fMRI Examples

Examples

While advancements in imaging technology have allowed researchers to evaluate structural issues in the brain, many neurological disorders can only be mapped through physiological markers. This makes functional MRI (fMRI)–which is both safe and quite accessible–incredibly useful for brain researchers and neurosurgeons.

By measuring activation of specific brain regions, noted by statistically significant changes in the BOLD signal, fMRI is utilized by neurosurgeons to map which brain tissues should be targeted in surgery. In practice, this involves patients performing a probe task during an fMRI scan. These probe tasks are tailored to the neurological disorder at hand; for instance, motor task activation correlates neatly with brain regions associated with motor control. Because functionally unique areas of the brain can vary between different patients, having patients perform probe tasks in concert with fMRI scans before surgery allows for tailored surgical planning.

The primary comparison of fMRI-based surgical planning is with the Wada test (the intracarotid amobarbital test), which is the standard method for pre-surgical lateralization–evaluating which brain hemisphere can be attributed to a specific function–in disorders such as epilepsy. In the Wada test, surgeons anesthetize each hemisphere while evaluating the performance of the other, requiring them to catheterize the internal carotid arteries. This procedure comes with a small risk of stroke, making fMRI surgical planning an appealing non-invasive alternative. While fMRI is inherently different from the Wada test as fMRI scans take both hemispheres into account, there have been promising results. In language lateralization, for instance, fMRI has had above 90% agreement with Wada test results in well-lateralized cases. Although not a replacement, fMRI can serve as an instrumental assistance to the Wada test in pre-surgical planning.

Studies have also shown that evaluating how close brain regions to be targeted in surgery are to cortically active regions can provide insight into the post-surgery risks of certain operations. Despite the promise here, there are a number of drawbacks, including the lack of standardization across pre-surgical fMRI analysis techniques. Additionally, researchers note that tumor-induced effects can muddle fMRI readings. While informative, fMRI readings in the context of surgical operations should be used with caution.

A fMRI can be used to diagnose mental illnesses by tracking the flow of oxygenated blood to brain regions. A higher concentration of oxygen flows to active brain regions since these neurons need their energy replenished with oxygen to continue firing. Yellow on the fMRI reading indicates higher brain activity and red on the fMRI reading indicates lower brain activity. This yellow to red scale is a continuous color scale. It is important to note that fMRI does not give great temporal resolution, because the oxygenated blood flows about 6 seconds after neuronal activity actually occurs in order to replenish lost energy from the neurons involved in the neuronal activity.

There are 2 main types of fMRI that are key to diagnosing mental illness: resting fMRI and task-related fMRI. For resting fMRI, the design is that the patient is told to close their eyes and not think of anything in particular. The patient then is scanned for 45-55 minutes. This shows what the patient’s brain is like at ‘resting state’ or when they or not performing any task in particular. In other words, resting-state fMRI tracks your brain activity when your Default Mode Network is activated. In many mental illnesses like ADHD, depression, and schizophrenia, the default mode network has very high activity and activates very easily.For task-related fMRI, the design involves the patient being told to perform certain tasks while the 45-55 minute fMRI takes place. There are 2 categories of tasks that patients are asked to perform. There is block design, where you may be asked to click a button every time a blue word appears on the screen for 15 minutes. For the next 15 minutes, you only click a button when you see red words appear on the screen. There is also event-related design, where the tasks are randomly switched during different intervals of time. You are asked to click for blue words for the first two trials, and the instructions are randomly changed to only clicking when you see red words.

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This shows the patient’s brain when it is in its ‘attentive state’. This occurs when we are focused on any task: playing video games, doing homework, etc. In other words, task-related fMRI tracks your brain activity when your Central Executive Network (CEN) is activated. Block design tests a patient’s ability to maintain attention. Event-related design tests a patient’s ability to switch your point of attention and immerse your focus on this new task. People with mental illnesses such as ADHD or schizophrenia have underactive CEN. In other words, they struggle to maintain focus, and switching points of attention is very hard for these patients without them getting lost in their thoughts, and thus losing focus.

A key indicator for mental illness can be found when comparing a patient’s resting and task-related fMRI scans in order to test a patient’s DMN and CEN activity It is important to note that higher activation of DMN reduces activity of CEN and vice versa. People with overactive DMN find themselves daydreaming and unable to be in the present moment. The difficulty to focus reflects itself through a lower CEN activity. In most mental illnesses, such as bipolar’s disease, depression, schizophrenia, and ADHD, there is a general trend where the patient’s resting state fMRI will show very high activity of DMN brain regions, and the patient’s event-related fMRI shows relatively lower activity of CEN brain regions in comparison of DMN regions.

The difference in diagnosis of these specific mental illnesses depends on which specific brain regions of the DMN are overactive, and which subsequent brain regions of CEN are underactive. To further specify which mental illness a patient suffers from, we can perform EEG readings after specific tasks. We have enough data on EEG where we can tell a patient’s mood response to specific tasks. By seeing how patients’ brains respond to different stimuli in real time, we can further characterize the specific symptoms of the patient and pin down a more accurate diagnosis.