Det medisinske fakultet

Strain rate imaging.

Myocardial deformation imaging by ultrasound / echocardiography

Tissue Doppler and Speckle tracking

by 

Asbjørn Støylen, Professor, Dr. med.

Department of Circulation and Medical Imaging,
Faculty of Medicine,
NTNU Norwegian University of Science and Technology

Contact address: asbjorn.stoylen@ntnu.no




Gateway to strain rate imaging for the novice researcher and curious clinician. The gateway. Old town bridge, Trondheim, Norway. It is also called "the portal of happiness".


Follow updates on twitter:  @strain_rate
Website updated: December  2018. Recent updates:

Link to website categories, Link to website index, Link to list of tables of normal values from the HUNT study, link to guide to clinical use.

What's new:




Spring is here again, and although it is difficult to go up in the mountains these days, I'm sure the cranes have returned just as last year.


Vector flow imaging. High frame rate. Image courtesy of Annichen S Daae.

October 2021: Added several paragraphs on integration flow vector imaging data in the basic pysiology section, the paragraphs are placed under the relevant paragrphs on the phases of the heart cycle.


October 2020: Added a discussion on regional myocardial work based on strain-pressure loops. The method is based on a standardised pressure loop calibrated for actual HR and BP. This loop will then be the same for all segments, and the differences in the strain pressure loops will be due to the differences in strain only. Thus, this is the emperor's new clothes again, and the concept of wasted work in dyssynchrony, can be demonstrated without strain pressure loops. Also the method applied to global myocardial work. A new method for estimating myocardial work, based on strain pressure loops, has aroused interest lately. Probably because regional strain pressure is just as noise susceptible as regional strain (and gives no more information), transition to global myocardial work (GMW) parallels transition from regional to GLS some years ago.  But as the method is based on a standard pressure loop calibrated by arm cuff BP, it do not confer more information than SV × SBP. And SV × SBP is sensitive to both increased cardiac ouptput, and increased load. But is it useful? It is not a measure of contractility, being the product of SV and BP, while contractility is the ratio of BP and SV. In my opinion, this is similar to a business, instead of considering supply (SV) vs demand (BP) ratio, considering supply × demand. So while SV is a result of supply (contractility) and demand (load), myocardial work is the product of load and SV. Which kind of business has a use for that? In clinical studies, GMW seems to decine (with GLS) in heart failure, but increase with increasing BP in hypertensives, despite decreasing GLS..........

Also added some results from the HUNT study, where measures are entered into an ellipsoid model of the LV showing that MAPSE contributes about 75% to the total SV (although this is not normative as the geometrical model has limitations. However, while MAPSE decreases with age, there is no compensatory increase in short axis function (which actually also declines), to explain maintained EF. EF is maintained with increasing age by the simultaneous decrease in LVEDV and SV, keeping the ratio constant. Also, MAPSE remains a constant percentage of the decreasing SV. The notion that circumferential function is the main contributor to SV, is bases on the lack of understanding that circumferential strain (except external CS) is partly due to wall thickening, which again is mainly due to longitudinal shortening.
LV myocardial volume do not increase with age. Due to the simultaneous increase in wall thickness and decrease in length, there is no increase in LV myocardial volume by age, when the age dependent increase in BP is taken into account.

September 2020:
Added a chapter on why there isn't, and can't be a gold standard for strain, as this rests on the choice of basic assumptions. As there is no universal algorithm for global strain, of course the concept of global strain as a universal measure of ventricular function has no exact meaning. It is only a theoretical concept.The problem goes beyond the inter vendor differences in speckle tracking. This is also discussed in the Global function section. There is no universal global strain and thus no reference, they are method dependent. This means that strain values cannot be validated, and different methods cannot be compared in terms of validity, and finally, normal values do only have validity within the method used.

Added a discussion of why speckle tracking strain will overestimate true wall shortening, by tracking the inward motion due to wall thickening (which is already accounted for as wall thickening is a function of wall shortening). This is probably the explanation also for the finding of a gradient of longitudinal layer strain, not the absurd notion of differential layer fibre function.

