Early tau detection in flortaucipir images: validation in autopsy-confirmed data and implications for disease progression

BACKGROUND

There is an increasing interest in utilizing tau PET to identify patients early in Alzheimer's disease (AD). In this work, a temporal lobe composite (Eτ) volume of interest (VOI) was evaluated in a longitudinal flortaucipir cohort and compared to a previously described global neocortical VOI. In a separate autopsy-confirmed study, the sensitivity of the Eτ VOI for identifying intermediate (B2) neurofibrillary tangle (NFT) pathology was evaluated.

METHODS

A total of 427 subjects received flortaucipir, florbetapir, MRI, and cognitive evaluation at baseline and 18 months. In a separate autopsy study, 67 subjects received ante-mortem flortaucipir scans, and neuropathological findings were recorded according to NIA-AA recommendations by two experts. Two VOIs: Eτ comprising FreeSurfer volumes (bilateral entorhinal cortex, fusiform, parahippocampal, and inferior temporal gyri) transformed to MNI space and a previously published global AD signature-weighted neocortical VOI (AD_signature) (Devous et al., J Nucl Med 59:937-43, 2018), were used to calculate SUVr relative to a white matter reference region (PERSI) (Southekal et al., J Nucl Med Off Publ Soc Nucl Med 59:944-51, 2018). SUVr cutoffs for positivity were determined based on a cohort of young, cognitively normal subjects. Subjects were grouped based on positivity on both VOIs (Eτ+/AD_signature+; Eτ+/AD_signature-; Eτ-/AD_signature-). Groupwise comparisons were performed for baseline SUVr, 18-month changes in SUVr, neurodegeneration, and cognition. For the autopsy study, the sensitivity of Eτ in identifying intermediate Braak pathology (B2) subjects was compared to that of AD signature-weighted neocortical VOI. The average surface maps of subjects in the Eτ+/AD_signature- group and B2 NFT scores were created for visual evaluation of uptake.

RESULTS

Sixty-four out of 390 analyzable subjects were identified as Eτ+/AD_signature-: 84% were Aβ+, 100% were diagnosed as MCI or AD, and 59% were APOE ε4 carriers. Consistent with the hypothesis that Eτ+/AD_signature- status reflects an early stage of AD, Eτ+/AD_signature- subjects deteriorated significantly faster than Eτ-/AD_signature- subjects, but significantly slower than Eτ+/AD_signature+ subjects, on most measures (i.e., change in AD_signature SUVr, Eτ ROI cortical thickness, and MMSE). The AD_signature VOI was selective for subjects who came to autopsy with a B3 NFT score. In the autopsy study, 12/15 B2 subjects (including 10/11 Braak IV) were Eτ+/AD_signature-. Surface maps showed that flortaucipir uptake was largely captured by the Eτ VOI regions in B2 subjects.

CONCLUSION

The Eτ VOI identified subjects with elevated temporal but not global tau (Eτ+/AD_signature-) that were primarily Aβ+, APOE ε4 carriers, and diagnosed as MCI or AD. Eτ+/AD_signature- subjects had greater accumulation of tau, greater atrophy, and higher decline on MMSE in 18 months compared to Eτ-/AD_signature- subjects. Finally, the Eτ VOI identified the majority of the intermediate NFT score subjects in an autopsy-confirmed study. As far as we know, this is the first study that presents a visualization of ante-mortem FTP retention patterns that at a group level agree with the neurofibrillary tangle staging scheme proposed by Braak. These findings suggest that the Eτ VOI may be sensitive for detecting impaired subjects early in the course of Alzheimer's disease.

Relationships Between Cognition and Neuropathological Tau in Alzheimer's Disease Assessed by 18F Flortaucipir PET

BACKGROUND

Tau neurofibrillary tangle burden increases with Alzheimer's disease (AD) stage and correlates with degree of cognitive impairment. Tau PET imaging could facilitate understanding the relationship between tau pathology and cognitive impairment.

OBJECTIVE

Evaluate the relationship between 18F flortaucipir uptake patterns and cognition across multiple cognitive domains.

