fMRI

Functional magnetic resonance imaging (fMRI) is a neuroimaging technique that provides a non-invasive method for mapping brain function^[1].

The "functional" in fMRI comes from the technique's ability to measure brain function via blood flow changes. The basis of this method is a process called Blood Oxygen Level Dependent (BOLD) contrast. This takes advantage of the fact that oxygenated and deoxygenated blood have different magnetic properties^[2].

When neurons in a particular brain region become more active, there is an increase in the demand for oxygen, leading to an influx of oxygenated blood. This activity leads to an initial decrease in the concentration of oxygenated hemoglobin and an increase in deoxygenated hemoglobin. However, this causes the body to respond by increasing the blood flow to the active area, delivering fresh oxygenated blood. This overcompensation leads to an increase in oxygenated hemoglobin and a decrease in deoxygenated hemoglobin, which is what the BOLD signal measures^[3].

This sequence of events takes some time to happen, so the BOLD signal doesn't instantaneously reflect the neural activity. This delay is called hemodynamic response delay or hemodynamic lag, and it's typically on the order of several seconds. The exact delay can vary depending on several factors, including the specific area of the brain and the individual's physiology. Because of this delay, fMRI is great for localizing brain activity but less ideal for measuring rapid changes in neural activity^[4].

fMRI is commonly used to study cognitive functions, such language processing, attention, memory, perception, emotion, and decision-making. It helps researchers understand how different brain regions interact and contribute to these cognitive processes. Clinical applications of fMRI include mapping brain function before surgery, studying neurological and psychiatric disorders, and evaluating treatment outcomes^[6].

fMRI is generally considered safe as it does not use ionizing radiation, unlike other imaging methods such as X-rays or CT scans. However, because it uses a powerful magnet, it's not suitable for people with certain types of implants (like pacemakers) or certain metal-containing objects in their bodies. Also, while the procedure itself is non-invasive, some people may experience discomfort due to the loud noise of the machine or claustrophobia^[7].

The first fMRI studies were published independently in 1992 by Dr. Seiji Ogawa and colleagues at Bell Labs and Dr. Kenneth Kwong and colleagues at Massachusetts General Hospital^[8].

Since then advancements in fMRI have come in several areas. On the hardware side, increases in the strength of the magnetic field have allowed for better spatial resolution^[9].

In 2020, Hyperfine Research has received FDA approval for the world's first portable MRI system. This system is a groundbreaking innovation in the field of neuroimaging, designed to be taken directly to a patient's bedside for scanning the patient's head and brain^[10].

Weighing only about one-tenth of a conventional, stationary MRI system, this device is roughly three feet wide and five feet tall, making it mobile enough to fit in an elevator and run on a standard power outlet. This dramatically increases the accessibility of MRI technology, making it possible to bring the scanner to the patient, rather than needing to transport the patient to a large, fixed machine^[11].

Hyperfine's portable fMRI employs common permanent magnets that don't require additional power or cooling. This enables the production of clinical contrast images and 3D renders using lower-power radio waves and magnetic fields^[12].

On the software side, improvements in statistical methods and machine learning algorithms have improved the ability to interpret the complex signals that come from the brain^[13].

Additionally, the use of fMRI has been greatly advanced by the development of task-based paradigms and resting-state analysis. Task-based paradigms involve having the subject perform a specific mental task while in the scanner, while resting-state analysis examines the spontaneous fluctuations in brain activity when the subject is not performing a specific task. These approaches have each provided unique insights into the organisation and function of the brain^[14].