|Year : 2017 | Volume
| Issue : 2 | Page : 133-139
Speckle tracking strain echocardiography: What sonographers need to know!
Ashlee M Davis1, David Adams1, Ashwin Venkateshvaran2, Fawaz Alenezi1
1 Department of Medicine, Division of Cardiology, Duke Cardiac Diagnostic Unit, Duke University, Durham, NC, USA
2 Department of Medicine, Division of Cardiology, Cardiac Diagnostic Unit, Stockholm, Sweden
|Date of Web Publication||28-Aug-2017|
Duke Cardiology, Duke Clinical Research Institute, Duke University Medical Center, Durham, NC 27705
Source of Support: None, Conflict of Interest: None
Introduction: Strain is a unitless measurement of dimensional or deformational change; speckle-tracking echocardiography is the most widely used technique to assess strain, with demonstrated clinical utility in a variety of settings. Objectives: This paper reviews the diagnostic and prognostic impact of echocardiographic assessment of left ventricle myocardial strain and what sonographers need to know in a daily practice. Methods: This paper have the most updated American society of echocardiography recommendations on a speckle tracking strain echocardiography, and included experiences of a large academic center, standardization as well as tips needed to perform a strain in a daily clinical practice. Conclusion: With good feasibility, reproducibility and evidence in support, speckle tracking strain echocardiography can be used as a standard echocardiography parameter in clinical practice.
Keywords: Left ventricle function, speckle tracking echocardiography, strain echocardiography
|How to cite this article:|
Davis AM, Adams D, Venkateshvaran A, Alenezi F. Speckle tracking strain echocardiography: What sonographers need to know!. J Indian Acad Echocardiogr Cardiovasc Imaging 2017;1:133-9
|How to cite this URL:|
Davis AM, Adams D, Venkateshvaran A, Alenezi F. Speckle tracking strain echocardiography: What sonographers need to know!. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2017 [cited 2020 Aug 11];1:133-9. Available from: http://www.jiaecho.org/text.asp?2017/1/2/133/213683
| Introduction|| |
In echocardiography, the term “strain” is used to describe local shortening, thickening, and lengthening of the myocardium. Strain imaging, also known as deformation imaging, is a technological advancement that has been developed as a means to objectively quantify regional myocardial function. The term strain refers to a unit-less measurement of dimensional or deformational change. Two strain techniques have dominated the research arena of echocardiography: (1) Doppler-based tissue velocity measurements frequently referred to as tissue Doppler and (2) two-dimensional (2D) displacement measurements by speckle tracking echocardiography (STE) or 2D strain. However, STE is the most widely used technique to assess strain, with demonstrated clinical utility in a variety of settings [Table 1].
Since the development of echocardiography, one goal has been to assess the function of the left ventricle (LV) as precisely and with as little variability as possible. The most popular of left ventricle (LV) has been the LV ejection fraction (LVEF). Although LVEF continues to be the most widely used measurement, it demonstrates significant limitations in providing a direct, objective assessment of LV function and is not sensitive to subclinical dysfunction. In addition, ejection fraction (EF) has been shown to demonstrate significant inter- and intra-observer variability. A few of the limitations in performing these measurements are frequent apical foreshortening, lack of endocardial border dropout, and geometric assumptions required for 2D calculations. The goal for a strain is to be an adjunctive assessment of LV function.
STE is available on many current day platforms, either as part of the standard configuration on high-end systems or as an additional off-line software application. As a sonographer, it helps to familiarize oneself with the STE application provided by one's ultrasound systems manufacturer, as different vendors choose to refer to their STE packages by different names. Today, global longitudinal strain (GLS) is the most studied and clinically used application of STE. The clinical advantage of GLS is that it can detect early myocardial dysfunction before any obvious cardiac dysfunction occurs, especially in diseases where high cardiovascular (CV) morbidity and mortally are reported, and traditional echocardiographic parameters like LVEF are normal. For the purposes of this review, we will be focusing on GLS.
| How Does Strain Work?|| |
Image processing algorithms on digital 2D echocardiography identify and track small, stable myocardial footprints (or speckles) generated by ultrasound-myocardial tissue interactions within a region of interest (ROI). Distances between speckles are tracked frame-to-frame over the cardiac cycle, and distances between speckles provide information about myocardial deformation. Simply, strain is the difference of the diastolic and systolic length [Figure 1].
|Figure 1: Schematic drawing of 2D speckle tracking showing the change in length from diastole to systole upper left corner pathologic specimen of longitudinal heart fibers. Lower left corner: 2D AP4ch image depicting targeted speckles. Center drawing: Change in length from diastole to systole. Right two images: Strain curve displays postspeckle tracking echocardiography analysis|
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By convention, positive values are assigned to lengthening, thickening, or clockwise rotation, whereas negative values are assigned to shortening, thinning, or counterclockwise rotation. There are three basic uses for 2D-STE: longitudinal, circumferential, and radial strain. Longitudinal strain refers to the shortening or lengthening of the myocardium from base to apex. The circumferential strain is the reduction of the circumference of the LV cavity. Radial strain is the measurement of LV wall thickening during systole. The longitudinal strain has emerged as the most clinically applicable and reproducible. The assessment of all three is beyond the scope of this document, therefore, we will be focusing on the longitudinal strain. It is important to note that in longitudinal strain greater degrees of deformation are expressed as a numerically lower (more negative) strain value.