METHODS

We acquired flortaucipir PET scans in 84 amyloid-positive control, mild cognitive impairment (MCI), and AD subjects. Flortaucipir standardized uptake value ratio (SUVr) values were obtained from a neocortical volume of interest (VOI), a precuneus VOI, and VOIs defined by the correlation between flortaucipir SUVr images and domain-specific cognitive tests. Cognitive assessments included Mini-Mental State Exam (MMSE), Alzheimer's Disease Assessment Scale-Cognitive Subscale (ADAS-cog), and a neuropsychological test battery (i.e., Wechsler Memory Scale-Revised Logical Memory (WMS-R), Trail Making Test, Boston Naming Test, Digit Symbol Substitution Test, Animal List Generation, WMS-R Digit Span, American National Adult Reading Test, Clock Drawing Test, Judgment of Line Orientation, and WMS-R Logical Memory II (Delayed Recall)) and the Functional Activities Questionnaire (FAQ). Correlation analyses compared regional and voxel-wise VOIs to cognitive scores.

RESULTS

Subjects included 5 controls, 47 MCI, and 32 AD subjects. Significant correlations were seen between both flortaucipir and florbetapir SUVrs and MMSE, ADAS-Cog, and FAQ. Cognitive impairment was associated with increased flortaucipir uptake in regionally specific patterns consistent with the neuroanatomy underlying specific cognitive tests.

CONCLUSION

Flortaucipir SUVr values demonstrated significant inverse correlations with cognitive scores in domain-specific patterns. Findings support the hypothesis that PET imaging of neuropathologic tau deposits may reflect underlying neurodegeneration in AD.

Positron Emission Tomography Imaging With [18F]flortaucipir and Postmortem Assessment of Alzheimer Disease Neuropathologic Changes

Importance

Positron emission tomography (PET) may increase the diagnostic accuracy and confirm the underlying neuropathologic changes of Alzheimer disease (AD).

Objective

To determine the accuracy of antemortem [18F]flortaucipir PET images for predicting the presence of AD-type tau pathology at autopsy.

Design, Setting, and Participants

This diagnostic study (A16 primary cohort) was conducted from October 2015 to June 2018 at 28 study sites (27 in US sites and 1 in Australia). Individuals with a terminal illness who were older than 50 years and had a projected life expectancy of less than 6 months were enrolled. All participants underwent [18F]flortaucipir PET imaging, and scans were interpreted by 5 independent nuclear medicine physicians or radiologists. Supplemental autopsy [18F]flortaucipir images and pathological samples were also collected from 16 historically collected cases. A second study (FR01 validation study) was conducted from March 26 to April 26, 2019, in which 5 new readers assessed the original PET images for comparison to autopsy.

Main Outcomes and Measures

[18F]flortaucipir PET images were visually assessed and compared with immunohistochemical tau pathology. An AD tau pattern of flortaucipir retention was assessed for correspondence with a postmortem B3-level (Braak stage V or VI) pathological pattern of tau accumulation and to the presence of amyloid-β plaques sufficient to meet the criteria for high levels of AD neuropathological change. Success was defined as having at least 3 of the 5 readers above the lower bounds of the 95% CI for both sensitivity and specificity of 50% or greater.

Results

A total of 156 patients were enrolled in the A16 study and underwent [18F]flortaucipir PET imaging. Of these, 73 died during the study, and valid autopsies were performed for 67 of these patients. Three autopsies were evaluated as test cases and removed from the primary cohort (n = 64). Of the 64 primary cohort patients, 34 (53%) were women and 62 (97%) were white; mean (SD) age was 82.5 (9.6) years; and 49 (77%) had dementia, 1 (2%) had mild cognitive impairment, and 14 (22%) had normal cognition. Prespecified success criteria were met for the A16 primary cohort. The flortaucipir PET scans predicted a B3 level of tau pathology, with sensitivity ranging from 92.3% (95% CI, 79.7%-97.3%) to 100.0% (95% CI, 91.0%-100.0%) and specificity ranging from 52.0% (95% CI, 33.5%-70.0%) to 92.0% (95% CI, 75.0%-97.8%). A high level of AD neuropathological change was predicted with sensitivity of 94.7% (95% CI, 82.7%-98.5%) to 100.0% (95% CI, 90.8%-100.0%) and specificity of 50.0% (95% CI, 32.1%-67.9%) to 92.3% (95% CI, 75.9%-97.9%). The FR01 validation study also met prespecified success criteria. Addition of the supplemental autopsy data set and 3 test cases, which comprised a total of 82 patients and autopsies for both the A16 and FR01 studies, resulted in improved specificity and comparable overall accuracy. Among the 156 enrolled participants, 14 (9%) experienced at least 1 treatment-emergent adverse event.

