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

To demonstrate the feasibility of 3D echo-planar spectroscopic imaging (EPSI) technique with rapid volumetric radial k-space sampling for hyperpolarized (HP) ¹³C magnetic resonance spectroscopic imaging (MRSI) in vivo.

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 ¹³C-labeled substances were performed in phantoms and rats using a double-resonant ¹³C/¹H volume resonator with 72 mm inner diameter.

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 [1-¹³C]pyruvate in a tumor-bearing rat, we obtained well-resolved MRSI datasets with a large matrix size of 12³ voxels covering the whole imaging FOV of (180 mm)³ within 6.3 s, enabling to observe metabolism in dynamic acquisitions.

After further optimization, the proposed rEPSI method may be useful in applications of HP ¹³C-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.