Nuclear and Molecular Imaging (PET, SPECT)

Nuclear and molecular imaging visualize disease at the metabolic and receptor level — beyond what structural CT/MRI can show. The clinical workhorses are FDG-PET (glucose metabolism), amyloid PET (β-amyloid plaques), tau PET (pathologic tau), DAT-SPECT (dopamine transporter density), and brain perfusion SPECT. Each answers a distinct clinical question.

FDG-PET (Fluorodeoxyglucose)

What It Measures

• Glucose metabolism. Higher uptake = more metabolic activity.
• Normal brain: high cortical uptake; basal ganglia and thalamus high; white matter low; cerebellum reference.

Dementia Patterns

Tumor Imaging with FDG-PET

• Recurrent tumor vs radiation necrosis: tumor recurrence has high FDG uptake; radiation necrosis has low/absent uptake.
• Glioma grading: high-grade glioma takes up FDG (often) vs low-grade (less).
• CNS lymphoma: typically highly FDG-avid; helps distinguish from toxoplasmosis (low uptake).
• Limitation: normal cortex is highly FDG-avid → contrast with tumor can be poor. Amino acid PET tracers (FET, MET) outperform FDG for glioma.

Epilepsy Localization

• Interictal FDG-PET: hypometabolism in the seizure focus.
• Used in temporal lobe epilepsy localization when MRI is non-lesional.

Amyloid PET

Tracers

• Florbetapir (Amyvid) — F-18.
• Florbetaben (Neuraceq) — F-18.
• Flutemetamol (Vizamyl) — F-18.
• (Historical Pittsburgh Compound B [11C-PIB] was research-only due to C-11 half-life.)

Interpretation

• Amyloid-positive: loss of normal gray-white contrast (cortical gray accumulates tracer to match white matter signal).
• Amyloid-negative: preserved gray-white contrast.
• Centiloid score for quantification (cross-tracer standardization).

Clinical Use

• Lecanemab and donanemab eligibility: anti-amyloid therapy requires confirmation of amyloid pathology before treatment. Amyloid PET OR CSF biomarkers (Aβ42/40 ratio, p-tau181) are both accepted routes per LEQEMBI / KISUNLA prescribing information. Validated plasma assays are emerging in triage workflows.
• Differential diagnosis of dementia: positive in AD; negative excludes AD pathology with high NPV.
• Atypical presentations: clarify whether AD pathology underlies logopenic PPA, posterior cortical atrophy, frontal-variant AD.
• Limitation: amyloid positivity is common in cognitively normal elderly (~30% by age 70) — interpretation must be clinically anchored.

Tau PET

Tracer

• Flortaucipir (Tauvid) — F-18. FDA-approved to estimate the density and distribution of aggregated tau neurofibrillary tangles in adults with cognitive impairment being evaluated for AD. (A positive scan supports but does not by itself establish AD as the diagnosis.)
• Newer second-generation tracers in development (PI-2620, MK-6240, RO-948).

Interpretation

• AD: medial temporal (Braak I/II) → temporoparietal (Braak III/IV) → diffuse cortical (Braak V/VI).
• Pathologic tau correlates better with clinical symptoms than amyloid does.
• Off-target binding: choroid plexus, basal ganglia (MAO-B), some pigmented lesions, hemorrhage.

Clinical Use

• Supports AD evaluation in selected cognitively impaired patients — interpret within the broader clinical and biomarker context, not as a standalone “AD confirmation.”
• Helps stage AD severity (Braak topography).
• Differentiates AD from primary tauopathies (PSP, CBD show different tau distribution — first-generation tracers are less sensitive for non-AD tau).
• Per the 2025 updated amyloid/tau PET appropriate use criteria, tau PET should be ordered in clinical scenarios where it will change management, not reflexively in every cognitive workup.

DAT-SPECT (Dopamine Transporter Imaging)

Tracer

• Ioflupane I-123 (DaTscan) — binds presynaptic dopamine transporter.

Interpretation

• Normal: bilateral symmetric “comma-shaped” striatal uptake.
• Abnormal (PD, DLB, MSA, PSP): asymmetric or reduced uptake; “period” or “full-stop” appearance (loss of putaminal tail).

