Final report: Validating a non-invasive imaging method to measure astroglial water transport in brain health and disease

Department News | June 30, 2022

Through our 2020 Small Grants Program, we allocated nearly $100,000 to a terrific set of diverse, one-year proposals from faculty, staff and trainees on a wide range of topics. This is the final report for the “Validating a non-invasive imaging method to measure astroglial water transport in brain health and disease” project led by Deidre Jansson, MSc, PhD, and Jeff Iliff, PhD. This project was funded by the Garvey Institute for Brain Health Solutions.

The purpose of the study was to determine the accuracy and specificity of Arterial spin labeling (ASL) — a non‐invasive perfusion technique used in MRI to track cerebral blood flow — in measuring vascular and glial‐dependent water transfer to establish whether it is a valuable clinical tool in Alzheimer’s disease (AD). This simple and safe technique, already approved for use in a clinical setting, has potential to circumvent current invasive approaches in people at risk for AD‐related dementias.

Below is their final report.

Summary of the work completed
This study was designed to validate a non-invasive imaging modality to measure glymphatic function in mice, to support the future utility in humans.

The initial study set to make use of the large 14T vertical MRI at South Lake Union because of the improved signal to noise ratios and the low cost to enable us to generate enough data to reach statistical significance. Unfortunately, the animals were unable to maintain normal respiration rates while being in a vertical position over long periods under anesthesia. Since respiration, heart rate and blood flow are known to have direct effects on glymphatic function, we concluded this would not be the ideal method to measure changes in glymphatic function. We switched to using the 4.7T at the Integrated Brain Imaging Center (IBIC) as it is a horizontal MRI and more suitable to performing longer time MRIs at physiological conditions.

The goal was to incorporate dynamic contrast-based MRI (the gold standard for measuring glymphatic function) in combination with arterial spin labelling and intravoxel incoherent motion (IVIM) MRI in wild-type and AQP4 and alpha-syntrophin knock out mice. These mouse models are known to have impaired glymphatic function using contrast-based MRI and other fluorescent imaging techniques.

We wanted to image 10 mice of each group, first with the non-invasive imaging, then with the injection of contrast. The critical issue was that the mice had to be under ketamine/xylazine anesthesia to facilitate glymphatic function (isoflurane is much easier to work with but inhibits glymphatic function).

This proved to be somewhat difficult to obtain, as again we ran into issues with the anesthesia and in maintenance of normal respiration while mice were restrained in the magnet. In the end we obtained contrast images of only 2-3 mice per group, and ASL and IVIM images of 5-6 mice per group.

We are currently conducting analysis on the images in hopes to have publication ready data by December 2022.

What did you learn because of this work?
We tested and optimized many different aspects of the procedure before moving to the transgenic experimental animals. Once we had established the MRI sequences using isoflurane and concluded that the non-invasive and invasive images could not be obtained in one go, we also then tested the anesthesia (K/X) to determine how long animals could remain anesthetized before needing another dose, and testing whether the animals could recover from the anesthesia to be re-dosed the following week. Unfortunately, although all conditions were set and working well- once we combined the anesthesia with time in the magnet, we ran into significant issues with animals remaining anesthetized or surviving anesthesia. Apparently, this is a common problem with K/X anesthesia, however, we did not have the option of using isoflurane.

The difficulty of this study has really highlighted the need for non-invasive techniques and for a better understanding of how these different anesthesia approaches might modulate physiological factors that change glymphatic function.

While in conducting this study we ran into significant issues that required changing initial plans, it was extremely informative for me (Dr. Jansson) as a new principal investigator using in vivo imaging methods and studying physiology. Being able to identify the problems and find alternatives, troubleshooting is a big part of research and in vivo methods especially. Now that I’ve had direct experience, I will be that much better prepared for the next project and be able to apply the knowledge I’ve gained.

What future activities might result from this award?
Although we did not obtain MRI images from the number of animals we had planned, we will still be able to examine the ASL and IVIM sequences in the same animals to determine whether we can detect changes in glymphatic function expected from the different genotypes.

The results from this project will be published as a small report in the next few months (Neurobiology of Aging has extended a call for fluid dynamics articles at the end of the year).

Once completed this data will inform on the validity of using the ASL and IVIM modality in human studies to measure glymphatic function.