Mohd Farid

Mohd Farid

Saturday, September 29, 2012

The Sonographer Attachment Programme At USA Report : Mayo Clinic: Advanced Echo =2D Strain=



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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