Functional Neuroimaging

fNI Fundamentals - Brain Buzz Basics

• Functional Neuroimaging (fNI): Techniques mapping brain activity by detecting physiological/metabolic changes (e.g., blood flow, oxygenation).
• Neurovascular Coupling (NVC): The core physiological basis.
  • ↑ Neuronal activity → ↑ Localized cerebral blood flow (CBF) & oxygen delivery.
• BOLD (Blood-Oxygen-Level Dependent) Effect: Cornerstone of fMRI.
  • Neural activation → ↑ CBF → ↓ Deoxyhemoglobin (paramagnetic) concentration → ↑ fMRI signal.

  ⭐ BOLD fMRI signal relies on the difference in magnetic susceptibility between oxyhemoglobin (diamagnetic) and deoxyhemoglobin (paramagnetic).

• Advantages: Non-invasive; in-vivo human brain mapping.
• Limitations: Indirect measure of neural activity; susceptible to artifacts; resolution limits.

fMRI Deep Dive - Mapping Mind Moves

• Principle: BOLD (Blood-Oxygen-Level-Dependent) contrast reflects neural activity.
  • Neurovascular coupling: ↑ activity → ↑ blood flow → ↑ oxyHb (↓ deoxyHb) → ↑ T2* signal.
• Sequence: Echo-Planar Imaging (EPI) for rapid acquisition.
• Types:
  • Task-based: Correlates brain activity with tasks.
    • Block design: Alternating task/rest.
    • Event-related: Detects responses to discrete stimuli.
  • Resting-state (rs-fMRI): Assesses spontaneous BOLD fluctuations; identifies networks (e.g., DMN).
• Clinical Uses:
  • Presurgical mapping: Eloquent cortex (motor, language, visual). Language lateralization uses Laterality Index (LI), $LI = (L-R)/(L+R)$.
  • Epilepsy: Non-invasive seizure foci localization.
• Paradigms:
  • Motor: Finger tapping.
  • Language: Verb generation.

⭐ In presurgical fMRI for language lateralization, a Laterality Index (LI) is often calculated to determine language dominance.

Nuclear Neuroimaging - Tracer Trackers

• Basic Principles:
  • PET (Positron Emission Tomography): Radiotracers emit positrons; annihilation creates two $\mathbf{511} \text{ keV}$ gamma rays detected simultaneously.
  • SPECT (Single Photon Emission Computed Tomography): Radiotracers emit single gamma rays; detected by rotating gamma cameras.
• PET Tracers & Uses:
  • $^{18}\text{F-FDG}$ (Fluorodeoxyglucose): Glucose metabolism (tumors, dementia, epilepsy).
  • $^{15}\text{O-water}$: Cerebral Blood Flow (CBF).
  • Amyloid (e.g., $^{18}\text{F-Florbetapir}$): Alzheimer's disease plaques.
  • Tau (e.g., $^{18}\text{F-Flortaucipir}$): Tau pathology in neurodegeneration.
• SPECT Tracers & Uses:
  • $^{99\text{m}}\text{Tc-HMPAO}$, $^{99\text{m}}\text{Tc-ECD}$: Cerebral Blood Flow (CBF) (stroke, dementia, epilepsy).
• Clinical Applications:
  • Dementia: Differentiating types (e.g., Alzheimer's vs. FTD patterns).
  • Epilepsy: Pre-surgical localization of seizure foci.
  • Brain Tumors: Grading, detecting recurrence, distinguishing from radiation necrosis.

⭐ Interictal FDG-PET in temporal lobe epilepsy typically shows hypometabolism in the epileptogenic zone, while ictal SPECT shows hyperperfusion.

Electrophys & DTI + Comparison - Signals, Pathways & Showdown

• EEG & MEG:
  • Signal: Neuronal electrical/magnetic activity.
  • Temporal Res: Excellent (ms).
  • Use: Epilepsy (seizure localization).
• DTI & Tractography:
  • Assesses: WM integrity (FA, MD).
  • Maps: Structural connectivity (links to function).
  • Use: TBI, stroke, tumors, surg plan.

Functional Neuroimaging Modality Comparison:

High‑Yield Points - ⚡ Biggest Takeaways

• fMRI uses BOLD signal (blood oxygenation) to map brain activity via hemodynamic response.
• PET with FDG assesses glucose metabolism; vital for tumor grading and dementia evaluation.
• SPECT evaluates regional cerebral blood flow (rCBF), aiding in ictal focus localization.
• DTI maps white matter integrity and tracts by measuring water diffusion anisotropy.
• MRS identifies brain metabolites (NAA, choline) for lesion differentiation and prognosis.
• Functional neuroimaging is key for pre-surgical planning and diagnosing neurological disorders.