Magnetic Resonance Evaluation of Blood Flow

Published on 24/02/2015 by admin

Filed under Cardiovascular

Last modified 24/02/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 1383 times

CHAPTER 17 Magnetic Resonance Evaluation of Blood Flow

Magnetic resonance imaging (MRI) is highly motion sensitive and can noninvasively offer accurate and reproducible quantification of blood velocity and flow. There are a variety of flow-sensitive MRI sequences, including phase contrast, time of flight, and arterial spin labeling, that allow visualization and varying degrees of quantification of blood flow. In this chapter, we focus on phase contrast MRI (also referred to as velocity-encoded cine MRI), which is routinely used in clinical practice for quantitative assessment of cardiovascular physiology and flow dynamics.

DESCRIPTION OF TECHNICAL REQUIREMENTS

Phase Contrast Magnetic Resonance Imaging

Velocity-encoded cine phase contrast MRI employs a bipolar gradient pulse to encode the velocity of moving protons. The two lobes of the bipolar gradient pulse are equal in strength but opposite in orientation, one positive, the other negative. A stationary proton will experience equal and opposite gradients that cancel one another and will have no resulting phase shift. However, a moving proton will not experience an equal but opposite second lobe of the gradient pulse and consequently will acquire a phase shift (Fig. 17-1). The angle of acquired phase shift is proportional to the velocity of the moving proton. An MRI sequence with this type of bipolar gradient is thus flow sensitive.

image

image FIGURE 17-1 Diagram of the effect of bipolar gradients on stationary and moving spins. Venc, velocity encoding value.

(Modified from Westbrook C, Roth CK, Talbot J. MRI in Practice, 3rd ed. Oxford, Wiley-Blackwell, 2005.)

To calculate the angle of acquired phase shift of a moving proton to determine its velocity, the flow-sensitive sequence is subtracted from a flow-insensitive sequence (i.e., a gradient sequence without a bipolar pulse). Stationary protons are subtracted out, leaving behind only the motion-induced phase shifts of moving protons. This subtraction gives rise to phase images, in which signal intensity is proportional to blood flow velocity. The unsubtracted combination of signal from the flow-sensitive and flow-insensitive sequences gives rise to magnitude images.

Multiple phase contrast acquisitions can be obtained over the cardiac cycle. With appropriate cardiac gating, these data can be segmented into time-resolved images of dynamic blood flow, which is referred to as velocity-encoded cine MRI. Compared with echocardiography, which also offers real-time evaluation of blood flow, velocity-encoded cine MRI is operator independent and consequently more reproducible.

Flow-Encoding Axes

Flow sensitivity occurs in the orientation of the applied bipolar gradient. For example, if the bipolar gradient pulses are applied in the z-axis, the resulting motion-induced phase shifts are induced along that axis, and the velocity of blood moving from the head to the feet is encoded. Bipolar gradients can be applied in all three dimensions, allowing flow encoding in any direction. However, increasing the number of flow-encoding axes also increases scan time.

Flow quantification is performed by prescribing an imaging plane orthogonal to the direction of flow within a vessel. During postprocessing, the borders of the vessel are delineated with a flexible region of interest for each segment of the cardiac cycle. This creates a cross-sectional area for each time point and defines the pixels that contain velocities representing intravascular flow. The spatial mean velocity is then calculated from these pixels and multiplied by the cross-sectional area for each time point in the cardiac cycle (Fig. 17-2). The result is blood flow calculated in milliliters per heartbeat.1

Pressure gradients can be estimated with the modified Bernoulli equation, ΔP = 4ν2, where ΔP is the peak pressure gradient in millimeters of mercury and ν is the peak blood flow velocity in meters per second. Unlike flow quantification, phase contrast imaging planes may be prescribed in a parallel or perpendicular orientation with respect to the direction of blood flow to capture the point of peak velocity of flow downstream from a stenosis (Fig. 17-3). It is important to select a relatively high velocity encoding (Venc) value because peak velocities associated with stenotic valves can exceed 5 m/sec.2

Imaging Limitations and Pitfalls

There are some pitfalls to be aware of in employing phase contrast MRI for blood flow quantification. Signal aliasing will occur if the peak velocity of blood flow surpasses the Venc value at any point in the cardiac cycle. This phenomenon takes place because positive velocities that surpass the Venc value will give rise to phase shifts that are interpreted as negative velocities (e.g., a phase shift of 185 degrees will be interpreted as −175 degrees). On phase images, areas of aliasing are easily identifiable: the sudden loss of signal in regions of maximum signal brightness (Figs. 17-5 and 17-6).

image

image FIGURE 17-5 Diagram of aliasing with low but not with high velocity encoding value (Venc).

(Modified from Westbrook C, Roth CK, Talbot J. MRI in Practice, 3rd ed. Oxford, Wiley-Blackwell, 2005.)

Underestimation of velocity and flow can occur if a vessel is not evaluated in a plane orthogonal to the direction of flow or if partial volume averaging occurs. In addition, peak flow velocity downstream of a stenosis, and thus the associated pressure gradient, can be underestimated for two reasons: (1) as the accuracy of peak velocity measurement is dependent on temporal resolution, the value may be underestimated by MRI compared with echocardiography, which has a higher temporal resolution; and (2) the precise, three-dimensional location of the peak velocity downstream from a stenosis may not be included in the two-dimensional phase contrast evaluation.3,4

TECHNIQUES