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68Ga-PSMA-11 dose reduction for dedicated pelvic imaging with simultaneous PET/MR using TOF BSREM reconstructions



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.


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.


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.


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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Fig. 1
Fig. 2
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Fig. 4



Glucose analog tracer labeled with fluorine-18, (2-deoxy-2-(18F)fluoro-d-glucose)


Tracer labeled with gallium-68, targeting the prostate specific membrane antigen (PSMA)

β (beta):

Regularization parameter in BSREM


Block sequential regularized expectation maximization




Maximum intensity projection


Multiparametric magnetic resonance imaging


Ordered subset expectation maximization


Prostate cancer


Prostate-specific membrane antigen


Standardized uptake value




Volume of interest


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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.


The University Hospital Zurich receives institutional grants from GE Healthcare.

Author information

Correspondence to Edwin E. G. W. ter Voert.

Ethics declarations


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

Ter Voert EEGV, Muehlematter UJ, Delso G et al (2018) Quantitative performance and optimal regularization parameter in block sequential regularized expectation maximization reconstructions in clinical (68)Ga-PSMA PET/MR. EJNMMI Res 8:70

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

Kranzbuhler B, Nagel H, Becker AS et al (2018) Clinical performance of (68)Ga-PSMA-11 PET/MRI for the detection of recurrent prostate cancer following radical prostatectomy. European Journal of Nuclear Medicine and Molecular Imaging 45:20-30


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• 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 (2020). https://doi.org/10.1007/s00330-020-06667-2

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  • 68Ga-PSMA-11
  • Positron-emission tomography
  • Magnetic resonance imaging
  • Radiation dosage
  • Pelvis