Published!: Rapid Cardiac Imaging with Hyperpolarized Carbon-13
Each month we will review/feature an article published by one of our very own MBP students in the biology and/or physics stream. This article showcases the research of Angus Lau from Dr. Chuck Cunningham's group [Link to Paper].
At the foundation of all biological molecules, carbon is indeed one of the most interesting and important nuclei that can be studied with MR. Unfortunately, carbon suffers from poor MR sensitivity: its MR-active 13C isotope is only 1% naturally abundant and its small magnetogyric ratio affords only a small population difference at ambient temperature. Dynamic nuclear polarization (DNP) is a technique that transfers spin alignment from electrons in free radicals to carbon nuclei. By increasing the 13C population difference in this way, MR signals can be enhanced by up to four orders of magnitude! The catch is that this non-equilibrium polarization decays irreversibly as the spins relax back to their Boltzmann configuration.
Real time in vivo imaging of metabolic intermediates is possible with the signal enhancement afforded by DNP. Metabolic imaging of the heart is of particular interest because metabolic changes often precede histological changes, potentially allowing earlier detection and characterization of diseased tissue in the heart. By injecting [1-13C]-pyruvate (99% 13C-enriched at the carboxyl carbon) intravenously, its conversion into lactate and bicarbonate in the heart can be monitored and mapped in 3D with 13C MR spectroscopic imaging.
One challenge with conventional whole-heart 3D MRI is the long scan time required for the large number of cardiac-gated excitations necessary for high resolution of the full volume, which must be completed within short breath-held periods. Faster regimes must be used so that the whole heart can be imaged before too much polarization in the 13C-labeled metabolites is lost. This paper demonstrates the successful implementation of large flip-angle spectral-spatial selective excitation acquired in a single-shot spiral k-space trajectory to obtain spatial mappings of pyruvate and bicarbonate spectra in 6 slices spanning the full volume of the heart. Compared to one study which required a 13-second acquisition for a single slice, this new pulse sequence allows imaging of the whole heart (6 slices) in ~6 seconds with comparable resolution. The shorter scan time allows three repetitions within a single 20-second breath-hold to improve signal-to-noise. Dynamic imaging was also performed with smaller flip angles to follow the relative concentration and spatial distribution of metabolites. In addition to extending the role of hyperpolarized 13C in cardiac metabolic imaging, this novel imaging methodology shows great promise for improving the feasibility of whole-heart MRI and was featured on the cover of this month's issue of Magnetic Resonance in Medicine.