Also added a discussion of why the strain product of the three strain components: do not, in real life Correspond to the volume ratio VS / VD, and cannot be used for determination of the (possible) systolic myocardial compression.


May 2020: I've revised and extended the chapters about the inter relations between the principal strains (longitudinal, transmural and circumferential). I've added the relations to the myocardial volumes, and extended the discussion about myocardial compressibility. As there is no gold standard for strain, and different sets of assumptions as well as specific methods will give different values. This also means that both the inter relations of strains, as well as the relations to the myocardial volumes and incompressibility calculations will vary, and at the present level of technology, strains cannot be used to decide if the myocardium is incompressible.

March 2020: I've added a more extensive explanation of the relation between strain and MAPSE, and strain rate and velocity to the paragraph on measures of global long axis function in the Basic physiological concepts in strain and strain rate section. Likewise, in the same section, the paragraph explaining diastolic strain rate, and relation to diastolic velocity, has been extended, with emphasis on the point that added value of diastolic strain rate to diastolic velocity is dubious.
October 2019: I have added the results from the HUNT3 study, giving the linear measures of strain in both longitudinal, circumferential and transmural direction, as well as the inter relation, indicating fairly little systolic compression of the myocardium. The concept of strain as a simultaneous deformation in three dimensions, and the explanation that the three normal strains are coordinates in three dimensions of this deformation (components of the 3-dimensional strain tensor), has previusly been extended both in the basic concepts of motion and deformation section and in the Basic concepts in myocardial strain and strain rate section.


Welcome

About the website:

What is this website about?

Mainly giving an overview of the concepts and use of of strain rate and strain: the fundamental concepts, the necessary technology and display modes, the relation of deformation to physiology and pathophysiology, and the clinical use.

The website is divided into sections to allow quicker downloading. Every section can be read separately and are accessible by links, or the complete website index.
The text is riddled with links. Following the links to see the reference, just click on the "back" button in your browser, and you return to the point in the text where you were.

The present section is mainly intended as an access portal, with a full website index with information about the content of the other sections. Also, I have added some general comments to strain and strain rate imaging, a short introduction explaining very briefly what deformation imaging (strain rate and strain) is about, and how it differs from the rest of echocardiography.
In addition, I have given some historical information about development of strain rate imaging.

The other sections in this website fall into four main categories:


The animations and video examples are all in *.gif animation format, so no special software in the form of various media players will be necessary for the animation. Also if downloaded (with due credits, of course), it can be embedded in a power point presentation and will run in all versions from office 2000 and onwards. It should then be treated as "picture", not a video, meaning that it is inserted in the file as a picture, and will then run without the media player. It also means that it will not be necessary to keep the loop separate outside the presentation, the animation is fully embedded in the powerpoint file, just like any other picture, and will run in a continuous loop when the picture is shown.


The internet is free, so feel free to use the examples found on this website in demonstrations and lectures. However, ordinary ethics dictates that credit should be given to the author (using the address: from: http://folk.ntnu.no/stoylen/strainrate).  For publishing, I and the Norwegian University of Science and Technology retain the copyright to all material published here. (For written publication, the acknowledgment should thus be: reproduced with permission from: http://folk.ntnu.no/stoylen/strainrate).
I do not consider the fact that a signature is embedded in some pictures sufficient acknowledgement, without the website address. Even if the images are taken from other websites that do acknowledge the source, I still require acknowledgment of this website as the principal source, the fact that there are publications with permission/acknowledgement available on the net, does not alleviate the duty to acknowledge the original source.

Using the material in papers without acknowledgement, and for published material without permission, I consider academic misconduct, in addition to being copyright violation.

The most extreme example of this kind I have seen so far, is a paper by the authors Dagianti A, Regna E, Laurito A, Malaj A, Gossetti B, Fedele F, that I recently came across in some journal called "Prevention and research". The paper had used nine figures coming from this website without copyright permission and with no acknowledgement. Repeated inquiries to the journal have elicited no response.




List of tables of normal values from the HUNT study with references.

