Introduction

Nuclear magnetic resonance (NMR) has an inherently low sensitivity. Hyperpolarization generally refers to methods that aim at improving the sensitivity of NMR by creating non-equilibrium nuclear spin distribution on the Zeeman levels. The hyperpolarized (HP) magnetization will decay according to T₁ and the time constraint emerging from the inevitable decay of polarization is a severe limitation of HP-NMR technology. ⁸⁹Y (½ spin g = -2.0864 MHz/T, 100 % natural abundance) has one of the longest T₁ values among the NMR active nuclei. The long T₁ of ⁸⁹Y (up to 600 s) translates into a much longer polarization lifetime than that of hyperpolarized 13C allowing the observation of chemical or biological processes on a longer timescale. ⁸⁹Y is a very attractive nucleus for the design of responsive magnetic resonance spectroscopy and imaging probes because of the sensitivity of the ⁸⁹Y NMR chemical shift to changes in the coordination environment of the Y³⁺ ion. Most importantly, Y³⁺ is a pseudolanthanide (its ionic radius is about the same as that of Ho³⁺) and so ligands developed for conventional Gd(III)-based (T₁-shortening) or Eu(III)-based (PARACEST) MRI agents can directly be applied with Y(III). Earlier we have shown that hyperpolarized ⁸⁹Y-DOTP, (DOTP = 1,4,7,10-tetraazacyclodecane-1,4,7,10-tetramethylenephosphonic acid) can be used as a ⁸⁹Y MR spectroscopy probe to measure pH (1). The low gyromagnetic ratio makes ⁸⁹Y one of the most challenging nuclei for conventional NMR and MRI. However, we have shown that ⁸⁹Y-DOTA (DOTA = 1,4,7,10-tetraazacyclodecane-1,4,7,10-tetraacetic acid, Fig.1) can be hyperpolarized by dissolution dynamic nuclear polarization (d-DNP) using commercially available hardware. We have achieved over 60,000-fold ⁸⁹Y NMR signal enhancement of ⁸⁹Y-DOTA after optimization of the DNP conditions. The ⁸⁹Y T1 was found to be 499 s at 9.4 T. (2, 3). Despite its great potential, HP ⁸⁹Y has not been tested in vivo. The goal of this project was to study the feasibility of detecting an ⁸⁹Y NMR signal in vivo after the injection of HP ⁸⁹Y-DOTA into rats.

Methods

Both in vitro phantom study and in vivo MR spectroscopy were performed at Agilent 4.7T animal MR scanner. We updated the ‘macroprescan_tn’ file to match the nuclear and atomic number of ⁸⁹Y nucleus. A custom-built ⁸⁹Y slotted surface coil (4) (Ø = 60 mm, 9.806 MHz) was placed under the phantom or the abdominal area of the rats for both radiofrequency (RF) excitation and data acquisition. In order to obtain the 9.806 MHz signals, the preamplifier was linked with a quarter wave cable. First, a glass-vial phantom containing gadolinium-doped (ProHance, 5 mM) ⁸⁹YCl₃ (2 M) was used for testing and calibrating the system. For in vivo studies, ⁸⁹Y-DOTA was synthesized as sodium salt (Fig.1). A HyperSense polarizer (3.35T, Oxford Instruments Molecular Biotools, UK) was used for ⁸⁹Y-DNP. ⁸⁹Y-DOTA (0.5 M in 3:1 w/w water:glycerol matrix doped with trityl OX063 polarizing agent at 15-mM) was polarized for 3 hours followed by dissolution with 4 mL of superheated water to give a solution of HP-⁸⁹Y-DOTA (11 mM final concentration). The HP-⁸⁹Y-DOTA solution was administered to healthy male Fischer rats (~200 g) by intravenous injection through the tail vein as a bolus (0.125 mmol/kg body weight, up to 4.0 mL, injection rate = 0.25 mL/s), immediately followed by a dynamic ⁸⁹Y MRS scan (pulse-and-acquire with 90^o hard pulse RF excitation and repetition time = 10 sec).

Results and Discussion

We designed a slotted surface radiofrequency (RF) coil for imaging ⁸⁹Y signal in a 4.7 T animal MR system. The coil has improved penetration and improved signal to noise ratio over the area of interest compared to regular surface coils (4). Due to the relatively long distance between the polarizer and the scanner and the effects of spatially fluctuating fringe magnetic fields from several high-field systems along the delivery path, relatively weak HP ⁸⁹Y signals were detected in both in vitro and in vivo experiments at the 4.7 T scanner. The chemical shift of ⁸⁹YCl₃ was set as reference (0 ppm). The capability of detecting ⁸⁹Y signals at the scanner was demonstrated by the chemical shift imaging (CSI) of thermally-polarized ⁸⁹YCl₃ phantom (FOV = 65 x 65 mm², matrix = 8 x 8, nominal flip-angle = 20^o, 10 averages), Fig 2. After confirming that HP ⁸⁹Y-DOTA could be detected at 4.7 T with the slotted surface coil with 10° pulse (Fig.3), we managed to detect in vivo signal of hyperpolarized ⁸⁹Y-DOTA at 109 ppm from rat abdominal area with 90° pulse (Fig.4).

Conclusion

In summary, we developed a slotted surface coil for detecting ⁸⁹Y resonance in a 4.7 T animal MR scanner, and showed ⁸⁹Y signal could be detected in phantoms containing thermally polarized ⁸⁹YCl₃ and HP ⁸⁹Y-DOTA solutions. We also demonstrated that in vivo HP ⁸⁹Y signal could be observed after the injection of HP-⁸⁹Y-DOTA into a rat. These results pave the way for the in vivo testing of responsive ⁸⁹Y complexes

Acknowledgements

UT Dallas Collaborative Biomedical Research Award; The Welch Foundation (I-2009-20190330); National Institutes of Health of the United States (P41 EB015908, S10 OD018468); The Texas Institute of Brain Injury and Repair.