What is the recommended imaging protocol for diagnosing dementia?

Proper Imaging for Dementia

Primary Recommendation

MRI brain without IV contrast is the preferred initial imaging study for all dementia evaluations, with CT head without contrast as an acceptable alternative when MRI is unavailable or contraindicated. 1, 2

Initial Imaging Approach by Clinical Scenario

Alzheimer Disease (Typical Presentation)

• MRI brain without IV contrast is the recommended first-line study to exclude treatable structural abnormalities (tumors, subdural hematomas, hydrocephalus) and demonstrate atrophy patterns in medial temporal lobes, lateral temporal and parietal cortices 1, 2
• CT head without contrast is an acceptable alternative when MRI is unavailable, though less sensitive for detecting hippocampal atrophy and vascular lesions 1
• Brain FDG-PET/CT and brain amyloid PET/CT are appropriate advanced imaging studies that support clinical diagnosis when combined with clinical history and other biomarkers 1
• IV contrast is not necessary for initial structural imaging in typical AD presentations 1

Alzheimer Disease (Atypical Presentation)

• MRI brain without IV contrast remains the primary structural imaging modality 1
• Brain amyloid PET/CT is particularly valuable to confirm or exclude AD pathology when clinical presentation is atypical 1
• Brain tau PET/CT may be appropriate to assess tau pathology distribution in atypical presentations 1
• Brain FDG-PET/CT helps demonstrate metabolic patterns that may differ from typical AD 1

Frontotemporal Dementia (FTD)

• MRI brain without IV contrast is recommended to assess for frontal and temporal lobe atrophy patterns and exclude mimics 1
• Brain FDG-PET/CT is highly recommended as it demonstrates characteristic frontal and temporal hypometabolism; combined with MRI, these modalities have high sensitivity and specificity for FTD diagnosis 1
• Brain amyloid PET/CT may be appropriate when differentiating early FTD from AD—a negative scan rules out AD and supports FTD diagnosis 1
• CT head without contrast is acceptable when MRI is unavailable 1

Dementia with Lewy Bodies (DLB)

• MRI brain without IV contrast or CT head without contrast to exclude structural mimics 1
• Brain striatal SPECT/CT (dopamine transporter imaging) is recommended to demonstrate dopaminergic loss in basal ganglia, which supports DLB diagnosis 1
• Brain FDG-PET/CT may show characteristic occipital hypometabolism 1

Vascular Dementia

• MRI brain without IV contrast is strongly preferred over CT because it is far more sensitive for detecting vascular lesions, white matter hyperintensities (assessed by Fazekas scale), cortical and subcortical infarcts, lacunar infarcts, and microhemorrhages 1, 2
• CT head without contrast can detect large infarctions but may miss significant vascular pathology; histopathologically verified vascular dementia cases with normal CT have been reported 1
• IV contrast is not needed for initial vascular dementia evaluation 1

Normal Pressure Hydrocephalus (NPH)

• MRI brain without IV contrast is recommended to demonstrate ventriculomegaly with Evans index >0.3, enlargement of temporal horns, callosal angle <90 degrees, and aqueductal flow void 1, 2
• CT head without IV contrast is appropriate to evaluate for ventriculomegaly out of proportion to sulci and exclude other pathologies 1
• Imaging features that predict positive shunt response include hyperdynamic aqueductal CSF flow on cine MRI 1

Mild Cognitive Impairment (MCI)

• MRI brain without IV contrast is recommended to assess hippocampal atrophy, which predicts conversion to AD with 80% accuracy 1
• Volumetric measurements of hippocampus, entorhinal cortex, and supramarginal gyrus can be used for risk stratification 1
• Brain tau PET/CT shows increased cortical tau deposition in MCI patients at high risk of converting to AD 1
• Brain perfusion and striatal SPECT/CT are not recommended for initial MCI imaging 1

Rapidly Progressive Dementia (RPD)

• MRI brain without and with IV contrast is the recommended initial study to detect infectious/inflammatory processes, demyelinating diseases, neoplastic and paraneoplastic processes, vasculitis, and prion-related diseases 1
• MRI brain without IV contrast alone is also appropriate 1
• CT head without IV contrast can demonstrate subdural hematomas, large masses, and hydrocephalus 1
• This is the only dementia scenario where IV contrast is recommended for initial imaging 1

Pre-Treatment and Post-Treatment Imaging for Antiamyloid Therapy

Pre-Treatment Imaging

• MRI brain without IV contrast is mandatory to identify imaging findings that may preclude treatment, particularly pre-existing microhemorrhages and superficial siderosis 1, 2
• Brain amyloid PET/CT is recommended to confirm presence of amyloid in brain parenchyma before initiating therapy 1
• Brain amyloid PET/CT has a lower false-negative rate than brain tau PET/CT for this purpose 1

Post-Treatment Monitoring

• MRI brain without IV contrast is the only recommended imaging modality for monitoring patients on antiamyloid monoclonal antibody therapy 1
• MRI is essential to detect amyloid-related imaging abnormalities (ARIA), including ARIA-E (edema) and ARIA-H (hemorrhage), which may require treatment modification or cessation 1, 2
• Brain amyloid PET/CT may be used secondarily to demonstrate clearance of amyloid from brain parenchyma 1
• IV contrast is not necessary to detect ARIA 1

Key Clinical Caveats

• Most patients over age 80 with cognitive impairment have mixed pathology, making pure diagnostic categories less common 2
• Imaging findings are supportive but not definitive—specific molecular biomarkers remain necessary to confirm Alzheimer disease diagnosis and guide disease-modifying therapy 2
• MRI is superior to CT for detecting vascular lesions, microhemorrhages, hippocampal atrophy, and rare dementia causes, though both are considered "usually appropriate" by ACR guidelines 1, 2
• Amyloid PET can be positive in cognitively normal individuals who never develop dementia, and a negative scan does not exclude non-AD neurodegenerative conditions 1
• MR spectroscopy, fMRI, and routine use of IV contrast are not recommended for initial dementia evaluation in most scenarios 1