Double diffusion encoding (DDE) of the water signal offers a unique ability to separate the effect of microscopic anisotropic diffusion in structural units of tissue from the overall macroscopic... Show moreDouble diffusion encoding (DDE) of the water signal offers a unique ability to separate the effect of microscopic anisotropic diffusion in structural units of tissue from the overall macroscopic orientational distribution of cells. However, the specificity in detected microscopic anisotropy is limited as the signal is averaged over different cell types and across tissue compartments. Performing side-by-side water and metabolite DDE spectroscopic (DDES) experiments provides complementary measures from which intracellular and extracellular microscopic fractional anisotropies (mu FA) and diffusivities can be estimated. Metabolites are largely confined to the intracellular space and therefore provide a benchmark for intracellular mu FA and diffusivities of specific cell types. By contrast, water DDES measurements allow examination of the separate contributions to water mu FA and diffusivity from the intraand extracellular spaces, by using a wide range of b values to gradually eliminate the extracellular contribution. Here, we aimed to estimate tissue and compartment specific human brain microstructure by combining water and metabolites DDES experiments. We performed our DDES measurements in two brain regions that contain widely different amounts of white matter (WM) and gray matter (GM): parietal white matter (PWM) and occipital gray matter (OGM) in a total of 20 healthy volunteers at 7 Tesla. Metabolite DDES measurements were performed at b = 7199 s/mm(2), while water DDES measurements were performed with a range of b values from 918 to 7199 s/mm(2). The experimental framework we employed here resulted in a set of insights pertaining to the morphology of the intracellular and extracellular spaces in both gray and white matter. Results of the metabolite DDES experiments in both PWM and OGM suggest a highly anisotropic intracellular space within neurons and glia, with the possible exception of gray matter glia. The water mu FA obtained from the DDES results at high b values in both regions converged with that of the metabolite DDES, suggesting that the signal from the extracellular space is indeed effectively suppressed at the highest b value. The mu FA measured in the OGM significantly decreased at lower b values, suggesting a considerably lower anisotropy of the extracellular space in GM compared to WM. In PWM, the water mu FA remained high even at the lowest b value, indicating a high degree of organization in the interstitial space in WM. Tortuosity values in the cytoplasm for water and tNAA, obtained with correlation analysis of microscopic parallel diffusivity with respect to GM/WM tissue fraction in the volume of interest, are remarkably similar for both molecules, while exhibiting a clear difference between gray and white matter, suggesting a more crowded cytoplasm and more complex cytomorphology of neuronal cell bodies and dendrites in GM than those found in long-range axons in WM. Show less
The altered iron concentration in many neurodegenerative diseases such as Alzheimer's disease (AD) has led to the development of MRI sequences that are sensitive to the accompanying changes in the... Show moreThe altered iron concentration in many neurodegenerative diseases such as Alzheimer's disease (AD) has led to the development of MRI sequences that are sensitive to the accompanying changes in the transverse relaxation rate. Heavily T-2*-weighted imaging sequences at high magnetic field strength (7 T and above), in particular, show potential for detecting small changes in iron concentration. However, these sequences require a long echo time in combination with a long scanning time for high resolution and are therefore prone to image artifacts caused by physiological fluctuations, patient motion or system instabilities. Many groups have found that the high image quality that was obtained using high resolution T-2*-weighted sequences at 7 T in healthy volunteers, could not be obtained in AD patients. In this study the source of the image artifacts was investigated in phantom and in healthy volunteer experiments by incorporating movement parameters and resonance frequency (f0) variations which were measured in AD patients. It was found that image degradation caused by typical f0 variations was a factor-of-four times larger than artifacts caused by movement characteristic of AD patients in the scanner. In addition to respiratory induced f0 variations, large jumps in the f0 were observed in AD patients. By implementing a navigator echo technique to correct for f0 variations, the image quality of high resolution T-2*-weighted images increased considerably. This technique was successfully applied in five AD patients and in five subjective memory complainers. Visual scoring showed improvements in image quality in 9 out of 10 subjects. Ghosting levels were reduced by 24 +/- 13%. (C) 2010 Elsevier Inc. All rights reserved. Show less
