Initial experiences of simultaneous in vivo metabolic imaging using MRI, PET, and hyperpolarized 13C MRSI from rat glioma models
JAE MO PARK1, Shie-Chau Liu1, Milton Merchant2, Taichang Jang2, Keshav Datta1, Praveen Gulaka1, Zachary Corbin2, Ralph E Hurd3, Lawrence Recht2, and Daniel M Spielman1

1Radiology, Stanford University, Stanford, CA, United States, 2Neurology and Neurological Sciences, Stanford University, Stanford, CA, United States, 3Applied Sciences Laboratory, GE Healthcare, Menlo Park, CA, United States

Synopsis

We demonstrated the feasibility of simultaneous investigation of in vivo metabolism using 1H MRI, time-of-flight 18F-FDG PET, and hyperpolarized 13C-pyruvate MRSI in C6 xenograft and ENU-induced brain tumor models. Volumetric images were acquired, and metabolic kinetics of FDG and pyruvate metabolism was investigated in the study.

Background

Multi-modal imaging has synergistic effects not only to improve image quality and accuracy in quantification, but also to understand complex in vivo metabolic processes1,2. In this study, we present our initial experiences of simultaneous in vivo metabolic imaging using 1H MRI, Time-Of-Flight (TOF) 18F-FDG PET, and hyperpolarized 13C-pyruvate MRSI in rat glioma models.

Methods

Two models of rat glioma were imaged using 1H MRI, 18F-FDG PET, and hyperpolarized 13C MRSI: male Wistar rats implanted with C6 glioma (N = 6, 260-393g, 10 days after C6 implantation) and male Wistar rats with N-Ethyl-N-nitrosourea (ENU)-induced glioma (N = 2, 383-444g, 116-123 days old). Each rat was anesthetized and catheterized in the tail vein before placing at the center of the GE 3T Signa PET/MR scanner. The imaging session started with a bolus injection of 1-mCi 18F-FDG (diluted to be 1ml), immediately followed by 1.5hrs of TOF PET acquisition. While PET data were collected proton MR images were acquired simultaneously using MRAC (for subject tissue attenuation correction), 2D T2-weighted fast spin echo (FSE), 3D T2-weighted CUBE, fluid-attenuated inversion recovery (FLAIR), and diffusion-weighed imaging (DWI) sequences. 13C MRSI data were also acquired during the PET scan after a bolus injection of 80-mM [1-13C]pyruvate, which was polarized for ~5hrs using GE SPINlab, using two imaging sequences: single time-point volumetric spiral chemical shift imaging3 (CSI, four spatial interleaves, acquisition time = 5.5s, field of view (FOV) = 64×64×48mm3, nominal spatial resolution = 4×4×4mm3), dynamic 2D spiral CSI4 (four spatial interleaves, temporal resolution = 3s, 20 time points, FOV = 64×64mm2, nominal spatial resolution = 4×4mm2, variable flip angle). Before the imaging session ended, T1-weighted spin echo (SE) images were obtained before and after Gadolinium injection (0.9ml, Gd:saline=1:2). The overall protocol is summarized in Figure 1. 1H-13C dual-tuned quadrature coil (Ø=80mm) was used for the MR(S)I, but the coil attenuation was not taken into account for the PET reconstruction. The dynamic PET data was reconstructed in 30-s bins (0-20min) and in 2-min bins (20-90min) using a fully 3D TOF iterative ordered subsets expectation maximization (OSEM) algorithm (24 subsets, 3 iterations, timing resolution=400ps). All the glioma rats were fasted for 20-24 hrs prior to 18F-FDG injection. Moreover, to investigate the effect of 80-mM pyruvate bolus injection on FDG uptake, three of the C6 glioma rats were scanned twice: ~1min after saline vs. pyruvate injections on two consecutive days (20hrs of fasting for both days). Temperature and respiration were maintained at ~36oC and 60 breaths/min, respectively, throughout the experiment.

