Table 3. Summary of the advanced MRI techniques applied in MS and NMOSD and discussed in the current review.
| Indices | Meaning | Abnormal | |
|---|---|---|---|
| T2-weighted and T1weighted images | |||
| T1/T2-weighted ratio | Myelin content [200], dendritic density [201], or neurite density [202] | ↓ | |
| DTI* | |||
| Fractional anisotropy (FA) | Axonal > myelin integrity | ↓ | |
| Mean diffusivity (MD) | Myelin > axonal integrity | ↑ | |
| Axial diffusivity (AD) | Axonal integrity | ↓↑ depending on timing | |
| Radial diffusivity | Myelin integrity | ↑ | |
| Diffusion along perivascular space index | Glymphatic function | ↓ | |
| NODDI† | |||
| Neurite density index (NDI) | Amount of neurites | ↓ | |
| Intracellular volume fraction (ICVF) | Amount of neurites | ↓ | |
| Orientation dispersion index (ODI) | Variability of neurite orientation (tissue complexity) | ↓ | |
| Isotropic volume fraction (isoVF) | Amount of free water | ↑ | |
| 1H-MRS | |||
| Choline (Cho) | Cell membrane marker | ↑ | |
| N-acetylaspartate (NAA) | Neuronal marker | ↓ | |
| Lactate (Lac) | Anaerobic metabolism | ↑ | |
| MT | |||
| Magnetization transfer ratio (MTR) | Myelin content | ↓ | |
*DTI: provides information on tissue integrity based on the movement of water molecules. The principle is that water molecules in the white matter do not move randomly but follow a preferential direction (i.e., fractional anisotropy), that overlaps the orientation of the white matter tracts [203,204]. The higher the fractional anisotropy, the higher the integrity of axons in the white matter. In addition, the presence of macromolecules such as myelin limits the mean diffusion of water molecules, so that increased mean diffusivity is usually considered an index of demyelination (although also the presence of the axonal membranes in part limits mean diffusivity) [203,204]. Other indices indicating the status of axons and myelin are the axial and the radial diffusivity, respectively [203,204]. However, this model has some limitations, such as its poor interpretability in case of crossing fibers or tissue organization (for example, in the cortex), but also the relatively non-specificity of findings, †NODDI: this model was recently proposed to overcome the limitations of DTI, and is obtained through the acquisition of multi-shell diffusion weighted images with axial echo-planar imaging (EPI) [205]. This is a three-compartmental model where brain tissue as made of an intracellular space (i.e., within the neurites membranes), extracellular space (i.e., around neurites, which includes glial cells or neural somas), and the cerebrospinal fluid (i.e., free water with isotropic diffusivity) [205].
MRI = magnetic resonance imaging, MS = multiple sclerosis, NMOSD = neuromyelitis optica spectrum disorders, DTI = diffusion-tensor imaging, NODDI = neurite orientation dispersion and density imaging, 1H-MRS = portion magnetic resonance spectroscopy, MT = magnetization transfer