By providing superior soft tissue contrast and detail, MRI has become a standard imaging technique for a host of central nervous system, musculoskeletal, cardiovascular, and oncologic disorders. As mouse and rat models have been developed to study these and other disorders (Beaumont et al., 2009
; Stuckey et al., 2008
; Valable et al., 2007
), rodent MRI has been used with increasing frequency in multiple fields. Furthermore, as greater emphasis has been placed on outcomes, the need for longitudinal survival models has increased in rodent experimentation. Nearly all survival rodent studies require repeated medication administration and/or frequent blood sampling. Therefore, the maintenance of intravascular access is a helpful adjunct to any rodent survival study.
While intravenous access can be obtained from various methods, multiple studies suggest that repeated vein puncture increases animal stress and alters circulating levels of glucose, corticosteroids, prolactin, epinephrine, growth hormone, insulin, and renin (Diehl et al., 2001
; Joint Working Group on Refinement, 1993
; Krinke, 2000
). Furthermore, vein puncture is technique-dependent and often requires a period of warming for vessel dilatation. Eliminating the morbidity associated with repeat vein puncture, intravascular access devices provide for easy administration of medications and repeated blood sampling. Despite reproducible success in larger animal models, only recently has long-term patency been shown in the rodent literature (De Jong et al., 2001
; Yang et al., 2005
In the case of fMRI, the ideal survival study should allow the same animal to be imaged at multiple time points. Such a study requires sedation that does not interfere with the BOLD signal, yet permits safe animal recovery following each scan. Dexmedetomidine, an alpha-2 adrenergic agonist, has been shown to be effective in multiple BOLD fMRI studies (Cho et al., 2007
; Cho et al., 2008
; Pawela et al., 2009a
). This sedative allows adequate sedation for fMRI imaging, affords the use of spontaneous breathing, and has a potent reversal agent with a quick onset of action. Currently, with the use of MRI-compatible subcutaneous vascular access ports and the sedative dexmedetomidine, we have demonstrated a safe and effective model for a rodent survival fMRI study. Furthermore, with reliable intravenous access, sedative doses can easily be adjusted to accommodate longer scanning times. This protocol circumvents the morbidity associated with mechanical ventilation, eliminates animal-to-animal variability, and reduces the overall number of animals required for a longitudinal study.
In this study on healthy rats, S1FL activation drop-off can be seen at time points of 4 and 8 weeks. At longer time points, the fMRI signal recovers to a relatively normal level. This progression corresponds with the natural progression of scar tissue formation following electrode placement, which peaks at about 4–8 weeks with subsequent gradual resolution. The phenomenon observed here could be the consequence of compression of the nerve trunk by developing scar tissue after electrode placement, thereby affecting the conductivity of nerve fibers via axonotmesis. Behavior tests were performed to test this hypothesis. Decreases in grip strength and in Von Frey monofilament response were observed at 4–8 weeks, consistent with this hypothesis. Behavior test results as well as fMRI results return to normal ranges at 12 weeks. These experiments support the idea that there is a coupling between scar tissue formation around peripheral nerves that have been mildly stressed by electrode placement and reduced neuronal activity.
As a control for axonotmesis, fcMRI of the insular cortex was introduced to the study. A highly consistent fcMRI insular network was acquired at the various time points after surgery. A similar insular network was reported previously using BOLD fMRI and independent component analysis (ICA) in a nonsurvival rat model (Zhang et al., 2010
; Liang et al., 2011
). Our study shows that this network pattern is conserved across the different time points. Due to the highly specific seed we used in this study, only the insular cortical network was demonstrated.
Based on the atlas (Paxinos and Watson, 2007), the network consists of the granular insular cortex (GI), the dysgranular insular cortex (DI), the agranular insular posterior cortex (AIP), and the piriform cortex. These areas are related to signal integration, synaptic plasticity, and memory (Blair et al., 2001
; Sweatt, 2001
), which can also be modified by task (Kida et al., in press 2011
). It is hypothesized that fcMRI of granular insular cortex can be used as a control in fMRI survival experiments of various kinds including cortical plasticity studies.
In this study, seven vascular access ports remained patent over a period of 12 weeks. This experiment is exciting in that long-term vascular access can be reproducibly achieved and used safely in a rodent survival MRI study. The applications of such a port extend far beyond the scope of peripheral nerve experimentation. Such access ports can be applied to any survival MRI study requiring repeated medication administration, intravenous contrast, or blood sampling.
- Use of vascular access ports for longitudinal rat fMRI studies established
- Repeated medication, sedation, contrast agent, or blood sampling allowed
- Axonotmesis, grip strength, Von Frey response, and voxel count correlated
- 9.4 T fMRI technology for rat models of disease enhanced Technology for study of CNS response to PNS injury and repair advanced