Purpose: Prior 13C MR imaging studies of hyperpolarized (HP) [1‐13C]pyruvate and 13C urea in the TRAMP murine model of prostate cancer have demonstrated the ability to monitor changes in metabolism and perfusion. Radiation therapy represents a common treatment of prostate cancer that could clinically benefit from an early assessment of therapeutic efficacy. The goal of this work was to investigate serial changes in perfusion (HP urea) and HP pyruvate metabolism in TRAMP tumors following exposure to increasing doses of radiation therapy in order to better understand the potential clinical value of HP 13C MR for predicting prostate cancer radiation therapy. Methods: Three TRAMP tumors (mean tumor size = 4.3 ± 1 cc) were exposed to varying doses of radiation by placing a single radioactive seed source using a Nucletron microSelectron‐HDR applicator on the surface of the tumor for ∼10 min. dwell periods. This resulted in doses of 14 Gy (close to the seed) to 5 Gy (deeper within the tumor). MR imaging studies were acquired serially at baseline, 1 day, 4 days and 8 days following therapy using a 14T, 600WB micro‐imaging spectrometer. Results: There are visually clear dose dependent changes in the lactate/pyruvate ratio over time. HP urea and total HP carbon significantly decreased in tumor regions receiving both high (p < 0.01; and 0.05, respectively) and intermediate doses (p< 0.01, for both) by 1 day after treatment. Whereas HP urea initially increased in the low dose regions and then decreased. For all three doses, the lactate/pyruvate ratios significantly decreased (p < 0.01 for all doses) by 8 days following treatment. Conclusions: Significant, dose‐dependent decreases in perfusion and pyruvate‐to‐lactate flux were observed after radiation therapy. Ongoing serial radiation studies of TRAMP tumors are investigating the relationship between perfusion and metabolic changes and therapeutic efficacy.

Hyperpolarized, C labeled biomarkers can provide unique biochemical and physiological information in-vivo and have been used in animal models to detect and characterize tumors (1,2). This is made possible by the unprecedented signal enhancement achieved by the dynamic nuclear polarization (DNP) technique (3). However, special pulse sequences are needed for MRSI applications because the hyperpolarized magnetization is rapidly and irreversibly depleted due to relaxation, RF pulse saturation and metabolism. MRSI studies at high fields pose further challenges because T2*, motion and flow related artifacts are increased. The increased chemical shift spread at high fields is beneficial in resolving the resonance peaks in MRSI but may cause spatial errors in the slice and readout dimensions. The wide spectral dispersion may also result in lower digital resolution and in aliasing of outlying peaks in the frequency dimension. In this project we developed a novel, single-shot, 3D imaging sequence for hyperpolarized MRI studies on high field systems and investigated its performance by obtaining C images of lactate, pyruvate and urea in a transgenic mouse prostate cancer (TRAMP) model. Experimental Methods and Materials The pulse sequence used for the single-shot, chemical shift specific method is shown in Figure 1. It is based on a 3D spinecho EPI sequence and, as in the GRASE (4) method, uses additional 180 degree pulses during the echo train to minimize the effect of T2* related signal loss and artifacts. Flyback EPI readout gradients are applied during the blipped gradient duration and the data acquired during the positive gradients to avoid the Nyquist ghost artifact. SLR pulses, with 6msec duration and 500Hz bandwidth, were designed to selectively excite only the resonance of interest. 3D images, with a data size of 16x12x12, were acquired in 153msec and provide high temporal resolution. The maximum gradient strength used was about 300mT/m. The experiments were done using a vertical, 14.1T Varian 600WB micro-imaging system equipped with 55mm 1000mT/m gradients and 40mm diameter proton and carbon RF coils. The animals were placed in a temperature controlled animal holder and anesthetized using isoflurane. An animal monitoring system (SA Instruments) was used to monitor respiration and trigger the scanner during all protocols. The proton coil was used for shimming and anatomical imaging and then the carbon coil was used for C imaging. A mixture of [1-C]-pyruvate and C-urea was polarized using an Oxford HypersenseTM DNP instrument and 400ul of the resulting dissolution mixture containing 80mM pyruvate and 74mM urea was administered via a jugular vein catheter. 44s after injection, C image datasets corresponding to lactate, pyruvate, and urea were acquired in 0.46s (153msec per image). After acquiring the image, a portion of the dissolution mixture was injected into a second catheter placed inside the RF coil and a 4mm slab spectrum taken to measure the level of polarization and normalize the image intensities for quantitative analysis. In some cases, we have used the signal from the kidney as an internal reference to normalize image intensities for analysis.

Results: Representative HP 13 C-spectra in untreated animals (pre-treated), those with no proliferation/growth after castration (androgen dependent), and those with proliferation/growth after castration (androgen independent) are shown in Figure 1. Qualitatively, we observed significantly lower hyperpolarized lactate and THC levels in androgen dependent versus androgen independent cancer. Specifically, the androgen independent phenotype had a significantly higher HP lactate/noise (632±180 vs 182±115, p<0.0004), THC (1060±281 vs 501±326, p<0.004), HP lactate/pyruvate (2.04±0.71 vs 0.713±0.27, p<0.002), and LDH activity (5.79±2.76 vs 1.45±0.30 nM-NADH/min/µg-protein/ml, p<0.003) relative to the androgen dependent phenotype. Figure 2 is a bar plot quantitatively summarizing the differences between androgen dependent and independent disease. Discussion: AIPC is defined as continued tumor progression in the face of castrate-levels of androgen. In this study we show that in the TRAMP model, it is also characterized by significantly elevated LDH activity that correlates with elevated HP 13 C-lactate levels in vivo. Additionally THC, a measure of HP pyruvate uptake, was also significantly elevated in androgen independent disease. These studies suggest that these HP biomarkers could be helpful in delineating disease status for improved therapeutic selection. References:

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 C metabolic imaging applications. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.

V. Zhang, M. Grinde, L. Tabatabai, J. Simko, M. Albers, D. Vigneron, and J. Kurhanewicz Radiology, University of California, San Francisco, San Francisco, CA, United States, Joint Bioengineering Program, University of California, Berkeley/San Francisco, San Francisco, CA, United States, Circulation and Medical Imaging, Norwegian University of Science and Technology, Trondheim, Norway, St.Olav University Hospital, Trondheim, CA, Norway, Pathology, University of California, San Francisco, San Francisco, CA, United States

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.

M. J. Albers, A. P. Chen, R. Bok, V. Y. Zhang, R. E. Hurd, Y. F. Yen, M. L. Zierhut, S. J. Nelson, D. B. Vigneron, and J. Kurhanewicz Department of Radiology, University of California San Francisco, San Francisco, CA, United States, Bioengineering Department, UC Berkeley / UC San Francisco, San Francisco, CA, United States, Global Applied Science Laboratory, GE Healthcare America, Menlo Park, CA, United States