Fast functional brain imaging using constrained reconstruction based on regularization using arbitrary projections

This work describes a novel method for highly undersampled projection imaging using constrained reconstruction by Tikhonov-Phillips regularization and its application for high temporal resolution functional MRI (fMRI) at a repetition time of 80 ms. The high-resolution reference image used as in vivo coil sensitivity is acquired in a separate acquisition using otherwise identical parameters. Activation studies using a standard checkerboard activation paradigm demonstrate the inherent high sensitivity afforded by the possibility to separate activation-related effects from "physiological noise.". In this first proof-of-principle of the constrained reconstruction based on regularization using arbitrary projections (COBRA) technique, experiments are performed in a single-slice mode, which allows for a comparison with fast single-slice echo-planar imaging (EPI) at equal temporal resolution. The COBRA method can be extended to three-dimensional (3D) encoding without severe penalty in temporal performance. Analysis of the global signal change demonstrates the excellent reproducibility of COBRA compared to standard EPI. Activation analysis is considerably improved by the possibility to remove electrocardiogram (ECG)-related and breathing-related signal fluctuations by physiological correction of each individual breathing and ECG cycle, respectively.