Technical Note: Transconvolution based equalization of positron energy effects for the use of 68Ge/68Ga phantoms in determining 18F PET recovery

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Abstract

Purpose

Avoiding measurement variability from 18F phantom preparation by using 68Ge/68Ga phantoms for the determination of 18F recovery curves (RC) in clinical quality assurance measurements and for PET/CT site qualification in multicentre clinical trials.

Methods

RCs were obtained from PET/CT measurements of seven differently sized phantom spheres filled either with 18F or with 68Ga. RCs for the respective other isotope were then determined by two different methods: In the first method, images were convolved with positron range transconvolution functions derived from positron annihilation distributions found in literature. This method generated recasted images matching images using the respective other isotope. In the second method, the PET/CT system's isotope independent (intrinsic) point spread function was determined from said phantom measurements and convolved with numerical representations simulating hot spheres filled with the respective other isotope. These simulations included the isotope specific positron annihilation distributions. Recovered activity concentrations were compared between recasted images, simulated images, and the originally acquired images.

Results

18F and 68Ga recovery was successfully determined from image acquisitions of the respective opposite isotope as well as from the simulations. 68Ga RCs derived from 18F data had a normalized root-mean-square deviation (NRMSD) from real 68Ga measurements of 0.019% when using the first method and of 0.008% when using the second method. 18F RCs derived from 68Ga data had a NRMSD from real 18F measurements of 0.036% when using the first method and of 0.038% when using the second method.

Conclusions

Applying the principles of transconvolution, 18F RCs can be recalculated from 68Ga phantom measurements with excellent accuracy. The maximal additionally introduced error was below 0.4% of the error currently accepted for RCs in the site qualification of multicentre clinical trials by the EARL program of the European Association of Nuclear Medicine (EANM). Therefore, our methods legitimately allow for the use of long-lived solid state 68Ge/68Ga phantoms instead of manually prepared 18F phantoms to characterize comparability of 18F measurements across different imaging sites or of longitudinal 18F measurements at a single PET/CT system.

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