Conclusions and Relevance

This study's findings suggest that PET imaging with [18F]flortaucipir could be used to identify the density and distribution of AD-type tau pathology and the presence of high levels of AD neuropathological change, supporting a neuropathological diagnosis of AD.

Comparison of regional flortaucipir PET with quantitative tau immunohistochemistry in three subjects with Alzheimer's disease pathology: a clinicopathological study

Background

The objective of this study was to make a quantitative comparison of flortaucipir PET retention with pathological tau and β-amyloid across a range of brain regions at autopsy.

Methods

Patients with dementia (two with clinical diagnosis of AD, one undetermined), nearing the end of life, underwent 20-min PET, beginning 80 min after an injection of ~370 mBq flortaucipir [¹⁸F]. Neocortical, basal ganglia, and limbic tissue samples were obtained bilaterally from 19 regions at autopsy and subject-specific PET regions of interest corresponding to the 19 sampled target tissue regions in each hemisphere were hand drawn on the PET images. SUVr values were calculated for each region using a cerebellar reference region. Abnormally phosphorylated tau (Ptau) and amyloid-β (Aβ) tissue concentrations were measured for each tissue region with an antibody capture assay (Histelide) using AT8 and H31L21 antibodies respectively.

Results

The imaging-to-autopsy interval ranged from 4–29 days. All three subjects had intermediate to high levels of AD neuropathologic change at autopsy. Mean cortical SUVr averaged across all three subjects correlated significantly with the Ptau immunoassay (Pearson r = 0.81; p < 0.0001). When Ptau and Aβ_1-42 were both included in the model, the Ptau correlation with flortaucipir SUVr was preserved but there was no correlation of Aβ_1-42 with flortaucipir. There was also a modest correlation between limbic (hippocampal/entorhinal and amygdala) flortaucipir SUVr and Ptau (Pearson r = 0.52; p < 0.080). There was no significant correlation between SUVr and Ptau in basal ganglia.

Conclusions

The results of this pilot study support a quantitative relationship between cortical flortaucipir SUVr values and quantitative measures of Ptau at autopsy. Additional research including more cases is needed to confirm the generalizability of these results.

Trial registration, NIH Clinicaltrials.gov NCT # 02516046. Registered August 27, 2015. https://clinicaltrials.gov/ct2/show/NCT02516046?term=02516046&draw=2&rank=1

A multicentre longitudinal study of flortaucipir (18F) in normal ageing, mild cognitive impairment and Alzheimer's disease dementia