Left ventricular dimensions
Systolic displacement, velocities, strain and strain rate
Diastolic velocities
 Comparison between methods and reproducibility

Website index:



This section:

Basic concepts

Basic concepts of motion and deformation

Deals with the basic concepts of motion (displacement and velocity) and deformation (strain and strain rate), and how these concepts are inter related. Previous sections from the maths section about mathematical relations as well as the extended discussion on Lagrangian vs Eulerian strain rate has been included. Further, how strain behaves in three dimensional objects, and how the different strain components are inter related in an incompressible object. It is important to realise that even if speckle tracking is in principle angle independent, strain measurement by speckle tracking is not.



Basic concepts in myocardial strain and strain rate

This section extends the basic concept of strain into the specific geometry of the left ventricle. It is important to understand that strain is about changes in myocardial dimensions. Thus strains the components of myocardial volume changes, and only expressions of geometry. The effects seen by strain rate imaging is thus explained by geometry, not by material prpoperties of the myocardium, over all geometry governs the changes and relations between strain components. This is true of all strains, longitudinal, transmural and circumferential as well as area strain. Also, the strain gradient across the wall seen both in transmural and circumferential strain is due to geometry, not differential fibre action.

The strain components are simply coordinates of the three dimensional deformation of the myocardium, and has nothing to do with material properties of the myocardium, such as anisotropy or fibre directions in a direct sense. Of course, the total deformation is a function of fibre shortening, and ultimately, among other things fibre architecture, but also of load and valve function. And it is of note, that while systolic deformation deformation continues to end ejection, myocardial relaxation starts at peak pressure.











Basic physiological concepts in strain and strain rate - relation to load and contractility - what does cardiac imaging actually measure?

The relation between function imaging and physiology - contractility, load, work and phases of the heart cycle. (Revised September 2017)


This section has been completely rewritten, taking into account a lot more of the early fundamental physiological research which still holds true, and which is important in understanding strain and strain rate imaging.  The basic relations to the phases of the heart cycle have been moved from the sections on global and regional systolic and diastolic function, while the clinical relations remains in the respective sections. This to present a more fully integrated physiology concept in this section,and an easier access to the clinical parts in the other sections.


It is important to realise that imaging, always shows the result of myocardial shortening. This is true of any imaging measure, whether it is fractional shortening, EF, longitudinal shortening, strain or strain rate. It is also true, irrespective of imaging method, MR, CT, MUGA or ultrasound. Myocardial contraction, on the other hand, generates force (tension). In an isolated myocyte or unloaded muscle preparation, this force generates shortening. Unloaded states, however, hardly exist in the intact heart. The opposite situation, the isometric contraction, generates force, but with no shortening. This actually exists during isovolumic contraction. In between, contraction generates force, but doesn't shorten until the force exceeds the resistance to shortening (load). Also, active contraction happens only during pre ejection, where there is no deformation (in fact about 80% of the systolic work is done during that phase), and first part of ejection, ejection and systolic deformation (chamber shortening, length and volume reduction etc), continues well into myocyte relaxation due to the inertia of the blood as discussed below.


Thus the notion that deformation indices can be load independent, is self contradictory, although different parameters may relate differently to load as discussed here. And the notion that different imaging methods (like MR) are less load dependent than others, is simply ridiculous.

However, as segment interaction is part of segmental load, it is in fact the load dependency (as well as the motion independency) of strain rate imaging is able to image regional inequalities in tension, which will result in inequalities in shortening. Here, however, timing is also of importance.

 


Technical aspects

Basic ultrasound for clinicians

This section is the basic ultrasound, B-mode and M-mode part of the previous "Basic ultrasound, echocardiography and Doppler for clinicians. Due to the size and number of illustrations, the page tended to load very slowly. It has now been split into this section on Basic ultrasound, and another on Doppler, including tissue Doppler.

 is intended as an introduction to basic ultrasound physics and technology for clinicians without technical  or mathematical background. A basic knowledge of the physical principles underlying ultrasound, will give a better understanding of the practical limitations in ultrasound, and the technical solutions used to solve the problems. This will give a clearer picture of the reasons for the problems and artifacts. Technical or mathematical background is not necessary, explanations are intended to be intuitive and graphic, rather than mathematical. This section is important for the understanding of the basic principles described in detail in the section on measurements of strain rate by ultrasound. Especially in order to understand the fundamental principles that limits the methods.The principles will also be useful to gain a basic understanding of echocardiography in general, and may be read separately, even if deformation imaging is not interesting.

