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phMRI with Simultaneous Measurement of Cerebral Perfusion and Blood-Cerebrospinal Fluid Barrier Function using Interleaved Echo-Time ASL

Charith Perera¹, Jack Wells¹, Ian Harrison¹, David Thomas^2,3,4, and Mark Lythgoe¹
¹Centre for Advanced Biomedical Imaging, University College London, London, United Kingdom, ²Neuroradiological Academic Unit, Department of Brain Repair and Rehabilitation, UCL Queen Square Institute of Neurology, University College London, London, United Kingdom, ³Leonard Wolfson Experimental Neurology Centre, UCL Queen Square Institute of Neurology, University College London, London, United Kingdom, ⁴Wellcome Centre for Human Neuroimaging, UCL Queen Square Institute of Neurology, University College London, London, United Kingdom

Synopsis

We employed an interleaved short/long echo-time ASL sequence to better understand the differential response of vessels associated with the blood brain barrier (BBB), and the relatively understudied blood-cerebrospinal fluid barrier (BCSFB) to pharmacological perturbation in the healthy and aged brain. We measured changes in both cortical perfusion and the BCSFB-ASL signal in response CO₂, caffeine, and vasopressin. Additionally, we demonstrated a marked decrease in BCSFB reactivity towards vasopressin in the aged vs adult brain. Together, these novel data highlight the value of this translational approach to capture simultaneous and differential pharmacological modulation of vessel tone at the BBB and BCSFB.

Introduction

The blood-brain-barrier (BBB) and blood-cerebrospinal fluid barrier (BCSFB) mediate the complex interplay between blood and the brain. Non-invasive CBF measurements obtained using arterial spin labelling (ASL) MRI provide a surrogate measure of the functionality of the BBB to, for example, facilitate a constant supply of oxygen and nutrients from the blood to the highly metabolically active brain cells. We have recently developed an ASL-MRI technique for the non-invasive assessment of BCSFB function: BCSFB-ASL uses an ultra-long echo time to quantify the rate of BCSFB-mediated delivery of endogenous arterial blood water to ventricular cerebrospinal fluid (CSF)¹. Importantly, the technique allows repeated measures with high temporal resolution which enables dynamic capture of the BCSFB’s functional response to drugs for the first time.
By alternating the ultra-long and short TEs used for BCSFB- and traditional-ASL, respectively, and keeping all other parameters constant, we are able to simultaneously capture two distinct physiological responses to a single drug dose or ‘challenge’: i) parenchymal perfusion; ii) rates of BCSFB-mediated labelled arterial blood water delivery to the CSF (a surrogate, non-invasive, measure of BCSFB function). This approach provides an efficient means to better understand the differential response of vessels associated with the BBB and BCSFB to pharmacological perturbation in the healthy and aged/diseased brain.

Methods

We first applied the interleaved-TE ASL method in the anaesthetized mouse brain (n=6) at 9.4T (Agilent), to investigate the effects of several selected challenges on vessels perfusing the choroid plexus and the cortex: vasopressin, CO₂, and caffeine (further details: Table 1). The interleaved-TE ASL protocol (Figure 3) utilized a FAIR-EPI labelling/readout with parameters: TI = 4000ms, TR = 12000ms, TE = 20ms and 220ms for traditional- and BCSFB-ASL, respectively, single 2.4mm slice centred on lateral ventricles, matrix size: 32x32, FOV: 20mm x 20mm.
We investigated the mechanistic link between BCSFB function and vasopressin in the aged brain by applying the interleaved-TE ASL technique to capture the phMRI response to vasopressin in a cohort of aged (23-months, n=14) and adult mice (5-months, n=14).
For each subject, a single ROI drawn across the cortex/lateral ventricles provided the mean average signal across the voxels for each slice-selective and non-selective image pair. For each image pair, the slice-selective intensity (M_L) had its corresponding non-selective intensity (M_C) subtracted to give a ΔM value, which was then divided by its corresponding Mc. In a subject-wise manner, ΔM/M_C values were divided by the mean ΔM/M_C value for the 10-minute baseline, providing a measure of relative, baseline-normalised perfusion (short TE) and BCSFB-ASL signal (ultra-long TE respectively).
Paired, 2-tailed t-tests were conducted when comparing challenge-responses to baseline. Unpaired, 2-tailed t-tests were conducted to compare adult and aged cohorts.

