With oxygenation-sensitive MRI, we can track myocardial oxygenation over time, and using breathing maneuvers, we can interfere with the physiology and therefore use this test for assessing microvascular function and overall coronary vascular function. Our technique utilizes an endogenous contrast agent in deoxyhemoglobin and vasoactive breathing maneuvers instead of a pharmacological stress agent to induce changes that we can then use to assess coronary vascular function. The needle and stress-free method allows us to assess vascular function in various diseases, and especially in diseases like heart failure with preserved ejection fraction and other forms of non-coronary ischemia.
This will allow for a diagnostic approach that will provide important clinical information for imaging people and for assessing the success of therapeutic interventions. When beginners are using this technique, they should make sure to practice the breathing maneuver with their participants outside of the scanner. As well, when performing the actual acquisition, the MRI technologist should watch the live-scanning window and the respiratory trace to make sure that the participants are accurately and adequately performing the hyperventilation and breath hold.
And now, Maggie Leo, our chief technologist here at the McGill University Health Center actually lead you through the procedure. To begin, ensure that every participant passes the MRI safety and compatibility questionnaire of the local institution, which should indicate questions on past medical and surgical history. Identify the presence of an implant, device, or metallic foreign body inside or at the surgical site of the participant.
If applicable, obtain a pregnancy test. Verify that the patient has abstained from vasoactive medication and caffeine for 12 hours before the MRI scan. Show the participant the instructional breathing maneuver video and perform a practice session of 60 seconds of paced hyperventilation, followed by a maximal voluntary breath hold with every participant outside of the MRI scanning room and provide feedback on the performance of the hyperventilation.
Instruct the participants to resume breathing when they have a strong urge to do so. Increase the repetition and echo time on the MRI console's standard bSSFP sequence and decrease the flip angle. Create two OS series, a baseline labeled OS base, and the continuous acquisition during which the breathing maneuver is performed labeled OS continuous acquisition.
In the OS continuous acquisition, increase the TE from one to 1.7. Increase the number of cardiac cycles until the acquisition time is nearly 4.5 minutes. For slice prescription, plan in an end systolic still frame of a long axis view.
Prescribe two short axis slices, one at the mid to basal, and the other at the mid to apical ventricular level. Adjust the sequence parameters as required for a given participant. Adjust the average gap in spacing between slices based on the size of the participant's heart and ensure proper slice location.
Adjust the field of view to avoid wrap artifacts if necessary. Make every effort to keep the field of view between 360 and 400 millimeters. Adjust the shim volume to be tight around the left ventricle in both the long and short axis views.
For sequence acquisition, approve the sequence and run it during the end-expiration breath hold. Ensure that this baseline OS sequence lasts approximately 10 seconds based on the heart rate and MRI scanner. Check both slices of the acquired series and look for any respiratory motion, poor slice location, or the presence of artifacts.
For troubleshooting, if the slice location is too basal or too apical, adjust the prescribed slices accordingly. If an artifact is present, check the phase and coating direction. Then make the field of view larger and adjust the shim volume around the left ventricle.
For the sequence planning, copy slice position and adjust volume from the OS baseline image as demonstrated earlier. Verify the image and slice positioning, then capture cycle. If applicable, open the livestream window.
In the control room, plug a device with the breathing maneuver instructions dot MP3 file into the auxiliary input or place it over the microphone projecting into the MRI scanner. Alternatively, manually guide the participant through the breathing maneuver using a stopwatch for timing and verbally provide instructions through the microphone connected to the MRI speaker system. Simultaneously press play for the OS continuous acquisition sequence on the MRI scanner for sequence acquisition and press play for the dot MP3 breathing instruction file as demonstrated earlier.
If manually guiding the participant through the breathing maneuvers, instruct them to breathe in and breathe out, then hold their breath for 10 seconds and to start hyperventilating as soon as they hear the metronome beep. Notify the participant at the 55-second time point of hyperventilation to take a deep breath in, breathe out, and hold their breath. Monitor the participant's performance of the paced hyperventilation through the control room window or MRI scanner camera to ensure adequate performance of the deep breathing.
If bellows are used, monitor the amplitude peaks on the respiratory gating viewer. If hyperventilation is not adequately performed after initial guidance, abort the acquisition and repeat the OS continuous acquisition sequence. Monitor for any small breaths taken by participants throughout the breath hold by monitoring the tracing of a respiration belt on the MRI console, or visually through the window or camera.
Once the participant starts breathing at the end of the breath hold, stop the acquisition. After the end of the acquisition, ask the participant if they experienced any adverse effects and allowed the participant to breathe normally for three minutes. The global B-MORE data was displayed visually and quantitatively.
Visually, pixel-wise color overlay maps were generated to augment quantitative measurements assessing myocardial oxygenation. Myocardial oxygenation reserve assessment depicted a stable global myocardial oxygenation in a healthy volunteer. A decrease in regional myocardial oxygenation was observed in a patient with a left anterior descending stenosis, while a global reduction in myocardial oxygenation was observed in a patient with heart failure.
Quantitatively, global B-MORE values were compared between healthy volunteers and patients with obstructive sleep apnea syndrome, coronary artery disease, ischemia with no obstructive coronary artery stenosis, heart failure with preserved ejection fraction, and post-heart transplant. To ensure good OS acquisition overall, there are three things to remember. One, that the images are performed in end-expiration for the OS breath hold.
Two, that the images themselves are free of any artifacts. And three, that the technologists are watching in the live scanning window to ensure accurate and adequate performance of the hyperventilation and breath hold. The results of breathing-enhanced oxygenation-sensitive MR can actually easily be combined with other markers from the MR scan such as tissue characteristics and function, and therefore serve as an overall as a very powerful diagnostic test that is cost efficient, safe, and extremely important for clinical decision making.
