Hyperpolarized Imaging

Hyperpolarizing Concentrated Metronidazole 15NO2 Group Over Six Chemical Bonds with More Than 15% Polarization and 20 Minute Lifetime

NMR hyperpolarization of uniformly 15N-labeled metronidazole-15N3 is demonstrated using SABRE-SHEATH (Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). In this antibiotic, a 15NO2 group is hyperpolarized via spin relays created by 15N spins in metronidazole-15N3, and the polarization is transferred from parahydrogen-derived hydrides over six chemical bonds. This efficient spin-relayed mechanism of polarization transfer is supported by measurements of polarization dynamics and T1 relaxation in micro-Tesla magnetic fields. In less than a minute of parahydrogen bubbling at ~0.4 μT, a high level of nuclear spin polarization P15N of ~16% is achieved on all three 15N sites of metronidazole-15N3 at up to ~41 mM concentration. This product of 15N polarization and concentration of 15N spins is ~6-fold better than any previous value for 15N SABRE-derived hyperpolarization. At 1.4 T, the hyperpolarized state of the 15NO2 group of this antibiotic persists for tens of minutes (T1~10 min), in part because its 15N spin has no detectable spin-spin couplings with protons of the molecule. A novel synthesis achieving robust uniform 15N enrichment of metronidazole is reported with 15% yield over three steps. This synthetic approach should be suitable for preparation of other 15N-labeled nitroimidazole derivatives, potentially enabling a wide range of in vivo metabolic probes from hypoxia sensing to theranostic imaging.

Dynamic metabolic imaging of hyperpolarized [2-13C]pyruvate using spiral CSI with alternating spectral band excitation

Hyperpolarized [2-13C] pyruvate permits the ability to follow the 13C label beyond flux through pyruvate dehydrogenase complex and investigate the incorporation of acetyl-CoA into different metabolic pathways. However, chemical shift imaging (CSI) with [2-13C] pyruvate is challenging due to the large spectral dispersion of the resonances. In this work, we presented a method that allows CSI of widely separated resonances without chemical shift displacement artifact, acquiring multiple frequency bands alternately to obtain dynamic time-course information. This approach enables robust imaging of downstream metabolic products of acetyl-CoA with hyperpolarized [2-13C] pyruvate.

Metabolic response of glioma to dichloroacetate measured in vivo by hyperpolarized (13)C magnetic resonance spectroscopic imaging​

The metabolic phenotype that derives disproportionate energy via glycolysis in solid tumors, including glioma, leads to elevated lactate labeling in metabolic imaging using hyperpolarized [1-13C]pyruvate. Although the pyruvate dehydrogenase (PDH)–mediated flux from pyruvate to acetyl coenzyme A can be indirectly measured through the detection of carbon-13 (13C)-labeled bicarbonate, it has proven difficult to visualize 13C-bicarbonate at high enough levels from injected [1-13C]pyruvate for quantitative analysis in brain. The aim of this study is to improve the detection of 13C-labeled metabolites, in particular bicarbonate, in glioma and normal brain in vivo and to measure the metabolic response to dichloroacetate, which upregulates PDH activity.

Measuring mitochondrial metabolism in rat brain in vivo using MR Spectroscopy of hyperpolarized [2-¹³C]pyruvate

Hyperpolarized [1-(13) C]pyruvate ([1-(13) C]Pyr) has been used to assess metabolism in healthy and diseased states, focusing on the downstream labeling of lactate (Lac), bicarbonate and alanine. Although hyperpolarized [2-(13) C]Pyr, which retains the labeled carbon when Pyr is converted to acetyl-coenzyme A, has been used successfully to assess mitochondrial metabolism in the heart, the application of [2-(13) C]Pyr in the study of brain metabolism has been limited to date, with Lac being the only downstream metabolic product reported previously. In this study, single-time-point chemical shift imaging data were acquired from rat brain in vivo. [5-(13) C]Glutamate, [1-(13) C]acetylcarnitine and [1-(13) C]citrate were detected in addition to resonances from [2-(13) C]Pyr and [2-(13) C]Lac. Brain metabolism was further investigated by infusing dichloroacetate, which upregulates Pyr flux to acetyl-coenzyme A. After dichloroacetate administration, a 40% increase in [5-(13) C]glutamate from 0.014 ± 0.004 to 0.020 ± 0.006 (p = 0.02), primarily from brain, and a trend to higher citrate (0.002 ± 0.001 to 0.004 ± 0.002) were detected, whereas [1-(13) C]acetylcarnitine was increased in peripheral tissues. This study demonstrates, for the first time, that hyperpolarized [2-(13) C]Pyr can be used for the in vivo investigation of mitochondrial function and tricarboxylic acid cycle metabolism in brain.

