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).
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).
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.