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Flow quantification
Cardiac MR can be used to calculate velocity and flow, providing additional quantitative information in the evaluation of the heart. Flow quantification is analogous to Doppler ultrasound in which moving blood can be given a positive or negative value, depending on the direction of flow.
!/images/ci/flowquan3t.jpg(Carotid ultrasound demonstrating flow within the internal carotid artery.)!
Similar to Doppler ultrasound, flow quantification provides physiologic information concerning blood flow.
!/images/ci/flowquan1t.jpg(MRI of great vessels.)!
Axial image at the level of the great vessels demonstrates reversal of flow in the left vertebral artery, consistent with subclavian steal.
!/images/ci/flowquan2t.jpg(MRI of the aortic valve.)!
Figure 3: Axial image at the level of the aortic valve shows a bicuspid aortic valve, a common cause of aortic stenosis.
Flow quantification in MR depends on a property called spin phase, in which phase differences in flowing blood are measured, with each phase being proportional to velocity. The data collected from phase-contrast measurement is then processed into two images: a magnitude image, which is used for anatomic orientation, and a velocity image, in which pixels of moving blood will have either a positive or negative value, depending on the direction of flow. By convention, black values show flow toward the viewer, whereas white values show flow away from the viewer.
Clinically, flow quantification is used most commonly to measure flow and velocity across valvular abnormalities. From these measurements, regurgitant fractions can be calculated. These measurements can be used to estimate the severity of a lesion and follow disease progression over time. Flow quantification is also used to evaluate shunts and can provide a measurement of flow across the shunt and shunt fraction.