Looking Inside without Hurting. Magnetic Resonance Imaging

At first glance it looks like a giant doughnut wrapped in hard plastic. A second look reveals that it is a magnetic resonator. That torus (the doughnut) is actually a giant coil made up of superconducting wires that transmit large electrical charges. Faraday said it in 1831: these electric charges produce a magnetic field. Certainly, a warning indicates in large letters that we should not approach with metals, as it is a giant magnet. With such a powerful magnet it is possible to induce the phenomenon of magnetic resonance in the nucleus of atoms, the most abundant of which, in biology, is hydrogen. Using electromagnetic wave emissions (radio frequencies) perfectly tuned to the resonance frequency, we can modulate the magnetic properties of these atoms and thus obtain information about the tissue. These are the fundamental foundations that make magnetic resonance imaging (MRI) possible, a technique that is today fundamental for the study of the human body in health and disease.

The National Laboratory of Magnetic Resonance Imaging (Lanirem) in UNAM’s Juriquilla campus has two MRI resonators for humans and another to study small animals. Human devices have a double duty; first, to provide public service as a diagnosis center, carrying out more than 4000 studies annually. Second, along with the animal resonator, clinical resonators are also used as tools for research in neurosciences.

Among MRI’s many advantages, three stand out. First, it does not use ionizing radiation nor cause tissue damage. Second, and derived from the first, it allows longitudinal studies to be carried out to see how the structures of the body change in shape, size, and characteristics. Finally, it is possible to sensitize the images so that they inform us of different biological processes, which allows us to infer characteristics not only of the anatomy, but of the tissue itself. For example, it is possible to obtain information about blood perfusion (the flow of blood to tissues and organs), the chemical composition of the tissue, the diffusion of water, etc.

MRI plays a fundamental role in neurosciences. Through this technology it has been possible to describe in greater detail the anatomy of the human brain and that of other species, as well as to capture its variability between subjects. At the end of the last century and during the first decade of the 21st, functional imaging made it possible to precisely locate the areas of the brain involved in different tasks. Through carefully designed experiments, it is possible to ask very specific questions that manage to demonstrate details of brain function. For example, beyond identifying the location of the visual cortex (which is very extensive but always anatomically located in the occipital lobe), functional resonance allows elucidation of regions within it responsible for specific functions, such as the recognition of faces or the identification of emotions in them. These experiments would have been very difficult, if not impossible, in other species and with other methods, since these are highly relevant and highly developed skills in humans.

CONNECTIVITY BETWEEN TWO BRAIN AREAS IS INFERRED WHEN THE SIGNAL FROM BOTH AREAS RISES AND FALLS IN CONCERT BETWEEN THEM

More recently, the study of functional brain networks has gained importance. This requires the person to remain at rest (but awake) while many images of the brain are taken over time. Connectivity between two brain areas is inferred when the signal from both areas rises and falls in concert between them. Through mathematical analysis, metrics are obtained that summarize the effectiveness of the flow of information within the brain. This way, it has been shown how functional networks change during life and also the way in which they are modulated through training, learning or through pharmacological treatments in some neurological diseases. On the other hand, diffusion-sensitive MRI-derived tractography methods are powerful tools for visualizing the infrastructure underlying intracerebral communication; they allow us to see the highways within the brain by which different regions talk to each other. Analogous to functional networks, these structural networks can be analyzed quantitatively in health and in disease. For example, many neurodegenerative diseases have changes in the brain’s white matter that alter structural networks, thus hindering the flow of information and resulting in cognitive or other deficits.

For both clinical and research purposes, new and better applications of MRI are developed every year. In this technological field, biology, physics, medicine, mathematics, statistics, computer science, and other disciplines link. No other imaging technique allows us to see any part of the human body and other animals with such anatomical detail (resolution), maintaining such a wide field of vision, exploiting multiple contrast-generating mechanisms that offer complementary information and doing everything innocuously and without causing any discomfort. UNAM proudly participates in the methodological advances, and clinical and research applications of MRI.

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