Functional imaging: what makes the brain tick?

Our brain is the command center of our body. This is where all information and impressions are collected and converted into responses and movements. Modern imaging techniques offer physicians and researchers unique insights into the actions of the human central nervous system. The functional imaging technique allows them to watch our brain in action.

The average human brain has about 100 billion brain cells, which are connected and create the central nervous system. There are an estimated 100 trillion connections, which make the human brain the most complex and most complicated organ in the known universe.

For centuries, researchers have tried to understand what our control center looks like, what happens inside and how it works. Imaging techniques are of vital importance to basic research since they allow scientists to see inside the body. They are also an integral part of the diagnostic process. There are several ways to examine the brain’s structure. Computed tomography (CT) is an imaging procedure that uses x-rays, which are detected via a scanner as they pass through the patient’s brain. Since the radiation is differentially absorbed, specific structures of the brain can be distinguished, allowing physicians to view bones of the skull, brain tissue, membranes, and blood vessels. The benefit of this technique: A CT scan doesn’t require long preparation time, making it quick and thus an essential tool in emergency departments.

Having said that, CT scans are relatively poor at identifying minor injuries or changes, making magnetic resonance imaging (MRI) the better choice. MRI scans are detailed images that can detect even minor structural variations.

Although experts know what a brain looks like, how does it actually work? Our brain collects all kinds of information, sensations, and impressions: we see a flower, smell freshly baked bread, and make a fist - these processes activate specific parts of the brain and ensure that it consumes energy in these areas.

Functional imaging techniques illustrate how the brain works. The metabolic activity of the nervous system is the most important factor in this setting. Experts can take various approaches and use different physical methods to view this activity. Single-photon emission computed tomography (SPECT) is often used to study the heart but also lends itself to neural diagnostics. It can be used to detect Alzheimer's disease, epilepsy or Parkinson's disease (apparatus and instruments for neurological diagnosis in the catalogue of MEDICA 2018).

By comparison, positron-emission tomography (PET) is a more widely used technique. It is an invasive technique that injects patients with a radioactive contrast agent that illustrates metabolic processes in the brain. It is most commonly used to measure blood flow (O-PET) and sugar (glucose) metabolism. The O-PET scan often uses a so-called subtraction technique in which the blood flow distribution during one brain state (a task performed without delayed stimulation) is subtracted from the distribution during another brain state (a task performed with delayed stimulation). The resulting activity reflects brain activity pertaining to certain task aspects (i.e., delay time). Both SPECT and PET can be used in combination with CT.

Magnetic resonance imaging was also essential for one of the most important processes. In addition to the structural MRI scans, this device is also capable of depicting functional brain activities. The amount of energy released during activities is supplied to the nerve cells via the blood vessels in the form of oxygen and glucose and ultimately used here. Functional magnetic resonance imaging (fMRI) takes advantage of this process by illustrating the respective oxygen content of the red blood cells via the so-called "BOLD" (blood oxygen level-dependent) effect. High oxygen levels suggest brain cells that are active at a specific site. The static images of this technique deliver a snapshot of the "firing" neurons. The brain activation is graphically represented by color-coding the strength of activation. Yellow indicates increased activity, while red points to weak activity. An underlay of the MRI image with the anatomical structure makes it possible to accurately correlate neural activity to a specific anatomical region of the brain.

In addition to its informative value, the cost-performance ratio makes non-invasive fMRI the preferred practical application over PET scans. The latter is more expensive and entails exposure to low levels of radiation for patients. Both techniques have a drawback: neural activity is only indirectly detected via the displayed metabolic activity. This mechanism is much slower than the working brain. Neural processes occur in the millisecond range, yet fMRI and PET scans take several seconds to record each image of the brain that add up to provide a more comprehensive picture. Electroencephalography (EEG) and the related magneto-encephalography (MEG) are arguably faster and well-known techniques for recording neural activity. However, one drawback of these brain imaging techniques with excellent temporal resolution is their poor spatial resolution and subsequent source localization of brain processes.

PET scans and fMRI are great methods to examine different brain mechanisms and analyze brain regions that control speech and movement. This often involves an event-related fMRI, where patients must follow instructions or perform cognitive tasks. It facilitates a precise localization of the activated brain regions. Other brain research areas also had success with this technique. Thanks to fMRI, we now know that the brain also processes illusory perception versus reality. The so-called cutaneous rabbit illusion evokes a sensory touch in test subjects that doesn’t actually take place. Another example is phantom pain after amputation that feels like pain coming from a body part that is no longer there. A fMRI study that included amputees with and without phantom limb pain enables scientists to now observe people's sensations and better understand the mechanisms that underlie and cause phantom pain. The activity linked to phantom limb sensation in the brain – and more accurately in the so-called somatosensory cortex - resembles the perception of a real hand. In this case, the brain virtually believes the amputated hand is still there. For the first time, brain imaging techniques provided objective evidence of these sensations. These findings can be important in both psychology and physical medicine and pave the way for new therapies that would not have been possible with mere structural imaging techniques. That being said, they also offer support during the treatment of patients. "Functional MRI is primarily used in the preoperative mapping of the cerebral cortex, as well as language lateralization in patients with epilepsy for example", says Dr. Alexander Smirnov of the Department of Neuroradiology at the N.N. Burdenko National Medical Research Center of Neurosurgery in Moscow in an interview with MEDICA.de.

Despite some flaws, functional imaging techniques are an important tool for medicine. They allow the treating physician to see the big picture because simply knowing the structure is not enough. It is crucial to pinpoint and understand the function and integrate it in future processes to ensure the best overall patient-centered care, including diagnosis, therapy, and aftercare. This also applies to research. These techniques will definitely also play an important role in the future when it comes to improving the treatment and care processes of mental and physical illnesses.