The Krebs tricarboxylic acid cycle (TCA) is central to metabolic energy production and is known to be altered in many disease states. Real time molecular imaging of TCA cycle in vivo will be important in understanding the metabolic basis of several diseases. Positron emission tomography (PET) using FDG-glucose (2-[18F]fluoro-2-deoxy-D-glucose) is already being used as a metabolic imaging agent in clinics. However, FDG-glucose does not reveal anything past glucose uptake and phosphorylation. We have developed a new metabolic imaging agent, hyperpolarized diethyl 1-13C 2,3-d2 succinate, that allows for real time in vivo imaging and spectroscopy of the TCA cycle. Diethyl succinate can be hyperpolarized using parahydrogen induced polarization (PHIP) in an aqueous solution with signal enhancement of 5000 compared to Boltzmann polarization. 13C magnetic resonance spectroscopy (MRS) and magnetic resonance imaging (MRI) were achieved in vivo seconds after injection of 10 to 20 μmol of hyperpolarized diethyl succinate into normal mice. The downstream metabolites of hyperpolarized diethyl succinate were identified in vivo as malate, succinate, fumarate and aspartate. The metabolism of diethyl succinate was altered after exposing the animal to 3-nitropropionate, a known irreversible inhibitor of succinate dehydrogenase. Based on our results, hyperpolarized diethyl succinate allows for in real time in vivo MRI and MRS with a high signal to noise ratio and with visualization of multiple steps of the TCA cycle. Hyperpolarization of diethyl succinate and its in vivo applications may reveal an entirely new regime wherein the local status of TCA cycle metabolism is interrogated on the time scale of seconds to minutes with unprecedented chemical specificity and MR sensitivity.

Background Magnetic resonance (MR) techniques using hyperpolarized 13C have successfully produced examples of angiography and intermediary metabolic imaging, but to date no receptor imaging has been attempted. The goal of this study is to synthesize and evaluate a novel hyperpolarizable molecule, tetrafluoropropyl 1-13C-propionate-d3 (TFPP), for detecting atheromatous plaque in vivo. TFPP binds to lipid bilayers and its use in hyperpolarized MR could prove to be a major step towards receptor imaging. Results The precursor, Tetrafluoropropyl 1-13C-acrylate (TFPA) binds to dimyristoylphosphatidylcholine (DMPC) lipid bilayers with a 1.6 ppm chemical shift in the 19F MR spectrum. This molecule was designed to be hyperpolarized through addition of parahydrogen to 13C acrylate moiety by Parahydrogen Induced Polarization (PHIP). 13C TFPA was hyperpolarized to Tetrafluoropropyl 1-13C-propionate (TFPP) to a similar extent to that of hydroxyethylacrylate (HEA) to hydroxyethylpropionate (HEP); 17% +/− 4 % for TFPP vs 20% for HEP; T1 relaxation times (45s ± 2 vs 55s ± 2) were comparable and the hyperpolarized properties of TFPP were characterized. HEA, like TFPA has a chemical structure with an acrylate moiety but do not have the lipid binding Tetrafluoropropyl functional group. Hyperpolarized 13C TFPP binds to lipid bilayer appearing as a second, chemically shifted 13C hyperpolarized MR resonance with further reduction in longitudinal relaxation time (T1 = 21s ± 1). In aortas harvested from Low Density Lipoprotein Receptor (LDLR) knock-out mice fed with a high fat diet for nine months, and in which atheroma is deposited in aorta and heart, 13C TFPP showed greater binding to lipid on the intimal surface than in normal diet control mice. When 13C TFPP was hyperpolarized and administered in vivo to atheromatous mice in a pilot study, increased binding was observed on the endocardial surface of the intact heart compared to normal fed controls. Conclusions Hyperpolarized 13C TFPP has bio-sensing specificity for lipid, coupled with 42,000 fold sensitivity gain in MR signal at 4.7 Tesla. Binding of TFPP with lipids results in the formation of a characteristic second peak in MR spectroscopy. TFPP therefore has the potential to act as an in vivo molecular probe for atheromatous plaque imaging and may serve as a model of receptor targeted bioimaging with enhanced MR sensitivity.

Silicon-based micro- and nanoparticles have gained popularity in a wide range of biomedical applications due to their biocompatibility and biodegradability in vivo, as well as their flexible surface chemistry, which allows drug loading, functionalization and targeting. Here, we report direct in vivo imaging of hyperpolarized (29)Si nuclei in silicon particles by magnetic resonance imaging. Natural physical properties of silicon provide surface electronic states for dynamic nuclear polarization, extremely long depolarization times, insensitivity to the in vivo environment or particle tumbling, and surfaces favourable for functionalization. Potential applications to gastrointestinal, intravascular and tumour perfusion imaging at subpicomolar concentrations are presented. These results demonstrate a new background-free imaging modality applicable to a range of inexpensive, readily available and biocompatible silicon particles.

Purpose:To assess the feasibility of a perfusion magnetic resonance (MR) imaging technique that uses Overhauser dynamic nuclear polarization (DNP) to provide contrast during the continuous delivery of hyperpolarized water in rats. Materials and Methods:Protocols approved by the local institutional animal care and use committees were followed. Twelve male Wistar rats were anesthetized and prepared by placing injection tubing in the subcutaneous layer (n = 3), peritoneum (n = 3), aorta (n = 3), or carotid artery (n = 3). Water was hyperpolarized by means of Overhauser DNP in the 0.35-T fringe field of a 1.5-T MR imaging magnet by using a custom-built system to continuously deliver radical-free hyperpolarized water to the subject. Fast gradient-echo and spoiled gradient-recalled-echo MR imaging sequences were used. The signal-to-noise ratio (SNR) of the images was calculated and compared. Results:Images showed greatly altered SNR and enhanced flow contrast at all injection locations. For subcutaneous and intraperitoneal injections, the water perfusion trajectory was observed for approximately 5 seconds after injection. Flow through a 4.2-cm length of artery was seen during intra-aortic injection. The right hemisphere of the brain was seen during injection into the right carotid artery. Images with hyperpolarized water had greatly altered SNR compared with images without injection or with the injection of nonhyperpolarized water, with a range of 13%-27% for the carotid and 444%-2900% for the other regions. Conclusion:Perfusion contrast for MR imaging can be obtained by continuously infusing hyperpolarized water, providing localized angiography or brain perfusion information in vivo for rat models.q RSNA, 2012Supplemental material: http://radiology.rsna.org/lookup /suppl

One of the most prolific chemical and anatomical imaging techniques of recent decades, magnetic resonance imaging (MRI), includes the ability to noninvasively assess neurochemical changes with magnetic resonance spectroscopy (MRS). Practical concerns are paramount in applying MRS, such as what the manufacturer provides with a routine MRI scanner, what methods are well tolerated by patients, and what has proved most diagnostically productive over a 25 year span of preliminary exploration of the technology. In this review, the authors explain the technical and neurochemical aspects of MRS and critically discuss its clinical neuroimaging applications.