Diffusion-weighted imaging (DWI):
The DWI method is a non-invasive functional imaging analysis of the internal structure and tissue components of lesions at the molecular motion level by detecting the diffusion movement of water molecules in vivo.It can detect the cerebrospinal fluid flow in the subarachnoid space of the optic nerve in a non-invasive manner.
It has been found that the flow range ratio of NTG patients was significantly lower than that of healthy control participants. This finding suggests that impaired cerebrospinal fluid dynamics may play a role in the pathophysiology of NTG.
Cio et al. found that there were global differences in structural connectivity and local graph theoretical measures in glaucoma patients, and these changes far exceeded the main visual pathways. The results support the hypothesis that whole brain structural reorganization occurs in patients with glaucoma, which is specific to structural connections.
This finding means that glaucoma can be categorized in the recently defined category of a brain disconnection syndrome.
The results provide indirect evidence that unknown factors may limit the reorganization of white matter after visual loss in glaucoma patients.
There are several limitations to DWI. First, DWI delineates that random Brownian motion of water in normal and pathological neural tissue environments changes. Reduced water diffusion has been shown to be sensitive to many diseases in the brain but is not specific to glaucoma. Therefore, the detection results of DWI may be affected by other factors and cannot be used as a biomarker of glaucoma neurodegeneration. Second, according to the partial volume effect and image slice direction, the diffusion limitation of the optic nerve may be ignored due to the small size of the optic nerve. Therefore, in the case of insufficient spatial sampling, the incidence of limited diffusion in the optic nerve may be underestimated. Thin slices (e.g., 3 mm) arranged parallel to the optic nerve should improve the sensitivity of optic nerve diffusion limitation. Third, although the sensitivity and accuracy of rs-EPI and rFOV-EPI are similar, their image quality in the orbital segment is significantly higher, and there are still artifacts in the optic canal segment and the intracranial segment.
Diffusion tensor imaging (DTI):
Based on conventional DWI, the new neuroimaging technology of DTI is an MRI method that goes by the assumption that water molecules diffuse with a Gaussian distribution to measure and obtain the anisotropy information of different tissues.
The dislocation of water molecules in nerve fibers and axons can be detected by the diffusion of water molecules along axons without a gadolinium contrast agent. Doing so can reveal abnormalities of the white matter structure and brain connection.
The DTI technology is the most commonly used method in MRI, and it can quantitatively measure the integrity of microstructure and tissue in vivo. It has been widely used to study the differences of white matter bundles in the visual pathway in glaucoma patients. Microstructural differences of white matter structures have been found in the optic nerve, optic tract, optic chiasm, optic radiation, and the occipital lobe of glaucoma patients.
It is believed that DTI may reveal early axonal injury with more sensitivity than a conventional MRI can.
Some researchers argue that DTI can be used to distinguish different glaucoma subtypes.
Compared to the established ophthalmic diagnostic methods, these new imaging techniques seems to enable earlier diagnosis of NTG.
Studies by Schreiber et al. further support the hypothesis that glaucoma can be considered a pan-neurodegenerative disease. The mechanism of brain injury outside the visual pathway in POAG patients is not the direct extension of visual pathway degeneration, but the primary neuropathological process of the transmission mechanism of neurodegenerative lesions. Giorgio et al. found that compared with the healthy control group, patients with NTG and POAG had significantly more gray matter atrophy in their visual system and non-visual brain areas, and significantly more changes in DTI-derived anatomical connections.
The diffuse structural and functional abnormalities of the brain of a patient with glaucoma may, at least in part, be unrelated to the increase of IOP and subsequent retinal degeneration.
Murai et al. have used DTI to report a significant correlation between optic radiation axon injury and decreased cerebral glucose metabolism in the striate cortex of POAG patients.
The DTI method has been widely used to evaluate microstructural abnormalities of white matter in the brain; however, it has some limitations. For example, due to the existence of organelles, cell membranes, and other barriers, water molecules often show non-Gaussian diffusion in biological tissues. Therefore, the practicability and sensitivity of the DTI model may not be completely optimal. In addition, DTI studies have shown damage to the integrity of the overall microstructure in the visual pathway of patients with glaucoma.
The mechanism behind the difference of directional diffusion rate in glaucoma DTI research is not clear, and the pathophysiology of glaucoma may be more complex than the deterioration of the axon and myelin sheath.
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