Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that gradually destroys memory and thinking skills. It is the most common cause of dementia, accounting for an estimated 60% to 80% of all cases. An MRI scan cannot definitively diagnose Alzheimer’s disease. However, the MRI is an indispensable and early tool in the complex diagnostic process, providing detailed images of brain structures. It serves to eliminate other potential causes of cognitive decline and offers supportive evidence of the structural changes typical of AD.
Ruling Out Other Causes
The primary and immediate reason a physician orders an MRI early in a diagnostic workup for cognitive decline is to perform a differential diagnosis. This structural imaging technique allows doctors to visualize the soft tissues of the brain with high resolution, helping to exclude conditions that can mimic the symptoms of dementia. Ruling out these non-AD causes is a necessary step, especially since some of them are treatable or reversible.
An MRI can clearly reveal structural abnormalities such as brain tumors or space-occupying lesions that could be pressing on brain tissue and causing cognitive symptoms. It is also highly effective at detecting vascular issues, including evidence of past strokes or mini-strokes (infarcts) that contribute to vascular dementia. These findings appear as areas of tissue damage or white matter lesions on the scan.
The scan can also identify conditions like normal pressure hydrocephalus, which is characterized by an abnormal buildup of cerebrospinal fluid leading to enlarged ventricles. Additionally, the MRI can find a subdural hematoma, which is a collection of blood typically resulting from trauma that puts pressure on the brain’s surface. Identifying and treating these alternative causes ensures that the symptoms are not misattributed to Alzheimer’s disease.
Identifying Patterns of Brain Atrophy
Once other structural causes are ruled out, the MRI’s second function is to look for physical signs that support a diagnosis of a neurodegenerative disease like Alzheimer’s. AD is characterized by the death of nerve cells, which results in a reduction of brain volume, known as atrophy. The MRI can precisely measure and visualize these structural changes within the brain.
The most characteristic structural change associated with Alzheimer’s is the atrophy of the hippocampus, a seahorse-shaped structure deep within the temporal lobe responsible for forming new memories. A thinning or shrinking of the hippocampus, visible on the MRI, is a strong indicator of the progression of AD, especially in late-onset cases. Doctors often use visual rating scales, such as the Scheltens scale, to assess the degree of medial temporal lobe atrophy.
Another structural change visible on an MRI is the enlargement of the ventricles, which are the fluid-filled spaces within the brain. This ventricular enlargement occurs as the surrounding brain tissue shrinks, causing the spaces to appear larger on the scan. While significant atrophy is highly suggestive of AD, structural shrinkage can also be seen in other forms of dementia or as part of the normal aging process.
Supporting Tests for Definitive Diagnosis
Since the MRI provides only structural information and context, achieving a definitive diagnosis of Alzheimer’s disease requires a multimodal approach. The process begins with a thorough cognitive and neurological assessment, which involves detailed clinical evaluations and tests of memory, problem-solving, and language. This step establishes the degree and pattern of a person’s functional and cognitive impairment.
A crucial modern component of the diagnostic pathway is biomarker testing, which aims to detect the biological hallmarks of AD: amyloid plaques and tau tangles. Biomarker testing can be performed by measuring the levels of amyloid-beta and tau proteins in the cerebrospinal fluid, collected via a spinal tap (lumbar puncture), or through new blood tests that measure certain protein fragments. Abnormal levels of these proteins—typically low amyloid-beta and high tau—are highly predictive of the disease.
Advanced imaging techniques, specifically positron emission tomography (PET) scans, provide the functional evidence that the structural MRI lacks. Amyloid PET scans use a radioactive tracer to visualize the buildup of amyloid plaques in the living brain. A tau PET scan can map the spread of tau tangles, which correlates with the severity of cognitive impairment. Specialists make a diagnosis of Alzheimer’s disease with high confidence by combining clinical symptoms, structural changes seen on MRI, and biological evidence from biomarkers and PET scans.