The advent of tau-targeted PET tracers such as flortaucipir (18F) (flortaucipir, also known as 18F-AV-1451 or 18F-T807) have made it possible to investigate the sequence of development of tau in relationship to age, amyloid-β, and to the development of cognitive impairment due to Alzheimer's disease. Here we report a multicentre longitudinal evaluation of the relationships between baseline tau, tau change and cognitive change, using flortaucipir PET imaging. A total of 202 participants 50 years old or older, including 57 cognitively normal subjects, 97 clinically defined mild cognitive impairment and 48 possible or probable Alzheimer's disease dementia patients, received flortaucipir PET scans of 20 min in duration beginning 80 min after intravenous administration of 370 MBq flortaucipir (18F). On separate days, subjects also received florbetapir amyloid PET imaging, and underwent a neuropsychological test battery. Follow-up flortaucipir scans and neuropsychological battery assessments were also performed at 9 and 18 months. Fifty-five amyloid-β+ and 90 amyloid-β- subjects completed the baseline and 18-month study visits and had valid quantifiable flortaucipir scans at both time points. There was a statistically significant increase in the global estimate of cortical tau burden as measured by standardized uptake value ratio (SUVr) from baseline to 18 months in amyloid-β+ but not amyloid-β- subjects (least squared mean change in flortaucipir SUVr : 0.0524 ± 0.0085, P < 0.0001 and 0.0007 ± 0.0024 P = 0.7850, respectively), and a significant association between magnitude of SUVr increase and baseline tau burden. Voxel-wise evaluations further suggested that the regional pattern of change in flortaucipir PET SUVr over the 18-month study period (i.e. which regions exhibited the greatest change) also varied as a function of baseline global estimate of tau burden. In subjects with lower global SUVr, temporal lobe regions showed the greatest flortaucipir retention, whereas in subjects with higher baseline SUVr, parietal and frontal regions were increasingly affected. Finally, baseline flortaucipir and change in flortaucipir SUVr were both significantly (P < 0.0001) associated with changes in cognitive performance. Taken together, these results provide a preliminary characterization of the longitudinal spread of tau in Alzheimer's disease and suggest that the amount and location of tau may have implications both for the spread of tau and the cognitive deterioration that may occur over an 18-month period.

Tau Positron-Emission Tomography in Former National Football League Players

BACKGROUND

Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease that has been associated with a history of repetitive head impacts. The neuropathological diagnosis is based on a specific pattern of tau deposition with minimal amyloid-beta deposition that differs from other disorders, including Alzheimer's disease. The feasibility of detecting tau and amyloid deposition in the brains of living persons at risk for CTE has not been well studied.

METHODS

We used flortaucipir positron-emission tomography (PET) and florbetapir PET to measure deposition of tau and amyloid-beta, respectively, in the brains of former National Football League (NFL) players with cognitive and neuropsychiatric symptoms and in asymptomatic men with no history of traumatic brain injury. Automated image-analysis algorithms were used to compare the regional tau standardized uptake value ratio (SUVR, the ratio of radioactivity in a cerebral region to that in the cerebellum as a reference) between the two groups and to explore the associations of SUVR with symptom severity and with years of football play in the former-player group.

RESULTS

A total of 26 former players and 31 controls were included in the analysis. The mean flortaucipir SUVR was higher among former players than among controls in three regions of the brain: bilateral superior frontal (1.09 vs. 0.98; adjusted mean difference, 0.13; 95% confidence interval [CI], 0.06 to 0.20; P<0.001), bilateral medial temporal (1.23 vs. 1.12; adjusted mean difference, 0.13; 95% CI, 0.05 to 0.21; P<0.001), and left parietal (1.12 vs. 1.01; adjusted mean difference, 0.12; 95% CI, 0.05 to 0.20; P = 0.002). In exploratory analyses, the correlation coefficients in these three regions between the SUVRs and years of play were 0.58 (95% CI, 0.25 to 0.79), 0.45 (95% CI, 0.07 to 0.71), and 0.50 (95% CI, 0.14 to 0.74), respectively. There was no association between tau deposition and scores on cognitive and neuropsychiatric tests. Only one former player had levels of amyloid-beta deposition similar to those in persons with Alzheimer's disease.

CONCLUSIONS

A group of living former NFL players with cognitive and neuropsychiatric symptoms had higher tau levels measured by PET than controls in brain regions that are affected by CTE and did not have elevated amyloid-beta levels. Further studies are needed to determine whether elevated CTE-associated tau can be detected in individual persons. (Funded by Avid Radiopharmaceuticals and others.).

Standardization of amyloid quantitation with florbetapir standardized uptake value ratios to the Centiloid scale

INTRODUCTION

Klunk et al. recently proposed a means of standardizing quantitation of amyloid burden from positron emission tomography scans to a common Centiloid scale, and we have applied that method to florbetapir.

