Diagnosing Alzheimer's: Imaging Tests

Reviewed by: HU Medical Review Board | Last reviewed: June 2019 | Last updated: August 2020

Diagnosing Alzheimer’s disease involves a variety of tests and exams to help rule out illnesses and determine the underlying causes of the presenting symptoms.

A definitive diagnosis of Alzheimer’s is only able to be done on autopsy, so use all possible screening and assessment tools to ensure that the diagnosis is as accurate as possible.1

In addition to taking a medical and symptom history and doing a physical exam and various neurological exams, various imaging tests are also done as part of a medical workup for Alzheimer’s disease. This helps provide valuable information to the physician in order for them to make an informed, accurate diagnosis.

Types of imaging used to help diagnose Alzheimer's

Different kinds of imaging can help provide doctors with different kinds of information.

Structural imaging shows things such as shape, position, and volume of tissues.2 This can show whether the brain or parts of the brain have shrunk at all, which may be early signs of Alzheimer’s disease.2 Magnetic resonance imaging (MRI) and computed tomography (CT) are kinds of structural imaging.

Functional imaging looks at cell activity in organs and how well cells are using sugar and oxygen.2 Positron emission tomography (PET) and functional MRI (fMRI) are types of functional imaging tests. Molecular imaging helps to find cellular or chemical changes that may be associated with certain types of diseases, using specialized targeted radiotracers. Tests that use molecular imaging include PET, fMRI, and single photon emission computed technology (SPECT).2

Differential diagnoses

Because there are other illnesses and conditions that can cause symptoms similar to those of Alzheimer’s disease, structural imaging tests are especially helpful in ruling out other conditions. Structural imaging can show whether a stroke has occurred, or whether there are any tumors, damage from head trauma, or if any fluid is on the brain.2 All of these things can present with symptoms similar to Alzheimer’s, and ruling these out can help physicians make a more accurate diagnosis.

Structural imaging

Computed tomography (CT) is a computerized x-ray imaging test where x-rays are aimed at a person and rotated around the body, making signals that are then processed by the machine’s computer, helping to produce images of sections of the body, akin to slices of images.3 Put together, these images create a three-dimensional portrait of the body and its structures. CT scans can be done with or without contrast.

Magnetic resonance imaging (MRI) is a kind of non-invasive imaging test that produces three dimensional images of the body part being scanned, such as the brain.4 It uses strong magnets and radiofrequency currents to create those images.4 A patient is put into a large magnet, often a tube, and needs to be still during the imaging. Sometimes a contrast dye or agent is injected in order to create an image that shows brightness in certain areas like the bloodstream, to help visualize fluid or structures. Sometimes the MRI can make people anxious or cause feelings of claustrophobia. Talk to the doctor about methods for reducing these feelings if they occur or if the person is prone to them. There may also be other alternatives, such as open MRI, although this is not available everywhere.

Functional imaging

When a positron emission tomography (PET) scan is done, a patient is intravenously given a small amount of a radioactive chemical, which is then absorbed by organs and tissues.5 The amount of the chemical is extremely small and may studies have demonstrated them to be non-harmful. Almost an hour after the injection of the chemical, the person is put on a flat exam table that is then placed in the PET scanner, which looks like a doughnut. The scanner detects the chemical that has been injected into the body and makes three-dimensional images of the body.

Functional magnetic resonance imaging, or fMRI, is used for mapping brain activity and any changes in brain activity.6 It is similar to an MRI and is non-invasive. A contrast dye is often injected intravenously, and the patient will be given certain tasks during the imaging test, as well as periods of rest. This allows the scan to take pictures of the brain at work and at rest, showing cellular activity.

Molecular imaging

Molecular imaging is the newest and most exciting new development in Alzheimer’s imaging tests. These kinds of tests are being looked at in helping to diagnose Alzheimer’s disease at its earliest stages, possibly even before changes occur to the structure of the brain.2 For example, there are molecules that can bind to amyloid plaques in the brain or neurofibrillary tangles. It might also be a way to monitor the progression of the disease and treatment effectiveness.

Active research is being performed to use PET to detect and diagnose Alzheimer’s disease as early as possible because early detection could make therapies even more effective. PET can also be used to diagnose other diseases that could be causing symptoms, such as Lewy body dementia, which is characterized by a loss of chemical called dopamine in the brain. If someone has neurofibrillary tangles but no amyloid, this indicates another type of neurodegenerative disease, called a tauopathy, that is not Alzheimer’s disease.

Many of these imaging techniques are being used in research and are not necessarily available or indicated for all patients, but it may be worth asking the doctor about. Molecular imaging produces pictures of bodily processes at the molecular and cellular level.7 In addition to PET scans and fMRI, another kind of molecular imaging is SPECT, or single photon emission tomography.

SPECT uses a SPECT scanner and a radiotracer that is injected into the bloodstream. The tracer accumulates in cells depending on the amount of blood flow they receive.7 In the brain, this can show increased or decreased brain activity that may be consistent with Alzheimer’s disease.

There are a variety of imaging tests used to help physicians make the diagnosis of Alzheimer’s disease. Depending on symptoms, doctors may use one or more of these tests. The good news is that these are non-invasive and painless, and provide a wealth of information that can help guide diagnosis and treatment.

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