"Juvenile development and ageing mediated changes in cortical structure and volume in the rat brain"

Luam Mengler, Artem Khmelinskii, Chrystelle Po, Marius Staring, Johan H.C. Reiber, Boudewijn P.F. Lelieveldt and Mathias Hoehn

Abstract

Introduction: Animal models can provide mechanistic insight into disease pathology and evolution, as well as a platform to test potential therapies. However, most pre-clinical research employs juvenile, 3 month old (300g) animals, which is not necessarily representative of the natural disease state.
We are conducting a lifespan study on rats, imaging the juvenile development as well as ageing processes of the brain with non-invasive techniques including functional and anatomical MRI and different PET-tracers, respectively.
An explorative study like this benefits from an automated evaluation technique that is based on individual structural changes rather than manual ROI analysis.
To overcome this limitation, we used elastic coregistration and individual deformation fields to address changes in the frontal cortex from MR images.

Material and Methods: From postnatal day 21 two groups of four male Wistar rats were housed pairwise. Food restriction (80% of ad libitum consumption) started at months 3 in order to minimize obesity, a risk factor for age-related diseases [1].
Animals were employed in MR experiments on a bimonthly basis. Group 1 was followed from the age of 3weeks up to 14 months, Group 2 from 10 until 20 months. MR experiments were conducted on an 11.7T Bruker BioSpec horizontal bore scanner. Animals were anaesthetized using 2% Isoflurane in 70:30 N2O:O2 and vital functions were monitored continuously.
T2 maps were chosen for their anatomical detail and quantitative reproducibility. Maps were calculated from an MSME (multi slice multi echo) sequence (10echoes) with TE=10ms, TR=5000ms, FOV=28x28mm and a resolution of 0.146x0.146mm in plane and 0.5mm slice thickness (without gaps). For every individual all T2 maps from the different ages were coregistered non-rigidly using a B-spline transform [2], and the corresponding deformation fields calculated. Deformation maps indicated volumetric changes of brain regions.
At specific time points (3 weeks and 3 months) a subset of rats was sacrificed for histological evaluation (cresyl violet).

Results: The deformation maps revealed a decrease in cortical thickness during juvenile development (3 weeks to 3 months). In parallel, quantitative evaluation of the frontal cortex showed a reduction of T2 relaxation time.
A further T2 reduction was observed after the age of 6 months, however, without significant changes in volume. Preliminary histological evaluation revealed a higher cortical cell density at 3 months when compared to 3 weeks of age.

Conclusions: Elastic coregistration is a useful tool for lifespan studies, providing unbiased information on volume changes on an individual basis. The deformation fields allow the creation of physiologically meaningful ROIs for quantitative analysis of imaging parameters.
A combination of reduced T2 and decreased cortical volume is implying an increasing tissue density (myelination and cell number) during development, lowering the relative amount of free water, and thus reducing the cortical volume. This was confirmed by histological evaluation.

Acknowledgements: This work was financially supported by BMBF (0314104) and ENCITE EU-FP7 (HEALTH-F5-2008-201842) program.

References:
[1] Mattson and Wan, Beneficial effect of intermittent fasting and caloric restriction on the cardiovascular and cerebrovascular systems, Journal of Nutritional Biochemistry, 16 (129-137), 2005.
[2] Klein & Staring et al., "elastix: a toolbox for intensity based medical image registration," IEEE-TMI, 2010.

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