Strain And Strain Rate Imaging
Myocardial
velocities measured with TDI may be overestimated by translational motion or
tethering of the myocardium, respectively. This limitation can be overcome by
measuring the actual extent of myocardial deformation (stretching or
contraction) by strain and strain rate imaging. Strain rate is the rate of
change in length calculated as the difference between two velocities normalized
to the distance between them it is expressed as seconds -1 . By
convention, shortening is represented by negative values and lengthening by
positive values for both strain and strain rate.
Tissue
tracking, also known as displacement, is similar to strain, except it is
integral of the tissue velocity over a given time. It represents the distance a
region of interest moves relative to its original location.
In
the normal heart, longitudinal strain values are similar from the base to the
apex, unlike tissue velocity, which is higher at the base than at the apex.
Every effort is made to ensure that the direction of tissue movement is less
than 30 degrees from the direction of the beam, but this is technically
challenging in the apical segmentsas the angle become wider. The narrow sector
angle approach on an individual wall obviates some of the above problems, which
precludes concurrent comparison of contralateral segments.
Strain
imaging is similar to measuring the myocardial velocity gradient, which is limited
to analyzing the myocardium that contracts in the direction that is parallel
with the ultrasound beam. However, better spatial resolution and a higher frame
rates (up to 200 frames/s) in strain rate imaging allow simultaneous
calculation of the strain rate of the myocardium within the selected sector,
which can be colour coded. A curved cursor can be placed along the entire
circumference of the LV
to analyze regional strain rate. However, accurate measurements of strain rate
depend on properly aligning the ultrasound beam so it is parallel with the
direction of myocardial motion.
Detection Of Myocardial
Ischemia
Regional
strain and strain rate are disturbed during the early stage of myocardial
ischemia. Some studies have suggested that strain imaging is more sensitive for
detecting acute ischemia than regional wall motion analysis. It has been shown
that the longitudinal peak regional strain and strain rate decreases as wall
motion worsens in patients with an acute myocardial infraction. During balloon
inflation, systolic strain imaging has been shown to be more sensitive for
detecting myocardial ischemia than TDI. This better sensitivity of strain rate
imaging has also been shown during dobutamine stress echocardiography. An
interesting observation during ischemia is regional delay in the onset of
myocardial motion, which is difficult to identify visually.
In
acute ischemia, the transition from regional systole to early diastolic
lengthening is delayed. When a time delay of more than 20% was used, it
identified ischemic myocardium during dobutamine stress echocardiography with a
sensitivity of 92% and specificity of 75%. However its main limitations are
additional time in analysis, gain dependency, and variability. Whether this
quantitative assessment provides incremental diagnostic value is not certain,
especially when the physician who is interpreting stress echocardiogram is
experienced in regional wall motion analysis.
Assessments Of
Myocardial Viability
The
myocardial velocity gradient can be used to differentiate viable from nonviable
myocardium in patients with an acute myocardial infarction treated with acute
reperfusion. It has been observed that myocardial contraction occurs even after
closure of the aortic valve, called post systolic shortening. This can be an
indication of asynchronous motion during the isovolumic relaxation period. TDI
and strain imaging are able to demonstrate this unusual cardiac motion. Post
systolic shortening of stunned myocardium may disappear with gradual infusion of
dobutamine. The presence of post systolic shortening during acute myocardial
ischemia also predicts functional recovery after reperfusion therapy.
Evaluation Of Cardiomyopathy
By
measuring regional myocardial function, TDI and strain imaging have potential
incremental value for the evaluation of cardiomyopathy and diastolic heart
failure. Thick walls due to athletic training would have normal TDI and strain
values , whereas thick walls due to infiltration or primary myopathy would have
reduced values. In addition, the pattern of regional dysfunction may be
different for various cardiomyopathies. TDI of myocardium provides different
information from that of strain imaging because TDI is affected by translation
as well as by actual movement of the tissue. A report from France demonstrated
that all components of strain were significantly reduced in hypertrophic
cardiomyopathy despite an apparently
normal EF. In patients with asymmetrical hypertrophic cardiomyopathy, longitudinal
septal strain was significantly lower than for other LV segments combined.
Speckle Tracking
Echocardiography
Speckle
Tracking is a method for quantifying myocardial motion in various plane using
2D images. Refection, scattering, and interference of the ultrasound beam in
the myocardial tissue produce a speckle formation. Myocardial regions with
unique speckle patterns in the gray scale 2D image can be tracked from frame to
frame throughout the cardiac cycle. This allows assessments of LV rotational
motion, often offered to as torsion or twist. The spiral shape of the LV
myocardial fibers results in a complex three dimensional (3D) torsion mechanism
for systolic contraction and untwisting for diastolic relaxation. The LV
myocardium consists of two layers. The subendocardial layer wraps around in the
direction of a right handed helix and subepicardial layer wraps around in the
direction of left handed helix. When viewed from LV apex, apical rotation is
counterclockwise and basal rotation is clockwise during systole. An analogy for
the LV contraction is the motion of wringing out a wet towel with your hands.
And the two hands twist the ends of the towel in opposite directions the
portion of the towel between the hands thickens and shortens longitudinally.
Speckle
tracking is an alternative method for quantification of LV systolic, and
potentially diastolic function. It also is another method for measuring strain
using 2D images instead of the TDI method described above. Speckle tracking
does not have the limitation of angle dependence that TDI derived strain
measurements have.
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