Chapter Fourteen
From Bulk Flow to Angiography and Cardiac MR

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.

Blood flowing faster in larger caliber vessels develops turbulence, particularly where vessel 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 (Figure 14-02).

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, whereas plug flow, as a special form of turbulent flow, is faster. All flow patterns comprise conceptual representations.

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.