|Year : 2020 | Volume
| Issue : 1 | Page : 22-28
Global Longitudinal Strain: A practical Step-by-Step Approach to Longitudinal Strain Imaging
Govindan Vijayaraghavan1, Sivasubramonian Sivasankaran2
1 Department of Cardiology, Kerala Institute of Medical Sciences, Thiruvananthapuram, Kerala, India
2 Department of Cardiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Thiruvananthapuram, Kerala, India
|Date of Submission||08-Apr-2019|
|Date of Acceptance||02-May-2019|
|Date of Web Publication||11-Apr-2020|
Dr. Govindan Vijayaraghavan
Kerala Institute of Medical Sciences, Thiruvananthapuram - 695 029, Kerala
Source of Support: None, Conflict of Interest: None
Global longitudinal strain imaging of the left ventricle is a simple bedside modality for objectively assessing the global and regional function of the major pumping chamber of the heart, the left ventricle. Currently available echo machines provide good quality speckle-tracking methods with good computational facilities providing standard, comparable bull's eye maps and parametric plots. This introductory chapter provides a step-by-step approach for the beginner to utilize this additional facility in day-to-day practice to precisely understand the left ventricular regional and global function for serial follow-up and prognostication. Pattern recognition is illustrated in the following article. Essentially, this article illustrates what the pictures mean and how to generate these meaningful echo pictures.
Keywords: Global longitudinal strain, peak systolic strain, postsystolic strain
|How to cite this article:|
Vijayaraghavan G, Sivasankaran S. Global Longitudinal Strain: A practical Step-by-Step Approach to Longitudinal Strain Imaging. J Indian Acad Echocardiogr Cardiovasc Imaging 2020;4:22-8
|How to cite this URL:|
Vijayaraghavan G, Sivasankaran S. Global Longitudinal Strain: A practical Step-by-Step Approach to Longitudinal Strain Imaging. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2020 [cited 2020 Jun 1];4:22-8. Available from: http://www.jiaecho.org/text.asp?2020/4/1/22/282195
| Introduction|| |
Longitudinal strain is an echocardiographic bedside method to assess the regional and global left ventricular function., We conventionally assess the left ventricular function by two-dimensional (2D) echocardiography. The Simpson's method of deriving ejection fraction of the ventricle is well accepted for all clinical reports and for research work. The old M-mode technique of measuring ejection fraction has been discarded by the American Society of Echocardiography (ASE) and is no longer used because of its unreliability. Laboratories, which use 3D echocardiographic machines, use the 3D voxel volumes of the left ventricle, which is more precise, as one can avoid foreshortening of the ventricle and geometric assumptions in calculation. Short of advanced 3D echocardiographic machines and magnetic resonance imaging, strain imaging has evolved as a cost-effective simple bedside tool, for objective evaluation and monitoring of global and regional ventricular function.
The regional wall motion is usually assessed by the operator by visualizing (eyeballing) the left ventricular endocardial motion as well as wall thickening. The left ventricle is divided into 17 segments representing the basal, mid, and apical segments of the interventricular septum, anterior wall, lateral wall, and the inferior wall with the apical cap. The interventricular septum is divided equally into inferior and anterior. The lateral wall along the anterolateral papillary muscle is the anterolateral wall and the rest is considered as the inferolateral segment of the left ventricle. The terminologies advocated by the ASE for wall motion are normal, hypokinetic, akinetic, dyskinetic, aneurysmal, or hyperkinetic. Visual assessment of wall motion requires individual expertise and is purely subjective. The wall motion could be labeled as one of the six types as termed above. As the left ventricle contracts in systole, it shortens in the longitudinal and circumferential direction (negative strain) and thickens in the radial direction (positive strain). The change in length or thickness is measured and expressed as a percentage of its diastolic length or thickness. This change in length or thickness is called the strain or deformation., Longitudinal shortening [Figure 1] is responsible for 60% of the left ventricular ejection and is the most important component of systolic function. This longitudinal shortening varies from −15% to −30% from base to apex. The circumference of the left ventricle becomes small with systole. This systolic shortening of circumference of the left ventricle is circumferential strain which measures to − 20%–−30% of its diastolic circumference. Left ventricular contraction causes radial thickening in systole. The thickening is measured as a positive strain of +30%–+60%. In the short-axis view during left ventricular systole, basal segment of the left ventricle rotates in an overall clockwise direction and apex rotates in a counterclockwise direction when viewed from apex to base. This squeezing movement of the left ventricle is essential for the systolic ejection and is termed as twist and has counterclockwise movement of about 65° at the apex and clockwise movement of the base for about 5° in a normal individual. This left ventricular torsion is followed by rapid untwisting, which contributes to ventricular filling because LV torsion is directly related to fiber orientation in the left ventricular wall. This article will deal with only the longitudinal strain.
