A new method to quantify tau pathologies with (11)C-PBB3 PET using reference tissue voxels extracted from brain cortical gray matter

Dominance of Tau Burden in Cortical Over Subcortical Regions Mediates Glymphatic Activity and Clinical Severity in PSP

BACKGROUND

Progressive supranuclear palsy (PSP) is a tauopathy that involves subcortical regions but also extends to cortical areas. The clinical impact of different tau protein sites and their influence on glymphatic dysfunction have not been investigated.

PATIENTS AND METHODS

Participants (n = 55; 65.6 ± 7.1 years; 29 women) with PSP (n = 32) and age-matched normal controls (NCs; n = 23) underwent 18 F-Florzolotau tau PET, MRI, PSP Rating Scale (PSPRS), and Mini-Mental State Examination. Cerebellar gray matter (GM) and parametric estimation of reference signal intensity were used as references for tau burden measured by SUV ratios. Glymphatic activity was measured by diffusion tensor image analysis along the perivascular space (DTI-ALPS).

RESULTS

Parametric estimation of reference signal intensity is a better reference than cerebellar GM to distinguish tau burden between PSP and NCs. PSP patients showed higher cortical and subcortical tau SUV ratios than NCs ( P < 0.001 and <0.001). Cortical and subcortical tau deposition correlated with PSPRS, UPDRS, and Mini-Mental State Examination scores (all P 's < 0.05). Cortical tau deposition was further associated with the DTI-ALPS index and frontal-temporal-parietal GM atrophy. The DTI-ALPS indexes showed a significantly negative correlation with the PSPRS total scores ( P < 0.01). Finally, parietal and occipital lobe tau depositions showed mediating effects between the DTI-ALPS index and PSPRS score.

CONCLUSIONS

Cortical tau deposition is associated with glymphatic dysfunction and plays a role in mediating glymphatic dysfunction and clinical severity. Our results provide a possible explanation for the worsening of clinical severity in patients with PSP.

Toward standardization of tau PET imaging corresponding to various tau PET tracers: a multicenter phantom study

OBJECTIVE

Tau positron emission tomography (PET) imaging is a recently developed non-invasive tool that can detect the density and extension of tau neurofibrillary tangles. Tau PET tracers have been validated to harmonize and accelerate their development and implementation in clinical practice. Whereas standard protocols including injected dose, uptake time, and duration have been determined for tau PET tracers, reconstruction parameters have not been standardized. The present study conducted phantom experiments based on tau pathology to standardize quantitative tau PET imaging parameters and optimize reconstruction conditions of PET scanners at four Japanese sites according to the results of phantom experiments.

METHODS

The activity of 4.0 and 2.0 kBq/mL for Hoffman 3D brain and cylindrical phantoms, respectively, was estimated from published studies of brain activity using [18F]flortaucipir, [18F]THK5351, and [18F]MK6240. We developed an original tau-specific volume of interest template for the brain based on pathophysiological tau distribution in the brain defined as Braak stages. We acquired brain and cylindrical phantom images using four PET scanners. Iteration numbers were determined as contrast and recover coefficients (RCs) in gray (GM) and white (WM) matter, and the magnitude of the Gaussian filter was determined from image noise.

RESULTS

Contrast and RC converged at ≥ 4 iterations, the error rates of RC for GM and WM were < 15% and 1%, respectively, and noise was < 10% in Gaussian filters of 2-4 mm in images acquired using the four scanners. Optimizing the reconstruction conditions for phantom tau PET images acquired by each scanner improved contrast and image noise.

CONCLUSIONS

The phantom activity was comprehensive for first- and second-generation tau PET tracers. The mid-range activity that we determined could be applied to later tau PET tracers. We propose an analytical tau-specific VOI template based on tau pathophysiological changes in patients with AD to standardize tau PET imaging. Phantom images reconstructed under the optimized conditions for tau PET imaging achieved excellent image quality and quantitative accuracy.