METHODS

Florbetapir and Pittsburgh compound B scans were acquired for 46 mixed clinical presentation subjects within 18 ± 20 days. Florbetapir and Pittsburgh compound B cortical standardized uptake value ratio (SUVr) values were well correlated for both standard Centiloid (R² = 0.894) and Avid (R² = 0.901) volume of interests (VOIs). The methods of Klunk et al. were applied to establish a conversion first from florbetapir SUVr values obtained using standard Centiloid VOIs to Centiloids and then from Avid VOIs (Joshi et al.) to Centiloids.

RESULTS

The equation for conversion of florbetapir SUVr from Avid VOIs to the Centiloid scale was as follows: Florbetapir Centiloids = 183 × SUVr_Avid - 177. The threshold that discriminated neuropathologically verified none or sparse versus moderate to frequent plaques in autopsy-confirmed data is 24.1 Centiloids.

DISCUSSION

These findings may allow improved tracer-independent amyloid quantitation.

Test-Retest Reproducibility for the Tau PET Imaging Agent Flortaucipir F 18

Alzheimer disease (AD) is characterized by β-amyloid (Aβ) plaques and tau neurofibrillary tangles. There are several PET imaging biomarkers for Aβ including ¹¹C-PiB and ¹⁸F-florbetapir. Recently, PET tracers for tau neurofibrillary tangles have become available and have shown utility in detection and monitoring of neurofibrillary pathology over time. Flortaucipir F 18 is one such tracer. Initial clinical studies indicated greater tau binding in AD and mild cognitive impairment patients than in controls in a pattern consistent with tau pathology observed at autopsy. However, little is known about the reproducibility of such findings. To our knowledge, this study reports the first data regarding test-retest reproducibility of flortaucipir F 18 PET. Methods: Twenty-one subjects who completed the study (5 healthy controls, 6 mild cognitive impairment, and 10 AD) received 370 MBq of flortaucipir F 18 and were imaged for 20 min beginning 80 min after injection and again at 110 min after injection. Follow-up (retest) imaging occurred between 48 h and 4 wk after initial imaging. Images were spatially normalized to Montreal Neurological Institute template space. SUVRs were calculated using AAL (Automated Anatomical Labeling atlas) volumes of interest (VOIs) for parietal, temporal, occipital, anterior, and posterior hippocampal, parahippocampal, and fusiform regions, as well as a posterior neocortical VOI composed of average values from parietal, temporal, and occipital areas. Further, a VOI derived by discriminant analysis that maximally separated diagnostic groups (multiblock barycentric discriminant analysis [MUBADA]) was used. All VOIs were referenced to a subsection of cerebellar gray matter (cere-crus) as well as a parametrically derived white matter-based reference region (parametric estimate of reference signal intensity [PERSI]). t test, correlation analyses, and intraclass correlation coefficient were used to explore test-retest performance. Results: Test-retest analyses demonstrated low variability in flortaucipir F 18 SUVR. The SD of mean percentage change between test and retest using the PERSI reference region was 2.22% for a large posterior neocortical VOI, 1.84% for MUBADA, 1.46% for frontal, 1.98% for temporal, 2.28% for parietal, and 3.27% for occipital VOIs. Further, significant correlations (R² > 0.85; P < 0.001) were observed for all regions, and intraclass correlation coefficient values (test-retest consistency) were greater than 0.92 for all regions. Conclusion: Significant test-retest reproducibility for flortaucipir F 18 was found across neocortical and mesial temporal lobe structures. These preliminary data suggest that flortaucipir F 18 tau imaging could be used to examine changes in tau burden over time.

Flortaucipir F 18 Quantitation Using Parametric Estimation of Reference Signal Intensity

