Targeted partial reconstruction for real‐time fMRI with arbitrary trajectories

Purpose A partial image reconstruction formalism is introduced for the targeted extraction of real‐time feedback from arbitrary trajectories when full image reconstruction in real time is computationally too demanding. Methods Explicit calculation and storage of linear combinations of lines of the reconstruction matrix by an incomplete basis change in spatial coordinates lead to translation of the expensive full reconstruction from a frame‐wise application to a region of interest (ROI)‐wise application. This step is independent from signal data and can be executed before the experiment. Subsequently, the results of the sum over fully reconstructed voxels can be evaluated directly. Data from a high‐speed fMRI acquisition was used to investigate the targeted partial reconstruction of a functional ROI atlas, incorporating an intravolume dephasing correction. The same data and ROIs were used for a comparison of the time series obtained with those obtained from already existing methods for compartment‐wise reconstruction. To examine real‐time feasibility, the reconstruction was implemented and tested for online reconstruction performance. Results The reconstruction yields results that are virtually identical to the standard reconstruction (i.e., the magnitude sums over the ROIs), with negligible discrepancies even after termination of the conjugate gradient algorithm at a feasible number of iterations. Notably, more discrepancies arise with existing compartment‐wise reconstructions. The online real‐time implementation evaluated 1 ROI within 2.8 ms in the case of a highly parallel 3D whole brain acquisition. Conclusion The high reconstruction fidelity and speed are satisfying for the exemplary application of real‐time functional feedback using a highly parallel 3D whole brain acquisition.