Abstract
In the first lecture, we reviewed the various brain imaging methods and their differences. Functional MRI (fMRI), now the most widely used, involves measuring small variations in cerebral oxygenation which, thanks to a neurovascular coupling mechanism, reflect the activity of neuronal circuits. FMRI offers a number of advantages : rapid, repeated, whole-brain measurements, without the need for injections. Although the measurement is indirect, it reflects neuronal discharges fairly faithfully and with a high degree of linearity (except in certain pathological cases). However, spatial resolution remains modest (of the order of a millimeter or a little less), and only allows us to visualize the average of populations of the order of several tens of thousands of neurons. Spatial resolution is also reduced, although analysis techniques can detect variations of the order of 100-200 milliseconds in the onset and duration of activation.
MRI is constantly progressing, notably with the use of parallel antennas with 32 or 64 channels, multiband sequences, parallel transmission and partial sampling techniques. Increasing the magnetic field improves the signal-to-noise ratio, which varies non-linearly. We have gone from 1.5 Tesla machines in the 1990s to MRIs at 3 T, then 7 T, 9.4 T, and finally, at NeuroSpin, to a unique prototype magnet 90 cm in diameter at 11.72 T, whose field rise was achieved in July 2019. These improvements not only concern functional MRI, but also the analysis of brain anatomy, at both mesoscopic (~1 millimeter) and microscopic (a few microns) scales. Different variants of diffusion MRI make it possible, for example, to assess the average diameter of axons, the average size of cells, and even their shape, without, however, seeing them individually.
