Imaging Renal Urea Handling in Rats at Millimeter Resolution using Hyperpolarized Magnetic Resonance Relaxometry

\textit{In vivo} spin spin relaxation time ($T_2$) heterogeneity of hyperpolarized \textsuperscript{13}C urea in the rat kidney was investigated. Selective quenching of the vascular hyperpolarized \textsuperscript{13}C signal with a macromolecular relaxation agent revealed that a long-$T_2$ component of the \textsuperscript{13}C urea signal originated from the renal extravascular space, thus allowing the vascular and renal filtrate contrast agent pools of the \textsuperscript{13}C urea to be distinguished via multi-exponential analysis. The $T_2$ response to induced diuresis and antidiuresis was performed with two imaging agents: hyperpolarized \textsuperscript{13}C urea and a control agent hyperpolarized bis-1,1-(hydroxymethyl)-1-\textsuperscript{13}C-cyclopropane-$^2\textrm{H}_8$. Large $T_2$ increases in the inner-medullar and papilla were observed with the former agent and not the latter during antidiuresis suggesting that $T_2$ relaxometry may be used to monitor the inner-medullary urea transporter (UT)-A1 and UT-A3 mediated urea concentrating process. Two high resolution imaging techniques - multiple echo time averaging and ultra-long echo time sub-2 mm$^3$ resolution 3D imaging - were developed to exploit the particularly long relaxation times observed

Development of Diffusion Weighted Magnetic Resonance Acquisition Techniques for Hyperpolarized Carbon-13 Metabolites and Applications to Cancer Detection and Characterization

Hyperpolarized carbon-13 magnetic resonance (MR) is a molecular imaging technique that allows for real-time measurements of in vivo metabolism. Hyperpolarized carbon-13 pyruvate can be used to better characterize cancer aggressiveness and response to therapy by monitoring the production of hyperpolarized carbon-13 lactate. In the work described herein, measure both the overall production of hyperpolarized carbon-13 lactate and its extra- and intracellular distribution. Understanding this distribution leads to an understanding of cancer aggressiveness and metastatic potential. Thus, measuring the production and the distribution of hyperpolarized carbon-13 lactate may allow for the differentiation of benign and metastatic cancers.We present bioreactor studies of two renal cell carcinoma (RCC) cell lines that show the efflux of hyperpolarized carbon-13 lactate play a significant role in the acquired signal and thus cannot be ignored. To study this distribution, we use a diffusion weighted pulsed gradient double spin echo sequence and a novel acquisition scheme for hyperpolarized carbon-13 metabolites. Given the highly dynamic signal changes characteristic of hyperpolarized carbon-13, we first verify the quantitative accuracy of the technique by measuring the diffusion coefficients of various hyperpolarized carbon-13 molecules in solution. Next, equipped with this technique, we measured the extra- and intracellular diffusion coefficients of various hyperpolarized carbon-13 metabolites in bioreactor studies with the same two RCC cell lines. We also assess the dynamic extra- and intracellular distribution of these hyperpolairzed carbon-13 metabolites in these cells and show that an inhibitor of the monocarboxylate transporter 4 (MCT4), which is responsible for the efflux of lactate and protons from the cytoplasm, increases the relative intracellular hyperpolarized carbon-13 lactate signal. Finally, we develop a methodology for diffusion weighted imaging and making apparent diffusion coefficient (ADC) maps of hyperpolarized carbon-13 metabolites on a clinical MR scanner. Using two novel acquisition techniques, we improve the accuracy and precision of these measurements. The work presented here will play an important role in assessing the tissue distribution of hyperpolarized carbon-13 metabolites, which will aid in the detection and characterization of cancer

Diffusion MR of hyperpolarized 13 C molecules in solution

We combined the high MR signal enhancement achieved using dissolution dynamic nuclear polarization (DNP) with a pulsed gradient double spin echo diffusion MR sequence to rapidly and accurately measure the diffusion coefficients of various hyperpolarized (13)C molecules in solution. Furthermore, with a diffusion-weighted imaging sequence we generate diffusion coefficient maps of multiple hyperpolarized metabolites simultaneously. While hyperpolarized experiments can measure rapid, non-equilibrium processes by avoiding signal averaging, continuous signal loss due to longitudinal relaxation (T(1)) complicates quantitation. By correcting for this signal loss, we demonstrate the feasibility of using hyperpolarized (13)C diffusion-weighted MR to accurately measure real-time (seconds) molecular transport phenomena. Potential applications include rapidly measuring molecular binding, cellular membrane transport, in vivo metabolite distribution and establishing a magnetic field independent hyperpolarized parameter

Hyperpolarized 13C-pyruvate magnetic resonance reveals rapid lactate export in metastatic renal cell carcinomas.

Renal cell carcinomas (RCC) are a heterogeneous group of tumors with a wide range of aggressiveness. Noninvasive methods to confidently predict the tumor biologic behavior and select appropriate treatment are lacking. Here, we investigate the dynamic metabolic flux in living RCC cells using hyperpolarized (13)C-pyruvate magnetic resonance spectroscopy (MRS) combined with a bioreactor platform and interrogated the biochemical basis of the MRS data with respect to cancer aggressiveness. RCC cells have significantly higher pyruvate-to-lactate flux than the normal renal tubule cells. Furthermore, a key feature distinguishing the localized from the metastatic RCC cells is the lactate efflux rate, mediated by the monocarboxylate transporter 4 (MCT4). The metastatic RCC cells have significantly higher MCT4 expression and corresponding higher lactate efflux, which is essential for maintaining a high rate of glycolysis. We show that such differential cellular transporter expression and associated metabolic phenotype can be noninvasively assessed via real-time monitoring of hyperpolarized (13)C-pyruvate-to-lactate flux

Dynamic UltraFast 2D EXchange SpectroscopY (UF-EXSY) of hyperpolarized substrates.

In this work, we present a new ultrafast method for acquiring dynamic 2D EXchange SpectroscopY (EXSY) within a single acquisition. This technique reconstructs two-dimensional EXSY spectra from one-dimensional spectra based on the phase accrual during echo times. The Ultrafast-EXSY acquisition overcomes long acquisition times typically needed to acquire 2D NMR data by utilizing sparsity and phase dependence to dramatically undersample in the indirect time dimension. This allows for the acquisition of the 2D spectrum within a single shot. We have validated this method in simulations and hyperpolarized enzyme assay experiments separating the dehydration of pyruvate and lactate-to-pyruvate conversion. In a renal cell carcinoma cell (RCC) line, bidirectional exchange was observed. This new technique revealed decreased conversion of lactate-to-pyruvate with high expression of monocarboxylate transporter 4 (MCT4), known to correlate with aggressive cancer phenotypes. We also showed feasibility of this technique in vivo in a RCC model where bidirectional exchange was observed for pyruvate-lactate, pyruvate-alanine, and pyruvate-hydrate and were resolved in time. Broadly, the technique is well suited to investigate the dynamics of multiple exchange pathways and applicable to hyperpolarized substrates where chemical exchange has shown great promise across a range of disciplines