Sensitive Molecular Magnetic Resonance Imaging Of Hyperpolarized Contrast Agents In Low Magnetic Fields

Abstract

Nuclear spin polarization <em>P</em> is a key factor in overall Magnetic Resonance (MR) sensitivity, and conventionally is of order 10^-6 owing to the tyranny of its induction by the strength of the detection magnetic field. But various hyperpolarization mechanisms applied externally to the detection field can temporarily increase nuclear spin polarization to near unity (<em>P</em> = 1). The resulting increased MR signal enables a variety of applications, including biomedical use of hyperpolarized (HP) contrast agents to assay cellular metabolism via Magnetic Resonance Imaging (MRI), typically ¹³C-labeled metabolites reporting on abnormal metabolism. In this work optimization of radiofrequency (RF) coils and hyperpolarizer automation are used to increase the detection sensitivity of hyperpolarized contrast agents (HCA) and improve their production. It is shown that low-field imaging can be more sensitive than corresponding high-field detection when using RF coils optimized to the resonant frequency. The feasibility of low-field molecular imaging of ¹H and ¹³C HCA with high spatial resolution (as fine as 94×94 μm²) is demonstrated with low-field 38 mm inner diameter RF coils at a static magnetic field strength <em>B</em>₀ = 0.0475 T, achieving signal-to-noise ratios suitable for <em>in vivo</em> imaging studies.