• Chapter 14 • Contents
  Introduction  Conventional Spin-Echo  Gradient Echo Sequences  Angiography  Time-of-Flight  Phase Contrast Maximum-Intensity Projection  Saturation Reduction Contrast-Enhanced MRA  Application  Techniques Cardiac MR Imaging  Synchronization  Static Studies  Flow Studies  Clinical Applications  Advanced Techniques

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Chapter Fourteen
From Flow to
Angiography and Cardiac MRI

14-01 Some Fundamentals

ulk or macroscopic flow of blood in vessels and of cerebrospinal fluid adds still another parameter to image contrast in MR imaging. This kind of fluid mo­tion is different from diffusion and perfusion and follows spe­ci­fic laws.

A number of contrast features of flow in MR imaging and in magnetic resonance an­gio­gra­phy (MRA) are rather complicated. The explanations in this chapter give a ge­ne­ral overview, without attempting to cover the complexity of the topic in detail (Fi­gu­re 14-01).

Figure 14-01:
Checking the vessels: Will there be laminar or plug flow? Or, worse, will plaques create turbulent flow?

Flowing blood and CSF can appear bright or dark, depending on their velocity, di­rec­tion and pattern of flow, and the pulse sequence used. In routine MR imag­ing nor­mal flow effects can mimic pathology. Thus, understanding their in­flu­en­ce upon ima­ge contrast is very important. This includes also knowledge of vas­cu­lar anatomy and comprehension of vascular dynamics.

Blood flowing through a small caliber vessel usually exhibits laminar flow (Fi­gu­re 14-02). Because of shearing forces, the blood closest to the vessel wall flows slowest. Blood velocity increases towards the center. Laminar flow is the pre­do­mi­nant kind of flow in the human body.

Figure 14-02:
(a) Laminar flow, (b) plug flow, and (c) laminar flow turning into vortex and turbulent flow after a vas­cu­lar stenosis. Laminar flow is relatively slow, plug flow, as a special form of turbulent flow, is faster.

Blood flowing faster in larger caliber vessels develops turbulence, in particular at pla­ces where the vessel's diameter changes, e.g., after stenoses or in vessels with ir­re­gu­lar lu­­men, and moves more randomly, which produces phase shifts among blood cells. The spins dephase and the blood signal intensity decreases.

Plug flow is a special case of turbulent flow with a flat flow profile; all fluid ele­­ments pos­sess the same velocity. Laminar flow is relatively slow, whereas plug flow, as a special form of turbulent flow, is faster. Note that all flow patterns in Figure 14-02 consist of conceptual schemes.

In general, flow velocity differs from ar­te­ries to veins. Pulsatile flow in ar­te­ries, to a lesser extent also in veins, is cyclical and irregular, depending on sys­tole and dia­stole of the heart. Thus, the appearance of the nature and velocity of the flow will depend on when during the cardiac cycle the image is taken. This kind of flow is often turbulent during parts of the cycle.