Improved SNR in phase contrast velocimetry with five-point balanced flow encoding

Abstract

Phase contrast velocimetry can be utilized to measure complex flow for both quantitative and qualitative assessment of vascular hemodynamics. However, phase contrast requires that a maximum measurable velocity be set that balances noise and phase aliasing. To efficiently reduce noise in phase contrast images, several investigators have proposed extended velocity encoding schemes that use extra encodings to unwrap phase aliasing; however, existing techniques can lead to significant increases in echo and scan time, limiting their clinical benefits. In this work, we have developed a novel five-point velocity encoding scheme that efficiently reduces noise with minimal increases in scan and echo time. Investigations were performed in phantoms, demonstrating a 63% increase in velocity-to-noise ratio compared to standard four-point encoding schemes. Aortic velocity measurements were performed in healthy volunteers, showing similar velocity-to-noise ratio improvements. In those volunteers, it was also demonstrated that, without sacrificing accuracy, low-resolution images can be used for the fifth encoding point, reducing the scan time penalty from 25% down to less than 1%.

Figure 1

Volume rendered aliasing spaces for 4-point balanced, 4-point referenced, and 5-point balanced encoding schemes viewed from 45° and -45° in the xy plane. Each shaded region has sphere rendered at the set V_enc. In certain directions, all methods can measure velocities higher than the prescribed V_enc, which is dependent on the encoding scheme. 4-point balanced encoding is particularly dependent on the velocity direction due to the rhombohedron shape, especially when compared to the cube and regular octahedron shapes of the 4-point referenced and 5-point balanced techniques, respectively.

Figure 2

Magnitude image showing phantom rotation (a) and corresponding left to right velocity images. 4-point balanced images (b) have the same first moment magnitude as the proposed 5-point scheme (c), while 4-point referenced images (d) have the same Venc as the 5-point exam. 4-point balanced images show significant aliasing, which is not evident in 5-point images. Visually, 5-point velocity images show improved VNR over 4-point referenced images.

Figure 3

Source velocity images from 4-point referenced, 4-point balanced, and 5-point acquisitions. Images from the 5-point exam show reduced noise compared to the equal V_enc exam (Referenced) and less velocity aliasing than the same first moment exam (Balanced).

Figure 4

In-plane vector plots during systole (t=143ms) and during peak flow reversal (t=469ms) for 5-point balanced encoding and 4-point referenced encoding. Systole images show the location of vector plots on the magnitude images. During systole, both encoding schemes produce visually similar images. During diastole, where there velocities are substantially lower, 5-point encoding shows substantially reduced noise for better visualization.

Figure 5

Source S/I velocity images for a single volunteer (a) and measured aliasing (b) as a function of resolution for accelerated 5-point encoding. On source images, arrows point to locations of uncorrected velocity aliasing, while images are labeled with the percent increase in scan time. The no aliasing area is indicated on the measured aliasing and is bounded by a 23.8mm spatial resolution and 331ms temporal resolution. Error is significantly more dependent on spatial resolution than temporal resolution in this case.