Technique of Speckle tracking Imaging

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Tissue Doppler and Speckle Tracking Imaging

As we mentioned in the section Doppler Tissue Imaging, echocardiographic assessment of the left ventricular function in routine clinical practice usually is visual. It can be explained by poor visualization of endocardial borders by older echo machines. With development of second harmonic imaging better visualization of endocardial borders was achieved, that allowed development of new advanced techniques for accurate quantitative assessment of regional and global left vnetricular function. The first of these methods was Integrated backscatter[1] Integrated backscatter was an accurate and objective method, but was limited to the parasternal left ventricular segments. Further technological development allowed measurement of myocardial Doppler tissue velocities as opposed to intra-cavitary velocities.[2][3] Doppler Tissue Imaging allowed evaluation of the myocardial velocities, that were of lower amplitude,than interacavitary velocities, and normally are not homogenous over the left ventricle. Basal myocardial velocities are higher than mid-ventricular, and mid-ventricular velocities are higher than apical: there is a base to apical gradient. Measurement of myocardial velocities permitted accurate calculation of other deformation parameters: strain and strain rate. Estimation of myocardial velocities with Tissue Doppler is based on the principles of traditional pulsed-wave and color Doppler echocardiography. A high-pass wall filter is used to measure blood flow velocities, and a low pass filter is used to estimate tissue velocities.[4]

Strain

Strain is the fractional change in the length of a myocardial segment, and is usually expressed as percentage. It may have positive and negative values which reflect lengthening or shortening.

Strain rate, SR, is the rate of change in strain and is usually expressed as 1/sec or sec-1. Deformation parameters: velocities, strain and strain rate can be assessed along the x, y and z directions, or along the anatomical coordinates of the left ventricle: longitudinal, radial and circumferential. Global strain reflects average longitudinal or circumferential strain over the left ventricle.[4]

Acquisition of the images should be performed with frame rate more than 100 frames /s, angle ≤15° and usually from the apical views.[4] Assessment of deformation parameters with Doppler Tissue Imaging can be affected by global heart motion and breathing, it is angle dependent and is best performed from apical views.

Today, Doppler Tissue Imaging is used mainly for the measurements of mitral annular velocities and is essential for the evaluation of diastolic function.

Additional application of Doppler Tissue Imaging – evaluation of mechanical dyssynchrony before cardiac resynchronization therapy [CRT] may be used in borderline cases.

Tissue velocities in the basal and mid-ventricular segments in the patient scheduled for CRT.


Speckle Tracking Imaging [Two Dimensional Strain]

First described in 2004,[5][6] based on the observation that ultrasound images contain many small particles, natural acoustic markers, which move together with the tissue and can be identified on adjacent frames. These natural markers are stable acoustic speckles, equally distributed within the myocardium.

Each marker can be identified and followed accurately during several consecutive frames.

The new marker location on sequential images is tracked and the local tissue velocity can be calculated as a marker’s shift divided by the time between 2 consecutive frames. Changes of the distance between neighboring elements reflect the tissue’s contraction or relaxation. Knowing tissue velocity at the previous and current position of the natural acoustic marker and the distance between them, strain rate and strain can be calculated.

For reliable results, echo images should be acquired at frame rate at least 40 frames per second or more. For calculation of longitudinal strain apical 4, 2 and 3 chamber views are obtained. First, apical 3 chamber view is selected: End-diastolic frame is selected according to the position of the aortic valve. The inner endocardial border is delineated manually by the operator, and then tracked by the software. If the automatic tracking is not satisfactory, the operator can correct it manually until best possible tracking is reached.

Inner and outer point line define region of interest, that can be changed manually. Inner line deifnes endocaridal border.

In the modern versions of the software and high quality images, manual corrections are often not necessary – almost fully automatic system.

2-chamber view, normal longitudinal strain

2-chamber view. Normal tissue velocities


3-layers strain

Longitudinal strain, strain rate and velocities reflect only longitudinal contraction of the myocardium. Left ventricular wall is composed of 3 layers of fibers: endocardial [inner] layer, mid layer and epicardial [outer] layer. Endocardial and epicardial fibers have longitudinal direction, mid-layer fibers are circumferential. Circumferential strain can be calculated by speckle tracking imaging from the short axis views at all myocardial levels: base, mid level and apex. Contraction of longitudinally and circumferentially oriented fibers results in systolic myocardial thickening, which can be characterized by radial strain.

Normally, longitudinal and circumferential endocardial strain is the highest and epicardial strain is the lowest: epicardial-to-endocardial gradient.[7] Basal strain is the lowest and apical strain is the highest: base-to-apex gradient.[7]

4-chamber view, normal longitudinal strain. Strain is the lowest in epicardial layer and the highest in endocardial layer: epicardial-to-endocardial gradient. Strain is the highest in apical and the lowest in the apical segments.

Torsion, Twist, Rotation

Normally, during systolic ejection , the apex the heart rotates counter-clockwise and the base rotates clockwise. This complex motion is determined by dominant contraction of epicardial fibers with larger radius of arm of moment. Rotation [degrees], can be measured with speckle tracking imaging from the short axis view images. During diastole untwisting occurs. The absolute apex-to-base difference in LV rotation is the twist angle [degrees].

