Developing In Vivo Perfusion Imaging Methods for Spinal Cord Using Hyperpolarized [13C]t-Butanol and [13C, 15N2]Urea
Ilwoo Park1, Jeremy Gordon1, Sarah Nelson1,2, and Jason Talbott1,3

1Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, United States, 2Bioengineering and Therapeutic Sciences, University of California San Francisco, San Francisco, CA, United States, 3Brain and Spine Injury Center (BASIC), University of California San Francisco, San Francisco, CA, United States

Synopsis

This study has demonstrated the feasibility of using hyperpolarized 13C MRI with [13C]t-butanol and [13C,15N2]urea for assessing in vivo perfusion in the cervical spinal cord. T-butanol rapidly crossed the blood-brain-barrier and diffused into spine and brain tissue, while urea predominantly remained in vasculature. The results from this study suggest that this technique may provide unique non-invasive imaging tracers that are able to directly monitor hemodynamic processes in the normal and injured spinal cord.

Introduction

Spinal cord injury (SCI) is a devastating neurological disorder affecting approximately 12,000 individuals in the United States each year1. Secondary injury, which occurs hours to months following initial primary traumatic insult, contributes to metabolic stress and progressive tissue damage, and serves as the main target for therapeutic intervention. Knowledge of spinal cord perfusion is important not only for the understanding of underlying physiology, but also for monitoring treatment. Clinical monitoring of spinal cord perfusion and secondary injury relies on secondary measures such as blood pressure and neurologic exam. Current non-invasive imaging methods for directly assessing spinal cord perfusion are significantly hampered by limited spatial resolution, physiologic motion and magnetic field inhomogeneity related to the bony spine2. Dissolution dynamic nuclear polarization (DNP) enables the acquisition of 13C MR data with a huge gain in sensitivity3. Recent studies using metabolically inactive 13C-labeled hyperpolarized (HP) compounds have demonstrated the feasibility of using this technique as a new perfusion MR method in preclinical cancer models4. The purpose of this study was to explore the feasibility of using HP 13C MRI with [13C]t-butanol and [13C,15N2]urea for evaluating in vivo perfusion of spinal cord in rodents as a first step in preparing for an in vivo study of traumatic injury. To our knowledge this is the first study that has demonstrated the acquisition of HP 13C perfusion imaging from the spinal cord.

Methods

Healthy female Long-Evans rats (n=3) were included in this study. All experiments were performed using a GE 3T scanner with a custom-designed 1H/13C-coil. The cervical lordosis was straightened with padding under the ventral neck to minimize partial voluming with non-spinal tissue (Figure 1D). 95μL [13C]t-butanol mixed with glycerol (50:50 by weight), 15mM trityl radical and 1.0mM Dotarem were polarized using a HyperSense® polarizer and dissolved in 4.5mL phosphate-buffered saline (PBS). 135μL [13C,15N2]urea mixed with glycerol (6M), 15mM trityl radical and 1.5mM Gd-DOTA were polarized using a SpinLab polarizer and dissolved in a mixture of 750μL PBS and ~13.5g of Tris-buffered aqueous solution. 2.7 mL HP [13C]t-butanol (100mM, pH=7.5) solution was injected through tail vein over 10s. At 7s from the start of injection, symmetric, ramp-sampled 13C EPI data were acquired every 4s for a total of 16 datasets (TR=1s, FOV=64×64mm, matrix=32×32, seven 6mm-slice, 30°-flip)5. Following t-butanol data acquisition, 2.7mL HP [13C,15N2]urea (80mM, pH=7.5) solution was injected and data acquired using the same method. T-butanol and urea magnitude signal were overlaid on T2-weighted FSE images and area-under-the-curve (AUC), maximum peak signal and time-to-maximum-signal were calculated for pixels corresponding to spine, supratentorial brain and blood vessels. In addition, preliminary ASL data was acquired using a standard clinical ASL sequence (TR/TE=5686/10.1ms, FOV=14cm, points=512, spiral readout with 8 arms, NEX=16, ten 6mm-slice).

