Strain rate imaging

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Strain rate imaging
This clip shows the use of XStrain software to calculate the longitudinal peak systolic strain (LPSS) in all the segments of the right ventricle (RV) during a systole
Purposemeasuring regional/global deformation of myocardium

Strain rate imaging is a method in echocardiography (medical ultrasound) for measuring regional or global deformation of the myocardium (heart muscle). The term "deformation" refers to the myocardium changing shape and dimensions during the cardiac cycle. If there is myocardial ischemia, or there has been a myocardial infarction, in part of the heart muscle, this part is weakened and shows reduced and altered systolic function. Also in regional asynchrony, as in bundle branch block, there is regional heterogeneity of systolic function. By strain rate imaging, the simultaneous function of different regions can be displayed and measured. The method was first based on colour tissue Doppler. [1] by using the longitudinal myocardial velocity gradient, already in use transmurally. [2] Later, the regional deformation has also been available by speckle tracking echocardiography, [3] [4] both methods having some, but different methodological weaknesses. Both methods, however, will acquire the same data (measurements may differ somewhat, however, being method dependent), and also can be displayed by the same type of display.

Contents

The point of deformation imaging, is that a passive segment in the myocardium for instance after an infarct, may move due to the action of an adjacent segment (tethering). Thus the displacement or velocity of a segment do not tell about the function of that segment. Deformation imaging, on the other hand, measures the differences of motion and velocity within the segment, which is equivalent to the deformation.

Basic concepts

Strain means Deformation, and is defined as relative change in length. The Lagrangian formula εL = (L-L0)/L0 = ΔL/L0, where L0 is baseline length and L is the resulting length, defines strain in relation to the original length as a dimensionless measure, where shortening will be negative, and lengthening will be positive. It is usually expressed in percent. An alternative definition, Eulerian strain defines the strain in relation to the instantaneous length: εE = ΔL/L. For a change over time, the Lagrangian strain will be: εL = Σ ΔL/L0, and Eulerian Strain εE = Σ (ΔL/L). The term was first used by Mirsky and Parmley in describing regional differences in deformation between normal and ischemic myocardium [5]

Strain rate is the rate of deformation. In ultrasound it is usually measured from the velocity gradient SR = (v2 - v1)/L where v2and v1 are the myocardial velocities at two different points, and L is the instantaneous distance between them. This is thus equivalent to the velocity difference per length unit (the spatial derivative of velocity) and has the unit s−1. Strain is then integrated from strain rate. This method, however, yields the Eulerian strain rate and strain. It has become traditional to use the Velocity gradient, but in integrating strain rate it is converted to Lagrangian strain by the formula εL = eεE - 1. [6]

Strain in three dimensions: Basically, any object or body is three dimensional, and can be deformed in different directions simultaneously. Strain can be described as a tensor with three principal strains (εx, εy and εz in a Cartesian coordinate system), and six shear strains components. In the heart, it has been customary to describe the three principal strain components as longitudinal (in the direction of the long axis of the ventricles), circumferential (in the direction of the ventricular circumference), and transmural (the deformation across the wall. Transmural deformation has also been called "radial", but this is unfortunate as in ultrasound in general the term radial describes "in the direction of the ultrasound beam"). However, as the heart muscle is incompressible, the three principal strain must balance; ((εx+1)(εy+1)(εz+1) = 1). [7] As the ventricle contracts in systole, there is longitudinal shortening (negative strain), circumferential shortening (negative strain) and transmural (wall) thickening (positive strain). Due to this, and the fact that the left ventricle in normal conditions contract with a relatively invariant outer contour, [8] [9] the longitudinal strain contains the main information, while transmural strain (wall thickening) is a function of wall shortening, wall thickness and chamber diameter, while circumferential shortening is mainly a function of wall thickening. It has been shown clinically that longitudinal strain rate and wall thickening are diagnostically equivalent. [10]

Methods

Strain rate imaging can be done by two principally different methods.

