- Artefact report
- Open Access
Significant artefactual noise in 90Y TOF-PET imaging of low specific activity phantoms arises despite increased acquisition time
EJNMMI Physics volume 6, Article number: 20 (2019)
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.
A Jaszczak Deluxe phantom was set vertically (Fig. 1). The hot rod insert (inner rods diameters 4.8, 6.4, 7.9, 9.5, 11.1, 12.7 mm) was partially filled with different solutions: first, with a 18F-FDG solution corresponding to a mean liver SUV = 2 in a 300-MBq 75-kg patient study; secondly, with a 2.65-GBq 90Y—300 ml DTPA water solution, giving a 18-mm height filling of the rods, which resulted in a ≈ 100 Gy mean absorbed dose in each filled sector slices. A 20-cm-thick attenuating medium, set under the phantom, modelled the transverse attenuation observed for the fully filled phantom set in conventional horizontal position.
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.
Hot rods were individually visualized in five sectors for the 18F setup and for the 100 Gy × 20 min/bed-90Y setup as well; this later setup was affected only by a little higher noise level (Fig. 2). A clear degradation of the hot rods intensities was observed for the 6.4, 7.9, and 9.5 mm rods in the 10 Gy × 200 min/bed setup while all sectors were dramatically impacted in the 2 × 5 Gy × 200 min/bed despite the exact acquisition time compensation in both setups.
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.
Availability of data and materials
There are available.
Conti M. Effect of randoms on signal-to-noise ratio in TOF PET IEEE Trans. Nucl. Sci. 2006;53:1188–93.
Walrand S, Hesse M, Chiesa C, Lhommel R, Jamar F. The low hepatic toxicity per Gray of 90Y glass microspheres is linked to their transport in the arterial tree favoring a nonuniform trapping as observed in posttherapy PET imaging. J Nucl Med. 2014;55:135–40.
Högberg J, Rizell M, Hultborn R, et al. Heterogeneity of microsphere distribution in resected liver and tumour tissue following selective intrahepatic radiotherapy. EJNMMI research. 2014;4:48.
van de Maat GH, Seevinck PR, Elschot M, et al. MRI-based biodistribution assessment of holmium-166 poly(L-lactic acid) microspheres after radioembolisation. Eur Radiol. 2013;23:827–35.
d’Abadie P, Hesse M, Jamar F, Lhommel R, Walrand S. 90Y TOF-PET based EUD reunifies patient survival prediction in resin and glass microspheres radioembolization of HCC tumours. Phys Med Biol. 2018;63:245010.
Van Elmbt L, Vandenberghe S, Walrand S, Pauwels S, Jamar F. Comparison of yttrium-90 quantitative imaging by TOF and non-TOF PET in a phantom of liver selective internal radiotherapy. Phys Med Biol. 2011;56:6759.
Nobody or organization to acknowledge.
This study was not funded.
Ethics approval and consent to participate
N/A (no human, no animal in the study)
Consent for publication
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
About this article
Cite this article
Hesse, M., Walrand, S. Significant artefactual noise in 90Y TOF-PET imaging of low specific activity phantoms arises despite increased acquisition time. EJNMMI Phys 6, 20 (2019). https://doi.org/10.1186/s40658-019-0259-6