Abstract
Objectives
When increasing the PET acquisition time to match the longer MRI protocol in simultaneous PET/MR, the injected PET tracer dose can possibly be lowered to reduce radiation exposure. Moreover, applying new commercially available time-of-flight (TOF) block sequential regularized expectation maximization (BSREM)–based reconstruction algorithms could allow for further dose reductions. The purpose of this study was to find the minimal dose of the tracer targeting the prostate specific membrane antigen (68Ga-PSMA-11) for a dedicated 15-min pelvic PET/MR scan that still matches the image quality of a reference 3-min scan at 100% (150 MBq) dose.
Methods
In this retrospective analysis, 25 patients were included. PET emission datasets were edited to simulate stepwise reductions of injected tracer dose. Reference TOF ordered subset expectation maximum (OSEM) and new TOF BSREM reconstructions were performed and differences in the resulting PET images were visually and quantitatively assessed.
Results
Visually, TOF BSREM reconstructions with relatively high regularization parameter (β) values are preferred. Quantitatively, however, high β-values result in lower lesion maximum standardized uptake values (SUVmax) compared to the reference. A β-value of 550 was considered the optimal compromise for the lowest possible 10% dose reconstructions, resulting in comparable visual assessment and lesion SUVmax.
Conclusions
This study indicates that the injected 68Ga-PSMA-11 tracer dose for a standard 3-min PET scan can be reduced to approximately 10% (15 MBq) when the PET acquisition time is matched to the 15-min pelvic MRI protocol, and when reconstructed with TOF BSREM using β = 550. This decreases the effective dose from 3.54 to 0.35 mSv.
Key Points
• Low-dose dedicated pelvic 68 Ga-PSMA-11 PET/MR reduces radiation exposure for patients.
• Retrospective study investigating the minimal dose needed for adequate image quality for 15-min PET frames over the pelvis showed using quantitative and qualitative analysis that a substantial dose reduction is possible without significant loss of image quality when using the TOF BSREM reconstruction algorithm.
• With the introduction of low-dose pelvic 68 Ga-PSMA-11 PET/MR, new potential applications of 68 Ga-PSMA-11 PET for local staging or investigation of equivocal MRI findings could become applicable, even for patients without confirmed prostate cancer.
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Abbreviations
- 18F-FDG:
-
Glucose analog tracer labeled with fluorine-18, (2-deoxy-2-(18F)fluoro-d-glucose)
- 68Ga-PSMA-11:
-
Tracer labeled with gallium-68, targeting the prostate specific membrane antigen (PSMA)
- β (beta):
-
Regularization parameter in BSREM
- BSREM:
-
Block sequential regularized expectation maximization
- FoV:
-
Field-of-view
- MIP:
-
Maximum intensity projection
- mpMRI:
-
Multiparametric magnetic resonance imaging
- OSEM:
-
Ordered subset expectation maximization
- PCa:
-
Prostate cancer
- PSMA:
-
Prostate-specific membrane antigen
- SUV:
-
Standardized uptake value
- TOF:
-
Time-of-flight
- VOI:
-
Volume of interest
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Acknowledgments
The authors would like to thank the study patients for their consent, T. Deller and K. Wangerin (GE Healthcare) for the technical support, GE Healthcare for the PETtoolbox, and the technicians of the “Wagi”-team (Marlena Hofbauer, Miguel Porto, Sofia Kaltsuni, Tobias Oblasser and Sabrina Epp, Melanie Thüringer, and Michele Hug) for their excellent work on the PET/MR.
Funding
The University Hospital Zurich receives institutional grants from GE Healthcare.
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Guarantor
The scientific guarantor of this publication is I.A. Burger.
Conflict of interest
This research was performed at the Department of Nuclear Medicine of the University Hospital Zurich which holds an institutional Research Contract with GE Healthcare. Author G. Delso is employed by GE Healthcare. Author I.A. Burger has received research grants and speaker honoraria from GE Healthcare. The other authors of this manuscript declare no relationships with any companies, whose products or services may be related to the subject matter of the article.
Statistics and biometry
No complex statistical methods were necessary for this paper.
Informed consent
All patients included gave written informed general consent for retrospective analysis of their data.
Ethical approval
This retrospective study has been approved by the cantonal ethics committee (BASEC-Nr. 2016-02230).
Study subjects or cohorts overlap
Some study subjects or cohorts could have been previously reported in one of our other (unrelated) studies about, e.g., 68Ga-PSMA-11 and PETMR like:
Rupp NJ, Umbricht CA, Pizzuto DA et al (2019) First clinico-pathological evidence of a non PSMA-related uptake mechanism for (68)Ga-PSMA-11 in salivary glands. Journal of Nuclear Medicine. https://doi.org/10.2967/jnumed.118.222307
Muller J, Ferraro DA, Muehlematter UJ et al (2019) Clinical impact of (68)Ga-PSMA-11 PET on patient management and outcome, including all patients referred for an increase in PSA level during the first year after its clinical introduction. European Journal of Nuclear Medicine and Molecular Imaging 46:889-900
Burger IA, Muller J, Donati OF et al (2019) (68)Ga-PSMA-11 PET/MR Detects Local Recurrence Occult on mpMRI in Prostate Cancer Patients After HIFU. Journal of Nuclear Medicine 60:1118-1123
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Pizzuto DA, Muller J, Muhlematter U et al (2018) The central zone has increased (68)Ga-PSMA-11 uptake: “Mickey Mouse ears” can be hot on (68)Ga-PSMA-11 PET. European Journal of Nuclear Medicine and Molecular Imaging 45:1335-1343
Muehlematter UJ, Rupp NJ, Mueller J, Eberli D, Burger IA (2018) 68Ga-PSMA PET/MR-Positive, Histopathology-Proven Prostate Cancer in a Patient With Negative Multiparametric Prostate MRI. Clinical Nuclear Medicine 43:e282-e284
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Methodology
• Retrospective
• Experimental
• Performed at one institution
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Svirydenka, H., Muehlematter, U.J., Nagel, H.W. et al. 68Ga-PSMA-11 dose reduction for dedicated pelvic imaging with simultaneous PET/MR using TOF BSREM reconstructions. Eur Radiol 30, 3188–3197 (2020). https://doi.org/10.1007/s00330-020-06667-2
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DOI: https://doi.org/10.1007/s00330-020-06667-2