The aim of this study was to investigate the limits of diagnostics and therapy planning for patients with prostate cancer using non-time-of-flight 18F/68Ga-PSMA PET/MRI under clinically challenging imaging conditions with small lesion sizes and low uptake. Lesion detectability and quantification accuracy were evaluated for different acquisition and reconstruction parameters in a systematic phantom study and subsequent on patient data.
PET/MRI measurements were performed using a small lesion NEMA phantom. PET data were acquired for nine different activity concentrations (AC). Data of a longer single-bed protocol in the pelvis or a shorter whole-body protocol were reconstructed using relative or absolute scatter correction (SC). PET images were analysed considering a ± 25% deviation range between imaged and true AC as acceptable. Thirteen PSMA-PET/MRI patients with primary lesions or lymph node metastasis < 12 mm in the pelvis were included in this study. The presence of the halo artefact was evaluated in six 18F-PSMA and seven 68Ga-PSMA PET/MRI patients. For 21 lesions (diameter 6.4-12.3 mm) in total, the AC was quantified.
For both radiotracers, the 9.7 mm sphere was still visible at 0.16 kBq/mL with emission times > 40 min. The 3.7 mm sphere was only detectable at 22 kBq/mL with emission times > 4 min. All spheres ≥ 6.5 mm provide acceptable quantification at an AC of 1.32 kBq/mL for 18F PET/MRI protocols of ≥ 12 min and 2.75 kBq/mL for 68Ga PET/MRI protocols. In phantom data, no halo artefact was observable and different SC methods had no impact on quantification. 4/6 18F-PSMA patients and 7/7 68Ga-PSMA patients showed a halo around the bladder using relative SC, which could be reduced in all patients using absolute SC. Comparing the minimum quantifiable AC (MQAC) from in the phantom study as a threshold to the patient data, all lesions provided acceptable quantification with values > MQAC (AC 3.3-108.5 kBq/mL) for equal reconstruction and acquisition parameters.
The results demonstrated that the detection of lesions in the sub-centimetre range and a reliable quantification of 18F/68Ga-PSMA uptake using standard acquisition and reconstruction parameters within clinical PET/MRI protocols is possible. This allows for an individual assessment of potential therapy options for each patient.
EJNMMI physics. 2026 Jun 15 [Epub ahead of print]
Maike E Lindemann, Walter Jentzen, Alina Küper, Pedro Fragoso Costa, Marcel Gratz, Lale Umutlu, James Nagarajah, Stephan G Nekolla, Ivo Rausch, Ken Herrmann, Harald H Quick, David Kersting
High-Field and Hybrid MR Imaging, University Hospital Essen, University of Duisburg-Essen, Essen, Germany. ., Department of Nuclear Medicine, University Hospital Essen, University of Duisburg-Essen, Essen, Germany., High-Field and Hybrid MR Imaging, University Hospital Essen, University of Duisburg-Essen, Essen, Germany., Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen, University of Duisburg-Essen, Essen, Germany., Department of Radiology and Nuclear Medicine, Radboud University Medical Center, Nijmegen, The Netherlands., Department of Nuclear Medicine, Klinikum Rechts Der Isar, School of Medicine and Health, Technical University of Munich, Munich, Germany., QIMP Team, Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria.