4 Deformation Analysis

Question 1

What are the definitions of displacement and velocity?

Answer 1

Displacement is the distance of movement of the myocardium.

Velocity is the speed of movement of the myocardium.

Question 2

What are the definitions of strain and strain rate?

Answer 2

Strain is the change in length as a percentage.

Strain = (change in length) / (original length) x 100

Strain rate is the speed at which the strain occurs

Average strain rate = strain / time

Question 3

What do positive and negative strain indicate?

Answer 3

Positive strain is relaxation (lengthening).

Negative strain is contraction (shortening).

Therefore, negative strain values are used to assess ventricular systolic function.

Question 4

How is myocardial deformation assessed?

Answer 4

The regional LV function is assessed by measuring the contractility, the ability of the region of the LV to develop a force, which causes the muscle to deform (shorten or lengthen) and move (velocity). The contractility is assessed by measuring the deformation and velocity. The myocardial displacement is assessed with strain. The myocardial velocity is assessed with TDI.

The contractility is dependent on the preload (the level of ventricular stretch, which is dependent on ventricular filling) and afterload (the force the heart works against to eject blood).

Question 5

What is the coordinate system in myocardial deformation analysis?

Answer 5

Displacement or velocity or strain is measured via a coordinate system. The coordinate system axes are longitudinal (the long axis of the ventricle), radial (the short axis of the ventricle) and circumferential.

Question 6

What are longitudinal, radial, circumferential and axial strain?

Answer 6

Longitudinal strain measures the change in length.

Radial strain measures the change in diameter.

Circumferential strain measures the change in circumference along the short axis of the heart.

Axial strain is the movement in relation to the probe.

Displacement = velocity x time.

Question 7

How are longitudinal and axial strain measured?

Answer 7

Longitudinal strain assesses deformation, measured using speckle echocardiography.

Axial strain assesses deformation, measured using TDI.

Question 8

What are the definitions of shear deformation and shear strain?

Answer 8

Shear deformation is the distortion of the heart when layers of the myocardium slide over each other due to the twisting and untwisting of the heart. Shear deformation, unlike strain, involves a change in the shape of the myocardium without a change in the volume.

Shear strain is the relative displacement, and the change in the angle, between the layers of the myocardium.

Question 9

What is torsion?

Answer 9

Torsion is the twisting of the heart, during systole, and untwisting, during diastole. Torsion causes shear stress within the layers of the myocardium. Torsion allows effective ejection and filling.

Torsion is the difference in rotation between the basal and apical, normalised to the distance between the base and the apex, measured in degrees/cm. Normal LV torsion is 10-20°/cm. LV torsion may be increased in HCM but may be decreased in HF.

Question 10

How do the apex and the base rotate during systole and diastole?

Answer 10

In systole, the heart twists due to the helical organisation of the myocardial fibres in the LV. The apex rotates clockwise and the base rotates anticlockwise whilst the mid-LV stays stationary. This is measured in degrees. LV twist is assessed by acquiring apical and basal short axis slices.

Question 11

What is the myocardial velocity gradient and how is it calculated?

Answer 11

MVG is the rate of change in velocity within the myocardium. MVG shows the difference in velocity between two positions in the myocardium over time.

MVG= ∆velocity/∆time

Question 12

How is TDI derived strain calculated?

Answer 12

The TDI derived strain rate is calculated using the velocities at two positions in the myocardium. Integrating the TDI derived strain rate over time results in the TDI derived strain.

Question 13

How is TDI derived strain accuracy improved via controls?

Answer 13

Decrease the sample volume size to increase the spatial resolution. This improves tracking which improves strain measurements.

Decrease the offset distance (the distance between the sample volume and the endocardium).

Question 14

How is TDI derived strain accuracy improved via algorithms?

Answer 14

Spatial averaging and temporal averaging are the averaging of the velocity over space and time. This decreases noise to improve signal quality but removes potentially important variations in velocity, and therefore strain.

Sample volume tracking uses tracking algorithms (e.g. speckle tracking) to track the myocardium to improve the accuracy of strain measurements.

Drift compensation corrects for the shifts in the baseline during TDI acquisition via processing algorithms. Drift is the deviation in the baseline due to small tracking and alignment errors.

Question 15

What are the limitations of TDI derived strain?

