Amyloid & Tau PET Imaging
Positron emission tomography (PET) targeting amyloid-beta (Aβ) and hyperphosphorylated tau has transformed the diagnostic approach to Alzheimer disease (AD) and related dementias. Three FDA-approved amyloid PET tracers and one FDA-approved tau PET tracer (flortaucipir/Tauvid) now allow in vivo detection and quantification of the two hallmark proteinopathies of AD. With the advent of anti-amyloid monoclonal antibody therapies (lecanemab, donanemab), molecular PET imaging has shifted from a primarily research tool to a gatekeeper for treatment eligibility, therapy monitoring, and biological disease staging under the 2024 revised Alzheimer’s Association diagnostic criteria.
Bottom Line
- Three FDA-approved amyloid PET tracers: florbetapir (Amyvid, 2012), flutemetamol (Vizamyl, 2013), and florbetaben (Neuraceq, 2014) — all received expanded indications in June 2025 for quantitative measurement and therapy monitoring
- Centiloid (CL) scale: Standardized quantitative metric across all tracers; positivity threshold ≈25 CL; donanemab clearance thresholds are <11 CL (single scan) or 11–25 CL (two consecutive scans)
- IDEAS study: Amyloid PET changed clinical management in >60% of MCI and dementia patients and improved diagnostic certainty by 50–60%
- Flortaucipir (Tauvid): Only FDA-approved tau PET tracer (May 2020; EMA August 2024); estimates density and distribution of neurofibrillary tangles; designated a Core 2 biomarker for disease staging
- Tau PET Braak staging: Recapitulates neuropathological Braak stages I–VI in vivo and informs biological staging (Stages A–D) under the 2024 criteria
- Anti-amyloid therapy eligibility: Confirmed amyloid positivity (PET or CSF) is mandatory; tau PET stratification guided treatment response in TRAILBLAZER-ALZ 2 (greatest efficacy in low/medium tau)
- CMS coverage expansion (2023–2025): Lifted one-scan-per-patient restriction; added separate reimbursement for radiopharmaceuticals; amyloid PET now accessible for both diagnosis and treatment monitoring
Amyloid PET: FDA-Approved Tracers
All currently approved amyloid PET tracers are F-18-labeled derivatives of thioflavin-T that bind fibrillar Aβ plaques. They replaced the earlier research tracer C-11 Pittsburgh Compound B (PiB), which had a 20-minute half-life limiting its use to centers with an on-site cyclotron.
| Tracer | Brand Name | Manufacturer | FDA Approved | Half-Life | Scan Window | Key Features |
|---|---|---|---|---|---|---|
| Florbetapir F-18 | Amyvid | Eli Lilly/Avid | April 2012 | 110 min | 30–50 min post-injection | First approved; most widely used in clinical trials; highest non-specific white matter binding |
| Flutemetamol F-18 | Vizamyl | GE Healthcare | October 2013 | 110 min | 90 min post-injection | Closest structural analog to PiB; highest gray-to-white matter contrast |
| Florbetaben F-18 | Neuraceq | Life Molecular Imaging | March 2014 | 110 min | 90–110 min post-injection | Stilbene derivative; lower non-specific cortical binding than florbetapir |
In June 2025, the FDA approved expanded indications for all three tracers, permitting quantitative measurement of amyloid plaque density and therapy monitoring — a major advance beyond the original qualitative (positive/negative) visual read indication.
Interpretation of Amyloid PET
Amyloid PET scans are interpreted through three complementary approaches. Visual (qualitative) read classifies a scan as positive when cortical gray matter tracer uptake approaches or exceeds white matter intensity, indicating moderate-to-frequent neuritic plaques (inter-reader agreement 85–95%). Standardized uptake value ratio (SUVR) normalizes regional cortical uptake to a reference region (cerebellum or pons); thresholds are tracer-specific and not directly comparable without harmonization. The Centiloid scale provides a universal quantitative metric:
Centiloid Standardization
- Definition: A universal quantitative scale harmonized to C-11 PiB, where 0 CL represents young healthy controls (no amyloid) and 100 CL represents typical AD dementia
- Positivity threshold: ≈25 Centiloids across all tracers
- Clinical utility: Enables direct cross-tracer comparison, longitudinal monitoring, and treatment response assessment
- Donanemab clearance thresholds: <11 CL on a single scan, or 11–25 CL on two consecutive scans, defines amyloid clearance and permits treatment discontinuation
- Re-accumulation rate: After clearance with donanemab, amyloid re-accumulates at ≈2.8 CL/year, remaining below positivity threshold for years
- Lecanemab: CLARITY AD showed reduction of −59.1 CL versus placebo at 18 months
IDEAS Study & Clinical Impact
The Imaging Dementia — Evidence for Amyloid Scanning (IDEAS) study was the landmark real-world study that led to expanded CMS coverage of amyloid PET.
