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New NIST-Traceable Standard for Accuracy in Quantitative Nuclear Medicine Imaging

About 2.5 million Positron Emission Tomography (PET) procedures were expected to be carried out in 2009 according to a recent Frost and Sullivan market report. The majority (over 90 %) of those studies utilize [18F]fluorodeoxyglucose (FDG)  as the imaging agent. As the technology has matured, PET is increasingly being used a quantitative technique to monitor disease response to treatment. The quantification accuracy of the acquired images strongly depends on the accuracy of the measurements of the radioactivity content of the 18F that is injected into the patient, as well as the accuracy of the calibration of the PET scanner. Re-entrant ionization chambers, often called "dose calibrators", are used in the clinics to perform both of these measurements. Until recently, however, there was no long-lived NIST-traceable standard for 18F, making it impossible to reference activity measurements of this radionuclide to a common standard. Working in collaboration with RadQual, LLC, researchers in the NIST Radioactivity Group recently developed a calibration methodology for a new 68Ge-based calibration source developed for this purpose. 

This source, designed and marketed by RadQual, LLC, was calibrated against the NIST standard for 68Ge with a combined relative standard uncertainty of about 0.5 %.  In a separate experiment, the same source geometry was used to make a direct comparison between the 68Ge and 18F NIST standards to determine a relative response factor for 18F in commercially-available dose calibrators. The nuclear medicine community now has the ability to use a long-lived source (271 d) to calibrate dose calibrators for 18F with a standard uncertainty of less than 1 %.

As a test of the usefulness of this new source, three calibrated sources were sent to M. D.Anderson Medical Center, the University of Washington Medical Center and the University of Oklahoma Medical Center for measurement in their respective dose calibrators (39 in all) at the dose calibrator manufacturers' recommended settings for 18F. The results showed that for the common dose calibrator brand, the recommended setting gave a measurement for 18F that was in error by almost +7 %. Another manufacturer's recommended setting for the same radionuclide gave a result that was low by between 5 and 9 %. Although similar results had been observed by NIST in earlier, small-scale experiments, this was the first time that such a large study has been conducted. With the availability of this new standard, clinical sites can now directly calibrate their dose calibrators for 18F in a way that is traceable to NIST.

The results of this work highlight the importance of standards in nuclear medicine imaging. The availability of a NIST-traceable standard for 68Ge with a link to the NIST 18F standard provides the nuclear medicine community with the tools it needs to realize the true quantitative potential of PET, thereby enhancing patient care.   

Brian Zimmerman