PET imaging of tau pathology in Alzheimer disease may benefit from the use of white matter reference regions. These regions have shown reduced variability compared with conventional cerebellar regions in amyloid imaging. However, they are susceptible to contamination from partial-volume blurring of tracer uptake in the cortex. We present a new technique, PERSI (Parametric Estimation of Reference Signal Intensity), for flortaucipir F 18 count normalization that leverages the advantages of white matter reference regions while mitigating potential partial-volume effects. Methods: Subjects with a clinical diagnosis of Alzheimer disease, mild cognitive impairment, or normal cognition underwent T1-weighted MRI and florbetapir imaging (to determine amyloid [Aβ] status) at screening and flortaucipir F 18 imaging at single or multiple time points. Flortaucipir F 18 images, acquired as 4 × 5 min frames 80 min after a 370-MBq injection, were motion-corrected, averaged, and transformed to Montreal Neurological Institute (MNI) space. The PERSI reference region was calculated for each scan by fitting a bimodal gaussian distribution to the voxel-intensity histogram within an atlas-based white matter region and using the center and width of the lower-intensity peak to identify the voxel intensities to be included. Four conventional reference regions were also evaluated: whole cerebellum, cerebellar gray matter, atlas-based white matter, and subject-specific white matter. SUVr (standardized uptake value ratio) was calculated for a statistically defined neocortical volume of interest. Performance was evaluated with respect to test-retest variability in a phase 2 study of 21 subjects (5-34 d between scans). Baseline variability in controls (SD of SUVr and ΔSUVr) and effect sizes for group differences (Cohen d; Aβ-positive impaired vs. Aβ-negative normal) were evaluated in another phase 2 study with cross-sectional data (n = 215) and longitudinal data (n = 142/215; 18 ± 2 mo between scans). Results: PERSI showed superior test-retest reproducibility (1.84%) and group separation ability (cross-sectional Cohen d = 9.45; longitudinal Cohen d = 2.34) compared with other reference regions. Baseline SUVr variability and ΔSUVr were minimal in Aβ control subjects with no specific flortaucipir F 18 uptake (SUVr, 1.0 ± 0.04; ΔSUVr, 0.0 ± 0.02). Conclusion: PERSI reduced variability while enhancing discrimination between diagnostic cohorts. Such improvements could lead to more accurate disease staging and robust measurements of changes in tau burden over time for the evaluation of putative therapies.

Relationships between flortaucipir PET tau binding and amyloid burden, clinical diagnosis, age and cognition

The advent of tau-targeted positron emission tomography tracers such as flortaucipir (18F-AV-1451, also known as 18F-T807) have made it possible to investigate the sequence of development of tau and amyloid-β in relationship to age, and to the development of cognitive impairment due to Alzheimer's disease. In this study, flortaucipir tau and florbetapir amyloid positron emission tomography were obtained for 217 subjects including 16 young and 58 older cognitively normal subjects, 95 subjects with mild cognitive impairment (Mini-Mental State Examination 24-30) and 48 subjects with clinically-defined possible or probable Alzheimer's disease (Mini-Mental State Examination >10). Images were evaluated visually and quantitatively by regional and voxel-based cortical to cerebellar standard uptake value ratios. For amyloid positron emission tomography positive (Aβ+) subjects, flortaucipir neocortical standard uptake value ratio was significantly higher with more advanced clinical stage (Alzheimer's disease > mild cognitive impairment > older cognitively normal) and was significantly elevated for Aβ+ mild cognitive impairment and Alzheimer's disease subjects relative to the respective Aβ- subjects. In contrast, florbetapir Aβ- older cognitively normal subjects showed an increase in flortaucipir standard uptake value ratios in mesial temporal lobe regions (amygdala, hippocampus/choroid plexus region of interest) compared to younger cognitively normal subjects, but no increased standard uptake value ratios in neocortical regions. Analysis of covariance with planned contrasts showed no differences in regional or composite posterior neocortical flortaucipir standard uptake value ratio as a function of diagnostic group among Aβ- older cognitively normal or clinically diagnosed Alzheimer's disease or mild cognitive impairment subjects. The pattern of flortaucipir distribution among Aβ+ subjects was reminiscent of the cross-sectional distribution of tau reported in post-mortem pathology studies, in that the most commonly affected regions were the inferior and lateral temporal lobes, the same regions where the first signs of increased retention appeared in Aβ+ cognitively normal subjects. However, there was large variability in extent/density of flortaucipir tau binding among Aβ+ subjects. Although high neocortical flortaucipir retention was consistently associated with an Aβ+ florbetapir positron emission tomography scan, not all Aβ+ subjects had elevated flortaucipir standard uptake value ratios. Finally, within the Aβ+ group, increasing levels of flortaucipir tau binding were associated with increased cognitive impairment, as assessed by Mini-Mental State Examination and Alzheimer's Disease Assessment Scale. These results suggest development of tau beyond the mesial temporal lobe is associated with, and may be dependent on, amyloid accumulation. Further, the results are consistent with the hypothesis that cortical tau is associated with cognitive impairment.

