Cerebrovascular MRI in the mouse without an exogenous contrast agent

Purpose: To assess the effect of a variety of anesthetic regimes on $T_2^{\ast }$ -weighted MRI of the mouse brain and to determine the optimal regimes to perform $T_2^{\ast }$ -weighted MRI of the mouse cerebrovasculature without a contrast agent.

Methods: Twenty mice were imaged with a 3D $T_2^{\ast }$ -weighted sequence under isoflurane, dexmedetomidine, or ketamine-xylazine anesthesia with a fraction of inspired oxygen varied between 10% and 95% + 5% CO₂ . Some mice were also imaged after an injection of an iron oxide contrast agent as a positive control. For every regime, whole brain vessel conspicuity was visually assessed and the apparent vessel density in the cortex was quantified and compared.

Results: The commonly used isoflurane anesthetic leads to poor vessel conspicuity for fraction of inspired oxygen higher or equal to 21%. Dexmedetomidine and ketamine-xylazine enable the visualization of a significantly larger portion of the vasculature for the same breathing gas. Under isoflurane anesthesia, the fraction of inspired oxygen must be lowered to between 10% and 14% to obtain similar vessel conspicuity. Initial results on automatic segmentation of veins and arteries using the iron oxide positive control are also reported.

Conclusion: $T_2^{\ast }$ -weighted MRI in combination with an appropriate anesthetic regime can be used to visualize the mouse cerebrovasculature without a contrast agent. The differences observed between regimes are most likely caused by blood-oxygen level dependent effects, highlighting the important impact of the anesthetic regimes on cerebral blood oxygenation of the mouse brain at rest.