Purpose 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. Materials and methods Dynamic 13C spectroscopy data were acquired in normal rats, wild type mice, and mice with transgenic prostate tumors (TRAMP) either within a single slice or using a one-dimensional echo-planar spectroscopic imaging (1D-EPSI) encoding technique. Rate constants were estimated by fitting a set of exponential equations to the dynamic data. Variations in fitted parameters were used to determine model robustness in 15 mm slices centered on normal rat kidneys. Parameter values were used to investigate differences in metabolism between and within TRAMP and wild type mice. Results The kinetic model was shown here to be robust when fitting data from a rat given similar doses. In normal rats, Michaelis–Menten kinetics were able to describe the dose–response of the fitted exchange rate constants with a 13.65% and 16.75% scaled fitting error (SFE) for kpyr→lac and kpyr→ala, respectively. In TRAMP mice, kpyr→lac increased an average of 94% after up to 23 days of disease progression, whether the mice were untreated or treated with casodex. Parameters estimated from dynamic 13C 1D-EPSI data were able to differentiate anatomical structures within both wild type and TRAMP mice. Conclusions The metabolic parameters estimated using this approach may be useful for in vivo monitoring of tumor progression and treatment efficacy, as well as to distinguish between various tissues based on metabolic activity.

In order to compare in vivo metabolism between malignant gliomas and normal brain, (13)C magnetic resonance (MR) spectroscopic imaging data were acquired from rats with human glioblastoma xenografts (U-251 MG and U-87 MG) and normal rats, following injection of hyperpolarized [1-(13)C]-pyruvate. The median signal-to-noise ratio (SNR) of lactate, pyruvate, and total observed carbon-13 resonances, as well as their relative ratios, were calculated from voxels containing Gadolinium-enhanced tissue in T(1) postcontrast images for rats with tumors and from normal brain tissue for control rats. [1-(13)C]-labeled pyruvate and its metabolic product, [1-(13)C]-lactate, demonstrated significantly higher SNR in the tumor compared with normal brain tissue. Statistical tests showed significant differences in all parameters (P < .0004) between the malignant glioma tissue and normal brain. The SNR of lactate, pyruvate, and total carbon was observed to be different between the U-251 MG and U-87 MG models, which is consistent with inherent differences in the molecular characteristics of these tumors. These results suggest that hyperpolarized MR metabolic imaging may be valuable for assessing prognosis and monitoring response to therapy for patients with brain tumors.

Hyperpolarized 13C offers high signal-to-noise ratios for imaging metabolic activity in vivo, but care must be taken when designing pulse sequences because the magnetization cannot be recovered once it has decayed. It has a short lifetime, on the order of minutes, and gets used up by each RF excitation. In this paper, we present a new dynamic chemical-shift imaging method that uses specialized RF pulses designed to maintain most of the hyperpolarized substrate while providing adequate SNR for the metabolic products. These are multiband, variable flip angle, spectral-spatial RF pulses that use spectral selectivity to minimally excite the injected prepolarized 13C-pyruvate substrate. The metabolic products of lactate and alanine are excited with a larger flip angle to increase SNR. This excitation was followed by an RF amplitude insensitive double spin-echo and an echo-planar flyback spectral-spatial readout gradient. In vivo results in rats and mice are presented showing improvements over constant flip angle RF pulses. The metabolic products are observable for a longer window because the low pyruvate flip angle preserves magnetization, allowing for improved observation of spatially varying metabolic reactions.