Hyperpolarized 13C metabolic imaging using dissolution dynamic nuclear polarization

This article describes the basic physics of dissolution dynamic nuclear polarization (dissolution‐DNP), and the impact of the resulting highly nonequilibrium spin states, on the physics of magnetic resonance imaging (MRI) detection. The hardware requirements for clinical translation of this technology are also presented. For studies that allow the use of externally administered agents, hyperpolarization offers a way to overcome normal magnetic resonance sensitivity limitations, at least for a brief T1‐dependent observation window. A 10,000–100,000‐fold signal‐to‐noise advantage provides an avenue for real‐time measurement of perfusion, metabolite transport, exchange, and metabolism. The principles behind these measurements, as well as the choice of agent, and progress toward the application of hyperpolarized 13C metabolic imaging in oncology, cardiology, and neurology are reviewed.

Multi-Channel Metabolic Imaging, with SENSE reconstruction, of Hyperpolarized [1-13C] Pyruvate in a Live Rat at 3.0 tesla on a Clinical MR Scanner

We report metabolic images of 13C, following injection of a bolus of of hyperpolarized [1-13C] pyruvate in a live rat. The data were acquired on a clinical scanner, using custom coils for volume transmission and array reception. Proton blocking of all carbon resonators enabled proton anatomic imaging with the system body coil, to allow for registration of anatomic and metabolic images, for which good correlation was achieved, with some anatomic features (kidney and heart) clearly visible in a carbon image, without reference to the corresponding proton image. Parallel imaging with sensitivity encoding was used to increase the spatial resolution in the SI direction of the rat. The signal to noise ratio in was in some instances unexpectedly high in the parallel images; variability of the polarization among different trials, plus partial volume effects, are noted as a possible cause of this

Kinetic modeling of hyperpolarized 13C1-pyruvate metabolism in normal rats and TRAMP mice

The purpose of the study was to investigate metabolic exchange between 13C1-pyruvate, 13C1-lactate, and 13C1-alanine in preclinical model systems using kinetic modeling of dynamic hyperpolarized 13C spectroscopic data and to examine the relationship between fitted parameters and dose–response.

T2 relaxation times of 13C metabolites in a rat hepatocellular carcinoma model measured in vivo using 13C‐MRS of hyperpolarized [1‐13C]pyruvate

NMR hyperpolarization of uniformly 15N-labeled metronidazole-15N3 is demonstrated using SABRE-SHEATH (Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). In this antibiotic, a 15NO2 group is hyperpolarized via spin relays created by 15N spins in metronidazole-15N3, and the polarization is transferred from parahydrogen-derived hydrides over six chemical bonds. This efficient spin-relayed mechanism of polarization transfer is supported by measurements of polarization dynamics and T1 relaxation in micro-Tesla magnetic fields. In less than a minute of parahydrogen bubbling at ~0.4 μT, a high level of nuclear spin polarization P15N of ~16% is achieved on all three 15N sites of metronidazole-15N3 at up to ~41 mM concentration. This product of 15N polarization and concentration of 15N spins is ~6-fold better than any previous value for 15N SABRE-derived hyperpolarization. At 1.4 T, the hyperpolarized state of the 15NO2 group of this antibiotic persists for tens of minutes (T1~10 min), in part because its 15N spin has no detectable spin-spin couplings with protons of the molecule. A novel synthesis achieving robust uniform 15N enrichment of metronidazole is reported with 15% yield over three steps. This synthetic approach should be suitable for preparation of other 15N-labeled nitroimidazole derivatives, potentially enabling a wide range of in vivo metabolic probes from hypoxia sensing to theranostic imaging.

