An Overhauser‐enhanced‐MRI platform for dynamic free radical imaging in vivo

Abstract

Overhauser-enhanced MRI (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 (ULF), as both RF power absorption and penetration issues-a consequence of the high resonance frequencies of electron spins-are mitigated. However, working at ULF 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 ULF 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.

High-sensitivity in vivo contrast for ultra-low field magnetic resonance imaging using superparamagnetic iron oxide nanoparticles

“…Further approaches to increasing SNR and CNR at ULF include the use of sensitive superconducting quantum interference devices (SQUIDs) ( 45 ) or hyperpolarization techniques that can boost nuclear spin polarizations by orders of magnitude ( 46 , 47 ). Both of these approaches face substantial challenges; SQUID-based MRI typically requires a time-consuming prepolarization step ( 19 ), and the toxicity of hyperpolarization agents must be considered ( 27 , 48 , 49 ).…”

“…The coil was tuned to 276 kHz using an external resonator board (parallel tune—series match) and resistively broadened to an s 21 bandwidth of 4.7 kHz. This bandwidth allows a maximum 11-cm field of view (FOV) when imaging hydrogen nuclei with our 1 mT/m gradient set ( 49 ).…”

Dynamic nuclear polarisation of liquids at one microtesla using circularly polarised RF with application to millimetre resolution MRI

“…In Overhauser dynamic nuclear polarisation (O-DNP), the polarisation of electron spins belonging to free radicals is transferred to that of nuclear spins of the target molecules in a solution [24]. However, O-DNP has hardly been used at a magnetic field range below a few millitesla [20,25,26,27], because its effectiveness at enhancing the thermal polarisation significantly degrades in ultra-low fields. When applying the RF for O-DNP, positive and negative enhancements occur simultaneously, and in turn cancel each other out [19,28].…”

Low‐field MRI: An MR physics perspective

“…In this article we reviewed some of the current trends in imaging with low‐field MR scanners that use standard linear gradient encoding for image formation, and standard transmit and receive methods. We have not considered more experimental arrangements such as, for example, a gradient‐less MR system, using ultralow‐field measurements combined with squid detection, use of Overhauser‐enhanced MRI, or fast field cycling approaches . Another avenue that has not been discussed here, but which has potential in low‐field scanners, is the use of specialized contrast agents, which, due to the field‐dependent relaxometry parameters, can show increased T 1 enhancement at lower fields …”