In a head-to-head comparison of nine different vendors, STE was found to have a superior variability as compared to LVEF. This small, but statistically significant intervendor variability in strain values was related to postprocessing differences. Current 2015, the American Society of Echocardiography guidelines recognize the heterogeneity for normal values between published reports and do not define normal ranges. However, as a guide, a value more negative than −20% is considered likely to be normal. Our experience and recent meta-analysis suggested that an absolute value <−15.4% considered abnormal cutoff. To be clear, more negative is better. Identifying a normal value for your echo laboratory is an excellent quality assurance exercise, as there is not a current standard.
| Speckle Tracking Echocardiography: “how-To”|| |
The first step to performing strain analysis module is assessing if your STE package is available on-cart or if you will be performing offline. On-cart analysis refers to the analysis of strain parameters on the echocardiography machine either during or immediately after the study has been performed. Offline analysis, on the other hand, is performed on a dedicated workstation after the study is complete. In the context of echo laboratories in India, the analysis is more commonly performed on-cart. More recently, however, image archives and offline analysis are being employed in larger laboratory to optimize workflow.
Irrespective of whether the analysis is performed on-cart or off-line, the process of STE analysis involves three or four basic steps. These include (i) the acquisition of optimal 2D images, (ii) image storage, (iii) image transfer to a dedicated workstation (if applicable), and (iv) poststudy analysis. For the purpose of simplicity, and to encourage a more routine use of STE by sonographers, we propose a step-by-step approach to these basic stages.
Specific steps will be slightly different depending on the ultrasound system being used. Please see your ultrasound manufacturers manual for specific step-by-step information.
| Image Acquisition|| |
To begin with, STE analysis cannot be performed in the absence of electrocardiogram gating. An optimal signal demonstrating three cardiac cycles with nearly identical heart rates is necessary in all subjects with normal sinus rhythm. STE analysis can be more challenging to perform in subjects with a significant heart rate variation or atrial fibrillation.
The quality of STE analysis depends on the 2D images acquired. Hence, it is important to fall back on the basics of imaging to acquire the best possible images. Optimize gains and employ harmonic imaging to better delineate the endocardial-blood pool interface. Time gain compensation and focus functions can also be optimized to improve image quality. Familiarize yourself with additional vendor-specific functions on the machine to improve 2D image quality.
It is possible to perform longitudinal strain analysis using either a single plane (AP4ch) or three-plane strain study (AP4ch, AP2ch, APLAX). As a rule, a three-plane strain study will provide more robust data than a single view, therefore, that is the current recommendation. Place the patient in the left lateral decubitus position. Avoid foreshortening by choosing the apical window that demonstrates a more bullet-shaped ventricle with the highest base to apex length. A rounded, thickened apex may suggest a foreshortened image. While acquiring these views care should be taken to ensure the myocardial speckle is well-visualized, there is no translational motion from movement or breathing, and there are no lung or rib artifacts in the picture. Having the patient, perform a breath hold at the same point for each image is imperative for good strain data. The focus for these images should be on obtaining high-quality gray scale images with proper gain settings in an LV focused view with the depth decreased to just below the mitral annulus [Figure 2]. Pay attention to the sector width and be careful not to eliminate any of the LV apex or walls from the imaging sector. Frame rates between 40 and 90 Hz are appropriate for most strain analysis software packages, but again review your manufacturer's recommendations. In general, lower frame rates may not provide enough temporal resolution for satisfactory speckle tracking, and frame rates that are too high may not have software capabilities to track. Three cardiac cycles are recommended, which will give you multiple beat options to choose from for analysis. Once all three 2D views have been obtained, a continuous wave (CW) Doppler image with aortic valve closure (AVC) click should be obtained. This Doppler image should be taken with the same breath hold performed.
|Figure 2: Apical 4ch, 2ch, and LAX with optimal image quality, proper frame rate, and good electrocardiogram. Three beat loops taken, left ventricle focused view with depth decreased to just below the mitral annulus. Frame rates are between 40 and 90 Hz. Sector width is wide enough to obtain all of the left ventricle walls. Endocardium and myocardium are both well seen using harmonic imaging|
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| Image Storage and Transfer|| |
Images are stored on the echo machine's internal hard disk in a raw data format, and on-cart analysis requires a simple selection of images to proceed with analysis.