One of the challenges of optimizing signal-to-noise ratio (SNR) and image quality in 13 C metabolic imaging using hyperpolarized 13 C-pyruvate is associated with the different MR signal time-courses for pyruvate and its metabolic products, lactate and alanine. The impact of the acquisition time window, variation of flip angles, and order of phase encoding on SNR and image quality were evaluated in mathematical simulations and rat experiments, based on multishot fast chemical shift imaging (CSI) and three-dimensional echo-planar spectroscopic imaging (3DEPSI) sequences. The image timing was set to coincide with the peak production of lactate. The strategy of combining variable flip angles and centric phase encoding (cPE) improved image quality while retaining good SNR. In addition, two aspects of EPSI sampling strategies were explored: waveform design (flyback vs. symmetric EPSI) and spectral bandwidth (BW ‫؍‬ 500 Hz vs. 267 Hz). Both symmetric EPSI and reduced BW trended toward increased SNR. The imaging strategies reported here can serve as guidance to other multishot spectroscopic imaging protocols for 13 Key words: hyperpolarized 13 C; EPSI; in vivo metabolism; 3D spectroscopic imaging; metabolic dynamics MR spectroscopic imaging of hyperpolarized 1-[ 13 C]-pyruvate (noted herein as 13 C-pyruvate) is a promising technique for mapping metabolic activity in vivo, as demonstrated in recent animal studies (1-9). This method uses dynamic nuclear polarization (DNP) and a rapid in-field dissolution process to produce a highly polarized metabolic contrast agent (10). In less than 1 min following injection, 13 C-pyruvate and its metabolic products 1-[ 13 C]-lactate (noted herein as 13 C-lactate), 1-[ 13 C]-alanine (noted herein as 13 C-alanine), and 13 C-bicarbonate can be mapped at relatively high spatial resolution. This technology is especially promising in oncology, where lactate levels have been shown to correlate with disease progression (11) and response to therapy (12).The primary goal of this study was to optimize detection signal-to-noise ratio (SNR) and image quality in metabolic images of both 13 C-lactate and 13 C-pyruvate in multishot acquisition. The work was focused on strategies to minimize detection and image quality limitations associated with multiple metabolites, each with very different dynamics. Previous, nonspectroscopic studies have reported flip angle and phase-encode order strategies to optimize SNR and reduce image artifacts in hyperpolarized gas imaging (13-16). The impact of image timing relative to a rapid variation of contrast medium concentration (17) has also been reported. In this work, we examined the timing and sampling strategies for spectroscopic imaging of hyperpolarized 13 C-pyruvate and 13 C-lactate, with a focus on the different dynamic responses of each.Following an injection of hyperpolarized 13 C-pyruvate, the in vivo pyruvate signal is typically many times larger than the lactate signal, and has a very different time course (3,4,8). Typically, the 13 C-pyruvate signal in...

The purpose of this study was to combine a three-dimensional NMR-compatible bioreactor with hyperpolarized 13 C NMR spectroscopy in order to probe cellular metabolism in real time. JM1 (immortalized rat hepatoma) cells were cultured in a three-dimensional NMR-compatible fluidized bioreactor. 31 P spectra were acquired before and after each injection of hyperpolarized [1-13 C] pyruvate and subsequent 13 C spectroscopy at 11.7 T. 1 H and two-dimensional 1 H-1 Htotal correlation spectroscopy spectra were acquired from extracts of cells grown in uniformly labeled 13 C-glucose, on a 16.4 T, to determine 13 C fractional enrichment and distribution of 13 C label. JM1 cells were found to have a high rate of aerobic glycolysis in both two-dimensional culture and in the bioreactor, with 85% of the 13 C label from uniformly labeled 13 Cglucose being present as either lactate or alanine after 23 h. Flux measurements of pyruvate through lactate dehydrogenase and alanine aminotransferase in the bioreactor system were 12.18 6 0.49 nmols/sec/10 8 cells and 2.39 6 0.30 nmols/ sec/10 8 cells, respectively, were reproducible in the same bioreactor, and were not significantly different over the course of 2 days. Although this preliminary study involved immortalized cells, this combination of technologies can be extended to the real-time metabolic exploration of primary benign and cancerous cells and tissues prior to and after therapy. There is growing interest in the changes in fluxes through a variety of metabolic pathways associated with the evolution and progression of cancer (1-7) and its response to therapy. Recently, dynamic nuclear polarization (DNP) spectroscopic techniques have been used to study these metabolic fluxes in real time (8,9). Focus has been placed on changes in pathways associated with lipid synthesis and degradation (10,11), bioenergetics (12,13), and redox potential (14,15). Increases in lactate dehydrogenase (LDH) activity (8,9), changes in glutaminolysis (2), and decreases in pyruvate kinase activity (1) have been associated with cancer. These studies have been conducted primarily in extracts of cell cultures (3) or by way of in vivo animal studies (16)(17)(18). Cell culture studies provide a controlled platform for long-term metabolic studies, but they are tedious since each measurement requires new cells. In contrast, in in vivo studies the same animal can be used for multiple metabolic measurements, but they are more costly, are biologically heterogeneous, require many animals to be sacrificed, and are inefficient for the initial screening of novel metabolic tracers.For the last 3 decades, cell perfusion or ''bioreactor'' systems have been developed in an attempt to address these fundamental issues (15,19,20). These three-dimensional culture perfusion systems have allowed the monitoring of steady-state metabolites and their changes with time (21,22), typically through the analysis of output parameters such as concentrations of lactate, glucose, and alanine in media (23). Studies have also been performed...

