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.