Tissue Doppler-based measurements of myocardial strain are possible and accurate for structures that move along the ultrasound beam, but are underestimated in other directions and even impossible for angles close to 90º. To overcome these limitations, the speckle tracking technique was introduced in 2004, offering a more user-friendly workflow and better reproducibility.1 This echocardiographic technique is based on frame-by-frame tracking of ultrasound speckles as the natural acoustic markers within the image. By tracking these speckles at frame rates varying from 40 to 150 frames/s-1, 2D tissue velocity and displacement can be accurately calculated over time, irrespective of the direction of motion. Furthermore, myocardial strain can be determined from the displacement of speckles in relation to each other, providing an angle-independent parameter of regional myocardial function. To obtain this information, the left ventricular myocardium is first traced using the click-to-point approach on the end-systolic frame, followed by automated definition of an epicardial and mid-myocardial line. Whereas apical planes are used to depict longitudinal and transversal strain, the parasternal short-axis views allow the differentiation of circumferential and radial strain. Velocity vector imaging is another technique based on speckle tracking, whereby the vector length depicts the amount of strain and the direction of vectors represents the direction in which tissue is moving.
Clinical applications of speckle tracking include the evaluation of ischemic heart disease and left ventricular dyssynchrony.1–3 Although tissue Doppler imaging and particularly velocity imaging have been used widely to predict response to cardiac resynchronization therapy (CRT), angle dependency, noise, artifacts, tethering, and myocardial translation are known to affect the reproducibility of these measurements and may in part explain the relatively high rate of non-responders to CRT among patients referred using this technique as a criterion for referral. In contrast, the ability of 2D radial speckle tracking to predict the increase in stroke volume after CRT has been demonstrated, and this technique is accepted as a robust and highly sensitive and specific technique.4 2D speckle tracking is also able to detect radial dyssynchrony in patients with apical dysfunction not otherwise detected by routine longitudinal tissue Doppler. The superiority of 2D speckle tracking, particularly based on radial strain rather than longitudinal tissue Doppler findings, may be related to a better tracking algorithm in the short-axis view than in longitudinal images obtained from apical windows.
Furthermore, 2D speckle tracking has also been used to assess left atrial (LA) function in patients with either idiopathic or ischemic dilated cardiomyopathy.5 Analysis of atrial longitudinal strain in the basal segment of the LA septum, the LA lateral wall, and the LA roof showed that peak systolic myocardial strain was significantly compromised in patients with idiopathic dilated cardiomyopathy (DCMP) versus ischemic CMP in all analysed atrial segments (p<0.001).
Despite these encouraging findings, experiences with strain imaging based on 2D speckle tracking are still limited, and there is a need for additional validation studies and data on the reproducibility of derived strain curves. The main limitation of 2D speckle tracking is speckle loss due to motion out of the imaging plane.
3D Speckle Tracking
Recently, 3D speckle tracking has been developed and tested.6 The great benefit of using 3D information is that there is no loss of speckles due to motion outside of the imaging plane. In order to create a full-volume data set of the left ventricle (LV), realtime 3D imaging technology is used, allowing 3D evaluation of LV wall motion. A pyramidal data set with a reasonable balance between spatial and temporal resolution that encompasses the entire LV is usually obtained by sequential acquisition of four subvolumes, with each covering a complete cardiac cycle.
During the acquisition, a five-plane depiction of the four- and two-chamber apical views and short-axis planes at the apex, mid, and base of the LV guides the user to update the acquisition process continuously. After imaging the apical four-chamber plane, the system automatically displays the orthogonal two-chamber and three short-axis planes (basal, mid, and apical). Inner and outer borders of the LV myocardium are determined automatically. A dynamic 3D cast can be displayed with parametric information such as strain mapped onto the endocardial surface using different colors. The frame rate depends on various scanning conditions, but is set at around around 20–30 frames/second, thus resulting in sufficient temporal resolution to accurately detect peak strain and displacement information. Another advantage of 3D speckle tracking is the evaluation of the motion of all myocardial segments in a single analysis step, which significantly reduces analysis time.
Workflow
To apply the 3D wall motion tracking to a data set, the user usually has to set three reference points in the apical four-chamber view: two at the mitral valve level and one at the apex (A-plane). This procedure has to be repeated for the orthogonal B-plane. With these six reference points, the system automatically displays the endocardial border. The epicardial border is defined manually or by setting a default ‘myocardial thickness.’ After the shape of the LV at end-diastole is accepted by the user, the 3D wall motion tracking process can be started. Results and analysis of myocardial function including a number of parameters are displayed within a few seconds. This analysis can be performed online during the examination or offline on a dedicated workstation.
Left Ventricular Volume and Ejection Fraction
In addition to the evaluation of LV wall motion, endocardial detection and tracking allows dynamic quantification of LV volume, which provides volume over time. One advantage of this technique is that it is based on 3D endocardial tracking information and not geometrical assumptions necessary to calculate LV volume from endocardial boundaries in 2D planes. Our initial validation data comparing LV volumes and ejection fraction derived from 3D speckle tracking with magnet resonance imaging showed excellent inter-technique agreement and high reproducibility of the new 3D speckle tracking technique.