Torsion is base-to-apex gradient in the rotation angle along the long axis of the LV and is measured in degrees per sm, actially is calculated by diviseion of twist angle to the distance from the base to the apex of the LV.

Torsion has an important role in cardiac mechanics.

Blue – apical counterclockwise rotation, violet – basal clockwise rotation, white – torsion

Deformation parameters may vary between species.[8]

Visual assessment of left ventricular function may be successfully accomplished by speckle tracking analysis.[9][10] Right ventricular strain and strain rate are not recommended now for routine evaluation of right ventricular function due to large degree of variability, lack of normative data and low reproducability[11] Left atrial deformation parameters strain and strain rate can be evaluated with 2D Strain Speckle Tracking Imaging from 4-chamber and from 2-chamber views similarly to the LV strain. Left atrial appendage and pulmonary veins should be excluded from delieation. This time, strain of the left atrium can be usefull for prediction of atrial arrythmias, and for prediction of maintanence of sinus rhythm after cardioversion. Routine assesment of left atrial strain is not recommended today .[4]. Higher right atrial strain was found in CRT responders. Today right atrial strain is not recommended for clinical use .[4]. Cardiac mechanics is widely investigated now with speckle tracking imaging in different pathologies.

References:

  1. Vered Z, Mohr GA, Barzilai B, Gesler C, Wickline S, Wear K, et al. Ultrasound integrated backscatter tissue characterization of remote myocardial infarction in human subjects. J Am Coll Cardiol 1989;13:84-91.
  2. Sutherland GR, Stewart MJ, Groundstroem KW, Moran CM, Fleming A, Guell-Peris FJ, et al. Color Doppler myocardial imaging: a new technique for the assessment of myocardial function. J Am Soc Echocardiogr 1994;7:441-58.
  3. Heidmal A, Stoylen A, Torp H, Skjaerpe T. Real-time strain rate imaging of the left ventricle by ultrasound. J Am Soc Echocardiogr 1998;11:1013-9.
  4. 4.0 4.1 4.2 4.3 4.4 Mor-Avi, et al. Current and Evolving Echocardiographic Techniques for the Quantitative Evaluation of Cardiac Mechanics: ASE/EAE Consensus Statement on Methodology and Indications J Am Soc Echocardiogr 2011;24:277-313
  5. Reisner SA, Lysyansky P, Agmon Y, Mutlak D, Lessick J, Friedman Z. Global longitudinal strain: A novel index of left ventricular systolic function. J Am Soc Echocardogr 2004;17:630–3.
  6. Leitman M, Lysyansky P, Sidenko S, Shir V, Peleg E, Binenbaum M, Kaluski E, Krakover R, Vered Z. Two-dimensional Strain–A Novel Software for Real-time Quantitative Echocardiographic Assessment of Myocardial FunctionJ Am Soc Echocardiogr 2004;17:1021-9
  7. 7.0 7.1 Leitman M, Lysyansky P, Lysyansky M, Friedman Z, Tyomkin V, Fuchs T, Adam D, Krakover R, Vered Z: Circumferential and longitudinal Sstrain in 3 myocardial layers in normal subjects and in patients with regional left ventricular dysfunction.
  8. Bachner-Hinenzon N, Ertracht O, Leitman M, Vered Z, Shimoni S, Beeri R, Binah O, Adam D. Layer-specific strain analysis by speckle tracking echocardiography reveals differences in left ventricular function between rats and humans. Am J Physiol Heart Circ Physiol. 2010 Sep;299(3):H664-72.
  9. Blondheim DS, Beeri R, Feinberg MS, Vaturi M, Shimoni S, Fehske W, Sagie A, Rosenmann D, Lysyansky P, Deutsch L, Leitman M, Kuperstein R, Hay I, Gilon D, Friedman Z, Agmon Y, Tsadok Y, Liel-Cohen N. Reliability of visual assessment of global and segmental left ventricular function: a multicenter study by the Israeli Echocardiography Research Group. J Am Soc Echocardiogr. 2010 Mar;23(3):258-64.
  10. Liel-Cohen N, Tsadok Y, Beeri R, Lysyansky P, Agmon Y, Feinberg MS, Fehske W, Gilon D, Hay I, Kuperstein R, Leitman M, Deutsch L, Rosenmann D, Sagie A, Shimoni S, Vaturi M, Friedman Z, Blondheim DS. A new tool for automatic assessment of segmental wall motion based on longitudinal 2D strain: a multicenter study by the Israeli Echocardiography Research Group. Circ Cardiovasc Imaging. 2010 Jan;3(1):47-53.
  11. Rudski LG, Lai WW, Afilalo J, Hua L, Handschumacher MD, Chandrasekaran K, Solomon SD, Louie EK, Schiller NB: Guidelines for the Echocardiographic Assessment of the Right Heart in Adults: A Report from the American Society of Echocardiography. J Am Soc Echocardiogr 2010;23:685-713.
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