Chronic traumatic encephalopathy: Diagnostic updates and advances

Chronic traumatic encephalopathy (CTE) is a progressive neurodegenerative disease that occurs secondary to repetitive mild traumatic brain injury. Current clinical diagnosis relies on symptomatology and structural imaging findings which often vary widely among those with the disease. The gold standard of diagnosis is post-mortem pathological examination. In this review article, we provide a brief introduction to CTE, current diagnostic workup and the promising research on imaging and fluid biomarker diagnostic techniques. For imaging, we discuss quantitative structural analyses, DTI, fMRI, MRS, SWI and PET CT. For fluid biomarkers, we discuss p-tau, TREM2, CCL11, NfL and GFAP.

An optimized reference tissue method for quantification of tau protein depositions in diverse neurodegenerative disorders by PET with 18F-PM-PBB3 (18F-APN-1607)

Positron emission tomography (PET) with ¹⁸F-PM-PBB3 (¹⁸F-APN-1607, ¹⁸F-Florzolotau) enables high-contrast detection of tau depositions in various neurodegenerative dementias, including Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD). A simplified method for quantifying radioligand binding in target regions is to employ the cerebellum as a reference (CB-ref) on the assumption that the cerebellum has minimal tau pathologies. This procedure is typically valid in AD, while FTLD disorders exemplified by progressive supranuclear palsy (PSP) are characterized by occasional tau accumulations in the cerebellum, hampering the application of CB-ref. The present study aimed to establish an optimal method for defining reference tissues on ¹⁸F-PM-PBB3-PET images of AD and non-AD tauopathy brains. We developed a new algorithm to extract reference voxels with a low likelihood of containing tau deposits from gray matter (GM-ref) or white matter (WM-ref) by a bimodal fit to an individual, voxel-wise histogram of the radioligand retentions and applied it to ¹⁸F-PM-PBB3-PET data obtained from age-matched 40 healthy controls (HCs) and 23 CE, 40 PSP, and five other tau-positive FTLD patients. PET images acquired at 90-110 min after injection were averaged and co-registered to corresponding magnetic resonance imaging space. Subsequently, we generated standardized uptake value ratio (SUVR) images estimated by CB-ref, GM-ref and WM-ref, respectively, and then compared the diagnostic performances. GM-ref and WM-ref covered a broad area in HCs and were free of voxels located in regions known to bear high tau burdens in AD and PSP patients. However, radioligand retentions in WM-ref exhibited age-related declines. GM-ref was unaffected by aging and provided SUVR images with higher contrast than CB-ref in FTLD patients with suspected and confirmed corticobasal degeneration. The methodology for determining reference tissues as optimized here improves the accuracy of ¹⁸F-PM-PBB3-PET measurements of tau burdens in a wide range of neurodegenerative illnesses.

PET-based classification of corticobasal syndrome

INTRODUCTION

Corticobasal degeneration (CBD) is the most common neuropathological substrate for clinically diagnosed corticobasal syndrome (CBS), while identifying CBD pathology in living individuals has been challenging. This study aimed to examine the capability of positron emission tomography (PET) to detect CBD-type tau depositions and neuropathological classification of CBS.

METHODS

Sixteen CBS cases diagnosed by Cambridge's criteria and 12 cognitively healthy controls (HCs) underwent PET scans with ¹¹C-PiB, ¹¹C-PBB3, and ¹⁸F-FDG, along with T1-weighted magnetic resonance imaging. Amyloid positivity was assessed by visual inspection of ¹¹C-PiB retentions. Tau positivity was judged by quantitative comparisons of ¹¹C-PBB3 binding to HCs.

RESULTS

Sixteen CBS cases consisted of two cases (13%) with amyloid and tau positivities indicative of Alzheimer's disease (AD) pathologies, 11 cases (69%) with amyloid negativity and tau positivity, and three cases (19%) with amyloid and tau negativities. Amyloid(-), tau(+) CBS cases showed increased retentions of ¹¹C-PBB3 in the frontoparietal areas, basal ganglia, and midbrain, and reduced metabolism in the precentral gyrus and thalamus relative to HCs. The enhanced tau probe retentions in the frontal gray and white matters partially overlapped with metabolic deficits and atrophy and correlated with Clinical Dementia Rating scores.