Ischemic Heart Disease
For the first time, 3D speckle tracking allows for the definition of longitudinal strain in short-axis views and radial and circumferential strain in long-axis views, offering additional information regarding regional strain impairment, and thus promising to improve the assessment of wall motion in patients with suspected coronary artery disease. Using 2D strain, Becker et al. reported significant differences in myocardial deformation parameters of non-infarcted segments compared with segments with transmural and non-transmural infarction.2 Circumferential strain values were 18.6±5.6% (no infarction), 12.8±6.7% (non-transmural infarction), and -8.1±-5.3% (transmural infarction), respectively (p<0.0001). As a limitation, the detection of non-transmural infarction versus no infarction was associated with a low specificity for all parameters.
In another study including 80 patients, combined assessment of short-axis and long-axis cardiac function allowed differentiation of transmurality of chronic infarction. In subendocardial infarction, 2D circumferential strain parameters were preserved but 2D longitudinal strain parameters were reduced, whereas in transmural infarctions both parameters were significantly reduced.3
The new speckle-tracking-based 3D strain imaging provides detailed regional wall motion based on the 16-segment model according to the American Society of Echocardiography (ASE) or the 17-segment model (AHA). Myocardial motion is detected in all three dimensions and the real vector of motion is represented. The display of myocardial mechanics as twist (net difference of apical versus basal rotation with normal peak values of 9.8±4.0°) or torsion (net difference between rotational angles normalized for LV longitudinal length (°/cm), which is influenced by positive and negative inotropic interventions and depressed after myocardial infarction with reduced ejection fraction, is now possible in a 3D model. These parameters can be visualized on the LV-shaped parametric display, as well as time curves demonstrating the segmental and global parameter change during the cardiac cycle. Furthermore, a polar map of segmental strain displays infarcted areas in darker colors and differentiates dyskinetic wall motion. This may also have the potential to define regional wall motion abnormalities during or after stress. Usually, qualitative visual analysis is used to determine the presence of stress-induced wall motion abnormalities. In contrast, 3D wall motion tracking offers a new, more objective analysis based on detailed quantitative information on regional myocardial strain, which can be implemented into stress echo protocols.
Left Ventricular Dyssynchrony
In contrast to tissue Doppler imaging and its known limitations in the assessment of LV dyssynchrony in patients referred for CRT, 2D strain imaging is able to detect radial dyssynchrony in patients with apical dysfunction otherwise not found by routine longitudinal tissue Doppler. This may be related to a better tracking algorithm in short-axis views than longitudinal images obtained from apical windows. Recently, differences in baseline LV dyssynchrony between responders and non-responders were noted only for radial strain, whereas no differences between these groups of patients were found in circumferential and longitudinal strain.7
Since 2005, a number of studies using speckle tracking to assess LV dyssynchrony have been published, showing that a cut-off value of 130ms (time difference in peak septal-to-posterior wall strain) predicts an immediate increase in stroke volume the day after CRT with 91% sensitivity and 75% specificity.4 2D strain imaging has been shown to be of important value not only in the evaluation of LV dyssynchrony, but also after failure of resynchronization therapy due to inadequate lead position.8,9 Furthermore, speckle tracking echocardiography has the potential to analyze alterations in rotational mechanics caused by LV dyssynchrony. In a study reported by Sade et al., LV dyssynchrony was associated with abnormal rotation of the apical and basal regions, whereby rotation was defined as the average angular displacement of six standard segments around the LV central axis in the short axis image, LV twist as a net difference of LV rotation at isochronal time points between the apical and basal short-axis planes, and LV torsion as LV twist divided by LV diastolic longitudinal length.10 Peak apical and basal rotation, peak LV twist and torsion, apical and basal rotation at aortic valve closure (AVC), and LV twist and torsion at AVC were significantly lower in patients with LV dyssynchrony than controls.
LV torsion (cut-off 0.1°/cm) and twist (cut-off 1°) at AVC had the highest sensitivity (90%) and specificity (77%) in predicting CRT responders. 2D speckle tracking was a big step forward in determining the timing of events, which plays a major role in defining dyssynchrony, but speckles may be moving out of the 2D imaging plane, thus affecting the analysis of delayed contraction occurring in 3D. The new 3D system allows myocardial mechanics to be displayed as rotation, twist, and torsion more accurately. This facilitates applications of 3D wall motion tracking in the selection and follow-up of patients who are candidates for CRT.
Conclusion
Although 2D strain imaging has just begun to be integrated into a standard echocardiographic study, the next step, which is 3D wall motion tracking or 3D speckle tracking imaging (3D strain imaging), opens a new door to define volumes and ejection fraction semi-automatically. Furthermore, all types of myocardial strain (radial, circumferential, longitudinal, transversal) and displacement are derived easily and presented as time curves or dynamic polar plots. New mechanical parameters such as twist and torsion can be included to define early myocardial involvement in the individual patient. Its clinical potential is likely to be revealed by future studies in a wide range of heart diseases.