed the CTQ both with the traditional 18F and with the 68Ge IQ phantoms. The ISV decreased from 74.0% to 20.5% in BAC and from 63.3% to 17.4% in SBR.
The experience with the 68Ge phantom prompted us to introduce a new type of 68Ge phantom.
Two spheres (one with a 6mm diameter and one with a lower SBR value respect the others) are added to the 6 classical spheres of the NEMA phantom. This phantom could be used to evaluate and optimize the new reconstruction algorithm (i.e. with resolution recovery), to evaluate the noise of the PET/CT scanner, to choose the best segmentation algorithm and to done cross-calibration between the PET/CT scanner and the dose calibrator (using a sealed source of 68Ge). This characteristics allow to introduce a new limit for the CTQ procedure: BAC ±3%, SBR ±5%, RC ±10% and dose calibrator ±3%.
Tesi di Specialità
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