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How to display cardiac motion and deformationbyAsbjørn Støylen, Professor, Dr. med. Department of Circulation and Medical Imaging, Faculty of Medicine, NTNU Norwegian University of Science and Technology |
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Contact
address: asbjorn.stoylen@ntnu.no
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The main aim of this section is to explain how the different displays (curves, colur M-mode and 3D displays) are generated, what they mean and how they relate. As functional imaging is display, the different methods for display are given here, in order to being able to interpret the different displays.All methods can give the information as numerical traces, parametric (colour) images (in 2D or Colour M-mode). 3-/4D reconstruction has some limitations using segmental strain, however. Basically, however, irrespectively of method, the fundamental indices of motion (velocity and displacement) and of deformation (strain rate and strain) are the same. Also, the display of the indices can be used irrespectively of the method for acquiring them. However, some of the methods set limits for how the display can be made, and this is explained here.
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Longitudinal shortening of the left ventricle. The absolute shortening is the MAPSE, while the relative shortening is the normalised MAPSE = MAPSE / L0. | Wall thickening and circumferential
shortening. Both are measurable from the short axis
loops. |
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Lookng at an M-mode of the ventriculoar base (the mitral annulus), the systolic motion of the base can be displayed as a motion curve, and the peak systolic displacement of the base (the MAPSE - Mitral Annulus Plane Systolic Excursion) can be measured. Lomngitudinal strain is then MAPSE/L0 as shown above | Cross sevtional M-mode. In
ventricles without regional dysfunction, both transmural
strain = wall thickening = (Ws - Wd)/Wd, and
circumferential strain = ( |
Global measures have proven useful, but the main point of strain
rate imaging is regional function.
It is also evident, that as the apex is stationary, there is no
displacement, and the whole of the ventricle is deformed,
as there is differential motion:
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AS we seen the systole results in a longitudinal shortening. This means that the basal parts of the heart has motion towards the apex, while the apex is stationary (almost). Thus, the whole ventricle shortens - deforms. | The deformation is evident when
looking at the M-mode in the septum, the tissue
lines (which in reality is M-mode of
speckle lines) moves more, the more basal they are, and
thus the convergent lines shows the differential motion
(deformation). |
This is speckle tracking without advanced software. |
It's important to realise that both motion and deformation
parameters can be derived in a variety of ways. M-mode
and pulsed tissue
Doppler records the motion (displacement and velocity,
respectively) at one point at a time. Tissue Doppler gives the
motion velocity of the tissue.
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Pulsed tissue Doppler of the mitral ring. These are the velocity traces of the longitudinal motion. The peak systolic velocity of the annular plane is a measure of (but not equal to) strain rate. | As motion decreases from apex
to base, velocities have to as well. Thus there are
differential longitudinal velocities, decreasing from
maximal at the base to zero (almost)
at the apex. The differential velocity also described as
the velocity gradient,
is equivalent to the local rate of deformation; the
strain rate. |
Displacement |
Strain |
Displacement (motion) curve
(derived by integrating
the velocity curve from colour Doppler). The
curve shows the motion upwards (towards the apex) during
systole, and downwards (away from the apex)during early
and late filling phases in diastole, and the similarity
to an M-mode curve of the mitral ring is evident. |
Strain (deformation curve. The
curve describes the shortening of the myocardium in
systole, meaning that as the length becomes less, the
strain is negative. This
follows from the
definition. Then, there is lengthening again
in diastole, mainly during early and late filling
phases. However, the curve
remains negative, as all lengths are shorter than the
end diastolic, which is maximum length. Looking at the
M-mode curves above, the
curve describes the difference between two M-mode or
displacement lines, which is a spatial derivation. However,
the curve is actually obtained by temporal integration
of the strain rate curve below, and then converting from
Eulerian
to Lagrangian strain. |
Tissue velocity |
Strain rate |
Normal tissue velocity curve. The
curve is the derivative of the displacement curve above,
or oposite the displacement curve is the integral curve
of this velocity curve. The similarity of this cirve to
the pulsed Doppler curve shown above is evident. There
is systolic velocity towards the apex (upwards) during
systole (S), and downwards (away from the apex) during
the early (E) and late (atrial A) filling phases. |
Normal strain rate curve processed from the same acquisition by spatial derivation of velocity gradient. Again, the strain rate is negative during systole, as there is shortening, and then positive strain rate during early and late filling phases, as there is lengthening. |
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A patient with an apical
infarct, especially evident in the inferolateral
wall. |
By colour M-mode initial
akinesia apically, hypokinesia in the middle segment
and basal normal shortening. |
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Reduced strain rate in an
infarct visualised by tissue velocity. The systolic
velocities can be seen to decrease normally in the
basal segment (white to lilac curve, red interval),
while the middle segment (lilac
to orange curve, cyan interval), and almost no
difference in the apical segment (orange to green
curve, yellow interval). The intervals correspond to
the strain rate, showing normal shortening in the
basal segment, hypokinesia in the middle segment and
akinesia in the apical segment In fact, inital
systole, shows reversal of velocity curves, thus
signifying positive strain rate (initial
dyskinesia). |
Strain rate curves from the
segments between the measurement points in the left
image. Thus, the amplitude of the strain rate curves
correspond to the width of the intervals between the
measurement curves, and for clarity, the curves have
the same colours as the intervals to the left. Here is apical: initial dyskinesia, reduced peak strain rate, but also post systolic shortening, midwall hypokinesia and basal normal strain rate. |
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Velocity and displacement
curves with some noise in the velocity curves, the
integration in obtaining displacement curves tend to
eliminate random noise. |
Top: strain rate curves
obtained by spatial derivation of
the velocity curves to the rigth. Bottom, strain
curves by integration of the strain rate curves
above. Again, integration tends to smooth the random
noise. However, that makes it especially vulnerable
to non random noise (clutter). |
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Left: Real velocity curves from two
points at a distance of 1.2 cm, right, strain rate
calculated from the velocity traces as the
velocity gradient SR= (v(x) - v(x+![]() ![]() |
(Motion (velocity), The
diastolic phases of early and late relaxation are seen
as being simultaneous from base to apex.