|Figure 1: Side-by-side display of diastolic (left) and systolic (right) images of the apical four-chamber view left ventricle showing the longitudinal shortening of the left ventricle shown by large arrows in a normal individual|
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| Longitudinal Strain|| |
The velocity of motion of the myocardium can be studied by applying low-velocity filters for the reflected signals in the echo machine which is known as the tissue velocity or the tissue Doppler imaging. Like M-mode evaluation, good resolution and high-frame rates can be achieved and hence the measures are more objective than the visual assessment. Pulse Doppler in this mode depicts the peak velocity of the region of interest where the sample volume is placed, whereas interrogation by color tissue Doppler mode will define the mean velocities of multiple sample volumes. Velocity and displacement are measures of wall motion, but when different parts of a segment move with different velocities, that segment will undergo deformation, which can be studied as the strain and strain rate imaging. For example, if the endocardial and epicardial velocities are different, the difference in velocity of both the segments divided by the initial thickness will give the strain rate or rate of deformation of that segment. Myocardial deformation is derived using either tissue Doppler imaging or by 2D speckle tracking. Using tissue Doppler strain imaging the myocardial movement velocities are recorded and the computer algorithm defines the difference in velocities between the edges of adjacent segments. Since the velocity difference between the two edges is compared, the translational movement is negated and the deformation of that segment is depicted, as shown in [Figure 2]. Electrocardiogram (ECG) is the timekeeper for the systolic and diastolic events. Along with the beginning of the QRS complex of the ECG, the strain curve shows a small positive wave indicating lengthening of the myocardium lasting few milliseconds and then a gradual negative wave. The negative wave represents the systolic shortening percentage of the myocardium in each segment. The maximal shortening occurs at the end systole/peak systole and is more than 15%. This occurs toward the end of aortic systolic velocity curve, at the peak of the T-wave of the ECG. The shortening percentage or strain increases as you move from base to apex. From the end systole, the curve gradually goes back to the resting level of 0% till end diastole, to begin the next cardiac cycle. After the peak systole, the curve may show a small wave in early diastole called the postsystolic wave. This wave normally is less in magnitude than the peak systolic strain. Normal ranges for these values are now available on publications. Strain or deformation represents the systolic shortening of a segment of the myocardium and is expressed as a percentage of the diastolic length. This mimics the ejection fraction, where it is expressed as a percentage of the diastolic volume ejected out in systole. However, peak systolic strain is more related to contractility and occurs in the early part of the systole. Doppler tissue tracking is unidimensional and measures the movement in relation to the transducer.
