Spatially encoded phase‐contrast MRI—3D MRI movies of 1D and 2D structures at millisecond resolution

This work demonstrates that the principles underlying phase‐contrast MRI may be used to encode spatial rather than flow information along a perpendicular dimension, if this dimension contains an MRI‐visible object at only one spatial location. In particular, the situation applies to 3D mapping of curved 2D structures which requires only two projection images with different spatial phase‐encoding gradients. These phase‐contrast gradients define the field of view and mean spin‐density positions of the object in the perpendicular dimension by respective phase differences. When combined with highly undersampled radial fast low angle shot (FLASH) and image reconstruction by regularized nonlinear inversion, spatial phase‐contrast MRI allows for dynamic 3D mapping of 2D structures in real time. First examples include 3D MRI movies of the acting human hand at a temporal resolution of 50 ms. With an even simpler technique, 3D maps of curved 1D structures may be obtained from only three acquisitions of a frequency‐encoded MRI signal with two perpendicular phase encodings. Here, 3D MRI movies of a rapidly rotating banana were obtained at 5 ms resolution or 200 frames per second. In conclusion, spatial phase‐contrast 3D MRI of 2D or 1D structures is respective two or four orders of magnitude faster than conventional 3D MRI. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Aortic pulse wave velocity measurements with undersampled 4D flow‐sensitive MRI: comparison with 2D and algorithm determination

Purpose:

To compare pulse wave velocity (PWV) measurements obtained from radially undersampled 4D phase‐contrast magnetic resonance imaging (PC‐MRI) with 2D PC measurements and to evaluate four PWV algorithms.

Materials and Methods:

PWV was computed from radially undersampled 3D, 3‐directionally velocity‐encoded PC‐MRI (4D) acquisitions performed on a 3T MR scanner in 18 volunteers. High temporal resolution 2D PC scans serving as a reference standard were available in 14 volunteers. Four PWV algorithms were tested: time‐to‐upstroke (TTU), time‐to‐peak (TTP), time‐to‐foot (TTF), and cross‐correlation (XCorr). Bland–Altman analysis was used to determine inter‐ and intraobserver reproducibility and to compare differences between algorithms. Differences in age and PWV measurements were analyzed with Student's t‐tests. The variability of age‐corrected data was assessed with a Brown‐Forsythe analysis of variance (ANOVA) test.

Results:

2D (4.6–5.3 m/s) and 4D (3.8–4.8 m/s) PWV results were in agreement with previously reported values in healthy subjects. Of the four PWV algorithms, the TTU, TTF, and XCorr algorithms gave similar and reliable results. Average biases of +0.30 m/s and ?0.01 m/s were determined for intra‐ and interobserver variability, respectively. The Brown‐Forsythe test revealed that no differences in variability could be found between 2D and 4D PWV measurements.

Conclusion:

4D PC‐MRI with radial undersampling provides reliable and reproducible measurements of PWV. TTU, TTF, and XCorr were the preferred PWV algorithms. J. Magn. Reson. Imaging 2013;37:853–859. © 2012 Wiley Periodicals, Inc.