It is important to realise that the last couple of years has seen tremendous improvements in both hardware (allowing a much higher data input to the scanner as well as processing technology), and software (allowing  more data processing at higher speed). It even allow using input data in a way that also improves the beamforming characteristics in processing, as they are used for the generation of a picture. Thus the simple principles of beamforming outlined here are an over simplification compared to the most advanced high end scanners. Thus, present technology is far more complex, and neither traditional beamforming, focussing nor image processing conforms to the simple principles described here, in the most advanced high end scanners, but they will still serve to give an idea. And simpler equipment still conform more closely to the basic principles described here.

The physical principles still apply, although due to the use of MLA and similar even more advanced techniques, the fundamental limitation of the speed of sound has been dealt with.

Basic Doppler ultrasound for clinicians

This section is the Doppler part of the former "Basic ultrasound, echocardiography and Doppler for clinicians". Due to the size and number of illustrations, the page tended to load very slowly. It has now been split into one section on Basic ultrasound, Doppler, and this one on Doppler, including tissue Doppler. In addition, the background paragraphs from the previous mathematics section on the derivation of the Doppler equation and  the phase analysis have been included here (but can be bypassed, of course, for those not interested).

Technical or mathematical background is not necessary, explanations are intended to be intuitive and graphic, rather than mathematical. This section is important for the understanding of the basic principles described in detail in the section on measurements of strain rate by ultrasound. Especially in order to understand the fundamental principles that limits the methods.The principles will also be useful to gain a basic understanding of Doppler echocardiography in general, and may be read separately, even if deformation imaging is not interesting.




Basic strain ultrasound for clinicians.

Principles and technology for strain and strain rate imaging by echocardiography.

the present section takes this further, into the technical aspects of how deformation is measured using the basic methods for deformation imaging and measurements.
 All ultrasound methods can be used to assess regional function. And, in fact, regional function is regional deformation.

Basically, 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 across some of the methods for acquiring them. However, the application of the methods can vary, so this chapter deals with the application of the different ultrasound methods to deformation assessment, similarities and differences.

And the fundamental limitations of ultrasound apply. The poorer the image quality, the less useful are any of the methods:
Garbage in - garbage out.

Thus all methods have limitations connected to the basic limitations of ultrasound, and each method have specific limitation as well, and finally there are specific limitations connected to both numerical measurement and to the deformation imaging in itself. The "limitations and pitfalls have been moved to a separate section to enable quicker uploading.

Finally, there is no universal algorithm for global strain, of course the concept of global strain as a universal measure of ventricular function has no exact meaning. It is only a theoretical concept, but in practice related to each specific manufacturer as discussed in the section on normalised displacement and global strain.



How to display (and understand) cardiac motion and deformation.

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.

Problems and pitfalls

Limitations and artefacts related to strain rate imaging
Basically, 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 across some of the methods for acquiring them.

The fundamental limitations of ultrasound apply. The poorer the image quality, the less useful are any of the methods:
Garbage in - garbage out.

In addition, there are specific problems that arise from the specific application of the basic methods to deformation measurement, as well as how the methods are dealt with, which creates new pitfalls. The present section have been re edited, so the main headings are now the problems and limitations themselves, and the effect on each of the different methods are described under those headings.

Mechanics of physiology and pathophysiology

Global systolic functional imaging

An understanding of the concepts of myocardial load and work as well as the various systolic measurements to each other and to timing in systole given in the section on what strain and strain rate actually measure is an advantage.
January 2018: I have added recent results from the HUNT study (417), to the global systolic function section showing that GLS, which presumably should reduce biological variability compared to MAPSE, do not do so.


Regional systolic functional imaging.

This is what strain rate imaging is really about. Those of you who know most echo, but are curious about the physiological background for regional functional imaging, may well jump directly here.

Diastolic functional imaging



Clinical use of deformation imaging

Is deformation imaging useful?

 

This section is intended as a guide for those who want to start using deformation imaging, but who don't want to go into the basic theory of deformation, display and physiology/pathophysiology first. For this reason, it is placed first in the sections, instead of the conventional last place after all the theory have been dealt with.
The intention of this section is to shown how to become friends with the method. It is usually possible to sniff out extra information not visible to the eye. In this section I will try to show that the use of deformation imaging not necessarily needs to be very complicated or time consuming, as well as suggesting how to interpret the findings in a quick, and hopefully easy way, and then giving some more information for those who wish to go further. The chapter is illustrated illustrated by examples both of pathology and artefacts. It is an integrated approach, using both velocity curves and colour SRI, in addition to strain rate and strain curves.

The section is intended to be self contained, but presupposes some familiarity with the concepts and the colour and curve displays. Of course, links are provided to the relevant more in depth chapters in the other sections. To understand more about the curves, I recommend the section on displays.

This section can also be downloaded as a pdf (without the animations, of course). The other sections can be used as reference over time.


References for all sections



Some general comments
An academic discussion. Northern fulmars in New Ålesund, Spitsbergen Professor and student.  Blue eyed shag and adelie chick- Peterman Island, Antarctica

What is strain rate imaging about?

Mainly imaging regional inequalities in myocardial function.




Colour curved anatomical M-mode (CAMM) of strain rate. Apical infarct to the right, compared to normal on the left. For explanation see main chapter as well as specifically about CAMM below.


Same infarct seen by curves (explained below) to the left and CAMM to the right.


The method of strain rate imaging by tissue Doppler was developed here at the Norwegian University of Science and Technology in Trondheim, Norway. It was the subject of two doctoral theses, one in technology (1) and one in medicine (2), and was a result of a successful cooperation between technical research (in strain and velocity gradients) and medical research (in long axis function of the left ventricle). One of the important point of my work with long axis function, was that this lead to Strain Rate Imaging being applied to longitudinal velocity gradients, thus making the rough method more robust, as well as all segments of the ventricle available for analysis. The method was originally validated in a mechanical model, in cooperation with the university of Leuven, Belgium (3) and described in a method article from Trondheim in 1998 (4) and 2000 (5). The basic publications dealt with feasibility (1998) (4), clinical validation by comparison with echocardiography (6) and with coronary angiography (7). Validation of strain measurements (from integrated strain rate) was done at Rikshospitalet, Oslo, Norway by comparison with ultrasonomicrometry (8)  and MR, in cooperation with Johns Hopkins Hospital (9). Early work on the feasibility of the method in myocardial infarction was also done at the university of Linköping and later at Leuven (10). An excellent early review paper was published by the Leuven group (11).

Why use strain and strain rate?

One example:
Paradoxical motion of basal basal inferolateral wall. Is this dyskinesia?
 No, strain rate imaging shows vigorous systolic shortening of the basal inferolateral wall. Details of this case can be seen here.


  1. The strain and strain rate subtracts motion due to the effects of neighboring segments (tethering). Tethering may both mask pathological deformation and impart pathological motion to normal segments and deformation imaging and is necessary to locate and show the true extent of pathology. And in some situations exclude regional pathology. This means that motion parameters (displacement and velocity) reflects global function, and should be applied to the mitral ring, while deformation imaging (strain and strain rate) shows regional function within the myocardium.
    1. However, strain and strain rate are more susceptible to noise, both by tissue Doppler and speckle tracking ( the weaknesses of the last being masked by smoothing), and a rough qualitative assessment of regional function can be done by assessing the offset between velocity curves, as well as by colour M-mode.
    2. Even if velocity curves indicate abnormal motion, the deformation parameters are necessary to localise the area of abnormal deformation.
    3. Deformation parameters are useful in doing a more comprehensive assessment of local contraction and relaxation, such as initial stretch, hypokinesia, post systolic shortening, both in infarcts / ischemia and in electrical asynchrony.
  2. Strain and strain rate are deformation per length, and thus are normalised for heart size, also in global deformation, meaning that it reduces biological variability due to size differences. Clinical evidence for the advantage of this is emerging, at least in children where variability in heart size is greatest. The advantage in adults is still uncertain.
  3. However, both systolic and diastolic deformation itself is load dependent, as is the case with all volume based measures of ventricular function. This, it appears, cannot be emphasized enough. But as the main value is in diagnosis of regional dysfunction, the segment interaction in combination with the load dependency enables us to make inferences about uneven contractility, i.e. regional dysfunction, even if the contractility cannot be measured directly. Thus, regional deformation imaging shows regional relative contractility.
  4. In addition, in different inotropic states, the changes in contraction will be caused by changes in contractility, and thus the measures are valid measures of contractility changes.
Deformation imaging has added a lot of knowledge of the regional myocardial properties, and the fundamental physiological knowledge gained from deformation imaging is method independent. Thus, the basic principles and the relation to pathophysiological knowledge comes first, in this section.

