Fig 1.
Flow chart of the fitted SVD method.
Images represent example coil sensitivities across the same slice of the brain (four of 32 shown). The four left images are magnitudes of the coil sensitivities, and the four right images are phases of the coil sensitivities. a) Relative coil sensitivities calculated by voxel-wise SVD in prescan space, b) Coil sensitivities after alignment to a virtual reference coil created through minimax optimization across prescan space, c) Fitted coil sensitivities in target image space, d) Combined phase image after alignment with fitted coil sensitivities.
Fig 2.
Fitted SVD method in a human using prescan data from a single subject.
a) Average quality ratio and b) coefficient of variation of quality ratio as a function of fit order and fit mask size. Example convex hull (grey) and voxels included in fit (white) for various mask thresholds c) 10, d) 20, e) 30. f) Example phase image, g) unwrapped phase image, and h) quality ratio map at the selected parameters (order 6, fit mask of 20, minimax mask of 20).
Fig 3.
Comparison of phase combination methods.
One example slice is shown for each method. Top row: raw phase image, Middle row: unwrapped phase image for easier visualization (singularities circled in white), Bottom row: quality ratio across a representative slice. a) Complex sum combination, b) VRC c) Fitted SVD method using a low resolution prescan, d) COMPOSER, e) Voxel-wise SVD combination, f) BCC, g) Fitted SVD method using parameter matched dataset. Single threaded runtime of each method increases left to right and can be found in Table 1. Note: the BCC method applies a rough mask to the region-of-interest during combination and this causes zeros in the exterior of the raw phase image.
Table 1.
Summary of coil combination methods quality and single threaded runtime when implemented in Matlab R2018a.
Fig 4.
Combination quality of an asymmetrical coil.
a) Quality ratio of data collected in an occipital parietal coil when combined with the fitted SVD method, b) Spatially unwrapped phase data after fitted SVD combination.
Fig 5.
Phase noise ratios in an asymmetrical coil.
Voxel-wise SVD was used as the reference method. Phase noise ratio combined using a) complex sum, b) VRC, c) BCC, d) the fitted SVD method using the B1+ prescan, and e) the fitted SVD method using the EPI timeseries as input. Hyperintensities correspond with phase singularities in a and b.
Fig 6.
Effects of motion on the fitted SVD method.
a) Raw phase image, b) unwrapped phase image, c) quality ratio map created with no motion between the prescan and the imaging. d) Raw phase image, e) unwrapped phase image, f) quality ratio map created with 3.5 mm motion between the
prescan and the imaging. No singularities were observed.