2.3 Four-Dimensional(4D) Auto Left Atrial Quantify
4D full volume data were acquired with a transthoracic 4D probe, and 4 consecutive cardiac cycle images from the apical 4ch view were stored in digital format, and the measurements were analyzed offline using the commercially available EchoPAC version 203 software (GE Healthcare). During the acquisition process, it is necessary to ensure that the complete LA is included, and the frame rate exceeds 12 volumes/second to obtain adequate temporal assessment and volume/EF/strain calculation.
Open 4D LA image, select Measure/volume/4D Auto LAQ in turn. First, adjust the frame control until the mitral valve closes at the Set Landmark stage. Adjust the position and angle of image according to 4 pictograms in the upper right corner of each quadrant, so that the vertical line intersects the MV center and the apex of the LA. Second, at the Review stage, the system will segment the whole volume data set to locate the endocardial borders for the LA. At the same time, the editing function includes undo/redo to locate the time frame of mitral valve opening (ES), end-diastole (ED) and preA accurately, which can be adjusted manually when the automatic tracking is not accurate. In our study, we exclude the pulmonary veins and the LA appendage. Finally, the Results stage in the worksheet and volume provides various parameters that have been calculated:
Volume and LAEF: LAVmin, LAVmax, left atrial volume at onset of contraction (LA VpreA), and LAVImax was calculated by dividing the LAVmax by BSA. Left atrial emptying volume (LAEV) was calculated by LAVmax minus LAVmin. Left atrial emptying fraction (LAEF) was calculated by dividing LA EV by LAVmax. The following left atrial strain parameters are derived:
1.Reservoir phase: LA receive pulmonary veins during LV contraction, and account for approximately 40% of the atrium devoted to stroke volume. Longitudinal strain (LASr) and circumferential strain (LASr-c) are calculated by the difference of the strain value at ES minus ventricular ED. The reservoir phase encompasses the time of left ventricular isovolumic contraction, ejection and isovolumic relaxation.
2.Conduit phase: In normal subjects, LA passively transferring blood to the LV during early diastole accounts for approximately 35% of the atrium contribution to stroke volume. Longitudinal strain (LAScd) and circumferential strain (LAScd-c) are measured as the difference of the strain value LA PreA minus ES.
3.Contraction phase: In normal subjects, LA passively transferring blood to the LV in late diastole, account for approximately 25% of the atrium contribution to stroke volume. longitudinal strain (LASct) and circumferential strain (LASct-c) are measured as the difference of the strain value at ventricular ED minus LA-PreA only in patients in sinus rhythm.
In addition, radial strain is not involved in our study, which is related to vendor-independent software, although radial strain may account for a small proportion of LA function. We also used the global strain rather than the segmental strain because of the thin wall, which makes it difficult to obtain accurate results.