Clinical Use

• Distinguishing parkinsonism from non-degenerative tremor:
  • Abnormal: PD, DLB, MSA, PSP, CBD.
  • Normal: essential tremor, drug-induced parkinsonism, psychogenic tremor, dystonic tremor, SWEDDs (scans without evidence of dopaminergic deficit).
  • Vascular parkinsonism: variable — may be normal when nigrostriatal terminals are spared (pure striatal lacunar disease), or abnormal when there is structural nigrostriatal involvement or coexisting degenerative disease.
• Does NOT distinguish PD from DLB / MSA / PSP — all show abnormal DAT.
• DLB diagnostic biomarker: abnormal DAT-SPECT is a core indicative biomarker for probable DLB.

Brain Perfusion SPECT

Tracers

• Tc-99m HMPAO (Ceretec).
• Tc-99m ECD (Neurolite).

Clinical Use

• Dementia screening: similar patterns to FDG-PET but less sensitive.
• Epilepsy localization:
  • Ictal SPECT: hyperperfusion at seizure focus during ictal injection; subtraction (SISCOM) localizes precisely.
  • Interictal SPECT: hypoperfusion at focus.
• Brain death confirmation: absence of brain perfusion (hollow skull sign) supports brain death; lacks specificity for some clinical scenarios.

MIBG Cardiac Scintigraphy (Sidebar — Not Brain Imaging)

• I-123 MIBG cardiac uptake: reduced in Lewy body diseases (DLB, PD) due to cardiac sympathetic denervation.
• Reduced uptake supports DLB / PD over AD.
• A core indicative biomarker for probable DLB (along with DAT-SPECT and REM sleep behavior disorder).

Amino-Acid PET (Brain Tumor)

• F-18 FET (fluoroethyltyrosine), C-11 MET (methionine), F-18 DOPA.
• Higher tumor-to-background ratio than FDG (normal brain doesn’t take up amino acids heavily).
• Superior to FDG for glioma grading, biopsy targeting, recurrence vs radiation necrosis, treatment response.
• Widely used in Europe; uptake in US growing.

Pitfalls and Pearls

• Amyloid PET positivity rises with age: ~30% of cognitively normal 70-year-olds are amyloid-positive → interpret in clinical context.
• DAT-SPECT does not distinguish PD subtypes: abnormal in PD, DLB, MSA, PSP, CBD; normal in essential tremor, drug-induced, vascular, psychogenic.
• Cingulate island sign on FDG-PET is highly specific for DLB over AD.
• “Hot frontal” FDG pattern = bvFTD.
• Asymmetric frontotemporal FDG hypometabolism contralateral to clinical signs = CBD.
• Midbrain hypometabolism on FDG + clinical postural instability = PSP.
• Cerebellar / brainstem hypometabolism = MSA-C.
• Tau PET correlates with cognitive decline better than amyloid does — amyloid is a “trigger,” tau is the “executor.”
• SWEDDs (scans without evidence of dopaminergic deficit) — normal DAT-SPECT in clinically suspected PD → review diagnosis (often dystonic tremor or essential tremor).
• FDG-PET for tumor is limited by high normal-brain uptake; amino-acid PET (FET, DOPA) is superior.
• Amyloid pathology confirmation is now part of dementia workup for lecanemab / donanemab eligibility — amyloid PET or CSF biomarkers (Aβ42/40 ratio, p-tau181) are both accepted. Confirm payer coverage before ordering PET.

References

1. McKeith IG, Boeve BF, Dickson DW, et al. Diagnosis and management of dementia with Lewy bodies: Fourth consensus report of the DLB Consortium. Neurology. 2017;89(1):88-100.
2. Jack CR Jr, Bennett DA, Blennow K, et al. NIA-AA Research Framework: Toward a biological definition of Alzheimer’s disease. Alzheimers Dement. 2018;14(4):535-562.
3. Kuo PH, Stuckey J, Lewis SJ, Ravina B. The use of DAT-SPECT in movement disorders. J Nucl Med Technol. 2019;47(4):241-246.
4. Albert NL, Weller M, Suchorska B, et al. Response Assessment in Neuro-Oncology Working Group and European Association for Neuro-Oncology recommendations for the clinical use of PET imaging in gliomas. Neuro Oncol. 2016;18(9):1199-1208.
5. Mason NS, Mathis CA, Klunk WE. Positron emission tomography radioligands for in vivo imaging of Aβ plaques. J Labelled Comp Radiopharm. 2013;56(3-4):89-95.