From the Guidelines
The best imaging modality for diagnosing Alzheimer's disease is positron emission tomography (PET) with amyloid or tau tracers, which can directly visualize the pathological protein deposits characteristic of the disease. This is based on the most recent and highest quality study available, which recommends the use of amyloid PET to be appropriate in the evaluation of a patient with persistent or progressive mild cognitive impairment (MCI), especially because a negative amyloid PET scan in a patient with amnestic MCI would strongly weigh against AD as the etiology 1.
Key Points to Consider
- Magnetic resonance imaging (MRI) is typically used first to assess brain structure, showing hippocampal and cortical atrophy patterns typical of Alzheimer's, while also ruling out other causes of cognitive decline like tumors or strokes.
- For amyloid PET, tracers such as florbetapir, florbetaben, or flutemetamol are used to detect beta-amyloid plaques, while tau PET using tracers like flortaucipir can visualize neurofibrillary tangles.
- These advanced imaging techniques are usually reserved for cases where the diagnosis remains uncertain after clinical evaluation and standard MRI.
- Functional imaging with FDG-PET can also be helpful by showing characteristic patterns of reduced glucose metabolism in temporoparietal regions.
- While these imaging techniques provide valuable diagnostic information, they should be used in conjunction with comprehensive cognitive testing, medical history, and clinical examination, as Alzheimer's remains primarily a clinical diagnosis supported by biomarkers.
Evidence Supporting the Recommendation
The Alzheimer's Association clinical practice guideline for the diagnostic evaluation, testing, counseling, and disclosure of suspected Alzheimer's disease and related disorders recommends the use of amyloid PET in the evaluation of patients with MCI 1. Additionally, a review and synthesis of practical recommendations for timely and accurate diagnosis of symptomatic Alzheimer's disease in primary care also supports the use of amyloid PET as a diagnostic biomarker 1.
Important Considerations
It is essential to consider the safety and tolerability of lumbar puncture for CSF collection in the evaluation of patients suspected of having AD, as well as the experience and proficiency of the clinician performing the procedure 1. Furthermore, the interpretation of results in the clinical context may be difficult, and a comprehensive evaluation of the patient's medical history, cognitive testing, and clinical examination is necessary to support the diagnosis of Alzheimer's disease.
From the FDA Drug Label
Amyvid is a radioactive diagnostic agent for Positron Emission Tomography (PET) imaging of the brain to estimate β-amyloid neuritic plaque density in adult patients with cognitive impairment who are being evaluated for Alzheimer's Disease (AD) and other causes of cognitive decline A negative Amyvid scan indicates sparse to no neuritic plaques, and is inconsistent with a neuropathological diagnosis of AD at the time of image acquisition; a negative scan result reduces the likelihood that a patient's cognitive impairment is due to AD A positive Amyvid scan indicates moderate to frequent amyloid neuritic plaques; neuropathological examination has shown this amount of amyloid neuritic plaque is present in patients with AD, but may also be present in patients with other types of neurologic conditions as well as older people with normal cognition.
The best imaging of the brain to diagnose Alzheimer's is Positron Emission Tomography (PET) imaging using a radioactive diagnostic agent such as florbetapir (IV), also known as Amyvid 2, 2, 2.
- Key points:
- Amyvid is used to estimate β-amyloid neuritic plaque density in adult patients with cognitive impairment.
- A negative Amyvid scan reduces the likelihood that a patient's cognitive impairment is due to AD.
- A positive Amyvid scan indicates moderate to frequent amyloid neuritic plaques, but does not establish a diagnosis of AD.
From the Research
Imaging Techniques for Diagnosing Alzheimer's Disease
- Various imaging techniques are used to diagnose Alzheimer's disease, including positron emission tomography (PET) and magnetic resonance imaging (MRI) 3, 4, 5, 6, 7
- PET scans can measure amyloid-β (Aβ) accumulation, which is an early hallmark of Alzheimer's disease, but Aβ accumulation stagnates as the disease progresses 3
- MRI scans can measure hippocampal volume and detect medial temporal atrophy, which is a biomarker for neurodegeneration 4, 5, 7
- Visual rating systems, such as the Visual Rating System for Medial Temporal Atrophy (VRS-MTA), can be used to score the severity of medial temporal atrophy on MRI scans 4
Biomarkers for Alzheimer's Disease
- Reduced hippocampal volume is a biomarker for neurodegeneration, but it is not specific to Alzheimer's disease 3
- Hypometabolism in temporoparietal regions is seen as a biomarker for Alzheimer's disease, but glucose uptake reflects astrocyte function rather than neuronal function 3
- Amyloid-β (Aβ) is the earliest hallmark of Alzheimer's disease and can be measured with PET, but it may not be a suitable biomarker for monitoring disease progression 3
- Tau accumulation can be measured with PET radiotracers and has shown promising results in both early diagnosis and longitudinal monitoring of Alzheimer's disease 3
Clinical Implications
- Clinicians should be aware of the large proportion of Alzheimer's disease patients who present without atrophy of the hippocampus as measured with clinical MRI methods 5
- Non-amnestic phenotypes are more common in patients without hippocampal atrophy compared to those with atrophy 5
- An integrated biological and clinical staging scheme can accommodate the fact that common copathologies, cognitive reserve, and resistance may modify relationships between clinical and biological Alzheimer's disease stages 6