Cardiac MRI lab #2 Background The heart moves significantly during contraction - with a motion greater than the size of the MRI pixel. If imaging is done during this motion,

significant blurring and artifacts will occur. The cardiac motion is repeated every heart beat so the ECG signal can be used as a gating trigger. Additionally, the heart moves with respiration, as it is located on top of the diaphragm. This respiratory motion motion is significantly less repeatable than the motion due to cardiac contraction, so often cardiac sequences are done during a breath-hold. The subjects' breath-hold duration puts a real world limit on the length of the scan. The most basic method of creating images of the heart over the cardiac cycle (cine imaging) is using prospective cardiac triggering (using the R-wave in the ECG as the trigger) and acquiring k-space in the segmented manner. See attached papers for background (especially figure 3.15 and 10.10). This method requires a balancing of the number of time frames acquired, the image resolution, the TR, and the overall scan time. Pre-lab exercises/questions 1. Read the posted papers on cardiac MRI, especially sections starting on 149-158. 2. Calculate the optimal flip angle for SSFP based on published values of T1 and T2 for blood myocardium and fat. Note that we will be scanning at 3.0 Tesla. Lab exercises 1. A subject will be placed into the scanner and ECG lead will be attached. Observe the ECG signal outside and inside the MRI scanner. Do you see any difference? What would be causing the difference if you see anything? (we can grab a screenshot of the ECG signal) 2. Localizer scan will be performed. A segmented, gradient echo sequence will be loaded and planned in short-axis orientation. A scan will be done with a ≈12 second breath-hold (giving instructions to subject). The scan will be repeated without breath-holding. Describe the difference in the image and explain the artifacts in terms of motion and k-space. 3. Suppose you want an image with twice the resolution. Adjust the resolution parameters (first image lines, then FOV). What does this do to the image and why? What will this do the scan time and SNR? 4. Suppose the subject can only do a breath-hold of 6 seconds. What can you do to keep the higher resolution, but reduce the overall scan time? Define a relationship between resolution TR, number frames, image resolution, and scan time. 5. Go back to the original scan loaded. Change the flip angle based on values you cannulated in the pre-lab. Calculate the ratio contrast to noise between blood and myocardium for the three flip angle values (assume the noise is the same in all). Which flip angle would you use? Why? 6. Switch to retrospective triggering. Set the time frame at the same number acquired previously. Then set this to the maximum number of frames possible. What did this do to the scan time? How does prospective triggering work? What is the limitation of it? 7. Acquire a set of short axis slices covering the left ventricle. Calculate the ejection fraction. 8. We will do a few other scans (Look-Locker, Phase Contrast Flow, Angiography as we have time)