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Tissue Doppler, Strain and Strain Rate in ischemic heart disease “How I do it”
© Mada et al.; licensee BioMed Central Ltd. 2014
Received: 21 July 2014
Accepted: 1 September 2014
Published: 18 September 2014
Echocardiography is the standard method for assessing myocardial function in patients with ischemic heart disease. The acquisition and interpretation of echocardiographic images, however, remains a highly specialized task which often relies entirely on the subjective visual assessment of the reader and requires therefore, particular training and expertise. Myocardial deformation imaging allows quantifying myocardial function far beyond what can be done with sole visual assessment. It can improve the interpretation of regional dysfunction and offers sensitive markers of induced ischemia which can be used for stress tests. In the following, we recapitulate shortly the pathophysiological and technical basics and explain in a practical manner how we use this technique in investigating patients with ischemic heart disease.
Assessing the impact of ischemic heart disease on cardiac function is one of the key tasks of routine clinical echocardiography. At virtually any stage of the disease process, echocardiography is of diagnostic or prognostic value, guiding further therapeutic decisions. The careful observation of global and regional ventricular function and morphology in resting images, allows describing and distinguishing acute ischemia, scar and infarct related remodeling while stress echocardiographic methods may be used to assess the severity of coronary stenosis or to identify viable myocardium. Offering this wealth of information, echocardiography has become an indispensable tool in managing patients with coronary artery disease.
The acquisition and interpretation of echocardiographic images, however, remains a highly specialized task which requires knowledge, skills and experience. Not all echocardiographic parameters are quantifiable. Moreover, typical echocardiographic tasks, such as the evaluation of regional myocardial dysfunction in ischemic heart disease, rely entirely on the subjective assessment of the reader and require therefore particular training and expertise [1–4].
Tissue Doppler and speckle tracking based myocardial deformation imaging allows quantifying myocardial function far beyond what can be done with sole visual assessment. In the following, we would like to explain how we use this technique in investigating patients with ischemic heart disease.
How to measure myocardial function
Comparison of recommended steps for tissue Doppler and speckle tracking based deformation imaging
- TDI images with insonation angle <15° (single wall acquisition if needed) and adequate velocity scale (to avoid aliasing)
- Optimized 2D images avoiding foreshortening and stationery artefacts (reverberation)
-Frame rate > 100 fps (ideally > 140 fps)
- Frame rate: 40–80 fps
- Acquire spectral Doppler traces of mitral and aortic valve for timing
- Acquire a spectral Doppler trace at least of the aortic valve as backup for timing
- Measure aortic and mitral valve opening and closure in order to define cardiac phases
-Order of image analysis: arbitrary
- Start analysis with the apical long axis view in order to define aortic valve closure. Use Doppler derived data if needed.
- Set the region of interest (ROI) shape and size oval somewhat smaller than the wall thickness. Position the needed number of ROIs it in the middle of the segments to be analysed. Track ROI position if needed.
- Contour the myocardium according to the procedure proposed by the vendor. Avoid including the bright pericardium.
- Evaluation of curve quality. Repositioning of the ROI if needed.
- Careful visual control of the proposed tracking. Repositioning of the contours if needed. Exclusion of suboptimally tracked segments if re-contouring is unsuccessful.
- Shape analysis of the curves
- Shape analysis of the curves
- Measurement of peak values and further post-processing if needed
- Measurement of peak values and further post-processing if needed
Advantages and limitations of tissue Doppler and speckle tracking based deformation imaging
- High temporal resolution
- Dedicated image acquisition needed.
- Robust in case of limited image quality
- One dimensional: measures deformation along the ultrasound beam direction
- Easy recognition of artefacts, easy assessment of data quality
- Noisy data, sensitive to stationary artefacts
- Fast qualitative analysis: curves readily available by moving the mouse pointer over the myocardium (no post- processing needed)
- Region of interest must be tracked to keep the same position within the myocardium
- Training needed
- Standard 2D images used for analysis
- Lower temporal resolution
- Strong dependence on image quality
- Two-dimensional analysis possible
- Intensive regularization and drift correction with limited options for user interaction
- Less noisy data
- Very user friendly
- Difficult assessment of data quality
- Easy post-processing, wealth of graphical displays
- Intervendor differences in parameter definitions and measurement algorithms
- Highly automated extraction of derived parameters
- Need for training underestimated
Tissue Doppler derived deformation
TDI is the classic approach to measure deformation in echocardiographic images . From TDI data, strain rate can be simply derived by calculating the regional velocity gradient. By integration of the latter, strain is obtained. Major advantages of TDI derived strain measurements are the very high temporal resolution and the relative robustness if image quality is limited. After some training, curves are easy to interpret and allow a good distinction between good quality data and artefacts. For simple curve shape analysis in clinical use, no post-processing is needed since curves are readily available by just moving the mouse pointer over the myocardium. Disadvantages comprise the angle dependency of the data and the need for training and experience when advanced data handling and interpretation are needed.
Speckle tracking derived deformation
Which function changes can we expect?
