Transthoracic Speckle Tracking Echocardiography for the Quantitative Assessment of Left Ventricular Myocardial Deformation

Summary

Speckle tracking echocardiography is an emerging diagnostic imaging technique for the quantitative assessment of global and regional myocardial performance. Standard view echocardiographic motion images are recorded and deformation parameters are subsequently measured by automated continuous frame-by-frame tracking and motion analysis of speckles within the B-mode images of the myocardium.

Abstract

The value of conventional echocardiography is limited by differences in inter-individual image interpretation and therefore largely dependent on the examiners' expertise. Speckle tracking Echocardiography (STE) is a promising but technically challenging method that can be used to quantitatively assess regional and global systolic and diastolic myocardial performance. Myocardial strain and strain rate can be measured in all three dimensions — radial, circumferential, longitudinal — of myocardial deformation. Standard cross-sectional two-dimensional B-mode images are recorded and subsequently postprocessed by automated continuous frame-by-frame tracking and motion analysis of speckles within the myocardium. Images are recorded as digital loops and synchronized to a 3-lead EKG for timing purposes. Longitudinal deformation is assessed in the apical 4-, 3-, and 2-chamber views. Circumferential and radial deformation are measured in the parasternal short axis plane.

Optimal image quality and accurate tissue tracking are paramount for the correct determination of myocardial performance parameters. Utilizing transthoracic STE in a healthy volunteer, the present article is a detailed outline of the essential steps and potential pitfalls of quantitative echocardiographic myocardial deformation analysis.

Introduction

Scientific and clinical scenarios in cardiovascular medicine are more and more addressed by continuous variables and cutoff values rather than simplistic "yes or no" algorithms. Imaging techniques have evolved to be able to assess cardiac function in ever increasing detail. Speckle tracking echocardiography (STE) is an emerging diagnostic tool for the quantitative evaluation of myocardial performance. While conventional echocardiography is limited by subjective image interpretation and a strong dependence on the individual examiner's expertise, STE has been introduced as a reproducible and more objective method to quantify global and regional systolic and diastolic function^1,2.

Left ventricular (LV) myocardial deformation — longitudinal and circumferential shortening as well as radial thickening in systole and vice versa in diastole — can be described measuring the parameters strain (ε) and strain rate (SR). ε is a dimensionless percent change in myocardial length. SR is a time derivate of ε³. These important indices of myocardial function have been shown to be able to identify myocardial ischemia⁴, predict response to cardiac resynchronization therapy⁵ and to detect subclinical myocardial dysfunction while conventional echocardiographic parameters still remain normal⁶. In a systematic meta-analysis, global longitudinal ε, the most frequently used quantitative LV systolic function parameter, has been shown to have superior prognostic value for the prediction of major adverse cardiac events then LV ejection fraction (EF), the current gold standard for the assessment of LV systolic function⁷. Even very subtle alterations such as the effect of short term metabolic changes on myocardial mechanics in asymptomatic patients can be detected utilizing STE⁸.

Technically, STE uses greyscale 2D or 3D B-mode motion images recorded in standard echocardiography views. Several consecutive cardiac cycles are recorded in apical 4-, 3- and 2-chamber views to measure longitudinal deformation and in the parasternal short axis view for circumferential and radial deformation⁹. Moreover, by capturing the short axis view at the level of the mitral valve, the papillary muscles and the apex, LV torsion can be assessed³. Subsequently to image acquisition and storage as digital loops, myocardial deformation is measured on an off-line work station or on the ultrasound device itself. The software detects unique myocardial pixel patterns in the recorded greyscale images, so-called "speckles" and traces them throughout the analyzed cardiac cycle. Vectors are measured and deformation parameters are subsequently calculated. This way regional and global myocardial deformation can be assessed in systole and diastole for both the left and right ventricle and atrium¹⁰.

Protocol

The protocol content has been ethically approved by the Witten/Herdecke University Ethics Committee.

