What are the common functional imaging testing modalities?

Common Functional Imaging Testing Modalities

The common functional imaging testing modalities include EEG, evoked potentials, and MEG, while MRI and CT are structural imaging modalities, not functional imaging tests. 1, 2, 3, 4

Functional vs. Structural Imaging: Critical Distinction

Functional imaging measures brain activity, blood flow, metabolism, or electrical/magnetic signals in real-time, while structural imaging visualizes anatomical structures. 3, 5

True Functional Imaging Modalities:

• Electroencephalography (EEG): Measures electrical activity of the brain with millisecond temporal resolution, commonly used for seizure evaluation and post-cardiac arrest prognostication. 1, 2, 4, 5

• Evoked Potentials (EP): Records electrical responses to specific stimuli; bilateral absence of N20 cortical response to median nerve somatosensory evoked potentials (SSEPs) predicts poor neurological outcome with 0% false positive rate after cardiac arrest. 1

• Magnetoencephalography (MEG): Measures magnetic fields generated by neuronal electrical currents, providing direct assessment of brain electrophysiology with high temporal resolution (milliseconds) and superior spatial localization compared to EEG. 3, 4, 5

• Functional MRI (fMRI): Measures blood oxygen level-dependent (BOLD) signals reflecting regional neuronal activity changes, with high spatial resolution (few millimeters) but poor temporal resolution (seconds). 3, 6, 5

• Positron Emission Tomography (PET): Measures metabolic activity and blood flow using radiotracers; FDG-PET demonstrates 60% sensitivity for frontotemporal dementia diagnosis and high accuracy for Parkinson's disease diagnosis. 7, 3, 5

• Magnetic Resonance Spectroscopy (MRS): Measures chemical shifts in metabolite ratios to identify tissue abnormalities, useful for distinguishing brain tumor recurrence from radiation necrosis. 3

Structural Imaging (NOT Functional):

• Magnetic Resonance Imaging (MRI): Provides anatomical detail of brain structures; the American College of Radiology recommends MRI as preferred imaging for new-onset seizures in non-emergent situations. 2, 8

• Computed Tomography (CT): Rapidly identifies structural pathology including hemorrhage, stroke, tumors, and hydrocephalus; recommended for emergent seizure evaluation with high-risk features. 1, 2

Clinical Applications by Modality

EEG and Evoked Potentials:

• Post-cardiac arrest prognostication: Generalized suppression <20 µV or burst-suppression patterns predict poor outcome with 3% false positive rate at 24 hours post-ROSC in normothermic patients. 1
• Seizure evaluation: The American Academy of Neurology recommends EEG as part of neurodiagnostic evaluation for first unprovoked seizure, with abnormal findings predicting increased recurrence risk. 2

MEG:

• Epilepsy surgery planning: Localizes seizure foci and irritative zones with precision comparable to intracranial EEG, now indicated as routine clinical tool for lesional and nonlesional epilepsy patients. 3, 4
• Functional mapping: Identifies primary sensory cortices, language areas, and eloquent cortex with millimeter-level accuracy. 4

PET and fMRI:

• Dementia diagnosis: FDG-PET shows frontotemporal and parietal hypometabolism in frontotemporal dementia with 78.5% positive predictive value; reserve for diagnostically ambiguous cases after structural MRI. 7
• Presurgical planning: fMRI identifies sensorimotor cortex and language areas for tumor resection planning. 3

Temporal and Spatial Resolution Trade-offs

EEG and MEG provide superior temporal resolution (milliseconds) but moderate spatial resolution (millimeters to centimeters), while fMRI and PET offer high spatial resolution (few millimeters) but poor temporal resolution (seconds to minutes). 5 Combining modalities achieves both high temporal and spatial resolution by measuring different components of brain activity—electromagnetic signals versus neurovascular coupling. 6, 5, 9

Common Pitfalls to Avoid

• Confusing structural with functional imaging: MRI and CT visualize anatomy, not function; they cannot measure real-time brain activity or metabolism. 2, 3
• Misinterpreting confounded EEG: Sedatives, hypothermia, hypotension, and neuromuscular blockade reduce EEG prognostic accuracy; wait 24 hours post-ROSC in normothermic patients without confounders. 1
• Ordering PET prematurely: Obtain structural MRI first; add FDG-PET only when diagnosis remains uncertain, as it identifies 50% of cases missed by MRI alone. 7
• Ignoring regulatory status: MEG and PET are Health Canada-licensed; fMRI and MRS use MRI platforms without separate licensing; PET radiotracers require Clinical Trials Application. 3