Functional imaging shows motion and deformation of the myocardium within the image plane. Any change in perfusion or the underlying metabolism of the myocardial fibres will remain invisible unless it affects their contractile state. Further, imaging cannot measure pressures or forces and is, thus, “blind” to loading conditions. Any observed myocardial deformation is consequently a product of the contractile state of the myocardial fibres, modulated by pre- and afterload. It is therefore important to understand, that all findings in functional cardiac imaging do not directly relate to myocardial contractility, but must be interpreted in the context of wall thickness, chamber geometry and loading conditions.
Ischemic function changes
The imbalance between oxygen demand and supply results in an impairment or loss of contractile function and, subsequently, cell death. Transient ischemia will cause reversible regional dysfunction. Even without structural damage, however, the dysfunction may last for hours after an ischemia which is called “stunning” [12, 13]. A chronic reduction of coronary flow leads to a proportional myocardial dysfunction named “hibernation”. Hibernating myocardium may also recover completely after restoration of blood flow, but degenerative changes and persistent dysfunction may occur after longer malperfusion . Coronary occlusion lasting more than a few minutes results in irreversible myocardial injury with later scar formation . Cardiac remodeling will occur depending on the amount of myocardial loss [16, 17].
The particular anatomy of the coronary arteries and the higher extravascular pressure make the subendocardial region most vulnerable to ischemia. Dysfunction will therefore firstly occur in the subendocardial fibres . To which extend layer-specific (dys-)function can be evaluated by echocardiography, remains to be determined.
Functional imaging is blind to stenosis of the epicardial vessels unless regional dysfunction develops due to ischemia. Pharmacological, physical or electrical stress tests are needed to induce such ischemia and belong, therefore, to the standard armamentarium of echocardiography in the clinical setting of ischemic heart disease [4, 19, 20].
Characteristic deformation patterns in disease
Two principal changes can be observed with deformation imaging.
Type 1: The reduction in systolic strain indicates myocardial dysfunction, but may also be attributed to changes in loading conditions or ventricular geometry. The reliable detection of abnormal strain amplitudes requires a robust and reproducible measurement method. Under clinical conditions, the variability of both regional tissue Doppler and speckle tracking measurements is relatively high, which makes regional strain amplitude measurements currently less suited for routine use. This is in contrast to global strain measurements which have been shown to be reproducible and robust measures of global ventricular function, superior to classical EF measurements, probably due to the largely automated assessment and extensive spatial averaging .
“How I do it” - Practical approaches in particular situations
Chronic ischemic disease
Patients with myocardial damage after myocardial infarction will usually present changes in global and regional function, both of which need to be described and followed. In these patients, a speckle tracking based analysis can be most easily integrated in the clinical routine.
Three standard grey scale image loops, with settings balanced for both spatial and temporal resolution (ca. 40–80 frames per second), covering the LV and including the mitral and aortic valve should be stored. Care must be taken that the LV is not foreshortened and that reverberation artefacts (bright, wedge-shaped, stationery artefacts) on the edges of the image sector are avoided. A clear artefact free display of the apex is desired. We acquire three consecutive heart beats, avoiding extrasystoles. In addition, a PW- or CW-Doppler trace of the LVOT is useful to obtain timing information.
This is usually an emergency room scenario where an immediate coronary intervention has highest priority. The echocardiographic examination should therefore be limited to the necessary minimum which is needed to clarify remaining clinical questions. Conclusions will be usually drawn from the immediate visual analysis of the image data.
Data acquisition and analysis
Ischemia testing aims at detecting transient regional dysfunction during an exercise or pharmacological stress which requires training and experience [1–4]. Functional imaging can help to objectify findings and increase the diagnostic and prognostic value of the test [23, 26, 27]. Stress testing is therefore one of the most important applications of functional imaging.
Tissue Doppler strain is robust for shape analysis, quickly analysable during data review and can easily deal with high heart rates due to its excellent temporal resolution. It is therefore our preferred method during stress echocardiography. We use a machine which allows the simultaneous acquisition of grey scale and colour tissue Doppler data while only the grey scale image is displayed. Care must be taken that the underlying colour Doppler sector has the size of the full image, while the frame rate should not fall far below 100 fps. We acquire apical four-, three- and two-chamber views as they allow to measure longitudinal strain in all segments. We perform stress echos according to current recommendations . Dobutamine is the preferred stressor since it allows convenient image acquisition over three consecutive cycles during a breath hold even at peak stress. Bicycle stress is also possible, but makes data acquisition more challenging. As in regular stress echo examinations, the acquisition of identical image planes at each stage is crucial.
Summary and perspective
Current deformation imaging software is sufficiently developed to allow fast and reliable analysis and to provide clinically helpful answers. As any quantitative imaging technique, deformation imaging relies on the assumption that pathologic processes cause functional changes and that these changes can be visualized with sufficient accuracy. Improvements in both, tissue Doppler and speckle tracking algorithms would be desirable to reduce noise and improve measurement accuracy . If further developments in 2D and 3D speckle tracking, such as layer specific strain, or if the assessment of all three normal strain components in 3D data sets results in an improved diagnostic value of echocardiographic examinations remains to be determined.
From a current perspective, strain and strain rate imaging by tissue Doppler and speckle tracking are helpful tools in the clinical assessment of CAD patients and should be part of every echocardiographic examination.
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