Significant artefactual noise in 90Y TOF-PET imaging of low specific activity phantoms arises despite increased acquisition time
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Volumes of usual PET phantoms are about four to sixfold that of a human liver. In order to avoid count rate saturation and handling of very high 90Y activity, reported TOF-PET phantom studies are performed using specific activities lower than those observed in liver radioembolization.
However, due to the constant random coincidence rate induced by the natural crystal radioactivity, reduction of 90Y specific activity in TOF-PET imaging cannot be counterbalanced by increasing the acquisition time. As a result, most 90Y phantom studies reported images noisier than those obtained in whole-body 18F-FDG, and thus advised to use dedicated noise control in TOF-PET imaging post 90Y liver radioembolization.
We performed acquisitions of the Jaszczak Deluxe phantom in which the hot rod insert was only partially filled with 2.6 GBq of 90Y. Standard reconstruction parameters recommended by the manufacturer for whole-body 18F-FDG PET were used.
Low specific activity setups, although exactly compensated by increasing the acquisition time in order to get the same number of detected true coincidences per millilitre, were impacted by significant noise. On the other hand, specific activity and acquisition time setup similar to that used in post 90Y liver radioembolization provided image quality very close to that of whole-body 18F-FDG.
This result clearly discards the use of low specific activity phantoms intended to TOF-PET reconstruction parameter optimization. Volume reduction of large phantoms can be achieved by vertically setting the phantoms or by adding Styrofoam inserts.
The unusual 32 positron emissions per million decays and the speckled pattern observed in 90Y TOF-PET liver images initiated the widespread paradigm that this modality requires dedicated noise control. This belief was consolidated by many studies using large volume phantoms involving low specific activities to avoid PET saturation resulting in doses as low as 5 Gy for some of them. In addition, the acquisition time increase was often too limited to get the same number of detected true coincidences per milliliter as in post 90Y liver radioembolization.
However, due to the constant random rate originating from the L[Y]SO crystal radioactivity jointly with the low 90Y positron yield, lower specific 90Y activities cannot be counterbalanced by increased acquisition times . Moreover, Monte Carlo simulations , ex vivo autoradiography  and 166Ho MRI  proved the speckled nature of the sphere distribution in the liver.
Two bed positions were acquired, and the times per bed position were 1.5 min for 18F and 20 min for 90Y, respectively. The 90Y phantom was further imaged for 200 min per bed position after a 10- and 20-fold factor decaying; for this last setup, two slices were summed together to exactly compensate the activity reduction. The Philips Gemini TF64 system had a TOF-fwhm of 550 ps. Standard FDG reconstruction parameters were used for all the acquisitions, i.e. 3 iterations × 33 subsets.
Despite a slightly higher noise level, the spatial resolution of the 90Y 100 Gy setup image using standard reconstruction parameters was very close to that of the WB 18F-FDG setup, explaining the recent success of TOF-PET-based EUD in dose-response prediction .
Although time acquisition compensated, the 10 Gy and 5 Gy setup images were impacted by significant artefactual noise which clearly rules out the use of low specific activity phantom setups intended to reconstruction parameter optimization.
Volume reduction of large phantoms can also be achieved by vertically setting the phantoms or by adding Styrofoam inserts . The phantom mean absorbed dose should always be reported.
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Both authors equally contributed to the study. Both authors read and approved the final manuscript.
This study was not funded.
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The authors declare that they have no competing interests.
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