IDEAS Study Key Findings (Rabinovici et al., JAMA 2019)
- Study population: 16,008 Medicare beneficiaries with MCI or atypical/uncertain dementia
- Management change: Amyloid PET results changed clinical management in 60.2% of MCI patients and 63.5% of dementia patients within 90 days
- Diagnostic certainty: Improved by approximately 50–60%
- Medication changes: Significant shifts in AD-specific medication prescribing based on amyloid PET results
- Negative amyloid PET: Led to investigation for alternative diagnoses and discontinuation of AD-specific therapies in many cases
- CMS coverage: Study results supported lifting the one-scan-per-patient restriction and broader reimbursement
Appropriate Use Criteria for Amyloid PET
The Alzheimer’s Association/SNMMI Workgroup published updated appropriate use criteria (AUC) in 2024–2025:
| Appropriate | Inappropriate |
|---|---|
| Persistent or progressive unexplained MCI | Cognitively normal patients without clinical concern |
| Atypical or mixed presentation with diagnostic uncertainty | Based solely on family history or APOE4 carrier status |
| Early-onset dementia (<65 years) | Typical late-onset AD with high clinical certainty |
| Anti-amyloid therapy eligibility determination | Severity assessment of established diagnosis |
| Monitoring amyloid clearance during/after treatment | Nonmedical purposes (insurance, legal, employment) |
| When results will change diagnosis or management | Asymptomatic patients outside clinical trials |
CMS Coverage & Cost
Amyloid PET costs an estimated $3,000–$6,000 per scan. CMS coverage has expanded substantially: the one-scan-per-patient restriction was lifted in 2023, payment for radiopharmaceuticals was unbundled in 2024, and separate reimbursement for Amyvid in hospital outpatient settings was added in January 2025. Access barriers remain in rural areas due to limited PET scanner availability and radiopharmaceutical supply logistics. Blood-based biomarker screening (plasma p-tau217, FDA-cleared May 2025) may reduce reliance on amyloid PET as a first-line test, reserving PET for confirmatory or intermediate-result cases.
Tau PET: Flortaucipir (Tauvid)
Flortaucipir F-18 (Tauvid) received FDA approval in May 2020 as the first and only tau PET tracer for clinical use. It is indicated for adults with cognitive impairment being evaluated for AD and estimates the density and spatial distribution of aggregated neurofibrillary tangles (NFTs) composed of paired helical filament tau. The European Medicines Agency (EMA) granted marketing authorization in August 2024.
Flortaucipir binds selectively to 3R/4R tau isoforms in AD neurofibrillary tangles, with lower affinity for 4R-only tau (PSP, CBD) and 3R-only tau (Pick disease). Known off-target binding occurs in melanin-containing structures (choroid plexus, substantia nigra) and MAO-B-rich regions (striatum, thalamus). Scans are acquired 80–100 minutes post-injection over a 20-minute window.
Tau PET Braak Staging
Tau PET recapitulates the Braak neuropathological staging of NFT progression in vivo. The 2024 revised AD criteria integrate tau PET staging as a Core 2 biomarker for biological disease staging:
| Braak Stage | Tau PET Distribution | 2024 Biological Stage | Clinical Correlation |
|---|---|---|---|
| I–II | Entorhinal cortex, hippocampus, medial temporal lobe | Stage B (Early) | Often preclinical or early MCI; limited neocortical involvement |
| III–IV | Spread to lateral temporal, parietal, and posterior cingulate cortices | Stage C (Intermediate) | MCI to mild dementia; correlates with episodic memory decline |
| V–VI | Widespread neocortical involvement including frontal, prefrontal, and occipital cortices | Stage D (Advanced) | Moderate to severe dementia; extensive cortical dysfunction |
Clinical Applications of Tau PET
- Biological disease staging: Core 2 biomarker in 2024 criteria; stages A (no tau uptake) through D (high neocortical uptake)
- Prognostication: Higher tau PET burden predicts faster cognitive decline and greater functional impairment
- Treatment stratification: TRAILBLAZER-ALZ 2 showed greatest donanemab efficacy in patients with low/medium tau (CDR-SB: 36% slowing) versus high tau (22% slowing)
- Differential diagnosis: Distinguishes AD from non-AD tauopathies (PSP, CBD) based on topographic pattern
- Atypical AD phenotyping: Posterior cortical atrophy shows posterior-predominant tau; logopenic aphasia shows left temporoparietal tau; early-onset AD shows greater and more widespread tau than late-onset AD
- Longitudinal monitoring: Tau accumulation rates are faster in amyloid-positive MCI and AD than in cognitively normal adults
Investigational Tau PET Tracers
Second-generation tau PET tracers offer improved selectivity and reduced off-target binding:
| Tracer | Developer | Status | Key Advantages |
|---|---|---|---|
| MK-6240 (florquinitau) | Cerveau/Merck | Investigational; widely used in clinical trials | High signal-to-noise ratio; minimal off-target binding; strong Braak staging concordance |
| PI-2620 | Life Molecular Imaging | Investigational; second-generation | Binds both 3R/4R tau (AD) and 4R tau (PSP, CBD); potential for non-AD tauopathies |
| RO948 | Roche | Clinical trials | Used in Roche anti-amyloid and anti-tau clinical trials |
| APN-1607 (florzolotau) | APRINOIA | Approved in China (2024) | Approved for clinical use outside the United States |
Prevalence of tau PET positivity is consistent across tracers: flortaucipir 78%, MK-6240 76%, RO948 75%. The CenTauR threshold of 18 in the meta-temporal region has been proposed for standardization across tau PET tracers.
