Higher spatial resolution and sensitivity in whole brain functional MRI at 7T using 3D EPI accelerated with variable density CAIPI sampling and temporal random walk

Abstract Purpose To develop an efficient 3D EPI encoding technique for high spatiotemporal resolution functional MRI. Methods To exploit spatiotemporal fMRI data structure, we introduce a variable density 2D CAIPI sampling in the spatial domain combined with time‐wise extra random encoding in the time domain, thus achieving pseudo‐regular sampling with a regular blip while allowing incoherent sampling in a complementary manner across time. This enabled temporally regularized reconstruction of highly accelerated functional data acquisition. The encoding scheme was then validated against temporally invariant CAIPI encoding by applying to locally confined and whole‐brain around the primary visual cortex, respectively, with increasing the spatial resolutions. Results For partial brain imaging, our proposed method achieved higher reconstruction accuracy, resulting in a substantial increase of SSIM compared to an alternative method for 0.64 mm‐isotropic resolution. When used for whole brain imaging at 0.56 mm‐isotropic resolution, our method showed a decreased spatial extent of activation and produced high‐quality images for a clear distinction between activated and non‐activated regions around calcarine fissure with high spatial specificity. Conclusion The proposed 3D EPI encoding scheme, which exploits coherent and incoherent sampling properties, can significantly improve the image quality while providing a good balance between sensitivity and specificity in the activated regions.