But of course, even with the fundamentals of physiology, an understanding of the technical aspects of the methods and especially the limitations and propensity to artefacts is important, thus for better background the fundamental technology sections on basic ultrasound, basic Doppler and the relation to deformation measurements is important.



Now, it is important to emphasize that both motion and deformation imaging are no longer simply tissue Doppler derived modalities. Both can be derived by tracking the motion of the myocardium in grey scale pattern, speckle tracking. The basic concepts are the same; general principles of motion (velocity and displacement) vs. deformation (strain rate and strain) apply, irrespective of method but the limitations may differ between the methods.

The terms: - Velocity imaging, - Displacement imaging, - Strain rate imaging, - Strain Imaging, should not be taken synonymous with tissue Doppler, but should be used irrespectively of the method employed, and the term "by tissue Doppler",  "by speckle tracking" or whatever application is used should be added, if studies are cited.

The fundamental concepts relating to geometry, pumping and physiology, however, are independent of methods (except where the methods relate differently to geometry), so even if examples are taken from one method, the underlying pathophysiology should be the same with other methods. The sensitivity for specific changes may vary in different situations according to the strengths of each method .


In the ideal world, any measurement would give the diagnosis once correct cut off an normal values are established. But this is not the ideal world, and image quality is far from perfect in most cases, which is a fundamental property of ultrasound. Thus, no single measurement is perfect, an echocardiographic examination always consists of using all the available, more or less circumstantial evidence, weighing findings against each other and arriving at a conclusion. This will usually be fairly certain in the hands of an experienced clinician, even if single measurements are not. This is a fundamental property of all echocardiography. With the limitations inherent in basic ultrasound and in the specific methods, clinical ultrasound will partly be a craft, not pure science. The careful weighing of the evidence in terms of the methods limitations is thus an integral part of the examination, and a knowledge of the methods themselves and the method specific limitations is essential.

This is also the case with deformation imaging, and this should be the basic approach, deformation imaging being part of the total evidence, and can serve as an aid to diagnosis, as shown in the section on how to use deformation imaging clinically.

Why is deformation imaging underutilised?

The number of publications is enormous, but the use in daily clinic is still limited despite giving additional information, as shown in "Is deformation imaging useful", the method, especially tissue Doppler seems to have gone into a partial eclipse:


Partial solar eclipse, March 20. 2015. Picture taken from Trondheim


This has some historical reasons:

Tissue Doppler derived deformation

When tissue Doppler derived deformation (strain rate imaging) arrived (4), we concentrated on semi quantitative assessment with colour M-mode (4, 6, 7). The colour WMS seemed to equivalent with B-mode WMS both in feasibility and accuracy (6, 7), but no better. This is hardly be surprising, as the expert WMS is a fairly precise method, an colour WMS is still semi quantitative. However, with increased understanding, it beacme evident that the colour M-mode offered additional information, both on the time course of the deformation (like initial akinesia and post systolic shortening), as well as being very robust against noise.

noise

However, the possibility to go from semi-quantitative assessment of regional function to quantitative measurement, seemed interesting. And with objective measurement, the experience dependency might be bypassed. So objective measurement became the goal. And principally peak vaues. But as strain rate is the result of a derivation, it is much more noisy than  velocity curves. Even if smoothing and the integration to strain took care of much of the random noise, non random noise (especially clutter) remained a problem.