Combined Amyloid & Tau PET Approaches
Dual-tracer imaging provides the most comprehensive in vivo assessment of AD neuropathology:
- A+T− (amyloid positive, tau negative): Earliest biological stage of AD (Stage A); individual may be cognitively normal or have subjective cognitive decline
- A+T+ (medial temporal): Early tau involvement (Stage B); often corresponds to MCI; 50% conversion to dementia within 5 years
- A+T+ (neocortical): Intermediate to advanced AD (Stages C–D); strong correlation with clinical dementia severity
- A−T+: Uncommon; may represent non-AD tauopathy, primary age-related tauopathy (PART), or rare false-negative amyloid PET
- A−T−: Effectively rules out AD as primary etiology; consider non-AD dementias (FTD, DLB, VCI)
The temporal ordering proposed by Jack and colleagues, confirmed in the DIAN cohort, suggests amyloid accumulation begins ≈20 years before symptom onset, tau pathology ≈15 years before onset, and neurodegeneration ≈5–10 years before onset.
Role in Anti-Amyloid Therapy Eligibility & Monitoring
Key Requirements for Anti-Amyloid Therapy
- Amyloid confirmation is mandatory: Either positive amyloid PET or CSF biomarkers (or plasma p-tau217, now FDA-cleared) must confirm Aβ pathology before initiating lecanemab or donanemab
- Baseline MRI required: Must exclude >4 microhemorrhages, macrohemorrhages, superficial siderosis, vasogenic edema, or severe white matter hyperintensities (Fazekas ≥3)
- APOE genotyping recommended: Homozygous APOEε4 carriers have ARIA-E rates up to 34.5% (lecanemab) and 41.7% (donanemab)
- Tau PET stratification: While not required for eligibility, tau burden predicts treatment response — TRAILBLAZER-ALZ 2 showed CDR-SB slowing of 36% in low/medium tau versus 22% in high tau
- MRI monitoring schedule (lecanemab): Prior to 5th, 7th, and 14th infusions, and at week 52; additional MRI if ARIA symptoms emerge
- Amyloid PET for treatment completion (donanemab): Scans at weeks 24 and 52 to assess clearance (<11 CL or 11–25 CL on two consecutive scans); treatment can be discontinued upon amyloid clearance
Molecular PET in Non-AD Dementias
Amyloid PET positivity is not exclusive to AD: 25–30% of cognitively normal adults over 65 are positive (preclinical AD by 2024 criteria), >50% of DLB patients are amyloid positive, and CAA shows widespread amyloid with occipital predominance. FTD is generally amyloid negative, and positivity is often exclusionary. Current tau PET tracers have limited sensitivity for non-AD tauopathies due to preferential binding to AD-type 3R/4R paired helical filament tau:
- PSP: Increased retention in basal ganglia, midbrain, subthalamic nucleus, and cerebellar dentate — lower signal than in AD
- CBD: Asymmetric uptake in premotor cortex, globus pallidus, and basal ganglia contralateral to most-affected side
- Behavioral variant FTD: Elevated signal in ≈50% of cases (reflecting tau subtype); significantly weaker than in AD
- MAPT mutation carriers: Mixed 3R/4R mutations show high tau PET signal; 4R-only mutations show low signal
Limitations of Current Molecular PET Imaging
- Off-target binding: Flortaucipir binds melanin (choroid plexus), neuromelanin (substantia nigra), and MAO-B (striatum, thalamus)
- Non-AD tau insensitivity: Current tracers have limited affinity for 4R-only or 3R-only tau isoforms found in PSP, CBD, and Pick disease
- Amyloid PET in aging: 25–30% of cognitively normal elderly are amyloid positive; positivity alone does not equate to clinical AD
- Cost: $3,000–$6,000 per amyloid PET scan; tau PET similar or higher cost; limited insurance coverage for tau PET
- Accessibility: PET scanners concentrated in academic medical centers; rural access remains poor; radiopharmaceutical distribution requires regional manufacturing
- No PET tracers are currently FDA-approved for TDP-43 or alpha-synuclein imaging, though ACI-12589 (alpha-synuclein tracer) showed promising results in MSA differentiation
