PET-MRI imaging with non-standard isotopes

PET-MRI presents unique challenges in quantitation. Here, the multi-site, multi-disciplinary research team assesses the performance of the GE Signa PET MRI scanner with various non-standard isotopes.

With PET becoming more widely used, the transport logistics have allowed faster shipments of radioisotopes to small imaging centers. The majority of PET studies in clinical routine are still being performed with 18F, because of its physical properties combined with efficient transportation logistics which widely increase its availability. The same holds for 68Ga and 90Y, of which the use is not dependent on the availability of a cyclotron. However, the physical properties of the PET radioisotopes are quite different from 18F. 18F almost exclusively decays via positron emission (96.8%) and with a relatively low maximum energy of the positron of 0.6335 MeV. The maximum and mean range of 18F are equal to 2.4 and 0.6 mm. The other 3% of decays is via electron capture. .

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Background: Fully integrated PET/MR systems are being used frequently in clinical research and routine. National Electrical Manufacturers Association (NEMA) characterization of these systems is generally done with 18F which is clinically the most relevant PET isotope. However, other PET isotopes, such as 68Ga and 90Y, are gaining clinical importance as they are of specific interest for oncological applications and for follow-up of 90Y-based radionuclide therapy. These isotopes have a complex decay scheme with a variety of prompt gammas in coincidence. 68Ga and 90Y have higher positron energy and, because of the larger positron range, there may be interference with the magnetic field of the MR compared to 18F. Therefore, it is relevant to determine the performance of PET/MR for these clinically relevant and commercially available isotopes. Methods: NEMA NU 2–2007 performance measurements were performed for characterizing the spatial resolution, sensitivity, image quality, and the accuracy of attenuation and scatter corrections for 18F, 68Ga, and 90Y. Scatter fraction and noise equivalent count rate (NECR) tests were performed using 18F and 68Ga. All phantom data were acquired on the GE Signa integrated PET/MR system, installed in UZ Leuven, Belgium. Results: 18F, 68Ga, and 90Y NEMA performance tests resulted in substantially different system characteristics. In comparison with 18F, the spatial resolution is about 1 mm larger in the axial direction for 68Ga and no significative effect was found for 90Y. The impact of this lower resolution is also visible in the recovery coefficients of the smallest spheres of 68Ga in image quality measurements, where clearly lower values are obtained. For 90Y, the low number of counts leads to a large variability in the image quality measurements. The primary factor for the sensitivity change is the scale factor related to the positron emission fraction. There is also an impact on the peak NECR, which is lower for 68Ga than for 18F and appears at higher activities. Conclusions: The system performance of GE Signa integrated PET/MR was substantially different, in terms of NEMA spatial resolution, image quality, and NECR for 68Ga and 90Y compared to 18F. But these differences are compensated by the PET/MR scanner technologies and reconstructions methods. Keywords: PET/MR, NEMA NU 2–2007, 18F, 68Ga, 90Y

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