1. Technical Requirements

1. Utilize an echocardiography device featuring speckle tracking technology equipped with an adequate sector array tissue harmonic imaging transducer.
2. During image acquisition, record and connect a standard 3-lead EKG directly to the echocardiography device in order to synchronize echocardiographic motion images to electromechanical activity. This is mandatory for timing purposes during subsequent postprocessing analyses. Connect the study subject to the EKG and unfreeze the ultrasound image to start detecting the EKG signal.
3. Record digital loops as explained in detail below (steps 2.1-2.5) and store the data as moving images in DICOM format on an external disk. Subsequently, transfer the files to an off-line workstation.
4. Perform post-processing analyses using appropriate software as outlined in detail below (steps 3.1-3.13).

2. Recording of Echocardiographic Digital Loops

1. Examine the patient in the left lateral decubitus position (patient lying on the left side with the left arm stretched above the head).
   NOTE: This part of the protocol requires the patient/study subject to be present.
2. Alternatively, when combining STE with stress echocardiography modalities such as a bicycle ergometer, ensure that the patient is in a 45-degree upright position. In this case, utilize a standard bicycle ergometry device and perform standard stress echocardiography testing as previously described¹¹. During the recording of echocardiographic images, tilt the ergometer to achieve a left lateral body position in order to minimize artifacts by interfering lung tissue.
3. Take special care to optimize image quality to guarantee accurate assessment of myocardial deformation. To do so, adjust the frame rate between 60 and 80 frames per second using the "adjust frame rate" option. Furthermore, pay attention to include all aspects of the myocardial structures that shall be analyzed throughout the entirety of the cardiac cycle.
4. Obtain cross-sectional two-dimensional greyscale B-mode images in standard apical long axis and parasternal short axis planes as described by the European Association of Cardiovascular Imaging and the American Society of Echocardiography¹². Record several consecutive cardiac cycles (actually only one is necessary, recording of at least three cardiac cycles is advised in order to be able to choose the one with the best image quality during subsequent post processing) in each of the following planes:
   1. For the assessment of longitudinal ε and SR, capture standard apical 4-, 3-, and 2-chamber views as previously described¹². To do so, position the transducer at the apex of the heart near the apical impulse (usually between the 3^rd and 5^th intercostal space and between the mid-clavicular and anterior axillary line). Aim towards the right shoulder and angulate the transducer until all anatomical structures of interest become visible.
   2. Record images in the parasternal short axis view at the level of the mitral valve, the papillary muscles and the apex to detect circumferential ε and SR as well as radial ε and SR as described elsewhere in detail¹². To do so, place the probe at the left parasternal border at the 2^nd or 3^rd intercostal space and angulate until you obtain a cross-sectional perpendicular view of the LV.
5. When combining STE with cardiac stress testing such as bicycle ergometry or any other functional testing modality requiring serial measurements (see step 2.2), repeat step 2.4 at each desired time point.

3. Postprocessing Analysis

NOTE: This part of the protocol includes the evaluation and interpretation of the recorded echocardiographic images. It does not require the patient to be present and can be performed at any time following the previous part of the procedure.