Metabolic imaging in the anesthetized rat brain using hyperpolarized [1‐13C] pyruvate and [1‐13C] ethyl pyruvate

Formulation, polarization, and dissolution conditions were developed to obtain a stable hyperpolarized solution of [1‐13C]‐ethyl pyruvate. A maximum tolerated concentration and injection rate were determined, and 13C spectroscopic imaging was used to compare the uptake of hyperpolarized [1‐13C]‐ethyl pyruvate relative to hyperpolarized [1‐13C]‐pyruvate into anesthetized rat brain. Hyperpolarized [1‐13C]‐ethyl pyruvate and [1‐13C]‐pyruvate metabolic imaging in normal brain is demonstrated and quantified in this feasibility and range‐finding study.

Application of subsecond spiral chemical shift imaging to real‐time multislice metabolic imaging of the rat in vivo after injection of hyperpolarized 13C1‐pyruvate

Dynamic nuclear polarization can create hyperpolarized compounds with MR signal‐to‐noise ratio enhancements on the order of 10,000‐fold. Both exogenous and normally occurring endogenous compounds can be polarized, and their initial concentration and downstream metabolic products can be assessed using MR spectroscopy. Given the transient nature of the hyperpolarized signal enhancement, fast imaging techniques are a critical requirement for real‐time metabolic imaging. We report on the development of an ultrafast, multislice, spiral chemical shift imaging sequence, with subsecond acquisition time, achieved on a clinical MR scanner. The technique was used for dynamic metabolic imaging in rats, with measurement of time‐resolved spatial distributions of hyperpolarized 13C1‐pyruvate and metabolic products 13C1‐lactate and 13C1‐alanine, with a temporal resolution of as fast as 1 s. Metabolic imaging revealed different signal time courses in liver from kidney. These results demonstrate the feasibility of real‐time, hyperpolarized metabolic imaging and highlight its potential in assessing organ‐specific kinetic parameters.

In Vivo Carbon-13 Dynamic MRS and MRSI of Normal and Fasted Rat Liver with Hyperpolarized 13C-Pyruvate

The use of in vivo 13C nuclear magnetic resonance spectroscopy in probing metabolic pathways to study normal metabolism and characterize disease physiology has been limited by its low sensitivity. However, recent technological advances have enabled greater than 50,000-fold enhancement of liquid-state polarization of metabolically active 13C substrates, allowing for rapid assessment of 13C metabolism in vivo. The present study applied hyperpolarized 13C magnetic resonance spectroscopy to the investigation of liver metabolism, demonstrating for the first time the feasibility of applying this technology to detect differences in liver metabolic states. [1-13C]pyruvate was hyperpolarized with a dynamic nuclear polarization instrument and injected into normal and fasted rats. The uptake of pyruvate and its conversion to the metabolic products lactate and alanine were observed with slice-localized dynamic magnetic resonance spectroscopy and 3D magnetic resonance spectroscopic imaging (3D-MRSI). Significant differences in lactate to alanine ratio (P < 0.01) between normal and fasted rat liver slice dynamic spectra were observed. 3D-MRSI localized to the fasted livers demonstrated significantly decreased 13C-alanine levels (P < 0.01) compared to normal. This study presents the initial demonstration of characterizing metabolic state differences in the liver with hyperpolarized 13C spectroscopy and shows the ability to detect physiological perturbations in alanine aminotransferase activity, which is an encouraging result for future liver disease investigations with hyperpolarized magnetic resonance technology.

Hyperpolarized 13C Lactate, Pyruvate, and Alanine: Noninvasive Biomarkers for Prostate Cancer Detection and Grading​

An extraordinary new technique using hyperpolarized 13C-labeled pyruvate and taking advantage of increased glycolysis in cancer has the potential to improve the way magnetic resonance imaging is used for detection and characterization of prostate cancer. The aim of this study was to quantify, for the first time, differences in hyperpolarized [1-13C] pyruvate and its metabolic products between the various histologic grades of prostate cancer using the transgenic adenocarcinoma of mouse prostate (TRAMP) model. Fast spectroscopic imaging techniques were used to image lactate, alanine, and total hyperpolarized carbon (THC = lactate + pyruvate + alanine) from the entire abdomen of normal mice and TRAMP mice with low- and high-grade prostate tumors in 14 s. Within 1 week, the mice were dissected and the tumors were histologically analyzed. Hyperpolarized lactate SNR levels significantly increased (P < 0.05) with cancer development and progression (41 ± 11, 74 ± 17, and 154 ± 24 in normal prostates, low-grade primary tumors, and high-grade primary tumors, respectively) and had a correlation coefficient of 0.95 with the histologic grade. In addition, there was minimal overlap in the lactate levels between the three groups with only one of the seven normal prostates overlapping with the low-grade primary tumors. The amount of THC, a possible measure of substrate uptake, and hyperpolarized alanine also increased with tumor grade but showed more overlap between the groups. In summary, elevated hyperpolarized lactate and potentially THC and alanine are noninvasive biomarkers of prostate cancer presence and histologic grade that could be used in future three-dimensional 13C spectroscopic imaging studies of prostate cancer patients​

