Differential centiloid scale normalization techniques: comparison between hybrid PET/MRI and independently acquired MRI

Objective

Centiloid (CL) scales play an important role in semiquantitative analyses of amyloid-β (Aβ) PET. CLs are derived from the standardized uptake value ratio (SUVR), which needs Aβ positron emission tomography (PET) normalization processing. There are two methods to collect the T1-weighted imaging (T1WI) for normalization: (i) anatomical standardization using simultaneously acquired T1WI (PET/MRI), usually adapted to PET images from PET/MRI scanners, and (ii) T1WI from a separate examination (PET + MRI), usually adapted to PET images from PET/CT scanners. This study aimed to elucidate the correlations and differences in CLs between when using the above two T1WI collection methods.

Methods

Among patients who underwent Aβ PET/MRI (using ¹¹C-Pittuberg compound B (¹¹C-PiB) or ¹⁸F‐flutemetamol (¹⁸F-FMM)) at our institution from 2015 to 2023, we selected 49 patients who also underwent other additional MRI examinations, including T1WI for anatomic standardization within 3 years. Thirty-one of them underwent ¹¹C‐PiB PET/MRI, and 18 participants underwent ¹⁸F‐FMM PET/MRI. Twenty-five of them, additional MRI acquisition parameters were identical to simultaneous MRI during PET, and 24 participants were different. After normalization using PET/MRI or PET + MRI method each, SUVR was measured using the Global Alzheimer’s Association Initiative Network cerebral cortical and striatum Volume of Interest templates (VOI) and whole cerebellum VOI. Subsequently, CLs were calculated using the previously established equations for each Aβ PET tracer.

Results

Between PET/MRI and PET + MRI methods, CLs correlated linearly in ¹¹C-PiB PET (y = 1.00x – 0.11, R² = 0.999), ¹⁸F-FMM PET (y = 0.97x – 0.12, 0.997), identical additional MRI acquisition (y = 1.00x + 0.33, 0.999), different acquisition (y = 0.98x – 0.43, 0.997), and entire study group (y = 1.00x – 0.24, 0.999). Wilcoxon signed-rank test revealed no significant differences: ¹¹C-PiB (p = 0.49), ¹⁸F-FMM (0.08), and whole PET (0.46). However, significant differences were identified in identical acquisition (p = 0.04) and different acquisition (p = 0.02). Bland–Altman analysis documented only a small bias between PET/MRI and PET + MRI in ¹¹C‐PiB PET, ¹⁸F‐FMM PET, identical additional MRI acquisition, different acquisition, and whole PET (– 0.05, 0.67, – 0.30, 0.78, and 0.21, respectively).

Conclusions

Anatomical standardizations using PET/MRI and using PET + MRI can lead to almost equivalent CL. The CL values obtained using PET/MRI or PET + MRI normalization methods are consistent and comparable in clinical studies.