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flow basic physics

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Flow Imaging Basic Physics Velocity and Flow Measurements Vpeak = m/sec Bicuspid aortic valve in patient with aortic coarctation Velocity and Flow Measurements Vpeak = m/sec Bicuspid aortic valve in patient with aortic coarctation Velocity and Flow Measurements Mitral valve flow; E wave and A wave Velocity and Flow Measurements E A Vpeak = m/sec Mitral valve flow; E wave and A wave Why use Flow Quantification ? Physically Measure: Clinically Assess:  Velocity  Flow  Volume       Cardiac Output Stroke Volume Shunt Fraction Regurgitant Fraction Peak Velocity Pressure Gradient How Does Flow Quantification Work ? It’s a Phase-Contrast Technique ! Protons moving through a magnetic field gradient produce a phase shift proportional to their velocity & direction However, stationary protons produce no phase shift Measure the difference in phase shift between moving and stationary protons ient d a r g etic n g a m Bo Bo_ Forward Flow positive velocity positive phase shift Bo+ ient d a r g etic n g a m Bo Bo_ Reverse Flow negative velocity negative phase shift Bo+ ient d a r g etic n g a m Bo Bo_ No Flow zero velocity zero phase shift Bo+ How is the Data Acquired ? • Flow compensated (S1) and flow encoded (S2) echoes collected in pairs • Cardiac triggering synchronizes data to pulsatile flow ECG R-R Interval R-R Interval echoes s1 s2 s1 s2 s1 s2 s1 s2 s1 s2 s1 = flow compensated echo s1 s2 s1 s2 s1 s2 s1 s2 s1 s2 s2 = flow encoded echo What does a Phase Contrast Image Look Like ? Three Types of Images ! • Rephased Image • Magnitude Image • Phase Image rephased magnitude of flow compensated echo (S1) flow is bright background visible magnitude difference in echo magnitudes (S2 – S1) flow is bright background suppressed phase difference in echo phases (g) forward flow is bright reverse flow is black background is midgray What Is VENC ? Velocity Encoding Sensitivity ! • Maximum velocity encoded by the sequence • Has both magnitude and direction VENC is the velocity when phase angle (g) reaches +/-180 degrees • max forward velocity gives max white pixel (+2048) • max reverse velocity gives max black pixel (-2048) • zero velocity (stationary) gives midgray pixel (0) Cardiovascular MRI : physical principles to practical protocols / Vivian S Lee ; Lippincott Williams & Wilkins, c2006 Is VENC Direction Important ? Yes, it Must Be Applied Along Flow Direction ! • In-plane VENC for flow within image plane • Thru-plane VENC for flow perpendicular to image plane In-Plane Flow: Sagittal Aorta Thru-Plane Flow: Axial Aorta How Critical is the Slice Placement for Quantitative Measurements of Velocity and Flow ? • Only through-plane VENC is suitable for quantitative analysis • Slice must be exactly perpendicular to flow (use projections) Incorrect Slice Angle Causes Underestimation of Vmax Lotz, J et al Radiographics 2002;22:651-671 Copyright ©Radiological Society of North America, 2002 Is VENC Adjustment Important ? Yes, it Must Be Appropriate for Peak Velocity ! • VENC should be adjusted just slightly greater than peak velocity • If VENC is way too high or too low, results may be inaccurate Optimal VENC: • • Peak Velocity is slightly less than VENC Maximum grey scale is fully used Velocity Aliasing: • • Peak Velocity exceeds VENC Maximum grey scale is exceeded Poor Dynamic Range: • • Peak Velocity is much less than VENC Maximum grey scale is incompletely used VENC Too High Reduces SNR and Causes Inaccurate Vmax Lotz, J et al Radiographics 2002;22:651-671 Copyright ©Radiological Society of North America, 2002 Phase Contrast Images with Various VENC‘s VENC optimal (good) (max black/white & no aliasing) VENC slightly too low (bad)  (very little aliasing) VENC much too high (bad) (washed-out black/white) VENC much too low (bad)  (very much aliasing) ... coarctation Velocity and Flow Measurements Mitral valve flow; E wave and A wave Velocity and Flow Measurements E A Vpeak = m/sec Mitral valve flow; E wave and A wave Why use Flow Quantification ?... Be Applied Along Flow Direction ! • In-plane VENC for flow within image plane • Thru-plane VENC for flow perpendicular to image plane In-Plane Flow: Sagittal Aorta Thru-Plane Flow: Axial Aorta... / 180 How is the Data Acquired ? • Flow compensated (S1) and flow encoded (S2) echoes collected in pairs • Cardiac triggering synchronizes data to pulsatile flow ECG R-R Interval R-R Interval

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