Kinetics of the Tau PET Tracer 18F-AV-1451 (T807) in Subjects with Normal Cognitive Function, Mild Cognitive Impairment, and Alzheimer Disease

We report kinetic modeling results of dynamic acquisition data from 0 to 100 min after injection with the tau PET tracer ¹⁸F-AV-1451 in 19 subjects.

METHODS

Subjects were clinically diagnosed as 4 young cognitively normal, 5 old cognitively normal, 5 mild cognitive impairment, and 5 Alzheimer disease (AD). Kinetic modeling was performed using Logan graphical analysis with the cerebellum crus as a reference region. Voxelwise binding potential ([Formula: see text]) and SUV ratio ([Formula: see text]) images were compared.

RESULTS

In AD subjects, slower and spatially nonuniform clearance from cortical regions was observed as compared with the controls, which led to focal uptake and elevated retention in the imaging data from 80 to 100 min after injection. BP from the dynamic data from 0 to 100 min correlated strongly (R² > 0.86) with corresponding regional [Formula: see text] values. In the putamen, the observed kinetics (positive [Formula: see text] at the tracer delivery stage and plateauing time-SUVR curves for all diagnostic categories) may suggest either additional off-target binding or a second binding site with different kinetics.

CONCLUSION

The kinetics of the ¹⁸F-AV-1451 tracer in cortical areas, as examined in this small group of subjects, differed by diagnostic stage. A delayed 80- to 100-min scan provided a reasonable substitute for a dynamic 0- to 100-min acquisition for cortical regions although other windows (e.g., 75-105 min) may be useful to evaluate.

Regional profiles of the candidate tau PET ligand 18F-AV-1451 recapitulate key features of Braak histopathological stages

SEE THAL AND VANDENBERGHE DOI101093/BRAIN/AWW057 FOR A SCIENTIFIC COMMENTARY ON THIS ARTICLE: Post-mortem Braak staging of neurofibrillary tau tangle topographical distribution is one of the core neuropathological criteria for the diagnosis of Alzheimer's disease. The recent development of positron emission tomography tracers targeting neurofibrillary tangles has enabled the distribution of tau pathology to be imaged in living subjects. Methods for extraction of classic Braak staging from in vivo imaging of neurofibrillary tau tangles have not yet been explored. Standardized uptake value ratio images were calculated from 80-100 minute (18)F-AV-1451 (also known as T807) positron emission tomography scans obtained from n = 14 young reference subjects (age 21-39 years, Mini-Mental State Examination 29-30) and n = 173 older test subjects (age 50-95 years) comprising amyloid negative cognitively normal (n = 42), clinically-diagnosed mild cognitive impairment (amyloid positive, n = 47, and amyloid negative, n = 40) and Alzheimer's disease (amyloid positive, n = 28, and amyloid negative, n = 16). We defined seven regions of interest in anterior temporal lobe and occipital lobe sections corresponding closely to those used as decision points in Braak staging. An algorithm based on the Braak histological staging procedure was applied to estimate Braak stages directly from the region of interest profiles in each subject. Quantitative region-based analysis of (18)F-AV-1451 images yielded region of interest and voxel level profiles that mirrored key features of neuropathological tau progression including profiles consistent with Braak stages 0 through VI. A simple set of decision rules enabled plausible Braak stages corresponding to stereotypical progression patterns to be objectively estimated in 149 (86%) of test subjects. An additional 12 (7%) subjects presented with predefined variant profiles (relative sparing of the hippocampus and/or occipital lobe). The estimated Braak stage was significantly associated with amyloid status, diagnostic category and measures of global cognition. In vivo (18)F-AV-1451 positron emission tomography images across the Alzheimer's disease spectrum could be classified into patterns similar to those prescribed by Braak neuropathological staging of tau pathology.