Probes and Imaging

All dynamic nuclear polarization (DNP) experiments were performed with one of three double resonant probes. All probes are based on orthogo- nal solenoid (NMR) and Alderman-Grant (EPR) resonators. Spectroscopic probe A is optimized for a large electron drive field (B1e), with a high fill

factor Alderman Grant resonator at 190 MHz and external solenoid at 276 kHz and was used for all spectroscopic measurements at 6.5 mT. Spectro- scopic probe B, used for field sweeps was optimized for high NMR sensitivity

over a wide range of frequencies, with an Alderman Grant resonator at 140 MHz. An imaging probe with a 33 mm diameter sample region, shown in Fig. 6.10(a) was built for ND Overhauser-enhanced MRI (OMRI) at B0= 6.5 mT

(fH = 276 kHz, fEPR = 190 MHz). A photograph of the phantom used for

imaging in Fig. 6.5 is shown in Fig. 6.10(b) All imaging was performed in our 6.5 mT open-access, human imaging scanner [256] with the OMRI balanced steady-state free precession (bSSFP) sequence shown in Fig. 6.10(c) [248].

Figure 6.10: Details of OMRI setup. (a) Double resonant Overhauser- enhanced MRI (OMRI) imaging probe. The external solenoid was used for nuclear magnetic resonance (NMR) acquisition at 276 kHz. An internal Alderman-Grant resonator was used to saturate the electron paramagnetic resonance (EPR) resonance at 190 MHz. (b) Phantom with vials of ND solution and vials of water, used for imaging in Fig. 6.5. Scale bar is 20 mm in length. (c) Balanced steady-state free precession (bSSFP) OMRI imaging sequence used for imaging. Relative timing of NMR pulses, EPR pulses, readout gradient (GRO) and phase encode gradient (GPE) are shown.

Imaging parameters were: repetition time (TR) = 86 ms, echo time (TE ) = 43 ms, acquisition time (Tacq) = 28 ms, phase encode time (TPE) = 22.5 ms

A Platform for in vivo

Overhauser-enhanced MRI

7.1

Abstract

Overhauser-enhanced Magnetic Resonance Imaging (OMRI) is an electron- proton double resonance imaging technique of interest for its ability to non- invasively measure the concentration and distribution of free radicals. In vivo OMRI experiments are typically undertaken at ultra-low magnetic field, as both RF power absorption and penetration issues—a consequence of the high resonance frequencies of electron spins—are mitigated. However, working at ultra-low magnetic fields causes a drastic reduction in MRI sensitivity. Here, we report on the design, construction and performance of an OMRI platform optimized for high NMR sensitivity and low RF power absorbance, exploring challenges unique to probe design in the ultra-low magnetic field regime. We use this platform to demonstrate dynamic imaging of TEMPOL in a rat model. The work presented here demonstrates improved speed and sensitivity of in vivo OMRI, extending the scope of OMRI to the study of dynamic processes such as metabolism.1

1_{Material in this chapter related to the rat head probe is adapted from}

D. E. J. Waddington et. al., An Overhauser-enhanced MRI platform for dynamic free radical imaging in vivo, NMR Biomed., 31, e3896 (2018).

7.2

Introduction

Free radicals play crucial roles in the maintenance of tissue health and in the pathogenesis of diseases including diabetes [276], ischemia-reperfusion injuries [277] and cancer [278]. Improvements to accurate diagnosis and ap- propriate treatment of diseases in which free-radicals play a role require the development of noninvasive methods for mapping the distribution of free radicals in vivo. Electron paramagnetic resonance (EPR) is the gold standard of direct free radical detection, enabling unambiguous identifica- tion of the unpaired electrons inherent to radical species, but short elec- tronic spin-spin lifetimes (T2e) limit resolution in EPR-based imaging ap-

proaches [279, 280]. Overhauser-enhanced MRI (OMRI) overcomes the reso- lution limits of EPR imaging by indirectly imaging free radicals with high- resolution MRI [78, 79, 281]. In OMRI, the Overhauser effect is used to transfer large spin polarizations from radical electrons to dipolar-coupled 1_H

nuclei, with subsequent 1_{H MRI used to image the free radical distribution}

via enhanced 1_{H spin polarizations.}

While progress in OMRI has seen free radicals used as bioprobes of tissue oxygenation, metabolism and viscosity [250, 282–285], sensitivity limits have prevented widespread use. The sensitivity of OMRI is limited in comparison to clinical MRI, as the high gyromagnetic ratio of electrons (28 GHz/T) means that OMRI is typically performed at ultra-low magnetic fields (ULF, < 10 mT) to reduce the specific absorbance rate (SAR) to safe levels during the application of RF EPR saturation pulses [157, 286]. Overcoming the tradeoff between SAR and nuclear magnetic resonance (NMR) sensitivity has thus been a focus of OMRI development [287]. OMRI probes based on single- loop surface coil resonators minimize SAR by restricting the size of the EPR coil but necessarily result in an inhomogeneous Overhauser enhancement profile due to spatial variation in B1e [288, 289]. Volume resonators built

for EPR saturation can yield high B1e homogeneity, but compromises often

need to be made to minimize SAR and maintain the quality factor (Q-factor) of the NMR detection coil [289–291]. With another approach, field-cycled OMRI, the applied magnetic field is rapidly ramped to allow EPR saturation

at ULF followed by high efficiency NMR acquisition at near-clinical field strengths [157]. These field-cycled scanners improve OMRI sensitivity at the cost of significantly more complex hardware, but are slowed by the need to refresh the Overhauser-enhanced signal between acquisitions.

Recent advances in balanced steady-state free precession (bSSFP) MRI sequences at ULF have increased the speed and sensitivity of OMRI [246, 248, 256, 292, 293], raising the possibility of in vivo radical imaging with high temporal resolution. A stable radical species of interest for dynamic track- ing with OMRI is TEMPOL (4-hydroxy-TEMPO). TEMPOL is a neuropro- tective antioxidant [294] whose permeability across the blood brain barrier increases in cases of oxidative stress [295, 296]. As TEMPOL reduction has previously been used to monitor redox status in animal models, dynamic tracking of exogenously administered TEMPOL may prove a valuable tool for monitoring neurological diseases in which oxidative stress plays a key role, such as ischemia-reperfusion injury and Alzheimer’s disease [297].

Here we report on an OMRI platform designed for dynamic imaging of TEMPOL in a rat model at ULF. Simulations and experimental results are presented to evaluate the performance of a custom OMRI probe based on a modified Alderman-Grant resonator and designed for high sensitivity, en- hancement, homogeneity, and low SAR. Further, we leverage a highly efficient 3D bSSFP OMRI sequence to image TEMPOL in vitro at concentrations as low as 10 µM. We conclude this work with an in vivo demonstration of our platform, presenting brain images of TEMPOL in a rat model. This platform will enable tracking of exogenously administered radicals to study the role of oxidative stress in neurological diseases.