Ultrahigh Resolution fMRI at 7T Using Radial-Cartesian TURBINE Sampling

Abstract

Purpose: We investigate the use of TURBINE, a 3D radial-Cartesian acquisition scheme in which EPI planes are rotated about the phase-encoding axis to acquire a cylindrical k-space for high-fidelity ultrahigh isotropic resolution fMRI at 7 Tesla with minimal distortion and blurring. Methods: An improved, completely self-navigated version of the TURBINE sampling scheme was designed for fMRI at 7 Telsa. To demonstrate the image quality and spatial specificity of the acquisition, thin-slab visual and motor BOLD fMRI at 0.67 mm isotropic resolution (16 mm slab, TRvol = 2.32 s), and 0.8 × 0.8 × 2.0 mm (whole-brain, TRvol = 2.4 s) data were acquired. To prioritize the high spatial fidelity, we employed a temporally regularized reconstruction to improve sensitivity without any spatial bias. Results: TURBINE images provide high structural fidelity with almost no distortion, dropout, or T2* blurring for the thin-slab acquisitions compared to conventional 3D EPI owing to the radial sampling in-plane and the short echo train used. This results in activation that can be localized to pre- and postcentral gyri in a motor task, for example, with excellent correspondence to brain structure measured by a T1-MPRAGE. The benefits of TURBINE (low distortion, dropout, blurring) are reduced for the whole-brain acquisition due to the longer EPI train. We demonstrate robust BOLD activation at 0.67 mm isotropic resolution (thin-slab) and also anisotropic 0.8 × 0.8 × 2.0 mm (whole-brain) acquisitions. Conclusion: TURBINE is a promising acquisition approach for high-resolution, minimally distorted fMRI at 7 Tesla and could be particularly useful for fMRI in areas of high B0 inhomogeneity.