Thus, curves and especially peak values seemed to be difficult to interpret, as it was difficult to see whether the curves were due to pathology or artefacts. In addition the fact that tissue Doppler was angle dependent, and that strain rate derived from a set of velocities was even more angle dependent, lead to a lot of erroneous speculation that it could for instance not be used in the apical segment, which of course is nonsense. And in this setting, it became evident that using tissue Doppler with reliance was just as experience dependent as B-mode.

Thus, inherent weaknesses in the tissue Doppler method, especially angle dependency and vulnerability to noise, especially to clutter, made the method little approachable, and this  led to the method being little used.

And in all this, colour tissue Doppler was not reinstated, despite it's ability to:
  1. Give information as wall motion score similar to B-mode, and
  2. Give additional information on timing beyond the B-mode information (also due to higher temporal resolution), and
  3. Visualise clutter artefacts in a way that makes timing information still available, and hence,
  4. Might guide the placement of the ROIs to extract reliable curves.
Thus, interpretation of artefacts, as well as timing is easy with colour M-mode. The curved M-mode will then give the possibility of assessing both if regional shortening is normal, and to look at the timing. Actually, the curved M-mode has better spatial resolution that strain rate or strain curves.

Also, in the user interface provided, all difficulties were eminently visible, but instead of being considered an advantage, it disgusted a lot.
The problem being both the experience dependency, initially poor user friendliness, and variability of results. The limitations of the methods may scare potential users as well. 

One of the main points about assessing regional function by deformation parameters, however, is that peak values is not the most important thing at all. As seen below, the colour M-mode and the shape of the curves gives almost all information qualitatively in a quick and easy way:

Both initial delay in shortening of a segment, reduced peak values and post systolic shortening are all phenomenons that tells about the relative reduction in rate of tension development (strain rate) and total tension, all measures of relative reduced contractility of a segment or region, as well as delay in relaxation, another assessment of both ischemia and contractility.

In my opinion, however, tissue Doppler seems to be able to fly, although it is a rather heavy bird:
Wandering albatross is heavy in taking off from water. Drake Passage. However, this bird is fully able to take off and fly.



Speckle tracking derived deformation

When speckle tracking was launched, it seemed to solve some of the problems from tissue Doppler.

 
The challenger. Musk oxen, Grønnedal, Greenland.

  1. It came with an alluringly user friendly interface which actually had nothing to do with speckle tracking per se). Especially for deriving peak values, which is attractively easy, but poor use of data, as it discards additional information about timing. 
    1. In addition, the low frame rate makes it less suited for timing, especially in the spline smoothed versions.
  2. Also, it was maintained that speckle tracking was angle independent, and able to track in all directions,
    1. Which is a half truth at best, as
      1. the axial resolution of B-mode is so much better than the lateral, this means that tracking is better in the axial direction.
      2. The line density decreases with depth, and so, of course do the  lateral resolution and
      3. With increasing frame rate of B-mode, the line density is reduced, and especially in 3D speckle tracking, where line density is severely reduced.
  3. There was an apparent reduction in noise, (which again did not have anything to do with speckle tracking per se, but with the applications using a generous amount of smoothing), and the smoothing made the effects of clutter less visible, and the method apparently more robust.
    1. It is true that speckle tracking is done in harmonic mode, while tissue Doppler is done in fundamental, removing some of the clutter, but
    2. Most of the clutter is smeared out due to the spline smoothing, which may lead to absurd examples. 
    3. The only method free of this that I know of, is the segmental tracking application developed by NTNU, described elsewhere, which can track segmental deformation by speckle tracking alone, or in combination with tissue Doppler.
  4. Thus, the segmental values obtained by this kind of speckle tracking are not true segmental values, results of a spline (or similar) function taking into account not only the tracking of speckles in the actual segments, but the over all tracking as well as the global deformation.  the segmental values are then partly splines of the global function. Given the obvious curvature dependency in addition, the method has a serious sensitivity issue for reduced segmental function. Thus the apparent lower vulnerability and better reproducibility of speckle tracking over tissue Doppler is due to more smoothing. When the same smoothing is applied to tissue Doppler. results become near identical.
  5. It also seems that measurements are software dependent, even software updates from the same vendor results in different values, and also that reproducibility, even within one software frame is lower than previously reported. Not unsurprising for those of us who have tried this in practice.  (377).
Same small apical infarct analysed by speckle tracking (left) and tissue Doppler (right), showing fairly low sesitivity of speckle tracking in this particular instance, as described in more detail here, and technical reasons here. 



