Measuring Flow
How can phase contrast techniques be used to measure flow?
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Because phase shift is proportional to velocity, phase-contrast MR methods can be used to quantify moving fluids such as blood in arteries and veins or cerebrospinal fluid within the skull or spine.
Phase-contrast MR can detect flow in the three cardinal directions along which bipolar encoding gradients are applied. This allows calculation of three velocity vector components: vx, vy, and vz. These components be displayed as separate images using using a gray scale to define direction (superior-to-inferior, left-to-right, or anterior-to-posterior). Alternatively, the velocities may be combined into a single magnitude image, where the intensity of each voxel is proportional to the speed (S) given by
Phase-contrast MR can detect flow in the three cardinal directions along which bipolar encoding gradients are applied. This allows calculation of three velocity vector components: vx, vy, and vz. These components be displayed as separate images using using a gray scale to define direction (superior-to-inferior, left-to-right, or anterior-to-posterior). Alternatively, the velocities may be combined into a single magnitude image, where the intensity of each voxel is proportional to the speed (S) given by
These two displays (velocity-encoded and magnitude images) are illustrated below.
Velocity-encoded (vz) image, with flows from inferior to superior depicted as white and flows from superior to inferior as black. AA=ascending aorta, DA=descending aorta
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Magnitude image, with flows in all directions represented. Brightness of each voxel is proportional to speed (S).
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Either retrospective or prospective cardiac gating may be employed, enabling the measurement of flows through different phases of the cardiac cycle. A region of interest cursor is placed over a particular vessel, producing a graph of maximum flow velocity calculated at that point as a function of time after the R-wave trigger.
Individual images from the flow study may be displayed or they may be viewed in a cine loop. |
Flow velocities as a function of time in descending aorta of a volunteer using cardiac gated phase-contrast MR.
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Sagittal phase-contast cine CSF flow study. Note normal reversal of flow around the brainstem and upper cord during the cardiac cycle.
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Axial PC study through the brain stem showing pulsatile motion of the basilar artery anteriorly and flow through the cerebral aqueduct posteriorly.
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Once the mean velocity (V) is computed across a vessel, the cross-sectional area (A) can be measured and flow (Q) calculated by the formula: Q = V x A.
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.
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