Introduction

In previous study, a D₂O perfusion imaging method was report to use two compartmental model to quantitative measure the fast (f1) and slow (f2) perfusion of tissue.¹ According to the spatial distribution, the slow compartment was anticipated as the perfusion of CSF and interstitial fluid. In literature, brain volume can be influenced intentionally by dehydration and a following hyperhydration.² The extension of ventricles and shrinkage of white matter in some regions have been observed in the dehydration state.^{3, 4} With a fluid infusion of 2% of body weight in the previous D₂O perfusion imaging experiment, the tissues are expected in a hyperhydration state. In this study, we conducted both D₂O and H₂O infusion experiments on spontaneously hypertension rat (SHR). In order to observe the fluid induced tissue change, the T2 relaxometry was conducted before and after fluid infusion.

Materials and Methods

SHR rats were scanned by D₂O and H₂O infusion. The SHR is a widely used model for neurodegenerative disorder such as dementia. The SHR has extension of ventricles, which has advantageous for observation of CSF characteristics. All rats were anesthetized by 1.5 % isoflurane and injected with isotonic D₂O or H₂O (2ml/100g) via tail vein at 7T Bruker Clinscan MRI scanner. 80 measurements were acquired by the following parameters: turbo-spin-echo (TSE) with TR/TE = 2000/14ms, matrix size = 256x128, FOV = 35mm, turbo factor = 3, slice number = 3, slice thickness = 1.5mm, distant factor = 10%, sampling interval = 34s. Spin echo images at identical slices were acquired before and right after infusion, with the following parameters: TR =2000ms, TE: from 20 to 200ms with an interval of 20ms.

Results

The D₂O perfusion imaging was analyzed by a two-compartmental parallel model to separate fast flow f1 (blood) and slow flow f2 (CSF and interstitial fluid). One representative rat was shown in fig.1 for its fast flow 1 and slow flow f2, and a corresponding T2 mapping was shown in fig. 2. It is noted that the f1 showed good contrast between GM and WM, and the f2 was enhanced in matching regions with prolonged T2 map, such as hippocampus, ventricles, and peri-ventricular regions. The T2 is shortened in some regions including corpus callosum and amygdala. For comparison, the T2 mapping of another SHR rat with H₂O infusion was shown in fig. 3, which is enormously similar to fig. 2. It is also noted that the GM of cortex and muscles showed insignificant change of T2 in hyperhydration state.

Discussion

The T2 alteration is clearly observed after massive fluid infusion on SHR rat. The T2 prolonged region spatially correlated with the slow flow f2. It is anticipated to be due to the extra CSF production and enlarged slow flow in these region. The reason of T2 shortening effect is not clear yet. We speculated that myelin water fraction might be increased by blood supply, and then leads to the T2 decrease. Further investigations by diffusion imaging and voxel-based morphometry may help for validation. The T2 change of D₂O and H₂O are almost identical. Therefore, the effect of negative relaxivity of D₂O should be negligible. The D₂O perfusion imaging method with massive fluid infusion may alter the tissue T2 in some specific regions, and the measured perfusion parameters may be influenced and reflect the physiology under hyperhydration. The challenge of hyperhydration may provide further information of hydrodynamics in brain, which could be related to the emerging studies of aquaporin function and glymphatic system. Therefore, the D₂O perfusion imaging may have potential for the future studies on neurodegenerative disorders, neuroinflammation, and developing brain.

Acknowledgements

No acknowledgement found.