Three-dimensional radial echo-planar spectroscopic imaging for hyperpolarized 13C MRSI in vivo

Purpose: To demonstrate the feasibility of 3D echo-planar spectroscopic imaging (EPSI) technique with rapid volumetric radial k-space sampling for hyperpolarized (HP) C-13 magnetic resonance spectroscopic imaging (MRSI) in vivo. Methods: A radial EPSI (rEPSI) was implemented on a 3 T clinical PET/MR system. To enable volumetric coverage, the sinusoidal shaped readout gradients per k-t-spoke were rotated along the three spatial dimensions in a golden-angle like manner. A distance-weighted, density-compensated gridding reconstruction was used, also in cases with undersampling of spokes in k-space. Measurements without and with HP C-13-labeled substances were performed in phantoms and rats using a double-resonant C-13/H-1 volume resonator with 72 mm inner diameter. Results: Phantom measurements demonstrated the feasibility of the implemented rEPSI sequence, as well as the robustness to undersampling in k-space up to a factor of 5 without advanced reconstruction techniques. Applied to measurements with HP [1-C-13]pyruvate in a tumor-bearing rat, we obtained well-resolved MRSI datasets with a large matrix size of 12(3) voxels covering the whole imaging FOV of (180 mm)(3) within 6.3 s, enabling to observe metabolism in dynamic acquisitions. Conclusion: After further optimization, the proposed rEPSI method may be useful in applications of HP C-13-tracers where unknown or varying metabolite resonances are expected, and the acquisition of dynamic, volumetric MRSI datasets with an adequate temporal resolution is a challenge.