In vivo andin vitro bi-exponential diffusion ofN -acetyl aspartate (NAA) in rat brain: a potential structural probe?

Abstract

Diffusion measurements were performed on the N ‐acetyl aspartate (NAA) signal in in situ brains (in vivo and post‐mortem) and on in vitro brain tissue at 37 °C using wide ranges of b‐values (from 0 up to 4.5 × 106 s/cm2 and 35.8 × 106 s/cm2 for the in vivo and the in vitro cases, respectively). In vivo and in vitro NAA signals attenuation due to diffusion was measured at fixed diffusion times (t D ). In the in vitro cases the effect of t D on the apparent diffusion coefficients (ADCs) of NAA was evaluated. From these experiments the following observations and conclusions were made: (1) NAA signal attenuation both in vivo and in vitro is not mono‐exponential and could be fitted by bi‐exponential fitting function; (2) analysis of the low b‐value range only (up to 0.5 × 106 s/cm2 ) gives a mono‐exponential decay (r = 0.999); (3) in both cases the obtained ADCs are sensitive to the diffusion time; (4) the ADCs of the pre‐ and post‐mortem cases are nearly similar; (5) the ADCs obtained from the bi‐exponential fitting function decrease when the diffusion time increases; and (6) both the fast and the slow diffusing components of NAA show a considerable restriction by what seems to be a non‐permeable barrier from which two compartments were identified, one having a size of 6–8 μm and the other of ∼1–2 μm in size. It seems conceivable that the two populations identified in the diffusion experiments represent primarily the NAA in the cell body (soma) and in the neurital space (axons and proximal dendrites). © 1998 John Wiley & Sons, Ltd.

Recent advances in diffusion MRI modeling: Angular and radial reconstruction

“…However, this Gaussian hypothesis is often violated when the diffusion is hindered, as is the case in the brain due to the presence of white matter fibers (Assaf and Cohen, 1998;Niendorf et al, 1996). The propagator formalism enable one to characterize the diffusion without a prior Gaussian assumption (Kärger and Heink, 1983).…”

“…(Assaf and Cohen, 1998;Callaghan et al, 1991;Cory and Garroway, 1990;Kuchel et al, 1997;Regan and Kuchel, 2003).…”

Diffusion NMR spectroscopy

“…The reason for this apparent discrepancy is unclear. Cohen and coworkers [46][47][48] have made fundamentally important contributions to the field of DW-MRS by their systematic studies in brain and nerve tissue. These authors have primarily focussed on the diffusional characteristics of N-acetyl-D-aspartate (NAA), using a large range of b-factors (up to 35 Â 10 4 s mm À2 ) and diffusion times (up to 300 ms).…”

Extracellular–Intracellular Distribution of Glucose and Lactate in the Rat Brain Assessed Noninvasively by Diffusion-Weighted 1H Nuclear Magnetic Resonance Spectroscopy In Vivo

“…Water and cerebral metabolites experience restrictions in their self-diffusion by membranes (van Zijl et aI., 1994;Pfeuffer et aI., 1998a), as demonstrated by NMR measurements at large diffusion weighting and varied diffusion times in immo bilized cells and the rat brain (Assaf and Cohen, 1998;Pfeuffer et aI., 1998d). Intracellular molecules show a restricted diffusion characteristic limiting the mean dis placement of diffusing molecules at larger diffusion times.…”