In vivo and in vitro bi-exponential diffusion of N -acetyl aspartate (NAA) in rat brain: a potential structural probe?
✍ Scribed by Yaniv Assaf; Yoram Cohen
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 236 KB
- Volume
- 11
- Category
- Article
- ISSN
- 0952-3480
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✦ Synopsis
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 Â 10 6 s/cm 2 and 35.8 Â 10 6 s/cm 2 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 Â 10 6 s/cm 2 ) 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 mm and the other of $1-2 mm 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).