What is the difference between fMRI, EEG and TD-fNIRS?

fMRI (functional magnetic resonance imaging), EEG (electroencephalography) and TD-fNIRS (time-domain functional near-infrared spectroscopy) are all non-invasive neuroimaging techniques that can be used to study brain activity, but they work in different ways and have different strengths and limitations.

• fMRI measures changes in blood flow and oxygenation in the brain, which are related to neural activity. It is a non-invasive technique that can provide high spatial resolution (millimeters) images of brain activity, but it has relatively low temporal resolution, requires expensive equipment, and restricts the environment in which measurements can be made. While fMRI is well-suited for precisely evaluating the spatial distribution of brain activity its use has been limited to academic and research environments because of the high cost ($1000+ / scan) and lack of availability.

• EEG measures the electrical activity of the cortex (outer region of the brain), which is generated by the synchronized firing of neurons. It can measure brain activity in real time with a high temporal resolution of milliseconds, however, its spatial resolution is relatively low due to the distortion that occurs when the electrical signals pass through the skull and scalp. High channel count EEG systems have been demonstrated to achieve spatial resolution on the  order of centimeters but this comes at the cost of long setup times and the sometimes unwieldy application of coupling gel. EEG is also sensitive to electrical noise from the environment and artifacts like electrical signals from ocular or muscle movement that can be orders of magnitude larger. The combination of mediocre spatial resolution, long setup times, and signal artifacts result in limited successful applications of EEG. 

• TD-fNIRS (imaging technique of Flow2) measures changes in the blood oxygenation in the brain, similar to fMRI. The technique is based on measuring changes in the absorption of near-infrared light by the brain, similar to how pulse oximeters work to measure your blood oxygenation on your wrist or finger tip. The temporal resolution of Flow2 and the DevKit allows for measuring maps of blood oxygenation in 100’s of milliseconds. This is better resolution than fMRI and fast enough to compensate for physiological artifacts like respiration and heart rate. The spatial resolution of Flow2 is on the order of centimeters and can be set up for recording in under a minute. This optical technique can measure ~15mm deep into the cortex. TD-fNIRS systems, unlike other types of fNIRS, measure time-of-flight of photons and are capable of both measuring absolute oxygenation and circumventing confounding factors like scalp, hair color, and movement.