Thus, for regional function, the flight capability of speckle tracking is low:
Molting gentoo penguin, Port Lockroy, Antarctica.
Jumping penguins. Paradise Harbour, Antarctica.

A direct comparison between speckle tracking and tissue Doppler in clinical examples can be seen here.


It is thus no surprise that the publications on speckle tracking lately, has been about global strain, which is a global function parameter. (Thus, a method that started out as a method for regional function, has devolved to being a global measure, due to the inherent weaknesses in the speckle tracking application). Here, it may prove useful, especially when compared with the most used and poorest method of them all; ejection fraction. It has been instrumental in shifting emphasis from ejection fraction (which in fact doesn't work in small ventricles), to long axis function. However, so far it has not been proven that global longitudinal strain adds information to that of simple longitudinal shortening . However, the value of long axis function was shown early in the 90ies (31, 32, 33, 34, 35, 64, 65, 66) by mitral annular plane displacement(MAPSE). It has not been definitely proven that normalising for LV length (which is global strain), is advantageous in adults, although of value in children (159, 214, 288).

Also, global strain is not load independent, and do not measure contractility.


The different advantages and disadvantages, which is extensively discussed in a separate section on measurements of strain and strain rate by ultrasound. It may also be a surprise to some, but spectral Doppler has a fairly low temporal resolution. Even if sampling rate can be as high as 1000, the spectral analysis requires a long sequence of frames, thus reducing the effective frame rate, the temporal resolution of both pulsed Doppler flow and tissue Doppler is usually around, and may even be below 100 FPS. This is described in slightly more detail in the basic Doppler section.

Modern ultrasound technology utilising the ability of new probes being able to transmit more data, as well as increased computing speed and capacity for more advanced post processing, has increased both line density and frame rate of B-mode, and this development will continue. This means that some of the reservations towards speckle tracking in terms of frame rate and lateral resolution will become less important in time. This, however, do not lend more validity to earlier results. And so far, it does not apply to 3D speckle tracking

Concerning nomenclature:

The terms: - Velocity imaging, - Displacement imaging, - Strain rate imaging, - Strain Imaging, should not be taken synonymous with tissue Doppler, but should be used irrespectively of the method employed, and the term "by tissue Doppler",  "by speckle tracking" or whatever application is used should be added, if studies are cited.

There is still a need for standardisation of nomenclature in the field. Especially for newer measures relating to deformation imaging, which are not covered by The ASE standards (146). The systolic mitral annular excursion is a useful measure of global systolic function. It has had various names, the term AVPD (atrioventricular plane descent) is unfortunate, as it don't separate beteen mitral and tricuspid excursions, event though they are different, the term MAE has been used from the beginning, but as TAPSE has been firmly established for the right ventricle, I think the term MAPSE (mitral annular plane systolic excursion) should be used in order to harmonise. The text and figures in this website have been adjusted accordingly.

The nomenclature about positive and negative deformation has been a mess.The original definition of strain makes shortening and shortening velocity negative values. For the longitudinal and circumferential functional measures, this means that the more the contraction, the lower the negative values. Then "increased" strain and strain rate would mean "less contraction", which is absolutely counter intuitive. Also, the literature has through all the time strain and strain rate has been evident, talked about "peak values" strain and strain rate. Consistent with the usage of negative values, that actually should have been "trough values". For transmural strain, the case is opposite, wall thickening is positive strain and peak values are really peak values. The new definitions paper (287) recommends the use of absolute numbers, which will make the discussions more intuitive, and I wholeheartedly concur.

However, as we have two reference systems for strain and strain rate: Lagrangian and Eulerian, I'm no fan of the term "natural strain" for Eulerian strain, I can't see why one reference system is more "natural" than another. Also using mathemathician's names should be symmetrical.

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Editor: Asbjørn Støylen Contact address: asbjorn.stoylen@ntnu.no