Protodiastolic downward motion can be seen befor AVC
(aortic valve closure) in the tow basal segments. |
Deformation (strain rate)
shows both early and late relaxation to be biphasic,
and in addition the peaks are not simultaneous in
the different levels of the myocardium.
Protodioastolic elongation can be seen to be present
in the midwall segment only, the protodiastolic
motion of the basal segment being a tethering
effect. |
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As the apex is stationary, while the base moves toward the apex in systole, away from the apex in diastole, the ventricle has to show differential motion, between zero at the apex and maximum at the base. Longitudinal strain will be negative (shortening) during systole and positive (lengthening) during diastole (if calculated from end systole). | M-mode lines from an M-mode along the septum of a normal individual. These lines show regional motion. It is evident that there is most motion in the base, least in the apex. Thus, the lines converge in systole, diverge in diastole, showing differential motion, a motion gradient that is equal to the deformation (strain). This difference in displacement from base to apex is also evident in the displacement image shown above. |
AS motion decreases from apex to base, velocities has to as well. Thus, there is a velocity gradient from apex to base, which equals deformation rate. | Spatial distribution of systolic velocities as extracted by autocorrelation. This kind of plot is caled a V-plot (247). It may be usefiul to show some of the aspects of strain rate imaging. The plot shows the walls with septal base to the left, apex in the middle and lateral wall base to the right. As it can be seen again the velocities are decreasing from base to apex in both walls. There is some noise resulting in variation from point to point, but the over all effect is a more or less linear decrease. The slope of the decrease equels the velocity gradient. (Image courtesy of E Sagberg). However, this shows only one point in time, and all values are simultaneous. |
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Normal subject. Strain rate
(top) shows the changes in deformation, while strain
(bottom) shows the deformation statur at any given point
in time. Thus, quick changes will only show up in strain
curves as changes in the direction of the curves. This
is especially evident when looking at the diastole. |
In this case with a large antero
apical infarct, changes in
segmental deformation from stretch to shortening during
ejection is evident with strain rate (top), but not in
the strain curves (bottom). Peak rate of change in
any phase can be measured by strain rate, not by strain.
Changes in strain (i.e. strain rate) can be
puzzled out qualitatively, if one looks at the
changes in direction of the curves (which in fact is
strain rate). The main impression from strain,
however, is the systolic stretch in the two apical
segments. |
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Velocity imaging. Velocities toward the probe is coded red, away from the probe is blue. Thus the ventricle is red in systole, when all parts of the heart muscle moves toward the probe (apex) and blue in diastole. | Strain rate imaging, strain rate is coded yellow to orange for shortening, cyan for lengthening but green in periods of no deformation, and is thus yellow in systole, cyan in the two diastolic phases early and late filling and green in diastasis. |
Curved M-mode showing
velocities. In this case, the curve is drawn from
the apex to the base, showing one wall. The shifts
between positive (red) and negative (blue) velocities
are clearly demarcated. |
Curved M-mode showing strain
rate ( the curve is the same as in the image to the
left, but the mode is shifted to display strain
rate). The pattern is different, due to the better
spatial resolution when deformation is imaged, as
discussed in details below.