CONCLUSIONS

PET-based classification of CBS was in accordance with previous neuropathological reports on the prevalences of AD, non-AD tauopathies, and others in CBS. The current work suggests that ¹¹C-PBB3-PET may assist the biological classification of CBS and understanding of links between CBD-type tau depositions and neuronal deteriorations leading to cognitive declines.

Clinical validity of increased cortical binding of tau ligands of the THK family and PBB3 on PET as biomarkers for Alzheimer's disease in the context of a structured 5-phase development framework

PURPOSE

The research community has focused on defining reliable biomarkers for the early detection of the pathological hallmarks of Alzheimer's disease (AD). In 2017, the Geneva AD Biomarker Roadmap initiative adapted the framework for the systematic validation of oncological biomarkers to AD, with the aim to accelerate their development and implementation in clinical practice. The aim of this work was to assess the validation status of tau PET ligands of the THK family and PBB3 as imaging biomarkers for AD, based on the Biomarker Roadmap methodology.

METHODS

A panel of experts in AD biomarkers convened in November 2019 at a 2-day workshop in Geneva. The level of clinical validity of tau PET ligands of the THK family and PBB3 was assessed based on the 5-phase development framework before the meeting and discussed during the workshop.

RESULTS

PET radioligands of the THK family discriminate well between healthy controls and patients with AD dementia (phase 2; partly achieved) and recent evidence suggests an accurate diagnostic accuracy at the mild cognitive impairment (MCI) stage of the disease (phase 3; partly achieved). The phases 2 and 3 were considered not achieved for PBB3 since no evidence exists about the ligand's diagnostic accuracy. Preliminary evidence exists about the secondary aims of each phase for all ligands.

CONCLUSION

Much work remains for completing the aims of phases 2 and 3 and replicating the available evidence. However, it is unlikely that the validation process for these tracers will be completed, given the presence of off-target binding and the development of second-generation tracers with improved binding and pharmacokinetic properties.

High-Contrast In Vivo Imaging of Tau Pathologies in Alzheimer's and Non-Alzheimer's Disease Tauopathies

A panel of radiochemicals has enabled in vivo positron emission tomography (PET) of tau pathologies in Alzheimer's disease (AD), although sensitive detection of frontotemporal lobar degeneration (FTLD) tau inclusions has been unsuccessful. Here, we generated an imaging probe, PM-PBB3, for capturing diverse tau deposits. In vitro assays demonstrated the reactivity of this compound with tau pathologies in AD and FTLD. We could also utilize PM-PBB3 for optical/PET imaging of a living murine tauopathy model. A subsequent clinical PET study revealed increased binding of ¹⁸F-PM-PBB3 in diseased patients, reflecting cortical-dominant AD and subcortical-dominant progressive supranuclear palsy (PSP) tau topologies. Notably, the in vivo reactivity of ¹⁸F-PM-PBB3 with FTLD tau inclusion was strongly supported by neuropathological examinations of brains derived from Pick's disease, PSP, and corticobasal degeneration patients who underwent PET scans. Finally, visual inspection of ¹⁸F-PM-PBB3-PET images was indicated to facilitate individually based identification of diverse clinical phenotypes of FTLD on a neuropathological basis.

PET-detectable tau pathology correlates with long-term neuropsychiatric outcomes in patients with traumatic brain injury

