Velocity Encoded CineMRI

More recently several authors have demonstrated that dynamic MR imaging can be envisioned as an attractive alternative to echocardiography [58,59]. In addition to being a noninvasive technique, MR imaging has several advantages, such as providing three-dimensional anatomical and functional data, dynamic evaluation of flow and velocity measurements, and potentially more accurate measurements of ventricular function than echocardiography [60]. The clinical use of velocity-encoded MR (VEC-MR) imaging techniques allows for the accurate estimation of velocity profiles across a valve or any vascular structure. This capability of obtaining velocity information at any point in space during the cardiac cycle allows MR imaging to provide similar flow information as duplex Doppler or color Doppler ultrasound [61,62]. MR imaging, however, does not encounter the same limitations and problems of penetration for accessing different portions of the heart and therefore provides a better visualization of morphology and flow velocity throughout the cardiovascular structures.

Flow-sensitive imaging techniques permit the measurement of flow expressed either as velocity or volume flow per unit time. The most popular flow-sensitive cine-MR technique used now is the phase change technique based on the principle that the phase of flowing spins relative to stationary spins along a magnetic gradient changes in direct proportion to the velocity of flow. This technique, referred to as "phase contrast," "phase shift" MR imaging or "velocity encoded cine-MR" imaging (VEC-MRI) allows quantification of blood velocity profiles at different points during the cardiac cycle [61,63]. The VEC-MRI technique is based upon acquisition of two sets of images, usually acquired simultaneously: one with and one without velocity encoding. The subtraction of the two images allows the calculation of a phase shift that is proportional to the velocity of flow along the direction of the flow compensation gradient. Images can be reconstructed in magnitude, providing anatomic information, and in phase, providing flow velocity information. The phase shift, proportional to the velocity is displayed as variations in pixel intensity on the phase map image. On this image, stationary tissue appears in gray, whereas flow in the positive or negative direction along the flow-encoding axis will appear as bright or dark pixels, respectively. Thus, one can visually differentiate antegrade from retrograde flows. Furthermore, the phase map image can be color-coded with different colors for either flow direction as in Doppler reinforcing the differentiation between antegrade and retrograde flows (Fig. 10). Velocity can be encoded either on planes perpendicular to the direction of flow by using slice-selective direction (through plane velocity measurement) or parallel to the direction of flow by using phased-encoded or frequency-encoded directions (in plane velocity measurement) or more recently in three dimensions. However, VEC-MRI has potential sources of error and limitations [64]. Because of the cyclic nature of phase, aliasing may appear if more than one cycle of phase shift occurs. To avoid the aliasing phenomenon, which occurs when the velocity range is lower than the predicted maximal velocity, the velocity threshold must be correctly selected before the acquisition so as to maintain the phase shift less than 180 degrees. Flow-related signal loss can be due to (i) loss of coherence within a voxel, resulting in the inability to detect the phase of the flow signal above that of noise, (ii) inappropriate selection of the velocity range, leading to poor detection of small vessels with slow flow, and (iii) turbulence occurring in valvular stenoses and regurgitations. The latter can be overcome by using sequences with short TE. Partial volume averaging can occur in cases of small vessels, improper alignment of the vessel, or narrow inflow stream, particularly when using inplane velocity measurement and a thick slice. Misalignment between the true and measured flow influences the measurement of flow velocity as determined by the equation

FIGURE 9 Example of color-coded Doppler images superimposed over a dynamic 2D echocardiogram. See also Plate 55.

Magnitude f t

Flow velocity j n Vfqj!"

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