Example of velocity-encoded cine-MRI images (VEC-MRI) depicting flow patterns in the large vessels on a transverse plane of the thorax.
Vmeas = Vtrue cos 6, where 6 is the angle of misalignment. However, for small angles, the error is small (a misalignment of 20° produces an error of only 6%).
VEC-MRI can be used to calculate absolute speed at each point in the cardiac cycle at given locations in the plane of data acquisition. The velocity can be measured for each pixel or within a region of interest (ROI) encircling the entire or parts of the vessel cross sectional area or across a valve annulus. Based on the measurement of the cross-sectional area of a vascular lumen or a valve annulus on the magnitude image, the product of area and spatial mean velocity (average value for all pixels in the cross-sectional area on the phase image) yields instantaneous flow volume for a specific time frame in the cardiac cycle. Integration of all instantaneous flow volumes throughout the cardiac cycle gives the volume flow per heartbeat. This technique has been evaluated in vitro as well as in vivo by several authors, and it has led to accurate measurements of aortic and pulmonary artery flow, representing the stroke volumes of the left ventricle and the right ventricle, respectively . It has been used as well in the calculation of pulmonary to systemic flow ratio, allowing noninvasive quantification of left-to-right shunts and separate measurement of right and left pulmonary flows. These measurements can furthermore apply to the evaluation and quantitative assessment of regurgitant as well as stenotic valvular lesions.
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