PC VIPR: a high-speed 3D phase-contrast method for flow quantification and high-resolution angiography

Abstract

Background and purpose: Three-dimensional phase-contrast (3DPC) is limited by long imaging times, limited coverage, flow artifacts, and the need to perform multiple additional 2D examinations (2DPC) to measure flow. A highly undersampled 3D radial acquisition (isotropic-voxel radial projection imaging [PCVIPR]) makes it possible to increase the product of volume coverage and spatial resolution by a factor of 30 for the same imaging time as conventional Cartesian 3DPC. This provides anatomic information over a large volume with high isotropic resolution and permits retrospective measurement of average flow rates throughout the volume.

Methods: PCVIPR acquires a reference and three flow-encoded acquisitions for each VIPR projection. Complex difference images were formed by combining information from all flow directions. Following retrospective definition of planes perpendicular to selected vessels, volume flow rates were determined by using phase-difference information. The accuracy of average flow measurement was investigated in a phantom and in six volunteers. Anatomic PCVIPR images acquired in three patients and three volunteers by using a 384(3) matrix were compared with conventional Cartesian 3DPC.

Results: The flow validation produced R2 = 0.99 in vitro and R2 = 0.97 in vivo. PCVIPR produced minimal streak and pulsatile flow artifacts. PCVIPR produced far higher resolution and volume coverage in comparable imaging times. The highest acceleration factors relative to 3DPC were achieved by using gadolinium-contrast material. Ultimately, acceleration factors are limited by signal-to-noise ratio.

Conclusion: PCVIPR rapidly provides isotropic high-resolution angiographic images and permits retrospective measurement of average flow rate throughout the volume without the need to prescribe multiple 2D acquisition planes.

Fig 1.

Comparison of 3DPC (A, B) with a PCVIPR (C, D) acquisition having an acceleration factor of 17. In this case, comparable imaging times were used and the acceleration is due to a factor of 2.5 in volume coverage and a factor of 7 reduction in voxel volume. No pulsatile artifact appears on the PCVIPR image (C) as it does on the 3DPC image (A, arrows).

Fig 2.

Comparison of 3DPC (A, B) with a PCVIPR (C, D) acquisition having an acceleration factor of 30. In this case residual contrast was present and the acceleration is due to a factor of 4.5 in volume coverage and a factor of 7 reduction in voxel volume.

Fig 3.

Comparison of 3DPC (A, B) with a PCVIPR (C, D) acquisition having an acceleration factor of 61. The acceleration is due to a factor of 4.5 in volume coverage, a factor of 7 reduction in voxel volume, and a factor of 2 reduction in imaging time. Visualization of some small vessels is decreased in the PCVIPR examination, which indicates insufficient SNR to support this acceleration factor.

Fig 4.

Basic PCVIPR in vitro average flow measurement validation on a pulsatile flow on the flow phantom. Ten flow rates were measured. The PCVIPR results are plotted against the computer-set flow rates. One standard deviation is plotted as the error bar on each data point. A linear relationship was reached, with a linear fit slope of 0.94 and an R² = 0.99.

Fig 5.

In vivo flow measurement validation by using 2D cine PC MR on the right ICA and the basilar artery of six volunteers. One standard deviation is used to plot the error bar on each data point. The correlation of the two methods is shown here with R² = 0.97.

Fig 6.

When no posterior communicating arteries exist in the volunteer, the basilar artery is the only blood supplier to the two PCAs (B). The flow rates from the two PCAs were measured as shown in C, whereas the flow rate of the basilar artery was measured at the location as shown in B. As shown in A, the sum of the two PCA flow rates were very close to that of the basilar artery, where 1 SD was used as the error bar.