BackgroundThe use of in vivo13C 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.Procedures[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).ResultsSignificant 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.ConclusionsThis 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.

Purpose:To investigate the signal-to-noise-ratio (SNR) and data quality of time-reduced three-dimensional (3D) proton magnetic resonance spectroscopic imaging ( 1 H MRSI) techniques in the human brain at 3 Tesla. Materials and Methods:Techniques that were investigated included ellipsoidal k-space sampling, parallel imaging, and echo-planar spectroscopic imaging (EPSI). The SNR values for N-acetyl aspartate, choline, creatine, and lactate or lipid peaks were compared after correcting for effective spatial resolution and acquisition time in a phantom and in the brains of human volunteers. Other factors considered were linewidths, metabolite ratios, partial volume effects, and subcutaneous lipid contamination. Results:In volunteers, the median normalized SNR for parallel imaging data decreased by 34 -42%, but could be significantly improved using regularization. The normalized signal to noise loss in flyback EPSI data was 11-18%. The effective spatial resolutions of the traditional, ellipsoidal, sensitivity encoding (SENSE) sampling scheme, and EPSI data were 1.02, 2.43, 1.03, and 1.01 cm 3 , respectively. As expected, lipid contamination was variable between subjects but was highest for the SENSE data. Patient data obtained using the flyback EPSI method were of excellent quality. Conclusion:Data from all 1 H 3D-MRSI techniques were qualitatively acceptable, based upon SNR, linewidths, and metabolite ratios. The larger field of view obtained with the EPSI methods showed negligible lipid aliasing with acceptable SNR values in less than 9.5 min without compromising the point-spread function.

This study investigated the application of an acquisition that selectively excites the [1-13C]lactate resonance and allows dynamic tracking of the conversion of 13C-lactate from hyperpolarized 13C-pyruvate at a high spatial resolution. In order to characterize metabolic processes occurring in a mouse model of prostate cancer, 20 sequential 3D images of 13C-lactate were acquired 5 s apart using a pulse sequence that incorporated a spectral–spatial excitation pulse and a flyback echo-planar readout to track the time course of newly converted 13C-lactate after injection of prepolarized 13C-pyruvate. The maximum lactate signal (MLS), full-width half-maximum (FWHM), time to the peak 13C-lactate signal (TTP) and area under the dynamic curve were calculated from the dynamic images of 10 TRAMP mice and two wild-type controls. The regional variation in 13C-lactate associated with the injected pyruvate was demonstrated by the peak of the 13C-lactate signal occurring earlier in the kidney than in the tumor region. The intensity of the dynamic 13C-lactate curves also varied spatially within the tumor, illustrating the heterogeneity in metabolism that was most prominent in more advanced stages of disease development. The MLS was significantly higher in TRAMP mice that had advanced disease.