Flow Imaging Data Acquisition Ecg-Triggering Strategies Prospective Triggering • Measures less than entire cardiac cycle • Very sensitive to arrhythmias and variable heartrates • Acquisition Window manually adjusted • Cine frame-rate determined by # of measured segments Retrospective Triggering • Measures through entire cardiac cycle • Arrhythmia rejection is available • Acquisition Window automatically adjusted • Interpolates to any desired cine frame-rate Most Common Ecg-Triggering Strategies ECG Trigger Pulses Measured Echoes Retrospective (most common) Prospective a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | N interpolated frames measured frames N interpolated frames N 56 N measured frames Undersampled cardiac cycle = 10-20% Temporal Resolution • • Insufficient temporal resolution causes inaccurate velocity and flow Always keep temporal resolution (TR) small as possible Lotz, J et al Radiographics 2002;22:651-671 Copyright ©Radiological Society of North America, 2002 K-space Segmentation • Multiple k-space lines measured per cardiac phase (segment) • More lines/segment = faster scan, but larger TR (worse temporal resolution) ECG Trigger Acquisition Window Acquisition Window TR Echoes Cardiac Phases aaaaaaaaaa aaaaa a a a a a a a a aa 1234512345 12345 10 10 Echoes 1-5 N Echoes 6-10 a a aa a 10 N repeat to fill K-space Multiple VENC’s • Multiple velocity-encodings measured per cardiac phase (segment) • More encodings/segment = more velocity info, but larger TR (worse temporal resolution) ECG Cardiac Phases rx rx rx rx rx rxyz rxyz rxyz rx rx rx rxyz 10 11 12 13 14 15 16 17 rx rxyz rx rxyz rx rx rxyz rx rx rxyz rx rxyz rx Venc rx 10 rxyz VENC’s TR r = reference (zero) velocity encoding x= velocity encoding in x direction y= velocity encoding in y direction z= velocity encoding in z direction Single VENC velocity in direction Rephased Images Phase Images VENC Scout velocities in direction Rephased VENC 150 VENC 90 VENC 200 aliasing 3-D VENC velocity in directions Rephased Right-Left Ant-Post Through-Plane 3-D VENC velocity in directions Rephased Head>Foot H >> F Through-Plane Ant-Post Typical Retrospective Triggering VENC VENC‘s Calc Phases 25 Calc Phases 20 TR 39 ms or TR 59 ms Segments Segments Time 20 sec Time 30 sec 3x more velocity info but less temporal resolution and longer scan time ... More lines/segment = faster scan, but larger TR (worse temporal resolution) ECG Trigger Acquisition Window Acquisition Window TR Echoes Cardiac Phases aaaaaaaaaa aaaaa a a a a a a a a aa 1234512345... Retrospective Triggering • Measures through entire cardiac cycle • Arrhythmia rejection is available • Acquisition Window automatically adjusted • Interpolates to any desired cine frame-rate Most Common... 10-20% Temporal Resolution • • Insufficient temporal resolution causes inaccurate velocity and flow Always keep temporal resolution (TR) small as possible Lotz, J et al Radiographics 2002;22:651-671