Measuring Glymphatic Flow in Man Using Quantitative Contrast-Enhanced MRI

In this technical report, the authors used MR myelography to demonstrate the feasibility of T1 mapping to quantify contrast concentration to analyze glymphatic flow in man. There is increasing interest in its use as a biomarker and potential therapeutic target in Alzheimer disease. SUMMARY: On the basis of animal models, glymphatic flow disruption is hypothesized to be a factor in the development of Alzheimer's disease. We report the first quantitative study of glymphatic flow in man, combining intrathecal administration of gadobutrol with serial T1 mapping to produce contrast concentration maps up to 3 days postinjection, demonstrating performing a quantitative study using the techniques described feasibility and providing data on pharmacokinetics.

P rotein aggregation is the pathologic signature of many neurodegenerative diseases, including Alzheimer's disease (AD). 1 The recently discovered glial lymphatic ("glymphatic") system 2 may play a critical role in protein removal, including soluble amyloid-␤ 3 and HPF-(Hyperphosphorylated-). 4 This perivascular glymphatic pathway drives exchange between CSF and interstitial fluid, and dysfunction may be a causal factor in the development of Alzheimer's disease. 5 In rodent models, glymphatic flow decreases with age, 6 increases during sleep, 7 and can be pharmacologically manipulated, 7 suggesting similar approaches for patients with AD.
Qualitative MR imaging using percentage signal enhancement on T1-weighted images has been reported in animal models 8 and humans. [9][10][11][12] However, the percentage signal change on a T1weighted image with contrast concentration depends on the intrinsic T1 of each tissue, the corresponding change in T2 * , and the imaging parameters selected. To be able to apply pharmacokinetic models requires consistent determination of contrast concentrations across tissue types and concentrations. Here we use the combination of an intrathecal administration of a gadolinium-based contrast agent (gadobutrol) with quantitative MR imaging to provide maps of contrast concentration versus time, measuring glymphatic flow throughout the brain in a human volunteer at 3T.

Patient History
A 55-year-old man referred for MR myelography was recruited for this Health Insurance Portability and Accountability Actcompliant, institutional review board (The Feinstein Institute for Medical Research)-approved study using an intrathecal injection of 0.5 mL of 1.0 mmol of gadobutrol. The patient gave written, informed consent.

Data Acquisition and Analysis
Quantitative T1-mapping data were acquired on a 3T Achieva TX MR imaging scanner (Philips Healthcare, Best, the Netherlands) with an 8-channel head coil using a multiple flip angle 3D spoiled gradient-echo sequence with TE/TR ϭ 2.8/20 ms; flip angles ϭ 2°, 5°, 10°, 20°, 40°; FOV ϭ 240 ϫ 228 ϫ 120 mm 3 ; acquired resolution ϭ 1.2 ϫ 1.2 ϫ 1.2 mm 3 . Acquisition time was 3 minutes 47 seconds for each flip angle using a sensitivity encoding factor of 2, to give a total time of 24 minutes. Scans were acquired at baseline and at 9 further acquisitions during the first 10 hours (60, 210, 240, 270, 370, 450, 500, 560, and 620 minutes), followed by acquisitions at 26, 50, and 79 hours. The subject was not restricted but was asked to remain supine for as much as possible during the first 10 hours but then was allowed to move normally for the remaining time. He had a normal night's sleep before being scanned the final 3 times.

Data Analysis: Theory
The method is based on the nuclear MR steady-state signal equation, where E 1 ϭ exp ͩ ϪTR T 1 ͪ , S i is the signal intensity for each flip angle ␣ i , and M 0 is a constant representing the equilibrium magnetization. This equation can be transformed to a linear form, from which the gradient E 1 and hence the tissue T 1 can be determined. 13 The T 1 value is related to the contrast agent concentration by Where T 10 is the native T 1 value of the tissue (s), and r 1 is the relaxivity of the contrast agent, assumed to be 5.1 l/mmol/s for gadobutrol.

Data Processing
Each volume was coregistered to the 20°flip angle acquisition from the first scan acquired before contrast injection using SPM12 (http://www.fil.ion.ucl.ac.uk/spm/software/spm12). T1 maps were generated using least squares fitting to the signal equation using custom Matlab (MathWorks, Natick, Massachusetts) scripts. The precontrast map provided the baseline T 1 values (T 10 ), enabling contrast concentration to be calculated at each subsequent time.
Brain segmentation was performed using SPM12 on the 20°f lip angle acquisition from the first scan acquired before contrast injection, resulting in native space gray matter, white matter, and CSF components. Cortical gray matter masks were defined by the intersection of the gray matter mask and the corresponding regions of the Montreal Neurological Institute structural atlas transformed to native space. Juxtacortical white matter and CSF masks were defined by the intersection of their native tissue components with the cortical gray matter mask expanded by 3 mm using only voxels with Ͼ95% probability of being the target tissue.

RESULTS
The time course of contrast concentration during 79 hours postinjection is shown in Fig 1. Contrast arrived in the cisterna magna between 1 and 3 hours postinjection and covered the entire subarachnoid space at 8 hours. The subarachnoid CSF concentration peaked between 10 and 15 hours at approximately 0.5 mmol/L. Cortical enhancement measurable from 4 hours peaked between 10 and 26 hours, with a maximum concentration of approximately 0.1 mmol/L. The largest concentration was noted in the temporal lobes and insula in the first 10 hours. Cortical contrast remained at 50 hours with near-complete clearance at 79 hours. Depending on location, white matter concentration peaked between 26 and 50 hours.
Concentration-time curves were calculated for cortical gray matter and juxtacortical white matter (Fig 2). Biologic half-life of gadobutrol in the subarachnoid CSF, based on the washout curve, was approximately 12 hours. A 2-compartment model provided a good fit to the cortical contrast concentration curve.

DISCUSSION
Most current literature on glymphatic flow is based on rodent models using highly invasive techniques. A few studies used MR imaging qualitatively to investigate glymphatic flow in rodents 8 and humans. [9][10][11][12] Our report improves on those studies by quantifying contrast concentration. We are only aware of a single study that has used similar quantification in rats at 9.4T. 14 Compared with rodents, contrast uptake and clearance are much slower in our subject but are still faster than would be expected due to only passive diffusion, which provides further evidence that intrathecal administration of drugs may provide access to the entire brain parenchyma. We observed minimal enhancement of the lateral ventricles at any time, suggesting that contrast entering the ventricular system is rapidly diluted.
Although only a single individual was imaged, the acquisition of serial quantitative data at many time points during 3 days represents a unique dataset. While cost and inconvenience to patients of an identical approach are prohibitive for routine use, our results suggest that an abbreviated protocol may suffice.
Regarding retention of gadolinium-based contrast agents, recent studies including long-term follow-up after intrathecal contrast injection have not identified adverse effects. 15,16 Gadolinium retention after intrathecal administration was not observed for gadobutrol at the same dose used in the current study. 11

CONCLUSIONS
We have demonstrated the feasibility of using T1 mapping to quantify contrast concentration to analyze glymphatic flow in man, for which there is increasing interest in its use as a biomarker and potential therapeutic target in AD.