2.5. Speckle tracking imaging study:
For the STI study, the second-harmonic B-mode images of apical (4-chamber, 2-chamber, and 3-chamber) and short-axis (at the mitral valve and apical level) views were obtained. The LV endocardial border was manually traced at the end-systolic frame and a speckle-tracking region of interest was automatically selected. The width of the region of interest was adjusted as necessary to accommodate the total thickness of the LV wall. The computer automatically tracked stable objects in each frame using the sum of absolute differences algorithm. After these steps, the workstation computed and generated strain curves. For assessment of LV rotational mechanics through scanning and recording from left para-sternal short-axis view of both basal and apical short-axis planes to quantify basal and apical LV rotations using the same probe, with a frequency range 1.7–2.0MHz at a high frame rate (range: 80–115 frame/s).
The basal level was marked as the plane showing the tips of mitral valve leaflets at its center with full-thickness myocardium surrounding the mitral valve. Then the transducer was positioned one or two intercostal spaces more caudal and slightly lateral from the basal site to be perpendicular to the apical imaging plane (14). The apical level was defined just proximal to the level of LV apical luminal obliteration at the end-systole. The cross-section must be as circular as possible. We must pay careful attention to ensure that full thickness of myocardium is imaged throughout the cardiac cycle.
To analyze twist and untwist parameters, from the basal and apical short axis, data set with a well-defined endo-cardial border and the regions of interest were adjusted to include all myocardial thickness without including the pericardium. The endocardial borders of both basal and short axis planes were manually traced and subsequently tracked by the software. If poor tracking quality was observed, the region of interest was readjusted until acceptable tracking was obtained. After processing, curves of basal and apical LV rotation, twist, twist rate, and untwist rate were automatically generated by the software (Excel; Microsoft Corporation, Redmond, Washington, USA). Twist was calculated as apical rotation relative to the basal rotation, with counterclockwise rotation as viewed from LV apex expressed as positive value and clockwise rotation as a negative value. Peak LV twist, peak LV twist rate (as first positive peak after R wave on ECG), and peak LV untwist rate (as the first negative peak after aortic valve closure) were recorded. Cardiac cycle length was measured as R–R interval. Time to peak twist rate was measured as time from R wave to peak twist rate, and time to peak untwist rate was measured as time from R wave to peak untwist rate.