From DWI and DTI to High-b-values q-Space Diffusion-Weighted MRI (QSI) in White Matter Associated Disorders:
We with others showed that diffusion-weighted MRI (DWI) is very effective for early detection of cerebral ischemia. This development has revolutionized the early diagnosis of stroke. In 1990 we also provided a clear demonstration that water diffusion in white matter is clearly anisotropic, which laid the foundation of diffusion tensor imaging (DTI).
DTI was introduced in 1994 by Basser and colleagues and is used extensively for mapping fibers in the WM of the CNS. All these methods assume mono-exponential signal decay. As we found that signal decay in diffusion experiments performed on neuronal tissues is not mono-exponential, we suggested the use of the q-space approach, developed by others, to analyze such data in neuronal tissue. We then extended this approach to q-space diffusion MRI (QSI). Here, in fact, the displacement and probability maps can be obtained. QSI was used to study both the CNS maturation and degeneration. In experimental autoimmune encephalomyelitis (EAE) very good anatomical correlation was found between the q-space displacement and probability maps and different histological staining. This is in fact a "virtual histology".
Although water diffusion in neuronal tissues was found to be anisotropic more than a decade ago, the relative importance of myelin in determining the observed water anisotropy is still under debate. Indeed, we have used high b-value q-space diffusion MRI (QSI), which emphasizes the slow diffusing component, to study CNS morphology in myelin deficient (md) and in Long Evans shaker rats (les) spinal cords. These studies that were performed to evaluate the contribution of myelin to the observed anisotropy, showed that the lack of myelin in the md spinal cord significantly affects the diffusion characteristics of the tissues. However, it was found that the lack of myelin affects the diffusion anisotropy of water in white matter in a diffusion time-dependent manner. Also, the differences between the two groups were more apparent at high b values q-space imaging (QSI) and at longer diffusion times. Therefore we suggested the term apparent anisotropy (AA) and apparent fractional anisotropy (AFA). It was also found that QSI is more sensitive to the difference between control and md groups than conventional DTI. The same results were very recently observed with the les model of myelin deficiency.
