MR Cardiac Flow Imaging

MR Cardiac Flow Imaging   You have now completed the course MR Cardiac Flow Imaging.  You should now be able to:  Perform velocity and flow measurements Describe the data acquisition process Identify the optimal VENC for cardiac flow imaging ​Identify flow direction Click the Next button to take the assessment. By the end of this course you will be able to: Perform velocity and flow measurements Describe the data acquisition process Identify the optimal VENC for cardiac flow imaging ​Identify flow direction Flow Quantification Methods Physical Measures: Velocity Flow Volume Clinically Assess: Cardiac Output Stroke Volume Shunt Fraction Regurgitant Fraction Peak Velocity Pressure Gradient Cardiac triggering synchronizes data to pulsatile flow   Flow compensated (S1) and flow encoded (S2) echoes collected in pairs Velocity Encoding Sensitivity Maximum velocity encoded by the sequence Has both magnitude and direction Chosen by the user VENC is the velocity when phase angle (g) reaches +/- 180 degrees max forward velocity gives max white pixel (+4096) max reverse velocity gives max black pixel (-4096) zero velocity (stationary) gives mid gray pixel (0) Why VENC Adjustment is Important: VENC must be appropriate for Peak Velocity VENC should be adjusted just slightly greater than peak velocity If VENC is too high or too low, results may be inaccurate   VENC = 100 cm/sec Maximum white pixel = +100 cm/sec forward velocity Maximum black pixel = -100 cm/sec reverse velocity Mid-gray pixel = zero velocity    Peak Velocity is slightly less than VENC Maximum grey scale is fully used   Peak Velocity exceeds VENC Maximum grey scale is exceeded Peak Velocity is much less than VENC Maximum grey scale is not completely used   Reduces Signal-to-Noise Ratio Causes inaccurate Vmax VENC optimal Max black/white & no aliasing VENC slightly too low Very little aliasing VENC much too high Washed-out black/white VENC much too low A lot of aliasing VENC direction must be applied along flow direction In-plane VENC for flow within image plane Thru-plane VENC for flow perpendicular to image plane Choice of Direction In-plane velocity encoding is typically used for flow visualization, whereas through-plane velocity encoding can be used to determine flow and transported volume per cardiac cycle.   (Blood) flow (in ml/s) (Blood) Volume transported through cross-section per cardiac cycle (in ml)       In-Plane Flow:  Sagittal Aorta Through-Plane Flow:  Axial Aorta Slice must be exactly perpendicular to flow (use 2 projections)   How are Velocity, Flow, and Volume Related? Velocity (cm/sec) Flow (cm³/sec) = Velocity (cm/sec) * Area (cm²) Volume (cm³) = Flow (cm³/sec) * Time (sec) Incorrect slice angle causes under estimation of Vmax   Position slice perpendicular to ascending aorta   Sometimes, it is helpful to run a CINE image first to determine the area of turbulent flow in the images.   Position the slice perpendicular to the pulmonary artery outside the turbulent flow.   Ensure that you are as far away from the pulmonary valve as possible, but prior to the branching of the pulmonary artery. Click the tabs below to view features of retrospective and prospective triggering. Retrospective Measures through entire cardiac cycle Arrhythmia rejection is available Acquisition Window automatically adjusted Interpolates to any desired cine Prospective Measure less than entire cardiac cycle Very sensitive to arrhythmias and variable heartrates Acquisition Window manually adjusted Cine frame-rate determined by # of measured segments Insufficient temporal resolution causes inaccurate velocity and flow Always keep temporal resolution (TR) as small as possible   Multiple k-space lines are measured per cardiac phase More lines/phase = faster scanning, but larger TR and worse temporal resolution Lines/phase also known as the parameter Segments   Multiple velocity-encodings measured per cardiac phase (segment) More encodings/segment = more velocity info, but larger TR (poor temporal resolution)     One VENC Calculated Phases 20 TR 28 ms Segments 3 Time 19 sec   Three VENCs Calculated Phases 20 TR 37 ms Segments 2 Time 28 sec Three velocities in one direction   Forward Flow Protons moving through a magnetic field gradient produce a phase shift proportional to their velocity & direction Reverse Flow However, stationary protons produce no phase shift No Flow Measure the difference in phase shift between moving and stationary protons Forward Flow Forward Flow Reverse Flow Reverse Flow No Flow No Flow Calculated Velocity Calculated Velocity Click the items below to view 3D VENC examples. Example 1 Example 1 One velocity in 3 directions    Rephased Ant-Post Right-Left Through-Plane Example 2 Example 2   Rephased Head>Foot Through-Plane Ant - Post