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Purpose Cerebrospinal fluid (CSF) plays an important role in the clearance system of the brain. Recently, low b‐value diffusion tensor imaging (low‐b DTI) has been reported to be useful in the observation of CSF flow; however, the precise flow property observed by low‐b DTI has not been fully investigated. Accordingly, a mathematical framework of low‐b DTI is proposed for investigating CSF and clarifying its pseudorandom flow. Theory The framework will show that the limit of the diffusion tensor as b‐value decreases to zero approximately represents the covariance of the velocity distribution of the CSF’s pseudorandom flow. Methods The low b‐value diffusion tensor (DTL) of whole‐brain CSF was obtained using diffusion‐weighted echo‐planar imaging. Seven healthy volunteers were scanned for intersubject analysis; three of the volunteers was consecutively scanned for repeatability analysis. Obtained DTL was visually assessed by ellipsoid‐representation map and was statistically evaluated by calculating mean diffusivity (MD) and fractional anisotropy (FA) in regions of interest (ROIs) representing intensive pseudorandom flow. Results Obtained DTL consistently shows large and anisotropic diffusivity in some segments of CSF, typically the ROIs around the foramen of Monro, the aqueduct, the prepontine cistern, the middle cerebral artery, and the Sylvian fissure throughout the study. The statistical analysis shows high repeatability and consistently high MD and FA in all the ROIs for all the volunteers. Conclusion From the viewpoint of the proposed framework, the high and anisotropic DTL in the ROIs indicates large covariance of velocity distribution, which represents intensive pseudorandom flows of CSF.

Purpose: To shorten acquisition of diffusion kurtosis imaging (DKI) in 1.5-tesla magnetic resonance (MR) imaging, we investigated the effects of the number of b-values, diffusion direction, and number of signal averages (NSA) on the accuracy of DKI metrics.Methods: We obtained 2 image datasets with 30 gradient directions, 6 b-values up to 2500 s/mm 2 , and 2 signal averages from 5 healthy volunteers and generated DKI metrics, i.e., mean, axial, and radial kurtosis (MK, K ¬ , and K ¦ ) maps, from various combinations of the datasets. These maps were estimated by using the intraclass correlation coefficient (ICC) with those from the full datasets.Results: The MK and K ¦ maps generated from the datasets including only the b-value of 2500 s/mm 2 showed excellent agreement (ICC, 0.96 to 0.99). Under the same acquisition time and diffusion directions, agreement was better of MK, K ¬ , and K ¦ maps obtained with 3 b-values (0, 1000, and 2500 s/mm 2 ) and 4 signal averages than maps obtained with any other combination of numbers of b-value and varied NSA. Good agreement (ICC > 0.6) required at least 20 diffusion directions in all the metrics.Conclusion: MK and K ¦ maps with ICC greater than 0.95 can be obtained at 1.5T within 10 min (b-value = 0, 1000, and 2500 s/mm 2 ; 20 diffusion directions; 4 signal averages; slice thickness, 6 mm with no interslice gap; number of slices, 12).

Background 17O‐labeled water (PSO17) is a contrast agent developed to measure brain water dynamics and cerebral blood flow. Purpose To evaluate the safety and feasibility of PSO17. Study Type Prospective study. Subjects A total of 12 male healthy volunteers (23.1 ± 1.9 years) were assigned to three groups of four subjects: placebo (normal saline), PSO17 10%, and PSO17 20%. Field Strength/Sequence Dynamic 3D fluid attenuated inversion recovery (FLAIR, fast spin echo with variable refocusing flip angle) scans of the brain were performed with 3‐T MRI. Assessment Contrast agents were injected 5 minutes after the start of a 10‐minute scan. Any symptoms, vital signs, and blood and urine tests were evaluated at five timepoints from preinjection to 4 days after. Blood samples for pharmacokinetic analysis, including half‐life (T1/2), maximum fraction (Cmax), time‐to‐maximum fraction (Tmax), and area under the curve (AUC), were collected at 13 timepoints from preinjection to 168 hours after. Regions of interest were set in the cerebral cortex (CC), basal ganglia/thalamus (BG/TM), and white matter (WM), and 17O concentrations were calculated from signal changes and evaluated using Cmax. Statistical Tests All items were compared among the three groups using Tukey–Kramer's honestly significant difference test. Statistical significance was defined as P < 0.5. Results No safety issues were noted with the intravenous administration of PSO17. The T1/2 was approximately 160 hours, and the AUCs were 1.77 ± 0.10 and 3.75 ± 0.36 in the PSO17 10% and 20% groups, respectively. 17O fractions calculated from MRI signals were higher in the PSO17 20% group than in the 10% and placebo groups. Significant differences were noted between all pairs of groups in the CC and BG/TM, and between PSO17 20% and both placebo and 10% groups in the WM. Data Conclusion PSO17 might be considered safe as a contrast medium. Dynamic 3D‐FLAIR might detect dose‐dependent signal changes and estimate 17O. Evidence Level 1 Technical Efficacy Stage 1

High-speed diffusion spectroscopic imaging based on an echo-planar technique is presented. A pair of diffusion gradients is applied prior to a rapidly oscillating magnetic field gradient which encodes both chemical shift and spatial information. By applying this technique to a phantom consisting of acetone and water, a diffusion spectroscopic image is obtained in about 15 min, about 64 times faster than the time required in the conventional method. The measured diffusion coefficients show good agreement with previously reported values. This kind of diffusion spectroscopic imaging is expected to provide a way to observe more specific metabolism.