Proceedings of the meeting - Department of Physics - University of ...

P35VALIDATION OFDNP-MR (DYNAMIC NUCLEAR POLARISATION-MAGNETIC RESONANCE)MEASUREMENT OF SUBSTRATE METABOLISM AND UPTAKE IN HEARTLowri Cochlin, Marie Schroeder, Damian Tyler, George Radda, Kieran ClarkeCardiac Metabolism Research Group, Department of Physiology, Anatomy and Genetics, Sherrington Building, University of Oxford, UK.Introduction“Molecular Imaging”, the coupling of imaging technologies withspecific molecular probes may revolutionize detection, diagnosisand treatment of disease.Whilst MRI is widely used in healthcare for measuring structureand function, the application of MR for metabolic imaging is limitedby intrinsically low sensitivity originating from the low magneticenergy of nuclear spins compared with their thermal energyat room temperature. In most MRI, the weak nuclear polarizationis compensated by the high concentration of hydrogen nuclei inbiological samples. Unfortunately, magnetic nuclei such as 13 Chave low natural abundance yet, such nuclei can provide considerablemetabolic information. To this end, a method for obtaininghighly polarized nuclear spins in solution has been designed byArdenkjær-Larsen et al 1 , opening up the possibility of imaging 13 Clabelled metabolites in-vivo and more importantly their conversionto other species. Ardenkjær-Larsen et al 1 built upon the theory ofDynamic Nuclear Polarisation (DNP) 2 ; a method of enhancingnuclear polarization in the solid state, to perform DNP-MRS andDNP-MRI.Here we have used DNP coupled with Ardenkjær-Larsen’s dissolutionmethod 1 to produce hyperpolarized for study in-vitro and invivo.Surface coil localised spectroscopy has been used monitorthe uptake and conversion of 13 C labelled pyruvate in the heart andchest of rats.MethodsIn this study 13 C 1 -pyruvic acid (isotope enriched to 99%) waspolarized at 1.2 K using microwaves at 93.88 GHz and 150 mW.Polarization build-up was monitored over 60 minutes until steadystatepolarization was reached.Hyperpolarized 13 C 1 -pyruvic acid was dissolved with sufficientaqueous solution of NaOH and TRIS buffer to yield a 79 mMsolution of 13 C 1 -pyruvate with pH 7.6 at approximately 38°C. Thedissolved solution was injected into an empty glass vessel insidethe magnet for in-vitro experiments, or via a rat tail vein cannula tostudy uptake in-vivo. Spectroscopy was performed in a 7T horizontalbore magnet interfaced to a Varian Inova console (VarianInc., Palo Alto, CA) and a butterfly RF coil.For in-vitro experiments, spectra were acquired with one 12° pulseevery 2 seconds for 2 minutes from the beginning of injection. Forin-vivo experiments, spectra were acquired with 5° pulses every 3seconds for 1 minute from the heart and chest region of four femaleWistar rats. All investigations conformed to Home OfficeGuidance on the Operation of the Animals (Scientific Procedures)Act, 1986 (HMSO) and to institutional guidelines.ResultsHyperpolarized 13 C 1 -pyruvate was injected into a glass vial toobserve the hyperpolarized signal, followed by a multi-averagedacquisition to determine the signal available when the sample hasreturned to thermal equilibrium. Figure 1 shows the 22100 timessignal enhancement achieved after dissolution compared to thermalequilibrium polarization. This corresponds to 0.0006% thermalpolarization, and 13.3% hyperpolarized polarization.Dissolution, transport and injection were achieved within 25 seconds,including 5-10 second injection duration. Accumulation ofhyperpolarized tracer and subsequent decay of signal was observedby real-time acquisition of spectra.Hyperpolarized 13 C 1-pyruvateThermal equilibrium 13 C 1-pyruvateFigure 1: Top: thermal equilibrium spectrum of 13 C 1 -pyruvateSNR 1.1 in 3 hours (SNR 0.025 corrected for 2048 averages)Bottom: hyperpolarized SNR 480 in 0.5 seconds (no averages).Figure 2 shows the arrival (during injection) and decay of thepyruvate signal injected into a glass vial. Figure3 shows the bolusof polarized pyruvate arriving at the heart and its subsequent metabolisminto lactate, alanine and bicarbonate.Peak heightexperimentalcalculated0 20 40 60 80 100 120 Time / sFigure 2: 12° pulse, 0.5 second acquisition (2048 complex points)repeated every 2 seconds for 2 minutes. Injection duration was 10seconds.Pyruvate-hydrateLactateTime (every 3s)AlaninePyruvateBicarbonate10 5 0 -5 -10Frequency (ppm)Figure 3: Heart and chest spectra, 5° pulse, 0.5second acquisition(2048 complex points) repeated every 3 seconds for 1 minute.Injection of 1 cm 3 over 8 seconds into the tail vein of a femaleWistar rat.ConclusionWe have demonstrated high levels of polarisation in-vitro and invivowhich has enabled us to visualise the uptake of polarizedpyruvate in-vivo and its metabolism to lactate, alanine and bicarbonatein real-time. Future work will focus on spectroscopic imaging,kinetic modelling of the metabolic processes demonstrated inFigure 3, and the eventual study of real-time metabolism in normaland diseased heart.References1. Ardenkjaer-Larsen, J.H., et al. Proc Natl Acad Sci U S A,2003. 100(18): p. 10158-63.2. Jeffries. Phys Rev. 106, 164-165 (1957).Acknowledgements:This study was supported by the British Heart Foundation andGeneral Electric.