In the setting of offline analysis, the echocardiography machine is often configured to automatically deliver images to the workstation on completion of the examination. If a manual transfer is necessary, copy the whole study onto a digital media drive in DICOM for subsequent analysis rather than individual images in lossy AVI or MPG formats.
| Image Analysis|| |
Once all views have been obtained and are deemed satisfactory for STE, image analysis can begin. Again, this may be done either on or off the cart depending on your laboratory's set up. In STE, the placement of AVC is very important and if placed incorrectly can drastically change the strain numbers. We recommend setting the AVC based on your CW Doppler. Using the Doppler image for this AVC, placement is recommended over the 2D image due to improved temporal resolution found in Doppler; however, it is important to note that this may be difficult in the case of irregular heart rates or changing R to R intervals. If it is not possible to set the AVC in a Doppler image, you may then place it according to your 2D APLAX image [Figure 3].
Step 1: Open the CW Doppler image with AVC click. Confirm that the starting point is placed at the peak of the R wave; this will serve as the starting point. Under the measurement, package chooses event timing and selects AVC. Place the AVC line on the aortic valve (AV) closing click seen just after the AV/left ventricular outflow tract outflow jet is concluded [Figure 3]. Note: the event of a very wide QRS complex you may consider placing your starting point at the beginning of the Q wave rather than the peak of the R. This can be helpful in exposing certain disease states (e.g., mechanical dyssynchrony). If you are setting the AVC based on a 2D image, this will be an option once you process the APLAX view. When prompted, scroll frame by frame until you see the AV close and place the AVC marker at that time point.
Step 2: Launch the strain analysis software. Once the strain analysis software package is launched, you will start with the APLAX view. Depending on your machine settings, STE is either semi-automated or automated image analysis, meaning that you may or may not need to place points to start the image analysis.
Step 3: Chose the beat you wish to use by trimming the three-cycle loop down to the single beat you wish to analyze. Once you have chosen the beat, you wish to use click APLAX on the STE package. Some vendors will show a systolic yo-yo view. This is designed to help you place points better by seeing the myocardium moving.
Step 4: Time to place your tracking points. Begin analysis with the LV-focused apical three-chamber view. In most systems, the STE application will automatically identify the end-systolic frame using timing previously derived from the AVC. Most vendors have a three-point system, but if performing analysis of the cart you may be asked to place multiple points similar to a Simpson's biplane. Tracking point placement is very important regardless of the software being used. When using the three-point method, annular points are to be placed at the intersection of the annulus and the LV wall. Apical point should be placed along the endocardial border of the apical wall [Figure 4]. In the case of offline analysis, begin at the mitral annular border and trace the endocardial surface of the LV, stopping at the opposite annular border. The system then identifies the endocardial, midmyocardial, and epicardial surface and provides an ROI. This ROI can be adjusted either larger or smaller by the user, for example, in a hypertrophic heart, one may need to increase the ROI versus in thinned myocardium postmyocardial infarction (MI), one may need to narrow the ROI. The ROI should encompass the entire thickness of the LV myocardium, without being either too large or too small. Adjusting the ROI will depend on the software package being used, some will adjust using a dial, others will have a point to grab and adjust. Try to avoid including the very bright pericardium.
|Figure 4: Point placement for speckle tracking echocardiography in all three views in semi-automated system. Points should be placed along the endocardial border of the apex and at the insertion of left ventricle wall and mitral annulus. Start with APLAX view|
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Step 5: Begin process analysis. The system will track myocardial speckles frame-by-frame and provides a dynamic image that allows the sonographer to visually assess if tracking is adequate. Certain systems provide a status bar that showcases the tracking status of all six segments. Assess for tracking the accuracy. If tracking is not acceptable, you may need to slightly edit the points to find more consistent speckle for the machine to track.
Once tracking is deemed accurate, approve the suggested tracking scheme. The system will then display regional and average longitudinal strain, both as numeric values and color-coded strain curves [Figure 5]. Given that myocardial segments shorten during systole, strain is represented as a negative value or a waveform below the baseline.
|Figure 5: Normal APLAX with strain curve display. On the left, there is a six-segment model, each segment is represented by a color which corresponds to the curves on the upper right. The lower right section shows a color m-mode map that also corresponds to the colors seen in the six-segment model. Bright red denotes the most negative normal values seen at the yellow line of aortic valve closure|
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Step 6/7: Repeat the placing of tracking points for the AP4ch and AP2ch views as prompted.