Tau deposits is a core feature of neurodegenerative disorder following traumatic brain injury (TBI). Despite ample evidence from post-mortem studies demonstrating exposure to both mild-repetitive and severe TBIs are linked to tau depositions, associations of topology of tau lesions with late-onset psychiatric symptoms due to TBI have not been explored. To address this issue, we assessed tau deposits in long-term survivors of TBI by PET with 11C-PBB3, and evaluated those associations with late-life neuropsychiatric outcomes. PET data were acquired from 27 subjects in the chronic stage following mild-repetitive or severe TBI and 15 healthy control subjects. Among the TBI patients, 14 were diagnosed as having late-onset symptoms based on the criteria of traumatic encephalopathy syndrome. For quantification of tau burden in TBI brains, we calculated 11C-PBB3 binding capacity (cm3), which is a summed voxel value of binding potentials (BP*ND) multiplied by voxel volume. Main outcomes of the present study were differences in 11C-PBB3 binding capacity between groups, and the association of regional 11C-PBB3 binding capacity with neuropsychiatric symptoms. To confirm 11C-PBB3 binding to tau deposits in TBI brains, we conducted in vitro PBB3 fluorescence and phospho-tau antibody immunofluorescence labelling of brain sections of chronic traumatic encephalopathy obtained from the Brain Bank. Our results showed that patients with TBI had higher 11C-PBB3 binding capacities in the neocortical grey and white matter segments than healthy control subjects. Furthermore, TBI patients with traumatic encephalopathy syndrome showed higher 11C-PBB3 binding capacity in the white matter segment than those without traumatic encephalopathy syndrome, and regional assessments revealed that subgroup difference was also significant in the frontal white matter. 11C-PBB3 binding capacity in the white matter segment correlated with the severity of psychosis. In vitro assays demonstrated PBB3-positive tau inclusions at the depth of neocortical sulci, confirming 11C-PBB3 binding to tau lesions. In conclusion, increased 11C-PBB3 binding capacity is associated with late-onset neuropsychiatric symptoms following TBI, and a close correlation was found between psychosis and 11C-PBB3 binding capacity in the white matter.

[18F]GTP1 (Genentech Tau Probe 1), a radioligand for detecting neurofibrillary tangle tau pathology in Alzheimer's disease

OBJECTIVE

Neurofibrillary tangles (NFTs), consisting of intracellular aggregates of the tau protein, are a pathological hallmark of Alzheimer's disease (AD). Here we report the identification and initial characterization of Genentech Tau Probe 1 ([¹⁸F]GTP1), a small-molecule PET probe for imaging tau pathology in AD patients.

METHODS

Autoradiography using human brain tissues from AD donors and protein binding panels were used to determine [¹⁸F]GTP1 binding characteristics. Stability was evaluated in vitro and in vivo in mice and rhesus monkey. In the clinic, whole-body imaging was performed to assess biodistribution and dosimetry. Dynamic [¹⁸F]GTP1 brain imaging and input function measurement were performed on two separate days in 5 β-amyloid plaque positive (Aβ+) AD and 5 β-amyloid plaque negative (Aβ-) cognitive normal (CN) participants. Tracer kinetic modeling was applied and reproducibility was evaluated. SUVR was calculated and compared to [¹⁸F]GTP1-specific binding parameters derived from the kinetic modeling. [¹⁸F]GTP1 performance in a larger cross-sectional group of 60 Aβ+ AD participants and ten (Aβ- or Aβ+) CN was evaluated with images acquired 60 to 90 min post tracer administration.

RESULTS

[¹⁸F]GTP1 exhibited high affinity and selectivity for tau pathology with no measurable binding to β-amyloid plaques or MAO-B in AD tissues, or binding to other tested proteins at an affinity predicted to impede image data interpretation. In human, [¹⁸F]GTP1 exhibited favorable dosimetry and brain kinetics, and no evidence of defluorination. [¹⁸F]GTP1-specific binding was observed in cortical regions of the brain predicted to contain tau pathology in AD and exhibited low (< 4%) test-retest variability. SUVR measured in the 60 to 90-min interval post injection correlated with tracer-specific binding (slope = 1.36, r² = 0.98). Furthermore, in a cross-sectional population, the degree of [¹⁸F]GTP1-specific binding increased with AD severity and could differentiate diagnostic cohorts.

CONCLUSIONS

[¹⁸F]GTP1 is a promising PET probe for the study of tau pathology in AD.

In vivo direct relation of tau pathology with neuroinflammation in early Alzheimer's disease

OBJECTIVE

Neuronal damage and neuroinflammation are important events occurring in the brain of Alzheimer's disease (AD). The purpose of this study was to clarify in vivo mutual relationships among abnormal tau deposition, neuroinflammation and cognitive impairment in patients with early AD using positron emission tomography (PET) with [¹¹C]PBB3 and [¹¹C]DPA713.