|Figure 2: The strain curve: Peak systolic strain corresponds to the aortic valve closure or peak of the T-wave. Small negative deflection after that is termed or postsystolic strain|
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| Speckle Tracking for Strain Imaging|| |
Conventionally, we were recording strain curves by tissue Doppler echocardiography from various segments of the myocardium. For accurate measurements, the incident ultrasound beam should be parallel to the myocardium interrogated. With the normal and abnormal anatomy of the left ventricle, this is not a practical proposition while using the tissue Doppler. Speckle tracking in Doppler echocardiography avoids this problem. In 2D speckle tracking, acoustic markers distributed in the myocardium are automatically identified and tracked, and depending on the frame rate, their velocities are calculated. Unlike tissue Doppler strain imaging, speckle tracking is no longer angle dependent and hence has less interobserver and intraobserver variability. This postprocessing computer algorithm uses the routine grayscale digital images. Movement of the echo-reflecting speckle patterns in the myocardium is utilized to image the shortening of every segment during various phases of the cardiac cycle. Within each region of interest in the myocardium, the image-processing algorithm automatically subdivides regions into blocks of pixels tracking stable patterns of speckles. Subsequent frames are then automatically analyzed by searching for the new location of the speckle patterns within each of the blocks using correlation criteria and the sum of absolute differences. The location shift of these acoustic markers from frame to frame representing tissue movement provides the spatial and temporal data used to calculate velocity vectors. Temporal alterations in these stable speckle patterns are identified as moving farther apart or closer together and create a series of regional strain vectors and the sum of it as strain curves. The strain measurements are done by different software by various vendors. This article describes the automatic functional imaging (AFI) protocol used by the General Electric (GE) echo machines.
| Longitudinal Strain Imaging Protocol|| |
Longitudinal strain imaging [Figure 3] needs good quality apical views, four-chamber views, long-axis views, and two-chamber views imaged with a good frame rate and can be completed in four steps.
|Figure 3: Speckle-tracking echocardiography showing the longitudinal strain of the basal, mid, and apical segments in the three-chamber, panel a, four-chamber, panel b and two-chamber, panel c|
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- Reduce the depth of the image and the width of the sector to get a good quality image without any dropouts. Increase the frame rate to more than 60 fps. Breath held in expiration and minimizes the motion artifacts. Apical long-axis, four-chamber, and two-chamber images are to be archived for three cardiac cycles. Aortic closure is marked at the peak of the electrocardiographic T-wave. At this stage, choose the software for strain analysis depending on the vendor. With the GE machines, the AFI software forms the measurement protocol. Start with the apical long-axis view and follow the machine instructions. The region of interest is marked by placing the points at the base of the left ventricle near the atrioventricular posterior and anterior annulus and finally the apex. The region of interest should be chosen to include only the myocardium. Avoid the pericardium and the valve annulus, especially in the left ventricular outflow. The left ventricular walls will be highlighted in colors depicting the basal, mid, and apical segments. Apical long-axis (three-chamber) view will show the anterior septum and the inferolateral wall. Four-chamber view shows the inferior septum and anterolateral wall. Two-chamber view shows the inferior wall and the anterior wall. Thus, all the 17 segments can be interrogated when the apical four-chamber echo window is good. If there are two or more segments in any imaging plane, which are not acceptable by the machine software for analysis, depicted as red by the machine, do not proceed. On completion of the analysis, the machine uses various types of color coding, one for the segments analyzed and the others for the strain parameters of interest. When the images display additional physiologic phenomena, it constitutes parametric imaging. The ingenious use of parametric images utilizes the color coding of the segment as the borders and the degree of strain as the color filling the segment, along with ECG to highlight the timing of the strain measured, say peak systole or postsystole, and generate the quad screen and bull's eye images