In Vivo 13Carbon Metabolic Imaging at 3T With Hyperpolarized 13C-1-Pyruvate​

We present for the first time dynamic spectra and spectroscopic images acquired in normal rats at 3T following the injection of (13)C-1-pyruvate that was hyperpolarized by the dynamic nuclear polarization (DNP) method. Spectroscopic sampling was optimized for signal-to-noise ratio (SNR) and for spectral resolution of (13)C-1-pyruvate and its metabolic products (13)C-1-alanine, (13)C-1-lactate, and (13)C-bicarbonate. Dynamic spectra in rats were collected with a temporal resolution of 3 s from a 90-mm axial slab using a dual (1)H-(13)C quadrature birdcage coil to observe the combined effects of metabolism, flow, and T(1) relaxation. In separate experiments, spectroscopic imaging data were obtained during a 17-s acquisition of a 20-mm axial slice centered on the rat kidney region to provide information on the spatial distribution of the metabolites. Conversion of pyruvate to lactate, alanine, and bicarbonate occurred within a minute of injection. Alanine was observed primarily in skeletal muscle and liver, while pyruvate, lactate, and bicarbonate concentrations were relatively high in the vasculature and kidneys. In contrast to earlier work at 1.5 T, bicarbonate was routinely observed in skeletal muscle as well as the kidney and vasculature​

Double spin-echo sequence for rapid spectroscopic imaging of hyperpolarized 13C

Dynamic nuclear polarization of metabolically active compounds labeled with 13C has been introduced as a means for imaging metabolic processes in vivo. To differentiate between the injected compound and the various metabolic products, an imaging technique capable of separating the different chemical-shift species must be used. In this paper, the design and testing of a pulse sequence for rapid magnetic resonance spectroscopic imaging (MRSI) of hyperpolarized 13C is presented. The pulse sequence consists of a small-tip excitation followed by a double spin echo using adiabatic refocusing pulses and a ‘‘flyback’’ echo-planar readout gradient. Key elements of the sequence are insensitivity to calibration of the transmit gain, the formation of a spin echo giving high-quality spectral information, and a small effective tip angle that preserves the magnetization for a sufficient duration. Experiments in vivo showed three-dimensional coverage with excellent spectral quality and SNR.

Hyperpolarized C‐13 spectroscopic imaging of the TRAMP mouse at 3T—Initial experience

The transgenic adenocarcinoma of mouse prostate (TRAMP) mouse is a well‐studied murine model of prostate cancer with histopathology and disease progression that mimic the human disease. To investigate differences in cellular bioenergetics between normal prostate epithelial cells and prostate tumor cells, in vivo MR spectroscopic (MRS) studies with non‐proton nuclei, such as 13C, in the TRAMP model would be extremely useful. The recent development of a method for retaining dynamic nuclear polarization (DNP) in solution permits high signal‐to‐noise ratio (SNR) 13C MRI or MRSI data to be obtained following injection of a hyperpolarized 13C agent. In this transgenic mouse study, this method was applied using a double spin‐echo (DSE) pulse sequence with a small‐tip‐angle excitation RF pulse, hyperbolic‐secant refocusing pulses, and a flyback echo‐planar readout trajectory for fast (10–14 s) MRSI of 13C pyruvate (pyr) and its metabolic products at 0.135 cm3 nominal spatial resolution. Elevated 13C lactate (lac) was observed in both primary and metastatic tumors, demonstrating the feasibility of studying cellular bioenergetics in vivo with DNP hyperpolarized 13C MRSI.