Phase-contrast Flow Measurement Accuracy
How accurate are MR methods to measure flow? What are the artifacts and pitfalls?
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The accuracy of phase-contrast methods varies by size and location of imaged vessel. In general phase-contrast measurements correlate well with other techniques including Doppler and indicator dilution methods. Several limitations and caveats are warranted, however:
Angled Imaging plane. Measurement of flow velocities is most accurate when the vessel is perpendicular to the plane of imaging and through-plane flow-encoding is used. Deviations of up to 15° are tolerable but beyond this, significant errors can occur.
Inadequate Temporal resolution. If the data sampling rate throughout the cardiac cycle is too low, flow may not be measured at its peak value. The average velocity will then be underestimated. For imaging the great vessels at least 16 frames per cardiac cycle should be sampled, with standard values of 30 being optimal.
Inadequate Spatial resolution. When the pixel size exceeds one-third of the vessel diameter, significant partial volume effects may occur with underestimation of flow and peak velocities. This is not a problem for larger vessels like the aorta, but is an important issue for smaller structures like the coronary arteries or cerebral aqueduct whose diameters may be 2-3 mm or less.
Irregular or Turbulent Flow. Phase contrast pulse sequences are optimized for linear (non-accelerated) flow. When velocities change suddenly in magnitude, direction, or both, accuracy of flow measurements declines.
Magnetic Field Inhomogeneities. Because flow velocities are computed from phase changes, any process that causes systemic phase offsets across the field-of-view will produce measurement errors. Magnetic field inhomogeneities due to susceptibility effects, gradient non-uniformities, or eddy currents may lead to either underestimation or overestimation of flows. Automated subtraction and phase-correction techniques may improve the results, and manual shimming is sometimes needed.
References
Bryant DJ, Payne JA, Firmin DN, et al. Measurement of flow with NMR imaging using a gradient pulse and phase difference technique. J Comput Assist Tomogr 1984;8:588-93.
Firmin DN, Nayler GL, Klipstein RH, et al. In vivo validation of MR velocity imaging. J Comput Assist Tomogr 1987;11:751-6.
Lotz J, Meier C, Leppert A, Galanski M. Cardiovascular flow measurement with phase-contrast MR imaging: basic facts and implementation. Radiographics 2002; 22:651-671.
Bryant DJ, Payne JA, Firmin DN, et al. Measurement of flow with NMR imaging using a gradient pulse and phase difference technique. J Comput Assist Tomogr 1984;8:588-93.
Firmin DN, Nayler GL, Klipstein RH, et al. In vivo validation of MR velocity imaging. J Comput Assist Tomogr 1987;11:751-6.
Lotz J, Meier C, Leppert A, Galanski M. Cardiovascular flow measurement with phase-contrast MR imaging: basic facts and implementation. Radiographics 2002; 22:651-671.
Related Questions
How can phase contrast techniques be used to measure flow?
How can phase contrast techniques be used to measure flow?