- At this phase, each of the three imaging planes will generate four pictures in the quad format, as shown in [Figure 4] and [Figure 5]. At the upper left side [Figure 4]a, you can identify the myocardial segments each in unique colors, the color-coded sector image. [Figure 4]b graphically shows the strain velocities of the various segments in relation to ECG, and each curve carries the same color as the segment they represent. Normally, all segments move in parallel, with the basal segments showing lesser excursion than the apical segments. In parametric imaging, additional features of the reflected beam such as attenuation and time delay are utilized to understand the physiologic components of the segment. [Figure 4]c depicts the parametric systolic frame with the color coding of segments drawn on the border lines and maximal strain achieved at the peak of the T-wave, the peak systolic strain image. The color coding is such that good peak systolic strain is depicted as red, suboptimal strain as various shades of pink, and positive strain as blue, meaning that segment is dyskinetic. [Figure 4]d is the curved M-mode of all the segments starting from one base to the other with apex at the center. Red color shows normal negative strain and blue color positive strain or expansion of the segment, and the color ribbon on either side of the picture shows the color coding of the individual segments. The illustrative picture shows the apical five-chamber view, and hence, the curved M-modes start with the basal anterior septum at the top and end with the basal lateral wall at the bottom
- This sequence of image analysis is done for six segments of the apical long-axis view, apical four-chamber view, and apical two-chamber view. Once all the three apical images have been interrogated and saved, the AFI software [Figure 6] will be able to display the bull's eye picture of 17 segments of the left ventricle with measured peak systolic longitudinal strain values. This image will give you the mean longitudinal strain of each imaging plane and the mean of the peak global longitudinal systolic strain [Figure 6]a. In a normal individual, the strain curves reach its peak strain, at peak systole, toward the aortic valve closure, in a coordinated manner. You can select any segment of the bull's eye and highlight the respective colored strain curve so that each curve could be analyzed in detail [Figure 6]b. The color of each segment also represents the strain. Like the color M-mode, normal strain will be depicted in red and reduction of strain will be seen as light pink, and if the segments lengthen instead of shortening, it will be seen as blue. One should always get a picture of the moving images of the three imaging planes and the bull's eye picture so that correlation of each segment could be done with the strain values and with the color of each segment depicted in the bull's eye. The measured average peak systolic strain of each imaging plane as well as the global average peak systolic strain will be depicted in this picture
- In addition to the peak systolic strain bull's eye map, you can choose to have a bull's eye map of postsystolic index (PSI) in a blue format, as well as peak strain (postsystolic strain [PSS]) or time to peak strain with strain curves to explain the respective bull's eye depiction. [Figure 7] shows the bull's eye map of the various myocardial segments in a patient with dyssynchrony. One can select bull's eye view of the peak systolic strain panel D or PSS index in various shades of blue [Figure 7]f or see bull's eye view of the time to peak velocity format [Figure 7]e, which will demonstrate the timing of the peak strain from the onset of QRS complex. The PSI is calculated as follows: ([maximum strain in cardiac cycle − systolic peak strain]/[maximum strain in cardiac cycle]) ×100, where the PSS is the maximum strain in cardiac cycle. PSS will identify areas where the contraction continued on to diastole in certain myocardial segments as a pathological phenomenon. [Figure 8], [Figure 9], [Figure 10] display the bull's eye pattern and quad screen formats in different patients with dysfunctional ventricles.
|Figure 4: The quad screen display of apical long axis view. (a) The color coding from the lateral basal to the medial basal segment. (b) The strain velocities of all these six segments with the electrocardiogram. All the curves run parallel, reflecting synchrony. (c) The parametric image in systole, where the segments are identified by the color-coded border lines with normal peak strain colored as red. (d) The curved M-mode display of the segmental strain of all the six segments with the lateral basal segment at the bottom and the medial basal segment at the top|
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|Figure 5: Quad screen display of speckle tracking imaging of the apical five chamber view which shows the dyssynchronous and abnormal strain pattern in a patient with diseased ventricle. 5a shows the color coding of the segments, and 5b shows the strain curves of the segments: Apical septum is violet colored and its strain curve is above the base line, reflecting dyskinesia. 5c shows the peak strain of the individual segments and the apical septum is depicted blue (positive strain or dyskinesia). 5d is the curved M mode of the individual segments. Basal segments are red in systole with positive strain whereas the middle apical region is blue because of positive strain or dyskinesia|
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|Figure 6: Bull's eye map of the peak systolic strain. (a) The quad screen format of all the three views used to generate the bull's eye image on the fourth quadrant. (b) The strain velocity curves of the individual myocardial segments are displayed in three quadrants representing the apical four-chamber, two-chamber, and three-chamber views. The trapezoid cursor in the fourth quadrant bull's eye map is on the inferior midsegment and the blue strain curve gets automatically highlighted in the strain curves for the apical three-chamber view|