Results and Discussion

Figure 1 shows examples of dynamic 13C t-butanol and urea data from regions around cervical spinal cord at C3-C4 vertebral level (Fig1A), supratentorial brain (Fig1B) and with high vascular signal (Fig1C). Slice locations are indicated on sagittal scout image (Fig1D). T-butanol and urea imaging revealed informative differences in the distribution in different tissues. T-butanol rapidly crossed the blood-brain barrier (BBB) and was freely diffusible in spinal cord and brain (Fig1A,1B). Unlike t-butanol, signal from urea predominantly came from vasculature (Fig1C) and was negligible in spine and brain tissue due to its limited ability to cross BBB6. The maximum SNR of t-butanol and urea in the spine was 16 ± 5 and 5 ± 0.4 (mean±SD), respectively, and the AUC 97 ± 30 and 26 ± 2 (mean±SD), respectively (Table1). The representative time courses of t-butanol and urea signal were plotted over time for different tissues (Fig2). The t-butanol and urea from blood vessels reached their maximum earlier than those from brain and spinal cord (Table1). The map of AUC for HP perfusion agents and preliminary ASL data from regions with different tissues (Fig3) indicate that this novel imaging method may provide complementary information to study perfusion in various tissue types. With the high spatial resolution of this HP 13C data (2x2mm in-plane pixel), it should be possible to visualize and compare hemicontusion lesions with a contra-lateral hemi-cord. The distinct physiological characteristics of t-butanol and urea may be used to estimate the perfusion and permeability of different tissue compartments in future studies with injury models4.

Conclusions

We have demonstrated the feasibility of using HP 13C MRI with [13C]t-butanol and [13C,15N2]urea for assessing in vivo perfusion in the cervical spine of uninjured rats. The results suggest that this technique may provide a unique non-invasive imaging tool that is able to monitor hemodynamic processes underlying spinal cord injury.

Acknowledgements

The first author was supported by a NCI training grant in translational brain tumor research (T32 CA151022) and Kure It Grant for Underfunded Cancer Research. The support for the research studies came from NIH grants R01EB007588, P41EB13598, R00 EB012064, R21CA170148 and R01CA154915.

References

1. Looby S, Flanders A. Spine trauma. Radiol Clin N Am. 2011;49(1):129-63. doi: 10.1016/j.rcl.2010.07.019.

2. Stroman PW, Wheeler-Kingshott C, et al. The current state-of-the-art of spinal cord imaging: methods. Neuroimage. 2014;84:1070-81.

3. Ardenkjaer-Larsen JH, Fridlund B, Gram A, et al. Increase in signal-to-noise ratio of > 10,000 times in liquid-state NMR. Proc Natl Acad Sci USA. 2003;100(18):10158-63.

4. von Morze C, Bok RA, Reed GD, et al. Simultaneous multiagent hyperpolarized (13)C perfusion imaging. Magn Reson Med. 2014;72(6):1599-609.

5. Gordon JW, Machingal S, Kurhanewicz J, et al, Proc. 23rd Intl. Soc. Mag. Reson. Med. 2015;4605.

6. Levin VA. Relationship of octanol/water partition coefficient and molecular weight to rat brain capillary permeability. J Med Chem. 1980;23(6):682-4.

Figures

Figure 1. Hyperpolarized 13C dynamic perfusion images from three axial slices containing spinal cord (A), supratentorial brain (B) and blood vessels (C) in a rat. T-butanol rapidly diffused into spine and brain tissue, while urea remained in vasculature. Slice locations are indicated on sagittal image (D).

Table 1. Summary of 13C t-butanol and urea quantification. All values are reported as mean ± SD.

Figure 2. The plots of hyperpolarized 13C t-butanol and urea signal over time exhibit different perfusion characteristics for spinal cord, supratentorial brain and blood vessel.

Figure 3. The area-under-the-curve from 13C t-butanol and urea imaging and ASL data overlaid on T2 FSE images for spinal cord, brain and blood vessel. The preliminary comparison with ASL data indicates that the hyperpolarized 13C t-butanol and urea perfusion data may provide complementary information to study hemodynamic properties of spinal cord injury.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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