MRI has been with us since the Second World War, first for organic chemistry, basic research and now medicine. It used to be called Nuclear Magnetic Resonance Imaging, because it is a technique for observing “spin” at the atomic level.
How has MRI served the cause of psychiatry and what are its specific advantages for research in psychiatry?
Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to image the anatomy and the physiological processes of the body in both health and disease. MRI scanners use strong magnetic fields, radio waves, and field gradients to form images of the body.
MRI is based upon the science of Nuclear Magnetic Resonance (NMR). Certain atomic nuclei can absorb and emit radio frequency energy when placed in an external magnetic field. In clinical and research MRI, hydrogen atoms are most-often used to generate a detectable radio-frequency signal that is received by antennas in close proximity to the anatomy being examined. Hydrogen atoms exist naturally in people and other biological organisms in abundance, particularly in water and fat. For this reason, most MRI scans essentially map the location of water and fat in the body. Pulses of radio waves are used to excite the nuclear spin energy transition and magnetic field gradients to localize the signal in space.
MRI is particularly important for brain imagery because it produces higher resolution of brain complexity compared to other imaging techniques. Two kinds of MRI neuromaging techniques should be considered:
- Structural MRI is a neuroimage of different parts of the brain. Today this examination technique is used routinely for the identification of brain vessel anomalies. Ischemia occurs when a vessel is obstructed by the sclerosis of the vessel itself (due to atheroma) or by what we call an “embolism.” An embolism is a piece of atheroma or coagulated blood (blood clot) which obstructs the small arteria. A hemorrhage corresponds to the expansion of blood beyond the vessel itself, creating a lesion within the organ. Today MRI is able to precisely identify the area of the brain which is concerned by the hemorrhagic or ischemic stroke. Structural MRI also gives the neuro-radiologist important information about the parameters of a brain tumor. These include: irregularities suggesting a malignant of benign tumor, size, localization for the assessment of neurosurgery, etc.. Structural MRI may also be very helpful in neurological disorders such as Parkinson’s disease, multiple sclerosis, infectious brain disorders, as well as in different types of Dementia: fronto- temporal dementia or DFT, Alzheimer’s disease, etc..
In the field of psychiatry, the interest of structural MRI has not yet been precisely defined. Currentlly, in psychiatric daily practice it allows us to distinguish a stroke or a tumor that rarely may mimic a psychiatric disease. In research we know now that some structural anomalies of the brain have been associated with schizophrenia, Bipolar disorder, depressive illness as well as in Obsessive Compulsive Disorder. But these anomalies have been observed by pooling a set of data gathered from groups of patients. That means that heterogeneity prevails in psychiatry and if anomalies exist they are tenuous and difficult to observe at the individual level.
- 2. Functional MRI is a technique which could contribute more to the characterization of psychiatric symptoms and syndromes. Functional magnetic resonance imaging or functional MRI (fMRI) is a functional neuroimaging procedure using MRI technology that measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases. The primary form of fMRI uses the blood-oxygen-level dependent (BOLD) contrast. It is a type of specialized brain and body scan used to map neural activity in the brain.
Other methods of fMRI obtaining contrast are arterial spin labeling (Duhameau et al. 2011), and diffusion MRI. The latter procedure is similar to MRI but uses the change in magnetization between oxygen-rich and oxygen-poor blood as the basic measure. This measure is frequently corrupted by noise from various sources and hence statistical procedures are used to extract the underlying signal. The resulting brain activation can be presented graphically by color-coding the strength of activation across the brain or the specific region studied.
These fMRI techniques contribute highly to the explanation of psychiatric disorders because they help correlate cognitive anomalies which exist in psychiatric diseases (such as schizophrenia, depressive illness or OCD) with brain metabolism dysfunction collecting both with an excellent temporal resolution (within the seconds) and good spatial resolution. For example, in OCD we clearly know that doubt or uncertainty is associated with dysfunction of brain areas in charge of executive functions including the prefrontal cortex and more specifically the orbitofrontal cortex. In patients suffering from schizophrenia also, cognitive dysfunction has been clearly shown. But the variety of these dysfunctions is so huge that it is rational to consider that a whole network is dysfunctional rather than one sensorial module in line with only one cerebral area (Dondaine et al. 2014). Because psychiatric diseases integrate both emotional (so called “hot” cognitions) and cognitive (so called “cold cognition”) aspects, it is now probable that severe psychiatric disorders are involved with integrating neural crossroads networks (such as the salience network), that can be identified by the use of fMRI studies, but also with brain stimulation tools (Downar et al 2016).