What imaging exams should I order to diagnose Alzheimer's disease?

Imaging Exams for Alzheimer's Disease Diagnosis

For suspected Alzheimer's disease, MRI brain without IV contrast is the primary recommended imaging study, followed by brain amyloid PET/CT and brain FDG-PET/CT for cases requiring further diagnostic clarification. 1

Initial Structural Imaging

MRI Brain Without IV Contrast (First-Line)

• Recommended sequences 1:
  • 3D T1 volumetric sequence with coronal reformations (for hippocampal assessment)
  • Fluid-attenuated inversion recovery (FLAIR)
  • T2-weighted imaging (or susceptibility-weighted imaging if available)
  • Diffusion-weighted imaging (DWI)
• Key findings to assess:
  • Hippocampal atrophy (using medial temporal lobe atrophy scale)
  • Global cortical atrophy
  • White matter changes (using Fazekas scale)
  • Exclusion of other pathologies (tumors, subdural hematomas, etc.)

CT Head Without IV Contrast (Alternative)

• Use when MRI is contraindicated or unavailable 1
• Should include coronal reformations to assess hippocampal atrophy
• Less sensitive than MRI but can still detect significant atrophy and rule out other causes of cognitive impairment

Advanced Functional/Molecular Imaging

When structural imaging is insufficient for diagnosis, consider the following in sequence:

1. Brain FDG-PET/CT

• Recommended as the first advanced imaging study after structural imaging 1
• Detects characteristic hypometabolism patterns in temporoparietal regions
• High accuracy for differentiating AD from other dementias
• More cost-effective than proceeding directly to amyloid imaging 1

2. Brain Amyloid PET/CT

• Recommended after FDG-PET/CT if diagnosis remains unclear 1
• Directly visualizes amyloid plaques in brain parenchyma
• Particularly useful in:
  • Atypical presentations of AD
  • Early-onset cases
  • Cases with diagnostic uncertainty after other evaluations
• Should be ordered by dementia specialists 1

3. Brain Tau PET/CT

• Recommended for atypical presentations of AD 1
• Shows tau deposition patterns that correlate with clinical symptoms
• Less widely available than amyloid PET/CT

Not Recommended for Routine Clinical Use

• Advanced MRI sequences (MR spectroscopy, functional MRI, diffusion tensor imaging) 1
• Brain perfusion SPECT/CT as initial imaging (may be used if FDG-PET unavailable) 1
• CT or MRI with IV contrast 1
• Quantification software (still evolving field) 1

Diagnostic Algorithm

1. Start with structural imaging:

   • MRI brain without IV contrast (preferred)
   • CT head without IV contrast (if MRI contraindicated)

2. If diagnosis remains unclear after specialist evaluation:

   • Proceed to brain FDG-PET/CT

3. If still uncertain after FDG-PET/CT:

   • Consider brain amyloid PET/CT (ordered by dementia specialist)

4. For atypical presentations:

   • Consider brain tau PET/CT in addition to above

Special Considerations

• For patients being considered for antiamyloid monoclonal antibody therapy, MRI brain without IV contrast and brain amyloid PET/CT are specifically recommended 1
• For monitoring treatment with antiamyloid monoclonal antibodies, MRI brain without IV contrast is essential to detect ARIA (Amyloid-Related Imaging Abnormalities) 1
• Hippocampal atrophy, while characteristic of AD, is not specific and can occur in other conditions 2

Pitfalls to Avoid

• Relying solely on structural imaging for diagnosis
• Using advanced MRI sequences outside of research settings
• Ordering amyloid PET/CT before trying FDG-PET/CT (cost considerations)
• Failing to use standardized visual rating scales (MTA, GCA, Fazekas) when interpreting structural imaging
• Over-interpreting nonspecific white matter changes as evidence of AD

Remember that imaging is one component of a comprehensive diagnostic approach and should be interpreted in the context of clinical findings and other biomarkers when available.