Results and Discussion

Three tumor-bearing axial slices from a representative ENU-induced glioma rat are shown in Figure 2. The tumor region was confirmed with 1H MRI and showed upregulated 18F-FDG uptake and increased 13C-lactate labeling. Dynamic 18F-FDG uptake with a temporal resolution of 30s and dynamic imaging of [1-13C]lactate and [1-13C]pyruvate with a temporal resolution of 3s (Fig. 3) were reconstructed with sufficient SNR. Whereas the 13C-lactate production was consistently higher in the tumor ROI than in normal-appearing ROI, FDG-uptake curves in the brain showed two regimes: an initial fast uptake regime and a slowly increasing regime. The glioma ROIs tended to have slightly lower FDG-uptake in the first regime and more uptake in the second regime than the normal-appearing brain ROIs (Fig. 4A). The rats, infused with an additional 80-mM pyruvate prior to 18F-FDG injection, showed slower uptake rate of 18F-FDG in normal-appearing brain (11.8 ± 1.6 Bq/ml/s) in the second regime compared to the PET data acquired after saline injection (8.5 ± 0.6 Bq/ml/s, P = 0.04, Fig. 4). Accordingly, the increase in image contrast between tumor and normal-appearing brain was observed in all the rats (4.35 % at baseline, 9.60 % with pyruvate, P = 0.016). The change of FDG kinetics due to pyruvate injection suggests that FDG uptake via the glucose transporter (GLUT) and pyruvate uptake via the monocarboxylic transporter (MCT) might be competing processes.

Conclusion

We demonstrated the feasibility of simultaneous investigation of in vivo metabolism using 1H MRI, 18F-FDG PET, and hyperpolarized 13C-pyruvate MRSI in both C6 xenograft and ENU-induced brain tumor models.

Acknowledgements

We appreciate funding supports from National Institutes of Health (R01 CA176836, R01 EB019018, S10 OD012283, P41 EB015891) of the United States. We also thank GE Healthcare, Nadia’s gift, and Gambhir-RSL grant.

References

1. Zhang X, Chen YE, Lim R, Huang C, Chebib IA and El Fakhri G, Synergistic role of simultaneous PET/MRI-MRS in soft tissue sarcoma metabolism imaging. Magn Reson Imaging. 2015 Oct; doi: 10.1016/j.mri.2015.10.027.

2. Gutte H, Hansen AE, Larsen MM, Rahbek S, Henriksen ST, Johannesen HH, Ardenkjaer-Larsen J, Kristensen AT, Højgaard L and Kjær A, Simultaneous Hyperpolarized 13C-pyruvate MRI and 18F-FDG PET (HyperPET) in 10 dogs with cancer. J Nucl Med. 2015 Nov;56(11):1786-92.

3. Park JM, Josan S, Jang T, Merchant M, Watkins R, Hurd RE, Recht LD, Mayer D and Spielman DM, Volumetric spiral chemical shift imaging of hyperpolarized [2-13c]pyruvate in a rat c6 glioma model. Magn Reson Med. 2015 May; doi: 10.1002/mrm.25766.

4. Park JM, Josan S, Jang T, Merchant M, Yen YF, Hurd RE, Recht L, Spielman DM and Mayer D, Metabolite kinetics in C6 rat glioma model using magnetic resonance spectroscopic imaging of hyperpolarized [1-(13)C]pyruvate. Magn Reson Med. 2012 Dec;68(6):1886-93.

Figures

Figure 1. Metabolic imaging protocol for glioma-bearing rat brain using 1H MRI, 18F-FDG PET, and hyperpolarized 13C-pyruvate MRSI.

Figure 2. (A) 1H MRI, (B) hyperpolarized 13C MRSI, and (C) 18F-FDG PET images of tumor-bearing axial slices from a representative ENU-induced glioma rat.

Figure 3. Dynamic metabolite maps of hyperpolarized [1-13C]pyruvate and [1-13C]lactate, measured every 3s, from a representative ENU-induced glioma rat.

Figure 4. 18F-FDG uptake curves from ROIs of brain tumor and normal-appearing brain from a representative C6 glioma rat. Normal-appearing brain showed slower uptake rate of 18F-FDG in normal-appearing brain after additional 80-mM pyruvate infusion prior to the 18F-FDG injection. The increase in image contrast between tumor and normal-appearing brain was observed in all the rats after the additional pyruvate injection.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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