What imaging or scan results can indicate Alzheimer’s disease in a geriatric patient?

Imaging Findings Indicative of Alzheimer's Disease

Primary Recommendation

MRI brain without IV contrast is the appropriate initial imaging study for suspected Alzheimer's disease, with hippocampal atrophy being the single most diagnostically valuable finding, predicting conversion to AD with 80% accuracy. 1

Structural MRI Findings

Medial Temporal Lobe Changes (Most Important)

• Hippocampal atrophy is the most sensitive structural marker, with volumes measuring 1.75 standard deviations below normal in very mild AD, 1.99 SD in mild AD, and 2.22 SD in moderate AD 2
• Hippocampal volume measurements provide the best discrimination between AD patients and controls, with sensitivity of 83% and specificity of 80-89% 1
• Entorhinal cortex volume loss occurs earliest in the disease process, preceding hippocampal changes 1
• Signal abnormalities on intermediate T2-weighted scans in the uncal-hippocampal or insular cortex are frequently and almost exclusively noted in Alzheimer's disease 3

Cortical Atrophy Patterns

• Lateral temporal and parietal lobe atrophy with ventricular enlargement represents more advanced disease 1
• Moderate and severe cortical atrophy with third ventricular to intracranial width ratio larger than 7% effectively discriminates demented patients from normally aging controls 3
• Supramarginal gyrus thickness and entorhinal cortex thickness measurements aid risk stratification 1

Advanced MRI Sequences

Diffusion-Weighted Imaging

• Elevated apparent diffusion coefficient (ADC) values in the hippocampi, cingulate, parahippocampal gyri, and temporal/frontal lobes distinguish AD from healthy controls 1
• Diffusion tensor imaging demonstrates reduced fractional anisotropy in the precuneus 1

MR Spectroscopy Findings

• Decreased N-acetylaspartate (NAA) to creatine (Cr) ratios correlate with cognitive decline 1
• Elevated myoinositol (myI) to Cr ratios and elevated choline (Cho) to Cr ratios predict AD development 1
• Decreased NAA to myoinositol ratios distinguish AD from mild cognitive impairment 1

Perfusion Imaging

• Arterial spin labeling shows hypoperfusion patterns in temporoparietal regions, similar to FDG-PET findings 1

Nuclear Medicine Imaging

Amyloid PET/CT

• Brain amyloid PET/CT is highly recommended for pretreatment imaging before initiating antiamyloid monoclonal antibody therapy, demonstrating the presence of amyloid in brain parenchyma 4
• Amyloid PET/CT results in a change in diagnosis of suspected etiology of cognitive impairment in 25% to 44% of patients, increasing diagnostic confidence 4
• Amyloid PET/CT is positive in up to 60% of patients with MCI, with correlation between amyloid burden and cognitive test performance 4

FDG-PET/CT

• Brain FDG-PET/CT is recommended to demonstrate characteristic hypometabolism patterns in temporal and parietal cortex 4
• Patients with MCI who progress to dementia have lower FDG uptake in temporal and parietal cortex, whereas a negative FDG-PET/CT indicates progression from MCI to dementia is unlikely 4
• Amyloid PET/CT and FDG-PET/CT are complementary and when combined have better accuracy at predicting conversion of patients with MCI to AD 4

CT Findings

• CT head without IV contrast is an acceptable alternative when MRI is unavailable or contraindicated, though less sensitive for detecting hippocampal atrophy and vascular lesions 5
• CT can detect large intracranial masses, subdural hematomas, or other structural abnormalities that may produce clinical symptoms mimicking AD 4

Imaging Protocol Recommendations

Standard Protocol

• MRI brain without IV contrast is the appropriate initial imaging study for all dementia evaluations 4, 5
• IV contrast is not necessary for pretreatment or initial diagnostic imaging in AD patients 4, 1
• Contrast-enhanced MRI is reserved for rapidly progressive dementia to evaluate for infectious, inflammatory, or neoplastic causes 6

Pre-Treatment Imaging for Antiamyloid Therapy

• MRI brain without IV contrast is mandatory before initiating antiamyloid therapy to identify pre-existing microhemorrhages and superficial siderosis 1, 5
• Brain amyloid PET/CT has a lower false-negative rate than brain tau PET/CT and is more highly recommended for pretreatment confirmation 4

Post-Treatment Monitoring

• MRI brain without IV contrast is the only recommended imaging study for posttreatment monitoring to detect amyloid-related imaging abnormalities (ARIA), which can manifest as ARIA-E or ARIA-H 4
• Brain amyloid PET/CT can demonstrate clearance of amyloid from brain parenchyma after treatment 4

Important Clinical Caveats

• Early-onset AD patients are less likely to manifest typical atrophy patterns, making them better candidates for amyloid PET imaging rather than relying solely on structural MRI 1
• Most patients over age 80 with cognitive impairment harbor more than one type of brain pathological change, making mixed etiology dementia common 5
• Specific molecular biomarkers remain necessary to confirm Alzheimer's disease diagnosis and for treatment with disease-modifying therapy, despite supportive MRI findings 5, 6
• MR spectroscopy, fMRI, brain perfusion SPECT/CT, and brain striatal SPECT/CT are not appropriate imaging tests for initial AD evaluation 4