Susan J. Kohler

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

An extraordinary new technique using hyperpolarized (13)C-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-(13)C] 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 (13)C spectroscopic imaging studies of prostate cancer patients.

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. Magn Reson Med, 2007.

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 13C‐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 13C‐1‐pyruvate and its metabolic products 13C‐1‐alanine, 13C‐1‐lactate, and 13C‐bicarbonate. Dynamic spectra in rats were collected with a temporal resolution of 3 s from a 90‐mm axial slab using a dual 1H‐13C quadrature birdcage coil to observe the combined effects of metabolism, flow, and T1 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 1.5T, bicarbonate was routinely observed in skeletal muscle as well as the kidney and vasculature. Magn Reson Med 58:65–69, 2007.

Imaging Considerations for In Vivo 13C Metabolic Mapping Using Hyperpolarized 13C-Pyruvate

One of the challenges of optimizing signal‐to‐noise ratio (SNR) and image quality in 13C metabolic imaging using hyperpolarized 13C‐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 13C metabolic imaging applications. Magn Reson Med, 2009.

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. Magn Reson Med 63:1137–1143, 2010.

Magnetic resonance imaging at 9.4 T as a tool for studying neural anatomy in non-vertebrates

This report describes magnetic resonance imaging (MRI) methods we have developed at 9.4 T for observing internal organs and the nervous system of an invertebrate organism, the crayfish, Cherax destructor. We have compared results acquired using two different pulse sequences, and have tested manganese (Mn(2+)) as an agent to enhance contrast of neural tissues in this organism. These techniques serve as a foundation for further development of functional MRI and neural tract-tracing methods in non-vertebrate systems.

Sodium magnetic resonance imaging and chemical shift imaging

Abstract The study of sodium in biological systems is critical for an understanding of physiology and pathophysiology. NMR has proven to be an excellent tool for the study of sodium because of the relatively high concentration of sodium in the extracellular fluids of biological tissues, the inherent sensitivity of the 23 Na quadrupolar nucleus to its environment, and the high contrast obtainable in 23 Na MR images. The challenge has been to improve the instrumentation and to develop the data acquisition techniques to the point where high quality images and CSI data sets could be obtained in an acceptable time. As techniques have improved it has been demonstrated that 23 Na MRI is indeed sensitive to tissue viability and pathologic state, yet clinical studies have not yet found unique information from 23 Na MR images alone. (56, 75) The use of multiple echoes to provide relaxation information has greatly increased the utility of 23 Na MRI in spite of the fact that intra- and extracellular sodium pools do not appear to be differentiable solely on the basis of the presence of a short component of T 2 . The generation of images based on relaxation time thresholds (70) may prove valuable in providing more tissue-specific information. The use of paramagnetic shift reagents or contrast reagents has proved useful for the differentiation of intra- and extracellular sodium pools, but toxicity of the currently available shift reagents may be problematic. In instances where toxicity is not a severe problem, shift reagents can be used to monitor fluid dynamics as well as to distinguish different sodium pools.

Accurate flip-angle calibration for 13C MRI.

13C imaging and spectroscopy in the presence of injected labeled compounds can vastly extend the capability of MRI to perform metabolic imaging. The details of imaging in the presence of injected compounds, however, pose new technological challenges. Pulse sequences, in general, rely on precise flip‐angle (FA) calibration to create high signal‐to‐noise ratio (SNR), artifact‐free images. Signal quantification also requires precise knowledge of the excitation FA. In MRI scans that rely on signal acquisitions from injected compounds, however, such FA calibration is challenged by low natural‐abundance 13C signal levels before injection, and by time‐varying signal following injection. A method to precisely set the FA at the 13C frequency based on FA calibration at the 23Na frequency is presented here. A practical implementation of a coil (a dual‐tuned, 23Na/13C low‐pass birdcage coil) suitable for this calibration in vivo is also documented. Accurate FA calibration is demonstrated at the 13C frequency for in vivo rat experiments using this approach. Magn Reson Med 58:128–133, 2007.

23Na chemical-shift imaging

The maintenance of a sodium gradient across the plasma membrane is of crucial importance to living systems. Short-term changes in the sodium gradient are essential to nerve impulse transmission and cell excitability, while long-term changes in the gradient occur as the cells’ high-energy phosphate metabolites become depleted and/ or cell death occurs. The study of sodium and sodium gradients in biological systems is thus critical for an understanding of their physiology. Over the past few years advances in 23Na NMR spectroscopy have made it an increasingly useful tool for the study of sodium in biological tissue. In particular, the development of paramagnetic shift reagents for cations has provided the means to distinguish intracellular and extracellular sodium pools and to track the breakdown of the sodium gradient as a function of ischemia or cell death (1-8). The success of spectroscopic studies using cationic shift reagents to differentiate intraand extracellular sodium pools and track changes in the sodium gradient on a global basis in vivo has led us to develop a chemical-shift-imaging technique for sodium so that spectroscopic information may be obtained on a regional basis. Although common for ‘H imaging studies, CSI experiments for 23Na have not been performed previously due to the difficulties created by the extremely short relaxation times and low concentrations of sodium in vivo. A recent preliminary report describes microimaging experiments using the shift reagent dysprosium tripolyphosphate (9). Using frequency-selective RF pulses to excite the well-resolved resonances of shifted and unshifted sodium, two images of the different sodium pools were sequentially produced. This technique depends upon the complete resolution of the sodium resonances, rarely obtainable with shift reagents other than the toxic tripolyphosphate, and requires separate experiments for each sodium species. We have developed CSI techniques for sodium to be used in conjunction with nontoxic paramagnetic shift reagents such as DyTTHA3-. These experiments allow the simultaneous generation of separate images of the intracellular and extracellular sodium pools, and therefore the direct monitoring of the distribution of sodium on a regional basis. The technique does not require complete resolution of the shifted and unshifted sodium species

Magnetic resonance imaging determination of 23Na visibility and T2∗ in the vitreous body

Abstract We have developed 23Na magnetic resonance imaging techniques which provide high-resolution three-dimensional multiecho images in a short time. Our protocols produce a complete three-dimensional set of usable ocular images with 2 × 2 × 2 mm voxels in 4 min by coadding images from eight echoes. Alternatively, longer acquisition times may be used and the echoes processed individually to yield eight T2-weighted image sets. We have used these techniques to examine a variety of ocular disorders, to determine spin-spin relaxation times on a voxel-by-voxel basis, and to evaluate factors affecting the sodium “visibility” in the vitreous of enucleated bovine eyes. We found that both changes in the sodium visibility and changes in localized T 2 ∗ values are of physiological significance, reflecting alterations in the state of the vitreous in the enucleated eye.