Leland Stanford Junior University

Michael Markl

Norbert Joseph Pelc

Roland Bammer

Marcus Tedrow Alley

Michael E Moseley

Burak Acar

Errors in qualitative phase contrast measurements due to gradient field heterogeneities are reduced by using either a generalized reconstruction algorithm or an approximate reconstruction algorithm. True velocities are calculated using measured velocity information and phase differences, first moments of gradients, and gyromagnetic ratio.

Correction of the effect of gradient field non-linearities in phase contrast MRI

A general mathematical framework is formulated to characterize the contribution of gradient non-uniformities to diffusion tensor imaging in MRI. Based on a model expansion, the actual gradient field is approximated and employed, after elimination of geometric distortions, for predicting and correcting the errors in diffusion encoding. Prior to corrections, experiments clearly reveal marked deviations of the calculated diffusivity for fields of view generally used in diffusion experiments. These deviations are most significant with greater distance from the magnet's isocenter. For a FOV of 25 cm the resultant errors in absolute diffusivity can range from approximately &#x2212;10 to +20 percent. Within the same field of view, the diffusion-encoding direction and the orientation of the calculated eigenvectors can be significantly altered if the perturbations by the gradient non-uniformities are not considered. With the proposed correction scheme most of the errors introduced by gradient non-uniformities can be removed.

Correction of the effect of spatial gradient field distortions in diffusion-weighted imaging

A novel technique for velocity measurements (PC-SSFP) is disclosed that combines CINE Phase Contrast (PC) MRI and balanced Steady State Free Precession (SSFP) imaging. Flow encoding is performed without the introduction of additional velocity encoding gradients in order to permit data acquisition with short TR comparable to repetition times of typical SSFP imaging sequences. Sensitivity to through plane velocities is instead established by inverting (i.e. negating) all gradients along the slice select direction. Velocity sensitivity (venc) can be adjusted by altering the first moments of the slice select gradients. Disturbances of the SSFP steady state are avoided by acquiring different flow echoes in consecutively (i.e. sequentially) executed scans, each over several cardiac cycles, using separate steady state preparation periods. Comparison of phantom measurements with those from established 2D-CINE-PC MRI excellent correlation between both modalities. Results of volunteer examinations exhibit advantages of PC-SSFP, which include the intrinsic high signal to noise ratio (SNR) of balanced SSFP and consequently low phase noise in measured velocities. An additional benefit of PC-SSFP is its lower reliance on in-flow dependent signal enhancement, therefore yielding more uniform SNR and better depiction of vessel geometry throughout the whole cardiac cycle in structures with slow and/or pulsatile flow.

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Palo Alto, CA, United States of America

Michael Markl