Flow Hemodynamics
Susanne Schnell1

1Northwestern University, United States

Synopsis

Phase contrast MRI and its utilization to measure blood flow will be explained. Spins that move during an MRI acquisition exhibit different imaging characteristics compared to stationary spins. Flowing spins, for example from flowing blood, appear as an artifact in the image. However, by understanding these characteristics of flowing spins, their appearance can be utilized for angiographic purposes. 2D Phase contrast imaging is sensitized to flow by using a series of bipolar gradients to affect the phase signal of spins that flow with a uniform velocity in the direction parallel to the gradients. By utilizing ECG gating, blood flow velocities can be measured in a time-resolved manner. 2D phase contrast can be extended to a time-resolved 3D volume acquisition with 3-directional velocity encoding, which is called 4D flow MRI. This encoding of velocity enables quantification of flow hemodynamics. Furthermore, some potential sources of error will be discussed, such as misalignment of flow, velocity aliasing and phase offset errors.

Target Audience

Anyone who is interested in how we can measure blood flow and determine hemodynamic flow parameters in vivo using MRI.

OUTCOME/OBJECTIVES

Learners will understand phase-contrast MRI (2D PC and 4D Flow MRI) and how to effectively acquire and analyze the data. They will be able to understand sources of errors and how to avoid them.

PURPOSE

Phase-Contrast MR imaging will be discussed for its role in quantitative angiography. Phase contrast imaging is sensitized to blood flow velocity, affecting the phase signal of flowing spins. This encoding of velocity enables quantification of hemodynamics.

METHODS

MRI techniques provide non-invasive and non-ionising methods for the highly accurate anatomical depiction of the heart and vessels throughout the cardiac cycle. In addition, the intrinsic sensitivity of MRI to motion offers the unique ability to acquire spatially registered blood flow simultaneously with the morphological data, within a single measurement. In clinical routine, flow MRI is typically accomplished by using methods that resolve two spatial dimensions in individual planes and encode the time-resolved velocity in one principal direction, typically oriented perpendicular to the two-dimensional (2D) section. In this lecture, time-resolved 2D Phase-Contrast MRI as well as time-resolved 3D Phase-Contrast MRI techniques (4D Flow MRI) will be introduced. Several quantitative measures that can be derived from velocity encoded MRI will be discussed (peak velocity, regurgitation, pulse wave velocity, pressure drop, and wall shear stress). In addition, sources of errors such as phase offsets and velocity aliasing will be discussed. Emerging techniques and novel applications will be explored. In addition, applications of these new techniques for the improved evaluation of cardiovascular and cerebrovascular disease (intra-cranial arteries and veins) will be presented.

CONCLUSIONS

In summary, flow imaging with MRI has undergone and continues to undergo a substantial transformation, from simple techniques measuring one-directional blood flow velocities at a specific location to a more compressive diagnostic tool that can assess 3D blood flow, or derive advanced metrics of cardio- and cerebrovascular hemodynamics, such as pressure drop or WSS.

Acknowledgements

No acknowledgement found.

References

1. Markl, M., S. Schnell, C. Wu, E. Bollache, K. Jarvis, A.J. Barker, J.D. Robinson, and C.K. Rigsby, Advanced flow MRI: emerging techniques and applications. Clinical Radiology, 2016. 71(8): p. 779-795.

2. Dyverfeldt, P., M. Bissell, A.J. Barker, A.F. Bolger, C.-J. Carlhäll, T. Ebbers, C.J. Francios, A. Frydrychowicz, J. Geiger, D. Giese, M.D. Hope, P.J. Kilner, S. Kozerke, S. Myerson, S. Neubauer, O. Wieben, and M. Markl, 4D flow cardiovascular magnetic resonance consensus statement. Journal of Cardiovascular Magnetic Resonance, 2015. 17(1): p. 72.

3. Lotz, J., C. Meier, A. Leppert, and M. Galanski, Cardiovascular Flow Measurement with Phase-Contrast MR Imaging: Basic Facts and Implementation. RadioGraphics, 2002. 22(3): p. 651-671.

4. Schnell, S., S.A. Ansari, C. Wu, J. Garcia, I.G. Murphy, O.A. Rahman, A.A. Rahsepar, M. Aristova, J.D. Collins, J.C. Carr, and M. Markl, Accelerated dual-venc 4D flow MRI for neurovascular applications. Journal of Magnetic Resonance Imaging, 2017. 46(1): p. 102-114.

Further Reading:

1. Bernstein, M.A., A. Shimakawa, and N.J. Pelc, Minimizing TE in moment-nulled or flow-encoded two-and three-dimensional gradient-echo imaging. Journal of Magnetic Resonance Imaging, 1992. 2(5): p. 583-588.

2. Bernstein, M.A., X.J. Zhou, J.A. Polzin, K.F. King, A. Ganin, N.J. Pelc, and G.H. Glover, Concomitant gradient terms in phase contrast MR: Analysis and correction. Magnetic Resonance in Medicine, 1998. 39(2): p. 300-308.

3. Pelc, N.J., M.A. Bernstein, A. Shimakawa, and G.H. Glover, Encoding strategies for three-direction phase-contrast MR imaging of flow. J Magn Reson Imaging, 1991. 1(4): p. 405-13.

4. Hahn, E.L., Detection of sea-water motion by nuclear precession. Journal of Geophysical Research (1896-1977), 1960. 65(2): p. 776-777.

5. Dijk, Pieter van. Direct cardiac NMR imaging of heart wall and blood flow velocity. Journal of computer assisted tomography 8 3 (1984): 429-3

6 . Bryant, D.J., J.A. Payne, D.N. Firmin, and D.B. Longmore, Measurement of Flow with NMR Imaging Using a Gradient Pulse and Phase Difference Technique. Journal of Computer Assisted Tomography, 1984. 8(4): p. 588-593.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)