3D‐EPI blip‐up/down acquisition (BUDA) with CAIPI and joint Hankel structured low‐rank reconstruction for rapid distortion‐free high‐resolution T2* mapping

Purpose This work aims to develop a novel distortion‐free 3D‐EPI acquisition and image reconstruction technique for fast and robust, high‐resolution, whole‐brain imaging as well as quantitative mapping. Methods 3D Blip‐up and ‐down acquisition (3D‐BUDA) sequence is designed for both single‐ and multi‐echo 3D gradient recalled echo (GRE)‐EPI imaging using multiple shots with blip‐up and ‐down readouts to encode B 0 field map information. Complementary k‐space coverage is achieved using controlled aliasing in parallel imaging (CAIPI) sampling across the shots. For image reconstruction, an iterative hard‐thresholding algorithm is employed to minimize the cost function that combines field map information informed parallel imaging with the structured low‐rank constraint for multi‐shot 3D‐BUDA data. Extending 3D‐BUDA to multi‐echo imaging permits mapping. For this, we propose constructing a joint Hankel matrix along both echo and shot dimensions to improve the reconstruction. Results Experimental results on in vivo multi‐echo data demonstrate that, by performing joint reconstruction along with both echo and shot dimensions, reconstruction accuracy is improved compared to standard 3D‐BUDA reconstruction. CAIPI sampling is further shown to enhance image quality. For mapping, parameter values from 3D‐Joint‐CAIPI‐BUDA and reference multi‐echo GRE are within limits of agreement as quantified by Bland–Altman analysis. Conclusions The proposed technique enables rapid 3D distortion‐free high‐resolution imaging and mapping. Specifically, 3D‐BUDA enables 1‐mm isotropic whole‐brain imaging in 22 s at 3T and 9 s on a 7T scanner. The combination of multi‐echo 3D‐BUDA with CAIPI acquisition and joint reconstruction enables distortion‐free whole‐brain mapping in 47 s at 1.1 × 1.1 × 1.0 mm 3 resolution.