Purpose

Impairment of gastrointestinal (GI) functions may lead chronic conditions such as gastroparesis, obesity, or gastroesophageal reflux diseases that are often resistant to pharmacological or dietary treatments. Vagus nerve stimulation (VNS) is an emerging electroceutical technique for remedying gastric disorders, utilizing the vagovagal circuitry that innervates the GI. To evaluate and optimize VNS for GI modulation, we developed a contrast-agent MRI protocol to characterize gastric motility and emptying as the imaging-based readout of gastric physiology.

Method

Twenty-one Sprague-Dawley rats (male, 230-330g) were studied according to our protocol approved by the Purdue Animal Care and Use Committee. Animals were trained for seven days to consume voluntarily ~10g of Gd-labeled Dietgel after an 18-hour fast. After eating the Gd-labeled gel on the day of imaging, the animals were anesthetized with 4% Isoflurane for 5 minutes. Of the 21 animals, 10 were imaged under normal physiology and 11 were imaged following the implantation of a bipolar cuff lectrode on the left cervical vagus nerve. Electrical pulses (biphasic square pulses with inter-pulse duration (IPD) = 50ms; pulse amplitude (PA) = 0.6mA; pulse width (PW) = 0.36ms; frequency = 10Hz; 20 seconds on and 40 seconds off) were delivered at the start of image acquisition and lasted throughout the 4-hour experiment, as illustrated in Fig. 1. The animals were scanned in a 7T small-animal MRI system (BioSpec 70/30, Bruker). They first received a subcutaneous (SC) bolus injection of 0.01mg/kg dexmedetomidine solution (0.05mg/ml, Zoetis), followed by a continuous SC infusion of dexmedetomidine at 0.03mg/kg/hr along with 0.3-0.5% isoflurane mixed with oxygen at a flow rate of 500ml/min 15 min after the bolus. Two sets of respiratory-triggered FLASH sequences were used to obtain T1-weighted abdominal images: One for assessing gastric volume with higher spatial resolution and larger spatial coverage (TR/TE = 124/1.4ms; FA = 90°; matrix size = 256 x 256; slice thickness = 1mm; FOV = 60 x 60mm; four averages; and 30 slices) and the other for assessing gastric motility with higher temporal resolution and more targeted spatial coverage of the antrum (TR/TE = 11.8/1.1ms; FA = 25°; matrix size = 128 x 128; slice thickness = 1.5mm; FOV = 60 x 60mm; and 4 slices). The two sequences were interleaved for 4 hours. Gastric volume, as well as antral contraction frequency, amplitude and the velocity of the peristaltic wave, were quantified from the two sets of images using a custom-built image processing pipeline described elsewhere [1]. The degree to which the pyloric ring opened was measured in terms of the cross-sectional area (CSA) of the lumenal content in the pyloric canal from the high spatial resolution scans. One-way ANOVA was used to assess the statistical significance.

Results

With a parameter setting informed by electrophysiological findings, the VNS was found to significantly promote gastric emptying (Fig. 2). This effect is attributed to the expansion of the pyloric ring (control vs. VNS: 1.37±0.18 vs. 2.54±0.47 mm2; p < 0.05) (Fig. 3A) without notable changes in antral contraction, in terms of its amplitude (control vs. VNS: 30.58±2.96 vs. 32.46±3.02 % occlusion), velocity (control vs. VNS: 0.67±0.01 vs. 0.67±0.03 mm/s), or frequency (control vs. VNS: 6.34±0.07 vs. 6.42±0.17 cpm) (Fig. 4). The expansion of the pyloric ring was significantly correlated with the rate of gastric emptying, confirming their relationships (Fig. 3B).

Conclusion

Here, we reported the use of contrast-enhanced MRI to assess the modulatory effect of VNS on the GI functions in a rodent model. We developed a robust and effective contrast-enhanced MRI technique, along with an animal feeding protocol, to non-invasively monitor gastric emptying and motility. This protocol overcomes the challenge that animals are typically not compliant to test meals, which in turn is a requirement for most human gastric studies. Such experimental protocol was further empowered by advanced image analysis to enable comprehensive assessment of gastric functions and physiology. In summary, the proposed method allows non-invasive and repeated preclinical imaging of gastric physiology and diseases given electroceutical treatments.

Acknowledgements

This work was supported in part by NIH SPARC 1OT2TR001965 and Purdue University.

References

[1] Lu K-H, Cao J, Oleson S, et al., Contrast enhanced magnetic resonance imaging of gastric emptying and motility in rats. IEEE Transactions on Biomedical Engineering. 2017;64(11):2546-2554.