Tissue Doppler imaging and speckle tracking, also called strain imaging, are both ultrasound modalities used to assess left ventricle myocardial function. Speckle tracking has only gained popularity recently and not everyone knows how it works.
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The evaluation of myocardial function can be tricky. This course will help you master an easy-to-follow process on how to acquire, optimize, and interpret left ventricular strain images. Using hands-on tips and tricks, you’ll cover global longitudinal strain (GLS) as well as important pearls and pitfalls as we go through several exciting real-life examples. Get ready to become an echo expert!
Tissue doppler imaging and speckle tracking echocardiography are two quantitative techniques that can accurately measure left ventricular strain in patients with heart disease. These two modalities use different physical techniques for strain measurement. Tissue Doppler imaging uses the Pulse Wave Doppler principle, and speckle tracking echocardiography uses two dimensional grayscale B mode images. In this MedMastery lesson, we will compare the use of these two foundational imaging modalities for strain imaging.
Let's begin with a Tissue Doppler Imaging or TDI. TDI is an ultrasound technique that uses the Doppler principle to measure the velocity of myocardial wall motion. In contrast to traditional Doppler, which measures the velocity of blood flow. The color scale shows the direction of the myocardial wall motion. Red means that the myocardial wall is moving up towards the probe. Blue represents the myocardial wall moving away or down from the probe.
The ECG tracing is also shown at the bottom to help track the patient's heart rhythm and rate. TDI provides an instantaneous measurement of tissue velocity at a specific location, and is used to quickly assess the diastolic function of the heart. Flow moving toward the probe is positive and flow moving away from the probe is negative. The major pitfall of TDI is that it can only track one specific myocardial area at a time. Also, like any Doppler based modality, TDI is bound by the same image limitations, including angle dependence of the ultrasound beam.
To obtain accurate myocardial tissue motion measurements, the direction of the myocardial tissue motion must be parallel, or at an angle of less than 30 degrees to the ultrasound beam. But in reality, myocardium motion will be in the direction of the red arrow, which can be much greater than 30 degrees. This change in angle can reduce the accuracy of TDI. TDI is also very susceptible to signal noise, and several studies have shown that TDI produces significant interobserver variability in measurement.
To overcome these limitations of TDI, speckle tracking echocardiography, or STE, was introduced as an advanced modality to detect left ventricular myocardial function. STE is based on tracking every snapshot of grayscale images obtained during echocardiography. These grayscale images are composed of several bright speckles produced from the interaction between the ultrasound beam and myocardial tissue. A software program is then used to identify the speckles and track them frame by frame during the cardiac cycle.
The change in their displacement is then calculated to assess myocardial deformation. Because STE tracks is speckles in two dimensions along the direction of the myocardial wall, and no Doppler measurements are required, 2D STE is relatively independent of angle and thus more reproducible, which is one of the key advantages over TDI. However, because of modalities tracking individual speckles along the 2D image, it requires high resolution images. It is also very important that the entire left ventricle myocardium and apex are clearly visible in the require apical images for proper speckle tracking. These include the apical three chamber, four chamber and two chamber views.
Using multiple chamber views allow us to get as close as possible to a 3D picture of myocardial strain. Differences in image quality, acquisition and post processing can lead to measurement variability. This is why you must practice and gain enough experience to be able to verify image quality before selecting images for tracking which will be covered in later chapters.