1. Utilizing the quantitative echocardiography analysis software, click 'File' and 'Open' and chose the desired echocardiographic study data. Select a patient/study and pick an echocardiographic plane that shall be analyzed.
2. Click the 'Q'-icon in the right lower corner of the selected image. Next, press the 'aCMQ' button on the left.
3. Chose the cardiac cycle of the highest image quality by using the green 'QRS' skip keys at the bottom of the screen. Use the keyboard space bar to play and pause the loop.
4. Select a region of interest (ROI) to be analyzed by confirming the echocardiographic view on the left side of the screen. Next, have the software automatically detect the timing of end-diastole and suggest a ROI.
   NOTE: A first speckle tracking analysis is subsequently being computed by the software. Segmental and global ε curves are displayed on the bottom of the screen.
5. Click 'Strain rate' below the graphs to visualize segmental and global SR.
6. Visually verify the tracking quality suggested by the software.
   NOTE: To do so, critically control whether all aspects of the myocardium to be analyzed are completely covered by the ROI during the entirety of the cardiac cycle. Avoid including surrounding non-myocardial tissues into the ROI.
7. If necessary, manually reposition the entire ROI or single aspects of it, or even re-draw the ROI completely (see 3.8-3.9) in order to guarantee exact measurements.
   NOTE: Optionally, set the ROI to be transparent to adjust the ROI coverage to the appropriate position and width of the myocardium.
8. In the apical 4-, 3-, and 2-chamber view, have the software automatically determine a possible ROI dividing the myocardium into seven segments.
   1. In case ROI re-definition is necessary, click 'Draw' on the left and start out tagging the endocardial border at three reference points: the two opposing insertion points of the AV valve and the LV wall starting with the basal infero-septal/ basal infero-lateral/basal inferior part of the valve finishing with the center of the apex. Ensure that both endpoints of the tracked-endocardium are on the same level completely excluding the valvular tissue.
   2. If repositioning is necessary in order to optimize position and width of the ROI, click 'Edit' on the left side of the screen. Move each segment margin as well as the endocardial and epicardial borders individually with the cursor. Utilize an orthogonal line pointing towards the apex for navigation/orientation when moving the ROI in its entirety.
   3. Finally, start the speckle tracking re-analysis by pressing the 'Compute' button on the left side of the screen.
      NOTE: The software now automatically detects "acoustic markers", which deflect myocardial ultrastructures corresponding to myocardial fiber organization in their movement throughout the contraction and relaxation of the myocardium. These acoustic markers are traced through the entire duration of a complete cardiac cycle. The necessary calculation may take seconds to minutes. ε and SR are calculated by the software and presented in a numeric and graphic manner.
9. In the parasternal view, have the software automatically suggest a predefined ROI. Adjust this ROI manually, dividing the myocardium into six segments.
   NOTE: The width of the ROI, should exactly match the thickness of the myocardium. Where necessary, optimize position and width of the ROI as described in 3.8.2. A dot in the center of the ROI may be utilized for navigation/orientation when moving the ROI in its entirety.
   1. Next, start the speckle tracking re-analysis by pressing the 'Compute' button on the left side of the screen.
10. Chose segmental and global ε and SR to be displayed in curves or the comprehensive bull's eye format. To do so, click the 'Preferences' button at the bottom left corner of the screen. Different types of waveforms and displaying options can be selected in this menu.
11. If manual repositioning of the ROI is not sufficient to achieve appropriate overall speckle tracking quality, start over from 3.1 and redefine the ROI or consider selecting a different cardiac cycle prior to continuing to the next step.
12. Save and export the data for subsequent statistical analyses. If desired, cine loops or still frames can be exported as illustrations. To do so, click 'Export' at the bottom left corner of the screen and select the desired format and file directory.

Results

The principle parameters for the quantitative assessment of myocardial performance are ε and SR. Technically, all cardiac chambers can be analyzed using STE. However, since speckle tracking methodology has been mostly used to study the LV, the focus of this article is on LV myocardial mechanics. Generally, longitudinal ε and SR are the most commonly assessed LV deformation parameters. Longitudinal ε and SR describe systolic shortening (and diastolic lengthening) of the myocardium. Hence, ...

Discussion

Significance of the technique with respect to alternative methods

The current gold standard for the echocardiographic assessment of LV systolic function is the LV ejection fraction (EF)¹³. However, determination of EF is based on a simplistic approach that is closely correlated to the radial component of myocardial contraction but does not take into consideration the important longitudinal and circumferential planes. Hence, EF oversimplifies the three-dimensional co...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Phillips iE33 ultrasound system	Philips Healthcare		http://www.umiultrasound.com/ultrasound-machine/philips/ie33
S5-1 broadband sector array transducer	Philips Healthcare		5-1 MHz, http://www.usa.philips.com/healthcare/product/HC989605412081/s5-1
QLAB Advanced Quantification Software Version 10.5	Philips Healthcare		Q-App: Automated Cardiac Motion Quantification (aCMQ), www.philips.com/QLAB-cardiology
Xcelera R3.3L1 (Version 3.3.1.1103)	Philips Healthcare		http://www.usa.philips.com/healthcare/product/HC830038/xcelera-r41-cardiology-information-management-system