Answer 15

Angle dependence, the ability of the movement of other structures to affect the movement of the myocardium, segmental variation, poor image quality and artefact and inter-operator variation.

Question 16

What are myocardial speckles and how are they created?

Answer 16

There is a set pattern of speckles within the myocardium. The visualisation of the speckles is increased by increasing the gain.

The light and dark areas within the myocardium are due to the interaction between the ultrasound and the structures. The structures cause the ultrasound to scatter. The ultrasound scatters interact with each other to cause areas of constructive interference (e.g. a peak of one wave interacting with a peak of another wave, causing a greater peak (or trough)), which causes bright speckles, and areas of destructive interference (a peak of one wave interacting with a trough of another wave, causing an area of zero amplitude), which causes dark speckles.

Question 17

Why and how are myocardial speckles tracked?

Answer 17

Speckle tracking, overlaid on the greyscale image, allows the user to visualise the movement of the myocardium and qualitatively assess the LV function.

It is not possible to track a single speckle, because when the myocardium contracts and relaxes, there is a change in the myocardial matrix structure, which causes a change in the intensity the and position of the speckles. To counteract this, the echo machine starts with the first frame of the loop and a small area in the position of interest is defined (the area is the kernel). The echo machine records the position of the speckles within the kernel with a level greater than the defined intensity. The echo machine then analyses the following frame in the loop. The kernel starts in the identical position and a search area around the kernel is defined. The echo machine searches for the original pattern of speckles, with the high intensity, in the original position. The kernel moves within the search area. A mathematical algorithm identifies the position with the speckle pattern which is the most like the original, and this is the position of the kernel. This is repeated for every frame of the loop. The echo machine tracks the movement of the kernel throughout the cardiac cycle. This is all repeated for other kernels because multiple kernels can be tracked simultaneously. Typically, there will be 3 kernels in the radial direction and 30-50 kernels in the longitudinal direction. The echo machine groups the kernels into the 17 myocardial segments.

Question 18

How are displacement and velocity, and strain and strain rate, calculated via speckle tracking echocardiography?

Answer 18

Considering two kernels, the positions of the two kernels are known. This allows the distance/displacement between the kernels to be calculated. The distance/displacement between the kernels (length) will change with kernel movement between frames. The change in length is calculated and the time between frames is known. The change in the length is the strain. The rate in the change in the length is the strain rate.

Question 19

How is speckle tracking echocardiography affected by frame rate?

Answer 19

The higher the frame rate, the higher the quality of the speckle tracking. A frame rate of >40 cycles/s is required.

Question 20

What are the advantages of speckle over TDI for myocardial movement and deformation analysis?

Answer 20

The advantages of speckle tracking, over TDI, are the angle independence and the ability to measure velocity and strain in any direction, not one.

Question 21

When is speckle tracking echocardiography indicated?

Answer 21

To assess torsion and regional wall motion in patients with cardiomyopathies or ischemic heart disease.

In an ischemic myocardium, there is a decrease in the speckle derived strain and the development of post-systolic shortening (shown by increased strain after AV closure). A ratio of >0.35 of post systolic shortening (strain) to peak systolic strain during stress is a marker of ischaemia.

Question 22

What are longitudinal velocity, strain and strain rate curves?

Answer 22

Speckle tracking offers regional data. The data is analysed in relation to global timings.

Longitudinal velocity curves show the velocity and times. In systole, the myocardium contracts and accelerates towards the apex, peaks and decelerates to zero. This is the S wave. In early diastole, the myocardium relaxes and moves in the opposite direction. This is the E wave. In diastasis, there is a pause between the E wave and A wave. In late diastole, there is a second myocardial movement towards the start. This is the A wave.

Myocardial strain curves offer information on longitudinal, radial and circumferential strain. In normal hearts, the longitudinal systolic strain is negative because the heart contracts and shortens. At end systole, the strain is the most negative because the heart is the shortest. In diastole, the strain returns to zero because the heart relaxes and lengthens. In contrast, the radial systolic strain is positive because the heart wall thickens. Therefore, longitudinal and radial strain curves are inverse.

Strain rate curves show the speed of strain so peak during systole and zero at end systole, and peak twice during diastole. Therefore, strain rate curves and velocity curves are inverse.