Centiloid scale conversion method for amyloid PET tracers

Answer

There is no single arithmetic SUVR-to-Centiloid conversion: the Centiloid scale is a calibrated 0-100 scale, and each tracer or analysis pipeline needs its own paired calibration against the standard PiB method 128a4cd0. In the published standard PiB example, CL = 100*(SUVr - 1.009)/1.067, with 0 CL anchored to 34 young controls and 100 CL to 45 typical Alzheimer disease subjects 128a4cd0. For another tracer the steps are: normalise to MNI space, take the cortical target and whole-cerebellum reference SUVR, regress against the standard method in a paired cohort, and apply the resulting linear equation 128a4cd0 c1d2cba8. For positivity, about 25 CL is a reasonable operational threshold 128a4cd0, an autopsy-based threshold of 24.4 CL matched visual reads in 10,361 participants bc7683a7, and the AMYPAD consensus uses below 10 as negative, above 30 as positive and 10-30 as intermediate c1d2cba8.

Summary

The protocol is calibration, not a fixed formula. The Level-1 standard requires replicating the PiB pipeline, with QC targets of mean AD-100 SUVr 2.05-2.13, mean YC-0 SUVr 1.05-1.09, slope 0.98-1.02, intercept -2 to +2 CL and R^2 > 0.98, then a Level-2 calibration with at least 25 paired cases 128a4cd0. Published tracer-specific equations include CL = 153.4 x SUVR_FBB - 154.9 for florbetaben 15feb54a, and for flutemetamol CL = (121.42 x SUVR) - 121.16 with SPM8, CL = (115.24 x SUVR) - 107.86 with PMOD and CL = (120.32 x SUVR) - 112.75 with FSL, all on whole-cerebellum reference 3dbf16f6. These are pipeline-specific and must not be swapped. On thresholds, the values are not one number but a band: read- and autopsy-matched values sit near 25 (24.4 and 25.9 CL), while the consensus for definite positivity is above 30 and a therapy-initiation threshold of 24-30 CL is reported 128a4cd0 bc7683a7 2a7998c6. The sources set these thresholds for different purposes (agreement with reads, pathological certainty, treatment eligibility), so they are not rival estimates of one quantity.

Evidence

• Klunk et al. 2015 (Centiloid Project) - standard protocol, anchors, QC targets and Level-2 calibration requirements; about 25 CL operational threshold within a range of about 12 CL for subtle and about 30 CL for established pathology; 25.9 CL best agreement with visual reads for florbetapir. 128a4cd0
• Rowe et al. 2017 - florbetaben equation CL = 153.4 x SUVR_FBB - 154.9, R^2 = 0.96 against PiB in 35 subjects, scans 90-110 minutes after injection. 15feb54a
• Battle et al. 2018 - flutemetamol equations for SPM8, PMOD and FSL pipelines. 3dbf16f6
• Fujishima et al. 2022 - MRI-less pipeline agrees with the standard pipeline (R^2 > 0.97) across five tracers; bias from -1.6% (florbetaben) to +3.3% (NAV4694). dc29411a
• Bourgeat et al. 2023 - the standard neocortical mask is suitable for all five tracers; a universal mask gave only small gains. fa9343b0
• JAMA Neurology 2025 (IDEAS) - autopsy-based threshold of 24.4 CL against visual reads in 10,361 participants at 294 facilities. bc7683a7
• AMYPAD consensus 2024 - below 10 CL excludes amyloid pathology, above 30 CL corresponds to pathological amounts, 10-30 intermediate. c1d2cba8
• Clinical review 2026 - generic formula CL = 100 x (SUVR - SUVR_young)/(SUVR_AD - SUVR_young); 24-30 CL as therapy-initiation threshold. 2a7998c6

Methodology

All eight verified facts are used. I separated the calibration protocol, tracer-specific equations, pipeline-validation studies and threshold studies, and did not pool thresholds because they were derived against different references (visual reads, autopsy, CSF, histopathology consensus). Equations are quoted per tracer and per pipeline and never converted between tracers. The 2026 review and the AMYPAD consensus are secondary syntheses.

Limitations

• Equations are specific to tracer, regions, timing and software; no source gives a universal one 128a4cd0.
• The flutemetamol paper states no positivity threshold itself 3dbf16f6.
• Florbetaben calibration used a small cohort of 35 subjects 15feb54a.
• The therapy-initiation range comes from a review and not from a primary derivation 2a7998c6.