The overall goal of the following experiment is to perform high-fidelity functional cardiac MRI of the heart at ultra high field strength of 7 Tesla. This is achieved by A.utilizing multi-channel RF coil technology specifically tailored to the ultra high field strength. B.careful subject positioning.
C.employing high order BO shimming and D.making use of the available ECG trigger devices. Cardiovascular magnetic resonance imaging is of proven clinical value with a growing range of indications. This imaging in particular is of profound relevance for the assessment of myocardial function.
Ultra feeds, such as 7 Tesla, provides of a large signal noise advantage which can be transferred into spay-shous-lou-shus that exceed today's limits. In turn, we expect new possibilities for myocardial tissue characterization and micro structure imaging. The advantages of 7 Tesla is sometimes offset by a number of practical obstacles and physics raley phenomenon, such as Moreover, ECG triggering can be significantly impacted by the magneto-hydrodynamic effect.
Recognizing these challenges, we propose a setup and protocol for functional at 7 Tesla. The proposed imaging protocol consists of a four-fold improvement in spatial resolution with today's clinical practice. Unlike clinical scanners, operating at 1 point 5 or 3 Tesla, the ultra-high field scanner is not equipped with a body coil, and the use of a local transceiver array is essential to signal excitation.
Thus, the patient table has to be prepared to accommodate the additional hardware required to operate the dedicated 32-channel transceiver RF coil. The coil used in this experiment consists of several power-splitter, phase-shifter, and transmit receive interface boxes, in addition to the two RF coil sections that will be placed below and on top of the subject. First, place the additional RF coil hardware at the top end of the patient table.
Link the individual boxes with the appropriate BNC cables. Connect the interface boxes to the four coil plugs on the patient table. Make sure that there is sufficient space on the table to guarantee the positioning of the subject within the isocenter of the magnet.
As shown here, this can be achieved by pre-defining a spot for the coil on the patient table, in the preliminary test with volunteers of different body height. Place the posterior coil array into the pre-defined spot on the patient table. Connect the coil with the appropriate interface box.
Next, connect the four modules of the anterior coil array with its interface, and place them aside to allow for the subject positioning. Inform the subject about the imaging procedure, as well as the potential risks of undergoing the examination, and obtain consent in writing. Before entering the MRI safety zone, perform the MRI safety and metal screening.
Since imaging is performed during breath-hold at the end of expiration, consistent breath-holding is integral to image quality. Coach the subject on breathing technique prior to scanning. Position the subject's heart central to the posterior coil array.
The head will usually be placed on top of the coil interface box connectors. Careful placement of the cables and appropriate use of cushioning is important, and ensures the subject's comfort and compliance. Attach the ECG electrodes and trigger device to the body.
Attach the pulse trigger device to the subject's index finger. The second trigger device allows for switching in the event of severe ECG signal distortions. Hand the safety squeeze ball to the subject.
Place the anterior coil on the subject's chest. Use headphones and ear buds to reduce the noise exposure and allow communication with the subject. Drive the subject into the scanner bore.
Check the communication systems and the well-being of the subject before proceeding. Hi, can you hear me? Are you okay?
We're gonna start the scanner shortly now. Use basic localizer scans to verify the correct positioning of the participant's heart in the isocenter. Reposition the subject as necessary.
Next, prescribe the shim volume so that it covers the heart entirely. Use a non-triggered flow compensated, 2D multi-echo flash shim sequence to calculate the third-order shim currents. After setting the currents, make sure that the shim volume and the shim currents remain fixed throughout the remainder of the examination.
For double-oblique slice planning employ a breath-held and ECG triggered 2D flash sequence. The breath is always held in expiration. First, plan the two-chamber localizer slice perpendicular on the axial scout, and parallel to the septal wall.
To optimize the image contrast employ high-flip angles or use a segmented cine acquisition. Second, plan the four-chamber localizer slice perpendicular to the two-chamber localizer through the mitrial valve and the apex of the left ventricle. Finally, acquire seven short access localizer slices perpendicular to the four-chamber localizer slice, parallel to the mitrial valve and perpendicular to the septal wall.
Adapt the field of view as needed. Perform the cine acquisitions using a high-resolution, breath-held, ECG-triggered, segmented, 2D flash sequence. Start with the left ventricular four-chamber view, also known as the horizontal long axis.
Plan the central slice through the center of the mitral and tri-cuspid valves, and the apex of the left ventricle. Cover the entire heart. Scan each slice during an individual breath-hold expiration.
Proceed with the left ventricular short axis slices. Plan them perpendicular to the horizontal long axis and parallel to the mitral valve. Cover the whole left ventricle from the base to the apex.
Make sure that the first slice is accurately positioned at the mitral valve in leaflet insertions. Again, acquire each slice with an individual breath-hold and expiration. This figure shows a typical ECG trace obtained from a volunteer outside of the magnet bore on the left, and in the isocenter of the magnet on the right.
The ECG is corrupted by interference with electromagnetic fields and by the magneto-hydrodynamic effect, or MHD Effect, for short. The MHD effect is pronounced during the cardiac phases of systolic aortic flow and is visible as a severe distortion of the ST segment in the ECG trace. This compromises R-wave recognition and synchronization of the data acquisition within the cardiac cycle.
This figure shows representative diastolic and systolic images of the long axis views obtained using the proposed protocol. Due to severe distortions of the ECG trigger signal, pulse triggering was utilized for this acquisition. The jitter in the trigger signal induced minor motion artifacts which are pronounced during sistol.
Here, representative short axis views are shown. The very high spatial resolution of one by one millimeter in plane is clearly visible. Even when employing a slice thickness as thin as four millimeters, the images provide ample signal to noise and contrast to delineate the myocardial walls.
In some cases, signal voids due to destructive interferences in the transmission field can be seen as well. The 7 tesla allows us to perform the Cneg rescisions with very high special resolution. Compared to 1.5 or 3 Tesla, we were able to improve the spatial resolution by a factor of three to four.
Our results show that fungshotic MRI examinations can be successfully conducted at 7 Tesla and we can demonstrate the potential of ultra-field cardiovascular imaging.