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In the strain rate CAMM; there
is evidence of an early shortening before MVO, the
MVO can be timed, the pre AVC stretch is visible,
the apical lengthening during IVR, and the
propagatopn of stretch waves are all visible
in the curved M-mode- Although the events can
be seen in the curves as well, the space-time
resolutions are much easier to discern in the CAMM. |
The strain CAMM (it's the same
line as in the SR CAMM to the left, only reprocessed
into strain) doesn't give much information at all.
Looking at the curves, it is evident that the whole
course of the curves lies below zero, thus showing
a red colour, although there is a colour cut off around
-10. The blue lines is reverberations, but
with low intensity, as they don't even disturb the
SR CAMM. |
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Top, strain rate curves from
septum of a patient with a large anteroapical
infarct. The three segments are covered by one ROI
each and the curves are thus the average for that
segment. The CAMM and traces have the same temporal
resolution, and thus are able to show all phases.
There is initial stretch at start ejection (1),
hypokinesia during ejection (2), and post systolic
shortening after ejection (3). I addition there is
reduced recoil after atrial systole (4) (as compared
to the more healthy region), which may indicate some
stiffening in the infarct area. the CAMM shows
intioal stretch to be present even partway into the
basal segment, hypokineasia to be preent in the
apical half (i.e. the apical plus half the midwall
segment, and post systolic shortening also in half
the wall. Thus the extent of the different abnormal
strain rates is better discriminated by the CAMM. |
Strain curves and CAMM from the
same wall / segments. This illustrates 1:first that the temporal resolution of strain rate disappears when converted to strain, as strain in each point is the cumulated deformation up to that timepoint. In this case only stretch during the whole of ejection is seen in the apical and midwall segments. The remaining hypokinesia is nor evident. Changes in strain (i.e. strain rate) can be puzzled out qualitatively, if one looks at the changes in direction of the curves (which in fact is strain rate). Then, secondly even post systolic shortening disappears in the CAMM image, because parametric display shows only that the curve is above or below the zero point. Thus the colour display becomes rather featureless regarding time curves, even compared to strain curves. |
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All the phases of the cardiac
cycle can be seen and the timing seems normal. |
In this case there is left
bundle branch block, with reverberations in the
lateral wall. Still the septal flash with lateral
stretch, the lateral wall shorteining with early and
late septal stretch and the septal post systolic
shortening (recoil) can be seen. The
image can guide the placement of ROIs, but the
information should still be used only qualitatively.
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The curved M-mode is a line
(one dimensional) that curves through the two
dimensional plane (left). The curvature gives
information about the spatial relations between the
pints on the line. |
Keeping the curvature
information enables the mapping of the points of the
line in two dimensions (middle). |
Using three standard planes, it is possible to
reconstruct a grid with the true curvature of the surface,
in this case a curved plane, that curves through three
dimensions (left). |
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3D velocity display in
systole and diastole, the same dataset as in the bull's
eye above. |
3D strain rate display in
systole and diastole, the same dataset as in the bull's
eye images above. |
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Strain rate 4D mapping. The problem with the moving loop is the same as in 2D display. During ejection there is a short period with homogenous colour, when all the ventricle shortens simultaneously. But during diastole, there is a continuously shifting array of colours, as different parts of the ventricle elongates at different times. the continuously shifting colours are not easy to interpret. In addition it won't show all of the surface simultaneously. Stopping the scrolling will allow closer inspection, but scrolling in space will show only one instance in time. (In this case it's mid systole). (Image courtesy of E. Sagberg.) | Strain rate 3D mapping in space. Stopping the frame in one point in systole shows a fairly even distribution of colour (yellow - shortening), meaning an image with normal systolic function and fairly free from artefacts. In order to see all of the surface, however, the 3D. image has to be rotated in space. . The image can also be scrolled in time, showing that it contains a full reconstructed 4D dataset and displaying the full time course of the data. Stopping the scrolling will allow closer inspection. |
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Inferior infarct in strain rate 3D mapping in time.There is an area of akinesia in the inferior wall, although this is not easily distinguished in the loop.(Image courtesy of E. Sagberg.) | Inferior infarct in strain rate 3D mapping in space. Stopping the loop in systole reveals the area of akinesia and hypokinesia,(Image courtesy of E. Sagberg.) | while the stopping in diastole shows the post systolic shortening in the same area.(Image courtesy of E. Sagberg.) |
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Strain rate 3D mapping of apical infarct. (Image courtesy of E. Sagberg.) | Systolic frame showing apical akinesia to hypokinesia in the apex(Image courtesy of E. Sagberg.) | Diastolic frame showing an area of post systolic shortening.(Image courtesy of E. Sagberg.) |
Apical |
Inferior
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The area distorsion in bull's
eye is evident comparing these strain rate images of the
same infarcts in bull's
eye and 3D. All images are from mid systole, the infarcted
area is shown in cyan, showing a- to dyskinesia, while
normally shortening myocardium is shown in yellow. |