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In-vivo distortion of through-plane flow by spiral phase-contrast imaging
Journal of Cardiovascular Magnetic Resonance volume 14, Article number: W61 (2012)
To explain previously unrecognised consequences of two sources of phase curvature over the vessel cross-section in spiral imaging i.e. off-resonance and the velocity-encoded phase-shift.
The effects of off-resonance frequency errors during spiral readouts [Yudilevich and Stark, 1987] are known. Here we explain previously unrecognised intra-voxel dephasing consequences of two sources of phase curvature over the vessel cross-section in through-plane flow imaging using spirals i.e. off-resonance and the velocity-encoded phase-shift, including the consequences for in-vivo measurements.
Laminar through-plane flow phantom (50cm/s) and popliteal artery studies were acquired at Venc=50cm/s with initial off-resonance offsets ± 0 10, 20, 40Hz representing <1ppm at 1.5T. Reference (“Ref”) and velocity-encoded (“Vel”) magnitude and phase images were obtained (as a cine in-vivo). Spiral FOV was 150 mm, 1mm resolution, duration 25.7ms; TE/TR 4.0/32.7 ms, FA 30°, 4 interleaves.
Figure 1 shows magnitude and phase images for Ref and Vel scans with corresponding velocity maps (VM). For low flow rates the off-resonance blurring at +40Hz and -40Hz is similar for both Ref. and Vel. images. However, high velocities cause an apparent “implosion” (Figure 1-I) and “explosion” (Figure 1-E) of the vessel for opposite off-resonance frequencies. From the distorted VMs, peak velocity was measured at 37.8, 48.6, 56.3cm/s for -40, 0, 40Hz off-resonance respectively.
For theoretical explanation, Figure 2a) depicts the ideal diametrical phase line profile across a tube velocity-compensated spiral imaging, red line = on-resonance and blue/green lines = phase curvature induced by ±40Hz off-resonance error. Figure 2b)-red shows velocity-encoded phase over the vessel with parabolic flow. Figure 2b)-blue shows the consequence of their addition: the increased radial slope worsens intra-voxel dephasing in all but the central pixels of a laminar flow (“implosion” effect, Figure 1-I). In Figure 2b)-green the opposing phase curvatures lead to a cancellation of the radial phase slope at some radius, which can lie outside the true lumen (“explosion” effect, Figure 1-E).
The distortion of the velocity distribution over the in-vivo vessel distorts peak velocity by ~20% at -40Hz off-resonance at these sequence parameters. Separate tests eliminated through-plane gradient fields as a cause, including eddy-current effects after the velocity-encoding pulses.
These effects on magnitude images and velocity distributions at <1ppm off-resonance are potentially difficult for >20ms spiral readouts in small vessel applications at least, perhaps more so near B0-distortions such as lungs. Shorter spirals and avoiding large intra-voxel radial phase shear are some-what incompatible with rapid flow work.
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Pierce, I.T., Gatehouse, P.D. & Firmin, D.N. In-vivo distortion of through-plane flow by spiral phase-contrast imaging. J Cardiovasc Magn Reson 14, W61 (2012). https://doi.org/10.1186/1532-429X-14-S1-W61
- Phase Curvature
- True Lumen
- Phase Slope
- Radial Phase
- Spiral Imaging