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MR Cardiac Function

This Cardiac Imaging lecture will provide the MRI Technologists with information on cardiac function imaging.  Imaging tips to optimize image quality will be discussed.
 
Course Objectives:
Perform cine images to assess wall motion in standard views
Identify different cine pulse sequences
Identify the importance of temporal resolution for cine imaging

MR Cardiac Function By the end of this course you will be able to:  Define Temporal Resolution Identify different imaging sequences used in cine imaging Optimize parameters for cine imaging techniques Use cine imaging to assess cardiac function of: Wall Motion Ejection Fraction Valvular Function Click the tabs below to view examples of cardiac wall motion and ejection fraction. Horizontal Long Axis Vertical Long Axis Left Ventricular Outflow Tract  Short Axis    Argus syngo.via Ventricular Function Coronal Aortic Outflow Tract Left Ventricular Outflow Tract Short Axis Click the tabs below to view additional information and examples of Temporal Resolution. TR is the time increment between Cardiac Phases The shorter the TR, the better the Temporal Resolution (good) The shorter the TR, the longer the Acquisition Time (bad) Good Temporal Resolution Poor Temporal Resolution Short TR Smooth wall motion Long TR Jerky wall motion Larger Segments  >  Larger TR (worse temporal resolution)  >  Smaller TA (faster scan)   Only 1 k-space line measured per cardiac phase Temporal resolution (TR) very short Acquisition time very long (many heartbeats) Multiple k-space line measured per cardiac phase Temporal resolution (TR) much longer Acquisition time much shorter (few heartbeats) Spatial Resolution 1.7 x 1.7 x 7 mm Segments 15, TR 40 ms Retrospective Gating, 25 Phases Grappa x 2, 5 sec Breath Hold   Spatial Resolution 2 x 2 x 4 mm Segments 16, TR 42 ms Prospective Gating, 18 Phases Grappa x 2, 25 sec Breath Hold All k-space lines measured per cardiac phase Acquisition time extremely short (one heartbeat) Temporal resolution (TR) extremely long Only used in extreme situations Vertical long axis view Patient too sick to hold breath (note diaphragm) Cardiac gated over 2 consecutive heartbeats         Real-time Cine Benefits:  Can be used non-triggered and free-breathing with multiple slices for very sick patients who are having extreme arrhythmia or cannot breath-hold   Real-time Cine has less temporal and spatial resolution than Segmented Cine Real-time Cine Temporal Resolution 120 ms Spatial Resolution 5.0 x 5.0 mm Segmented Cine Temporal Resolution 40 ms Spatial Resolution 1.8 x 1.8 mm 90 x 192 matrix, iPAT x 3 Temporal resolution ~60 ms Very fast heartrate (120 bpm)  Sedated pediatric patients Unable to breath-hold 8 Slices in 8 heart beats Ejection fraction calculated Pediatric Patients: MR scanning has not been established as safe for imaging fetuses and infants less than two years of age.  The responsible physician must evaluate the benefits of the MR examination compared to those of other imaging procedures.   Radial K-space trajectory Higher spatial and temporal resolution No phase wrap at smaller FoV Isotropic in-plane resolution Less sensitive to motion Radial streaks more pronounced near edge FoV FOV Phase = 100% Trajectory = Radial Adjust Radial Views for desired scan time         Warping of grid pattern shows myocardial strain Sliding of grid pattern shows pericardial slippage   Same as spoiled GRE (FLASH) cine sequence Extra RF pulse for grid tags applied once per heartbeat   Click the tabs below to view examples and parameters for Line Tags and Grid Tags.            Minor variations in R-R interval Extreme variations in R-R interval Fibrillation, Bigeminy, excessive PVC’s +/- 300 ms Variation   No Arrhythmia Rejection +/- 300 ms Variation   With Arrhythmia Rejection The user defines an acceptance range for R-R variations   Heartbeats falling outside the range are rejected and must be repeated   This process lengthens the total acquisition time         Set acquisition window slightly shorter than shortest R-R interval Resulting cine images are not blurred, but represent an incomplete cardiac cycle   Extreme arrhythmias Blurred Retrospective-gated segmented cine   Non-gated real-time cine is insensitive to extreme arrhythmias 1 heartbeat ► Set Acquisition Window to 1 x RR 2 heartbeats ► Set Acquisition Window to 2 x RR Non-gated ► Turn off ECG Triggering & adjust Phases for desired Acquisition Time   1 Heartbeat 2 Heartbeats Non-Gated Dark bands running through image Inhomogeneous blood pool signal Pulsation ghosts in areas of high flow Caused by local field inhomogeneities (metal, shim, patient)         You have now completed the course, MR Cardiac Function.  