Step 8: After all three views have been analyzed; a global report will be generated. This will include a global bullseye as well as regional strain curves [Figure 6]. These images should be acquired and included in the patient's permanent record. This bullseye view can be very helpful in demonstrating certain patterns in specific disease states covered later in this article. All views will also be averaged to create a GLS value.
|Figure 6: Two types of bullseye display showing global longitudinal strain for each view, as well as global longitudinal strain average. On the left: 17-segment model, outer ring shows basal segments, second ring shows mid segments, inner ring shows apical segments, and center point shows apical cap. On the right, all three views (AP4, AP2, APLAX) are displayed with corresponding segmental curves as well as a 17 segment bullseye. Bright red denotes normal strain values with the most negative numbers. Both displays also report global longitudinal strain for each view and an average global longitudinal strain over all three views|
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| Future in Tracking|| |
Automated intelligence software is becoming more and more popular among ultrasound system manufacturers, and the idea of eliminating human error is driving that development. For that reason, as STE programs improve, it is likely that in the near future there will be no placing of points whatsoever [Table 2].
| Technical Factors That Influence Strain|| |
The strain may be influenced by image quality, choice of a segmental model, selection of image clips, selection of landmarks and contouring, selection of the ROI, timing placement during the cardiac cycle, recognition of poor tracking, and technical differences between vendors.
| Clinical Factors That Influence Strain|| |
Strain may be influenced by racial/ethnic/international differences, age and sex, hemodynamic factors, CV risk factors, medications, dialysis, pregnancy, and endurance athletics.
| When Should We Do Strain?|| |
One of the most exciting and clinically relevant uses for strain echocardiography is its ability to detect changes in myocardial contractility in the very early stages. As stated previously one of the major goals of an echocardiogram is the assessment of LV function. Many disease states, small changes in the LV function have major impacts on the patient's care and prognosis. GLS is recommended as a routine measurement in patients undergoing chemotherapy, in patients receiving cardiac resynchronization therapy, heart failure with preserved EF, coronary artery disease: detection of myocardial ischemia and viability. Another use for strain is its ability to differentiate various hypertrophic cardiomyopathies (HOCMs) such as HOCM, athlete's heart, and amyloidosis. This list is sure to expand as we learn more about the benefits of STE as an addition to our routine echocardiogram.
| Patterns in Disease States|| |
There are certain patterns seen in various disease states, which can help us to identify or differentiate between look alike diseases. Strain pattern recognition is constantly evolving, but here are a few of the most established and agreed on patterns.
- MI [Figure 7]
- Chemotherapy [Figure 8]
- Mechanical dyssynchrony [Figure 9]
- Amyloidosis [Figure 10]
- HOCM [Figure 11].
|Figure 7: Patient with myocardial infarction in distal left anterior descending. Each trace represents one left ventricle segment. In normal wall motion, all segments move negatively below the baseline and come to a peak at the aortic valve closure line, which is shown here in green. In this patient, the purple and green segments in all three views, which represent apical segments, are stretching positively above the baseline. This corresponds with dyskinetic apical segments. This is also shown in the bullseye plot by the blue colors in the center. Also, note that the global longitudinal strain is −8.1% which is abnormal|
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|Figure 8: Strain examination performed in a patient undergoing chemo treatment for breast cancer. Many patients receiving certain types of chemo which are considered cardiotoxic receive routine echocardiograms throughout their treatment. Including strain and a calculated left ventricle ejection fraction should be done to serially track their heart function quantitatively. This patient received an echo before starting chemo, again at 3 months, and once more at 5 months of treatment. Included in the figures are the calculated left ventricle ejection fraction and global longitudinal strain at each time point. (a) Initial pretreatment echo with normal left ventricular ejection fraction and global longitudinal strain. (b) Three months of treatment left ventricle ejection fraction remained normal, global longitudinal strain decreased. (c) Five months of treatment patients left ventricle ejection fraction now decreased and global longitudinal strain severely decreased|
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|Figure 9: Classic dyssynchrony pattern seen in a patient with left bundle branch block. Three components must be present to be considered a classic dyssynchrony pattern: (1) Early stretch above baseline, (2) early contraction below baseline, (3) late contraction postaortic valve closure. Blue arrow = Early stretch, Yellow arrow = Early contraction, Red arrow = Late contraction|
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|Figure 10: Strain pattern seen in a patient with confirmed amyloidosis. Yellow, teal, and red curves represent basal segments; white, green, and purple lines represent apical segments. Bullseye pattern shows bright red segments at the center of the bullseye with light pink segments around the edges, demonstrating the same pattern. This pattern is highly suggestive of Amyloidosis|
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|Figure 11: Patient with confirmed hypertrophic cardiomyopathy. Yellow and teal curves represent reduced septal segments. All other curves remain normal. Bullseye shows light pink segments at septum with all other walls as bright red. This pattern is c/w hypertrophic cardiomyopathy|
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
[Table 1], [Table 2]