Calibrated bridging of 60- and 90-min 18F-flutemetamol PET on digital SiPM PET/CT: a within-subject agreement study

This study evaluated whether delayed static 18 F-flutemetamol PET acquired at 60 min post-injection on digital SiPM PET/CT can be quantitatively bridged to the conventional 90-min Centiloid (CL) scale while preserving visual interpretability. Methods: In the derivation cohort, 63 participants underwent paired 60- and 90-min static 18 F-flutemetamol PET acquisitions on digital SiPM PET/CT systems. Three experienced readers independently performed visual assessment (VA) at each time point. Raw quantitative agreement between acquisition windows was first evaluated using SUVR and direct VIZCalc-derived CL values. A study-derived SUVR₆₀-to-CL₉₀ conversion equation was then developed and evaluated using Bland–Altman analysis, Lin’s concordance correlation coefficient (CCC), and leave-one-out cross-validation. The same equation was additionally applied without refitting to an independent internal validation cohort of 34 paired examinations. Results: The majority VA was concordant between 60 and 90 min in 62/63 examinations (98.4%; Cohen’s κ = 0.968). Raw SUVR₆₀ and SUVR₉₀, as well as raw CL₆₀ and raw CL₉₀, were highly correlated but showed systematic time-window differences and limited direct numerical agreement. After calibration, CL₉₀(est.60) showed excellent agreement with CL₉₀ in the derivation cohort (CCC = 0.997; bias − 0.01 CL; 95% limits of agreement, − 5.94 to 5.93 CL). In the independent internal validation cohort, CL₉₀(est.60) retained strong agreement with CL₉₀ (CCC = 0.997; bias − 1.78 CL; 95% limits of agreement, − 8.00 to 4.43 CL), with 30/34 examinations (88.2%) within ± 5 CL. CL-based classification showed high agreement with majority VA across prespecified thresholds. Voxel-wise analysis demonstrated time-window-dependent differences predominantly characterized by higher white matter signal at 90 min. Raw 60- and 90-min quantitative values for 18 F-flutemetamol PET are highly correlated but should not be regarded as directly numerically interchangeable. However, 60-min SUVR can be bridged to the conventional 90-min CL scale using a protocol-specific conversion equation, with preserved visual agreement and strong internal validation performance. These findings support the feasibility of a shortened delayed imaging protocol on digital SiPM PET/CT, provided that quantitative interpretation uses appropriate calibration and borderline cases are interpreted with caution.