You should now be able to:  Define Temporal Resolution Identify different imaging sequences used in cine imaging Optimize parameters for cine imaging techniques • Steady-State-Free-Precession (SSFP) • GRE Cine • Real-Time Cine • Grid Tag Cine   SSFP   (TrueFISP) Higher SNR & CNR Wall motion more clear   GRE   (FLASH) More sensitive to inflow effects Flow jets more clear SSFP (TrueFISP) GRE (FLASH) Less through-plane inflow enhancement More through-plane inflow enhancement (Valve orifice more easily seen) Grid Lines Click the tabs below to view examples of Radial Segmented Cine and Radial Real-time Cine images compared to Cartesian images. Cartesian Segmented Cine FoV 360 mm Anisotropic spatial resolution Radial Segmented Cine Better Spatial Resolution FoV 180 mm Isotropic Spatial Resolution Cartesian Real-time Cine   Radial Real-time Cine No Phase Wrap Small FoV Good spatial resolution 2.3 x 2.3 mm Good temporal resolution 65 ms   Select Arrhythmia Detection by Time   Click on Captured Cycle to adjust Target RR to Average Cycle   Adjust Trigger Window based upon RR variability (larger for more variability)   Increase Segments slightly to allow for rejected beats without long breath-hold          Optimize the Frequency Adjust Volume 1. Click the Adjust Volume button. This button is located on the Position Toolbar in VE11 and the Flyout Toolbar in Numaris X. 2. Click in the Confirm freq. adjustment checkbox (VE11) or select Always in the Confirm Frequency field (NumarisX). 3. Adjust the green box to cover only the heart and great vessels. Exclude the lungs and liver as much as possible. 4. Repeat the TrueFISP sequence. VE11 Numaris X Frequency Scout Sequence 1. Run the Frequency Scout. 2. Note the Frequency Offset Value of the image with the best quality. 3. Enter that value in the TrueFISP Measurement field. • Enter the Frequency Offset Value of theTrufi Delta Freq. parameter on the Physio/Cardiac parameter card. VE11 Numaris X The Theory Signals or images can be recovered from a significantly smaller amount of samples than conventionally required Benefits of Compressed Sensing • Decreased scan time • Acquire free-breathing, high-resolution cardiac cine images • Acquire the entire cardiac cycle with adaptive triggering in real time with a single breath-hold • Capture the whole cardiac cycle for precise quantification Ingredients • Sparsity - the signal to be recovered should have a sparse representation • Incoherence - under-sampling should result in a more noise-like aliasing • Non-linear Iterative Image Reconstruction - enforces sparsity while preserving consistency • Sequence – TrueFISP • UI Control Parameter – CS on the Resolution/ Acceleration Card • Triggering – Retrospective, prospective, or adaptive • Reconstruction – Slice-by-slice; ~ 15 seconds for a real-time cine • Processing - Inline VF/syngo.via MR Cardiac Analysis • Covers the complete cardiac cycle • Need to define the number of calculated phases • Calculated phases need to be defined as a value similar to the number of acquired phases • The number of acquired phases is shown as a tooltip on the Calculated Phases parameter Adaptively triggered CS Cine produces 2 series per slice • The first series contains the acquired frames • The second series contains a “Calculated Phases” number of frames   -  These frames are interpolated - Linearly • This method may not be suitable for highly variable heart rates • Adaptive triggering deselected then CINE imaging performed using prospective triggering • Adjust Acquisition Window to patient-specific R-R interval using Captured Cycle button • Set acquisition window to a few seconds to cover several heartbeats Adpatively Triggered CS CINE Prospectively Triggered CS CINE Temporal Resolution = 10.4 ms Acquisition Duration = 5.77 seconds Temporal Resolution = ~3.5 ms Acquisition Duration = 16 seconds > 300 Images   • Multi-shot protocols are stored with retrospective gating and without Arrhythmia Detection • If required, either Arrhythmia Detection or ECG/Trigger (prospective triggering) can be selected • Retrospective gating involves acquiring CINE data continuously from trigger to trigger Changing the number of shots per slice affects the Temporal Resolution (TR) • 8-shot retro protocol (8 heart beats per slice) • Temporal resolution approximate 10 ms • Calculated number of phases 100 • Spatial resolution 1.3 x 1.3 x 6 mm