Results

Administration of saline vehicle revealed no significant changes in cortical perfusion or the BCSFB-ASL signal. As expected, hypercapnia induced increases in relative cortical perfusion (21%, p=0.0038, Figure 1 a-b). A similar increase in the BCSFB-ASL signal from baseline was also observed (21%, p=0.031, Figure 1c-d).
A marked vasopressin-induced decrease in the BCSFB-ASL signal was observed (46%, p=0.0004, figures 1g-h). However, the 15% increase in cortical perfusion only bordered significance (p=0.055, Figure 1e-f)
Caffeine stimulated significant decreases in the BCSFB-ASL signal (41% average, p=0.0004, Figure 1k-l). Changes in cortical perfusion were not observed (p=0.25, Figure 1i- j).
Vasopressin induced a marked decrease in the BCSFB-ASL signal in the adult cohort, averaging 27% (Figure 2a-b, p=0.00004). In contrast, aged mice displayed an average 0.7% increase (n.s., p=0.93). This drastic impairment in the response of aged mice vs their adult counterparts was statistically significant (p=0.013, Figure 2b).

Discussion

Here we present a method to capture dynamic responses of the BCSFB to pharmacological modulation. By employing an interleaved-TE ASL sequence, we are also able to measure simultaneous dynamic changes in cortical perfusion.
Dampened dilatory responses to CO₂ have been implicated in many pathological conditions affecting the brain microvasculature, such as cognitive decline in ageing and dementia^2–5. Our results indicate a significant, CO₂-driven increase in CP perfusion of a similar magnitude to that observed in the cortex.
Measurements of caffeine-induced CBF changes in humans and rat models have shown CBF decreases^6,7. We provide dynamic data assessing the immediate effects of caffeine in the mouse brain for the first time, observing marked decreases only in the BCSFB-ASL signal.
It has become increasingly evident that the-BCSFB undergoes numerous morphological and functional changes within the ageing brain^8–10, with wide-ranging evidence that an increased levels of endogenous vasopressin may underlie decline^11–15. The observed impairment to a vasopressin challenge observed in the aged brain highlights the potential for novel measurements of BCSFB reactivity to provide upstream biomarkers of age-related pathological processes.

Conclusion

In conclusion, our results illustrate the value of an interleaved-TE ASL MRI approach to quantify pharmacologically-induced changes to vessels associated with the BBB in the cortex and the BCSFB in the choroid plexus. The marked impairment of the response to a vasopressin challenge within the aged mice highlight the capability of such measurements to be utilised for probing altered functionality and pathophysiology in the aged or diseased brain, providing a potential novel biomarker of age-related cognitive decline.

Acknowledgements

No acknowledgement found.

References

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10. Serot, J. M., Foliguet, B., Béné, M. C. & Faure, G. C. Choroid plexus and ageing in rats: A morphometric and ultrastructural study. Eur. J. Neurosci. 14, 794–798 (2001).

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Figures

Figure 1 - Interleaved-TE ASL data obtained simultaneously for relative cortical perfusion and relative BCSFB-ASL signal. Averaged time courses for relative cortical perfusion (column 1) and the individual animal averages (column 2). Relative BCSFB-ASL signal averaged time courses (column 2) are shown alongside their respective individual animal averages (column 3). Rows represent each drug or challenge: row 1 - CO₂ (a-d), row 2 - vasopressin (e-h), row 3 - caffeine (i-l). Further details of the drug challenges can be found in Table 1.

Figure 2 - Adult (n = 14) vs aged (n = 14) response to vasopressin.

a) Relative BCSFB-ASL signal obtained using Interleaved-TE ASL, showing both adult and aged averaged time courses (vasopressin administration at 10 mins).

b) Adult (n = 14) vs aged (n = 14) response to vasopressin, individual animal averages (baseline and post-vasopressin challenge).

Figure 3 - Interleaved-TE ASL acquisition. Schematic showing the individual images acquired within a single 48 second repetition of our Interleaved-TE ASL protocol: 2 pairs of controlled and labelled images are obtained at echo times of 20 ms and 220 ms.

Table 1 - Drug and gas challenge administration details. Details of the mouse strain and sample sizes used for our phMRI experiments, alongside the doses and administration routes for each of the selected challenges, as well as the anaesthetic regimes. Timings of the experimental phases have been shown: baseline, challenge (“on” time), and recovery (where applicable).

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)

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