Visualizing Non-Gaussian Diffusion: Clinical Application of q-Space Imaging and Diffusional Kurtosis Imaging of the Brain and Spine

Abstract

Recently, non-Gaussian diŠusion-weighted imaging (DWI) techniques, including qspace imaging (QSI) and diŠusional kurtosis imaging (DKI), have emerged as advanced methods to evaluate tissue microstructure in vivo using water diŠusion. QSI and DKI have shown promising results in clinical applications, such as in the evaluation of brain tumors (e.g., grading gliomas), degenerative diseases (e.g., speciˆc diagnosis of Parkinson disease), demyelinating diseases (e.g., assessment of normal-appearing tissue of multiple sclerosis), and cerebrovascular diseases (e.g., assessment of the microstructural environment of fresh infarctions). Representative metrics in clinical use are the full width at half maximum, also known as the mean displacement of the probability density function curve, which is derived from QSI, and diŠusional kurtosis, which is derived from DKI. These new metrics may provide information on tissue structure in addition to that provided by conventional Gaussian DWI investigations that use the apparent diŠusion coe‹cient and fractional anisotropy, recognized indices for evaluating disease and normal development in the brain and spine. In some clinical situations, sensitivity for detecting pathological conditions is higher using QSI and DKI than conventional DWI and diŠusion tensor imaging (DTI) because DWI and DTI calculations are based on the assumption that water molecules follow a Gaussian distribution, whereas hindrance of the distribution of water molecules by complex and restricted structures in actual neural tissues produces distributions that are far from Gaussian. We review the technical aspects and clinical applications of QSI and DKI, focusing on clinical use and in vivo studies and highlighting diŠerences from conventional diŠusional metrics.

Mean Diffusional Kurtosis in Patients with Glioma: Initial Results with a Fast Imaging Method in a Clinical Setting

“…Moreover, imaging with a rapid DKI technique in clinically unstable patients, such as those after a stroke or trauma, has the potential to provide new insights into the pathophysiology and longitudinal progression of these diseases. [22][23][24][25] …”

Application ofq-Space Diffusion MRI for the Visualization of White Matter

“…The quality of QSI depends on the range and steps of b-values applied, with higher accuracy resulting in more time-consuming scans. DKI was developed to reduce the time required for the imaging (Jensen et al, 2005) by estimating kurtosis values with only three low b-values, rather than precisely calculating the actual displacement distribution by full-scale QSI. DKI enables detection of microstructural changes during brain maturation and aging Figure 9.…”

“…DKI reduces the long acquisition times involved in full-scale QSI by using a rather simplified DWI sequence with a limited number of low b-values (ϳ2000 s/mm 2 , 3 steps). The major limitation of DKI is that the estimation equation is only valid for low b-values (ϳ3500 s/mm 2 ) (Hori et al, 2012b). However, the diffusionrelated signal decay curve in white matter is multiexponential and the accuracy of the PDF depends upon data with high b-values (ϳ10,000 s/mm 2 ) (Cohen et al, 2002).…”

“…Despite the impressive ability of QSI to visualize CNS myelin, its time-consuming nature hinders its clinical application. Diffusional kurtosis imaging (DKI) was developed to compensate for this limitation (Jensen et al, 2005). Kurtosis is a metric of non-Gaussianity that represents the deviation from a Gaussian curve.…”

Fast and Robust Estimation of Diffusional Kurtosis Imaging (DKI) Parameters by General Closed-form Expressions and their Extensions

“…However, several reports of clinical applications reveal important features of DKI, such as its potential usefulness in diagnostic imaging to evaluate neurological issues in aging 4 and disorders including stroke, 5 Alzheimer's disease, 6 schizophrenia, 7 glioma, 8,9 Parkinson disease, 10 and attention deficit hyperactivity disorder. 11 Recent trials on DKI in such regions as the liver 12 and spine 13 have also been reported. DKI may also benefit conventional DTI by aiding more accurate esti-mation of DTI parameters.…”