S2089
Physics - Image acquisition and processing
ESTRO 2026
Biograph mCT) and 3T PET/MRI system (Siemens Biograph mMR). The phantom was scanned with sphere concentrations of 2.8--9.0 kBq/ml mimicking hot lesions. 10-minute scans were repeatedly acquired in list-mode with different background activity in water (zero and background-to-sphere activity concentration ratios 1:8 and 1:4). For PET/MRI, custom attenuation maps (µ-maps) were derived from CT data of the phantom and installed on the system for in-line use. Reconstruction parameters were systematically varied. PET datasets were analyzed using CT-defined volumes- of-interest placed in spheres and background using PMOD. Recovery coefficients (RC) and contrast-to- noise-ratios (CNR) were computed for each sphere concentration according to NEMA guidelines. Results: In PET scans with zero background activity and clinical protocols, all spheres were discernible, and RCs ranged 0.03--0.40 (18F-FDG) and 0.07--0.25 (68Ga) in micro-spheres vs. 0.43--1.00/0.36--0.88 in NEMA spheres (Fig2a). The algorithms OSEM-PSF (PET/MRI) and OSEM-PSF-TOF (PET/CT) at 4-mm filter performed best; matrix size had little effect. Spheres as small as 6 mm were detectable with 2 to 3-minute scans for 18F- FDG at 1:8 background activity (CNR ≥ 5 ("Rose criterion"), Fig2b). For 68Ga, 8-mm spheres were detectable with 3-minute scans on PET/CT, whereas at least 7 minutes was needed on PET/MR to approach a CNR=5. 4 and 5-mm micro-spheres did not meet the detectability criterion on either system in the presence of background activity. Conclusion: A customized NEMA phantom for evaluating PET image quality in sub-centimeter uptake volumes was developed and successfully tested with two common PET tracers. Ongoing work will evaluate repeatability and robustness. The phantom facilitates comprehensive image quality analysis for small lesions, particularly relevant for next-generation PET systems and QIBs in radiotherapy.
Keywords: Quantitative imaging, PET, phantom study
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