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|Figure 7: The bull's eye images: (a-c) The strain curves of the three apical views. There are hypokinesia and poor strain (thick vertical arrow) and exaggerated postsystolic strain (oblique thin arrow) in the inferior and inferolateral segments. (d) The peak systolic strain which is poor in the inferior and inferolateral segments. Time to peak is delayed in these segments, which is displayed in the time to peak bull's eye map in panel e. Panel f shows the postsystolic strain map showing delayed contraction|
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|Figure 8: Quad screen formats of ventricular strain imaging. (a) The time to peak bull's eye map. The septal basal segments with normal activation are colored green, rest being yellow, and red in a patient with extensive coronary artery disease multisegmental delay and scarring with poor strain curves. (b) The bull's eye map of postsystolic strain in a patient with viable myocardium. Basal anterior septum and anterior wall are intensely blue with good postsystolic strain|
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|Figure 9: Quad screen formats of strain imaging in left ventricular endomyocardial fibrosis (a -c) the apical 4 chamber, 2 chamber and 3 chamber views respectively with the bull's eye map of the peak systolic strain shown in (d). (e-h) the strain waves of the apical views with the bull's eye plot of the peak systolic strain. Note that the reduced strain values of the fibrosed and calcified apical segments and the blue color indicating the systolic lengthening of apical segments|
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|Figure 10: Bull's eye format of peak global longitudinal strain from the apical views in a patient with myocarditis. (a-d) In the initial phase, note the markedly reduced strain values of all myocardial segments. The inferolateral basal and mid segments show systolic lengthening (blue). (e-h) The strain curves of the same patient after 1 month showing near complete recovery of all myocardial segments with average global longitudinal peak systolic strain value improved from -4.4% to -14.9%|
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| Ejection Fraction and Global Longitudinal Strain|| |
There is a wide criticism for the method of assessment of left ventricular function by ejection fraction. M-mode echocardiographic calculation of ejection fraction is not used now and is replaced with 2D Simpson's biplane method. Foreshortening is a major problem for apical imaging and may underestimate ejection fraction. 3D method is ideal but not universally available in every echo laboratory. Global longitudinal strain (GLS) is more reliable for assessing the systolic function in normal individuals as well as in patients with heart failure. The GLS strain is labeled as mildly reduced when it is between −15% and −12.5%, moderately reduced when it is between −8.1% and −12.5%, and severely when it is <−8.0% [Table 1]. This assessment was found to be statistically more reliable than the use of ejection fraction in assessing the prognosis of patients with cardiac failure. [Figure 8]b represents a patient with dilated cardiomyopathy with markedly reduced GLS of −4.5%, of course with poor prognosis for the patient. [Figure 9] is from a patient who presented to us with cardiac failure and left ventricular ejection fraction of 19%. His average peak GLS value was only −4.4%, obviously low ejection fraction correlated with the average low strain value. The segmental abnormality could be observed from the accompanying strain curves. In a month's time, he improved remarkably and the global average strain increased to −14.9%, near normal value, and the ejection fraction improved to 46%. The strain curve gives additional information about those segments which are yet to recover, an information required for follow-up imaging. This is superior to the information given by the global ejection fraction.
|Table 1: Grading of ventricular function by ejection fraction and global longitudinal strain compared|
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| Fallacies during Strain Imaging|| |
Quad screen image of the three imaging planes depends on the quality of each image you archive and analyze. If the images have many dropouts, we should repeat acquiring those images without any lapse of time. If the heart rate changes due to delay in archiving, the machine cannot calculate the bull's eye. If there are two or more segments in any imaging plane, which are not acceptable for analysis (will be depicted in red color during initial analysis), do not proceed but get better quality images for strain analysis. Always repeat the image archiving and analysis to see whether the same operator gets comparable bull's eye results. In the beginning, this will require a lot of patience on the operator. Unless you have successfully completed at least 50 imaging sessions on patients and got comparable bull's eye pictures on repeat imaging in the same patient, do not proceed to report the strain imaging. This is the long learning curve, which improves the precision and utility of strain imaging.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]