Emerging Developments
- Alpha-synuclein PET (ACI-12589): Significant uptake in cerebellar white matter and middle cerebellar peduncles in MSA, distinguishing from controls, Parkinson disease, DLB, AD, and PSP
- Blood-based biomarker triage: Plasma p-tau217 (FDA-cleared May 2025; AUC 0.93–0.96) as a screening tool before PET, reducing costs and improving accessibility
- AI-assisted PET interpretation: Deep learning algorithms for automated quantification may reduce reader variability
- Second-generation tau tracers: PI-2620, MK-6240, and others offer improved signal-to-noise ratios and broader tau isoform coverage for non-AD tauopathies
Summary: Amyloid vs Tau PET Comparison
| Feature | Amyloid PET | Tau PET |
|---|---|---|
| FDA-approved tracers | 3 (florbetapir, flutemetamol, florbetaben) | 1 (flortaucipir/Tauvid) |
| 2024 criteria role | Core 1 biomarker (diagnostic) | Core 2 biomarker (staging/prognostic) |
| Primary clinical indication | Confirms amyloid pathology; treatment eligibility | Disease staging; prognostication; treatment stratification |
| Quantitative scale | Centiloid (positivity ≈25 CL) | CenTauR threshold of 18 (meta-temporal); Braak staging |
| Pattern in typical AD | Diffuse cortical (frontal, parietal, posterior cingulate, temporal) | Braak progression: entorhinal → temporal → parietal → frontal |
| Longitudinal change | Slow accumulation; reaches plateau in advanced AD | Faster in MCI and AD than cognitively normal; tracks clinical decline |
| Utility in non-AD | Positive in CAA, DLB (>50%); negative in most FTD | Limited sensitivity for non-AD tauopathies (PSP, CBD) |
References
- Rabinovici GD, Gatsonis C, Apgar C, et al. Association of amyloid positron emission tomography with subsequent change in clinical management among Medicare beneficiaries with mild cognitive impairment or dementia. JAMA. 2019;321(13):1286-1294.
- Jack CR, Andrews JS, Beach TG, et al. Revised criteria for diagnosis and staging of Alzheimer’s disease: Alzheimer’s Association Workgroup. Alzheimers Dement. 2024;20(8):5143-5169.
- Schwarz AJ, Yu P, Miller BB, et al. Regional profiles of the candidate tau PET ligand 18F-AV-1451 recapitulate key features of Braak histopathological stages. Brain. 2016;139(Pt 5):1539-1550.
- Sims JR, Zimmer JA, Evans CD, et al. Donanemab in early symptomatic Alzheimer disease: the TRAILBLAZER-ALZ 2 randomized clinical trial. JAMA. 2023;330(6):512-527.
- van Dyck CH, Swanson CJ, Aisen P, et al. Lecanemab in early Alzheimer’s disease. N Engl J Med. 2023;388(1):9-21.
- Klunk WE, Engler H, Nordberg A, et al. Imaging brain amyloid in Alzheimer’s disease with Pittsburgh Compound-B. Ann Neurol. 2004;55(3):306-319.
- Johnson KA, Schultz A, Betensky RA, et al. Tau positron emission tomographic imaging in aging and early Alzheimer disease. Ann Neurol. 2016;79(1):110-119.
- Risacher SL, Apostolova LG. Neuroimaging in dementia. Continuum (Minneap Minn). 2024;30(6):1761-1789.
- Cummings J, Apostolova L, Rabinovici GD, et al. Lecanemab: appropriate use recommendations. J Prev Alzheimers Dis. 2023;10(3):362-377.
- Jack CR, Knopman DS, Jagust WJ, et al. Tracking pathophysiological processes in Alzheimer’s disease: an updated hypothetical model of dynamic biomarkers. Lancet Neurol. 2013;12(2):207-216.
- Gordon BA, Blazey TM, Su Y, et al. Spatial patterns of neuroimaging biomarker change in individuals from families with autosomal dominant Alzheimer’s disease: a longitudinal study. Lancet Neurol. 2018;17(3):241-250.
- Smith R, Capotosti F, Schain M, et al. The alpha-synuclein PET tracer [18F] ACI-12589 distinguishes multiple system atrophy from other neurodegenerative diseases. Nat Commun. 2023;14(1):6750.
- Braak H, Braak E, Bohl J. Staging of Alzheimer-related cortical destruction. Eur Neurol. 1993;33(6):403-408.
- Hampel H, Elhage A, Cho M, et al. Amyloid-related imaging abnormalities (ARIA): radiological, biological and clinical characteristics. Brain. 2023;146(11):4414-4424.