Quantification of Mouse Heart Left Ventricular Function, Myocardial Strain, and Hemodynamic Forces by Cardiovascular Magnetic Resonance Imaging

This CMRI protocol facilitates non-invasive in vivo quantification of mouse cardiac functional parameters, including ejection fraction, E over A ratio, global longitudinal strain, and hemodynamic forces. All functional cardiac parameters can be obtained from a single cardiac MRI examination and no complex tagging or dense scans are needed to quantify strengths or hemodynamic forces. Evidence suggests that global longitudinal strain and hemodynamic forces early diagnostic markers of heart failure.

To begin, place the mouse in the supine position on the mouse cradle. Hook the incisors of the mouse in the bite bar on the mouse cradle and adjust the nose cone to fit properly. Visually check if the breathing is stable, below 100 breaths per minute.

Use petroleum jelly to insert the rectal temperature probe and tape the fiber optic cable of the temperature probe to the mouse cradle. Place the respiratory balloon on the lower abdomen of the mouse and secure it with tape. Insert two ECG electrode needle subcutaneously in the thorax at the height of the forepaws and gently tape them down to prevent movement.

Place the radio frequency or RF coil over the mouse and connect the coil cables. Then, place the cradle in the magnet bore. Finally, check if the ECG signal is still stable.

Adjust the ECG and respiratory gating parameters within the ECG and respiratory signal monitoring software such that trigger points are generated at the R-peaks as the signal turns green only during the flat portion of the respiratory signal. To minimize ECG gating errors, set a blanking period of 10 to 15 milliseconds shorter than the R-R interval and keep updating this during the entire session. Based on the initial scout, perform a gated single frame gradient echo scout scan with five slices in three orthogonal directions.

To this end, position the stacks of slices on the approximate location of the heart. Perform a gated single for a multi-slice, short axis scout scan. To this end, use the previous gradient echo scout to position four to five slices in a mid left ventricular position, perpendicular to the long axis of the heart to find an initial estimate of the short axis views.

Then, in the sagittal view, check if slices are perpendicular to long axis. For the subsequent scans, adjust the number of cardiac frames or end frames such that the product of end frames and repetition time is about 60 to 70%of the R-R interval. Perform a gated single slice gradient echo scan to generate the long axis 2-chamber scout.

To this end, use the short axis and initial gradient echo scout scans to position a slice perpendicular to the short axis view, running parallel to the connection points between the left and right ventricle. Move this slice to the middle of the left ventricle and check in the coronal image of the gradient echo scout if the slice is aligned with the left ventricular long axis such that it is placed through the apex. Or form another gated single slice gradient echo scan to generate the 4-chamber scout scan.

To this end, position a slice perpendicular to the 2-chambers scouts scan and align to the center of the long axis such that the slice goes through the mitral valve and the apex. In the short axis views, adjust the slice such that it is placed parallel to the posterior and anterior ventricular wall and between the two papillary muscles. Check if the slice remains in the center of the ventricle throughout the entire cardiac cycle.

For a systolic function measurements perform a gated sequential multi-slice, short axis gradient echo scan. To this end, position a mid ventricular slice perpendicular to the left ventricular long axis in the 2-chamber and 4-chamber views in the center of the heart, and increase the number of slices to cover the heart from base to apex. For the following retrospectively gated scans, turn off all prospective cardiac and respiratory gating functionality.

Make a note of the cardiac and respiratory rate before and after each retrospectively gated scan and use these values for reconstruction purposes later. Perform three sequential single slice retrospectively gated gradient echo scans in short axis for quantification of the E'A'ratio, and 2-chamber and 4-chamber views, necessary for quantification of myocardial strain and hemodynamic force values. If the 2-chamber and 4-chamber scout orientations are suboptimal, adapt the orientations before performing the 2-chamber and 4-chamber scans.

Finally, perform a retrospectively gated, a single slice gradient echo scan in a 3-chamber view. To this end, position a slice perpendicular to the mid ventricular short axis view and turn the slice 45 degrees to pass from the anterior wall to the papillary muscle closest to the posterior wall. Inspect the basal short axis slice to see if the slice passes through the mitral and aortic valve.

Inspect the long axis 4-chamber view to determine if the slice is going through the apex. Open the reconstruction software retrospective, and load the raw data file corresponding to the retrospectively gated MRI scan. Inspect the raw navigator signal and note that the higher signal peaks represent the respiratory frequency and the lower signal peaks represent the heart rate.

Additionally, check whether the automatically detected heart rate corresponds to 10%of the observed values during each scan. If not, manually adjust these values because automated detection failed. Press Filter to perform the navigator analysis, which separates the heart navigator from the respiratory navigator.

Set the number of CINE frames to 32, and press sort k-space. Choose appropriate settings for a compressed sensing regularization and press reconstruct. Once the reconstruction has finished preview the CINE movie to evaluate the reconstruction.

Export DICOM images for further analysis with Export DCM. For a volumetric assessment of the left ventricle, select the multi-slice short axis scan images, and load it into the plugin for volumetric measurements. Use the contour tools to segment the endomyocardial borders in the end-systolic and end-diastolic frames.

For a diastolic measurements, select the mid ventricular short axis CINE images and load these into the plugin for volumetric measurements. Use the contour tools to segment the endocardial border for all frames. Compare the segmentation of neighboring frames as well as the generated volume time curves to ensure smooth transitions of the segmentation throughout the cardiac cycle.

Notice the distinct E and A filling phases. Export the left ventricular endomyocardial volumes and the corresponding timestamps, and load the values into the custom-built script provided in the supplemental material to calculate the E'A'ratio. For strain and hemodynamic force calculations, select the 2-chamber, 3-chamber, and 4-chamber long axis CINE images and load them into the plugin for volumetric measurements.

Use the contour tools to segment the endocardial border for all frames in all three orientations. Compare the segmentation of neighboring frames to ensure smooth transitions of the segmentation throughout the cardiac cycle. Once the contours are drawn in the plugin for volumetric measurements, run the plugin for the strain and hemodynamic force analysis.

Assign each of the acquired data sets to the corresponding labels for 2-chamber, 3-chamber, and 4-chamber views, and execute the strain analysis. For hemodynamic force analysis, draw the diameter of the mitral valve at the end-diastolic frame in all three orientations, and draw the diameter of the aorta in the 3-chamber long axis image. Representative high frame rate reconstructions of retrospectively gated scans using a custom-built post-processing software is shown.

From the resulting images, volume time curves during the cardiac cycle were determined as well as the corresponding first derivative curves for calculation of systolic and diastolic function parameters, respectively. The two, three, and four-channel view CINE images were analyzed using image analysis software to determine endocardial global longitudinal strain or GLS changes across the cardiac cycle and corresponding GLS values as a measure for myocardial strain. For each animal, it's also possible to produce a hemodynamic force time profile, which follows a consistent pattern of positive and negative peaks that represent the magnitude and direction of the hemodynamic force during the cardiac cycle.

Descriptive results of all outcome parameters were summarized. It is crucial that the ECG and respiratory signal monitoring software consistently detects the R-peaks. Otherwise, triggering is suboptimal, which could increase the scan time and lower the image quality.

For optimal quality of the cardiacs in images, it is important to find the best trade-off between total imaging time, number of cardiac frames, and the degree of regularization during reconstruction.