METHODS

Twenty patients with early AD and 20 age-matched normal control (NC) subjects underwent a series of PET measurements with [¹¹C]PBB3 for tau aggregation and [¹¹C]DPA713 for microglial activation (neuroinflammation). Inter- and intrasubject comparisons were performed regarding the levels of [¹¹C]PBB3 binding potential (BP_ND) and [¹¹C]DPA713 BP_ND in the light of cognitive functions using statistical parametric mapping (SPM) and regions of interest (ROIs) method.

RESULTS

The [¹¹C]PBB3 BP_ND was greater in the temporo-parietal regions of AD patents than NC subjects, and a similar increasing pattern of [¹¹C]DPA713 BP_ND was observed in the same patients. Correlation analyses within the AD group showed a positive direct correlation between [¹¹C]PBB3 BP_ND and [¹¹C]DPA713 BP_ND in the parahippocampus. Pass analysis revealed that cognitive impairment was more likely linked to the level of the parahippocampal [¹¹C]PBB3 BP_ND than that of [¹¹C]DPA713 BP_ND.

CONCLUSIONS

The pattern of abnormal tau deposition was very similar to that of neuroinflammation in patients with early-stage AD. Specifically, the direct positive correlation of tau pathology with neuroinflammation in the parahippocampus suggests that neuronal damage in this region is closely associated with microglial activation. Consistently, tau aggregation in this region matters more than neuroinflammation regarding the cognitive deterioration in AD.

In vivo binding of a tau imaging probe, [11 C]PBB3, in patients with progressive supranuclear palsy

Background

[¹¹C]pyridinyl‐butadienyl‐benzothiazole 3 is a PET imaging agent designed for capturing pathological tau aggregates in diverse neurodegenerative disorders, and would be of clinical utility for neuropathological investigations of PSP.

Objectives

To explore the usefulness of [¹¹C]pyridinyl‐butadienyl‐benzothiazole 3/PET in assessing characteristic distributions of tau pathologies and their association with clinical symptoms in the brains of living PSP patients.

Methods

We assessed 13 PSP patients and 13 age‐matched healthy control subjects. Individuals negative for amyloid β PET with [¹¹C]Pittsburgh compound B underwent clinical scoring, MR scans, and [¹¹C]pyridinyl‐butadienyl‐benzothiazole 3/PET.

Results

There were significant differences in binding potential for [¹¹C]pyridinyl‐butadienyl‐benzothiazole 3 between PSP patients and healthy control subjects (P = 0.02). PSP patients exhibited greater radioligand retention than healthy control subjects in multiple brain regions, including frontoparietal white matter, parietal gray matter, globus pallidus, STN, red nucleus, and cerebellar dentate nucleus. [¹¹C]pyridinyl‐butadienyl‐benzothiazole 3 deposition in frontoparietal white matter, but not gray matter, was correlated with general severity of parkinsonian and PSP symptoms, whereas both gray matter and white matter [¹¹C]pyridinyl‐butadienyl‐benzothiazole 3 accumulations in the frontoparietal cortices were associated with nonverbal cognitive impairments. Autoradiographic and fluorescence labeling with pyridinyl‐butadienyl‐benzothiazole 3 was observed in gray matter and white matter of PSP motor cortex tissues.

Conclusions

Our findings support the in vivo detectability of tau fibrils characteristic of PSP by [¹¹C]pyridinyl‐butadienyl‐benzothiazole 3/PET, and imply distinct and synergistic contributions of gray matter and white matte tau pathologies to clinical symptoms. [¹¹C]pyridinyl‐butadienyl‐benzothiazole 3/PET potentially provides a neuroimaging‐based index for the evolution of PSP tau pathologies promoting the deterioration of motor and cognitive functions. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Tau imaging detects distinctive distribution of tau pathology in ALS/PDC on the Kii Peninsula

Objective

To characterize the distribution of tau pathology in patients with amyotrophic lateral sclerosis/parkinsonism dementia complex on the Kii Peninsula (Kii ALS/PDC) by tau PET using [¹¹C]PBB3 as ligand.

Methods

This is a cross-sectional study of 5 patients with ALS/PDC and one asymptomatic participant with a dense family history of ALS/PDC from the Kii Peninsula who took part in this study. All were men, and their age was 76 ± 8 (mean ± SD) years. Thirteen healthy men (69 ± 6 years) participated as healthy controls (HCs). Dynamic PET scans were performed following injection of [¹¹C]PBB3, and parametric PET images were generated by voxel-by-voxel calculation of binding potential (BP*_ND) using a multilinear reference tissue model. [¹¹C] Pittsburgh compound B (PiB) PET, MRI, and cognitive tests were also performed.

Results

A voxel-based comparison of [¹¹C]PBB3 BP*_ND illustrated PET-detectable tau deposition in the cerebral cortex and white matter, and pontine basis including the corticospinal tract in Kii ALS/PDC patients compared with HCs (uncorrected p < 0.05). Group-wise volume of interest analysis of [¹¹C]PBB3 BP*_ND images showed increased BP*_ND in the hippocampus and in frontal and parietal white matters of Kii ALS/PDC patients relative to HCs (p < 0.05, Holm-Sidak multiple comparisons test). BP*_ND in frontal, temporal, and parietal gray matters correlated with Mini-Mental State Examination scores in Kii ALS/PDC patients (p < 0.05). All Kii ALS/PDC patients were negative for [¹¹C]PiB (β-amyloid) except one with marginal positivity.

Conclusion

[¹¹C]PBB3 PET visualized the characteristic topography of tau pathology in Kii ALS/PDC, corresponding to clinical phenotypes of this disease.

Nuclear medicine of the cerebellum

Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) with different radiotracers enable regional evaluation of blood flow and glucose metabolism, of receptors and transporters of several molecules, and of abnormal deposition of peptides and proteins in the brain. The cerebellum has been used as a reference region for different radiotracers in several disease conditions. Whole-brain voxel-wise analysis is not affected by a priori knowledge bias and should be preferred. SPECT and PET have contributed to establishing the cerebellum role in motion, cognition, and emotion control in physiologic and pathophysiologic conditions. The basic abnormal imaging findings include decreased or increased uptake of flow or metabolism tracers in the cerebellum alone or as part of a network. Decreased uptake is generally observed in primary structural damage of the cerebellum, but can also represent a distant effect of cerebral damage (crossed diaschisis). Increased uptake can be observed in Freidreich ataxia, inflammatory or immune-mediated diseases of the cerebellum, and in status epilepticus. The possibility is also recognized that primary structural damage of the cerebellum might determine distance effects on other brain structures (reversed diaschisis). So far, SPECT and PET have been predominantly used in clinical studies to investigate cerebellar changes in neurologic and psychiatric diseases and in connection with pharmacologic, transcranial magnetic stimulation, deep-brain stimulation, or surgical treatments.

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.

Tau-imaging in neurodegeneration

Pathological cerebral aggregations of proteins are suggested to play a crucial role in the development of neurodegenerative disorders. For example, aggregation of the protein ß-amyloid in form of extracellular amyloid-plaques as well as intraneuronal depositions of the protein tau in form of neurofibrillary tangles represent hallmarks of Alzheimer's disease (AD). Recently, novel tracers for in vivo molecular imaging of tau-aggregates in the brain have been introduced, complementing existing tracers for imaging amyloid-plaques. Available data on these novel tracers indicate that the subject of Tau-PET may be of considerable complexity. On the one hand this refers to the various forms of appearance of tau-pathology in different types of neurodegenerative disorders. On the other hand, a number of hurdles regarding validation of these tracers still need to be overcome with regard to comparability and standardization of the different tracers, observed off-target/non-specific binding and quantitative interpretation of the signal. These issues will have to be clarified before systematic clinical application of this exciting new methodological approach may become possible. Potential applications refer to early detection of neurodegeneration, differential diagnosis between tauopathies and non-tauopathies and specific patient selection and follow-up in therapy trials.