Tissue Doppler

The Tissue Doppler method is based on the colour Doppler, giving a velocity field with velocity vectors along the ultrasound beam over the whole sector. It measures the velocity gradient between two points along the ultrasound beam with a set distance. [1] It gives the same result as the velocity gradient. [6] This method has been validated experimentally in a mechanical model, [11] in an animal model, [12] and in patients against echocardiography, [13] coronary angiography [10] and MR [14] [15] The method is limited to one direction; along the ultrasound beam, can thus mainly be used from the apical window, and for longitudinal strain and strain rate measurements only. It is sensitive to angle deviation between the velocity vector (direction of motion) and the ultrasound beam, and is sensitive to noise, especially clutter noise. It has a high temporal resolution, at the cost of a relatively low lateral spatial resolution.[ citation needed ]

Speckle tracking

Speckle tracking echocardiography is based on grey scale echocardiography (B-mode), and the fact that the reflected echo from the myocardium shows a speckle pattern that is a mixture of small scatters and interference patterns. The pattern being random, each region of the myocardium (called a "kernel"), has a unique speckle pattern, and that this speckle pattern is relatively stable, at least from one frame to next. By this, the movement of a kernel from one frame to the next, can be tracked by a "best match" search algorithm. The most commonly used is the "sum of absolute differences", [4] shown to be similarly accurate as Cross-correlation. [16] [17] The method thus tracks a kernels motion from one frame to the next. From the frame rate, the velocity vector can be calculated, both in magnitude and direction. From this, a velocity field again can be generated over the whole sector, as with tissue Doppler, and strain rate can be derived, and then strain can be integrated. Alternatively strain can be measured directly from the change in distance between speckles. [18] [19] (resulting in Lagrangian strain directly), and strain rate derived temporally (it then has to be converted to Eulerian strain rate). The speckle tracking methods varies with non-commercial and commercial systems. Speckle tracking has been shown to be comparable to tissue Doppler derived strain, [20] and has been validated against MR [15] [19] [21]

The method tracks independently of the beam directions, and can thus track in two dimensions. It is also said to be independent of the angle error inherent in the Doppler algorithm. However, as the radial resolution (along the beam) is far better than the lateral resolution which also decreases with depth, both the angle independence and the tracking ability across the sector is lower. Also, instead of angle independency, the resulting strain values are dependent on the ROI (Region Of Interest) size and shape. Finally, in order to achieve tracking quality, the values are in most commercial applications smoothed by a spline smoothing function along the ROI, so the regional measurements are not pure regional, but rather to a degree, spline functions of the global average. In addition, the method has a lower sampling rate due to the limited frame rate of B-mode, which reduces tracking validity, especially at high heart rates.[ citation needed ]

Colour-coded left atrial strain assessed by speckle tracking during a representative cardiac cycle.
Systemic sclerosis (SSc) patient: 2D longitudinal strain of the right ventricle. The values of the basal, mid and apical segment of the free lateral wall were taken for further analysis.

Display

Longitudinal strain rate and strain: multiple simultaneous traces from three different regions in the septum. Left: strain rate, Right: Strain. As thelongitudinal systolic deformation is shortening, the systolic strain and strain rate is negative. The strain curves shows the gradual decrease in length during systole, and then the gradual lengthening during diastole, but strain rate fremans negative during the whole heart cycle, as the ventricular length is shorter than at end systole. Strain rate is the rate of deformation, and is negative during systole, when the ventricle shortens. Strain rate, however, becomes positive when the ventricle lengthens. Thus the more rapid phase shifts show details of the lengthening, displaying that it is not homogeneous. Multiple traces.png
Longitudinal strain rate and strain: multiple simultaneous traces from three different regions in the septum. Left: strain rate, Right: Strain. As thelongitudinal systolic deformation is shortening, the systolic strain and strain rate is negative. The strain curves shows the gradual decrease in length during systole, and then the gradual lengthening during diastole, but strain rate fremans negative during the whole heart cycle, as the ventricular length is shorter than at end systole. Strain rate is the rate of deformation, and is negative during systole, when the ventricle shortens. Strain rate, however, becomes positive when the ventricle lengthens. Thus the more rapid phase shifts show details of the lengthening, displaying that it is not homogeneous.
Strain rate colour curved anatomical M-mode. The yellow line is drawn from the base of the septum, through the apex and down the lateral wall as can be seen on the small images on the left. The rectangular area then represents the colour M-mode, where the vertical axis is distance along the line, while the horizontal axis is time. The image represents one heart cycle. Strain rate values are yellow to red for negative strain rate (shortening) and cyan to blue for positive strain rate (lengthening). Green represents area and time of no deformation. Strain rate CAMM.jpg
Strain rate colour curved anatomical M-mode. The yellow line is drawn from the base of the septum, through the apex and down the lateral wall as can be seen on the small images on the left. The rectangular area then represents the colour M-mode, where the vertical axis is distance along the line, while the horizontal axis is time. The image represents one heart cycle. Strain rate values are yellow to red for negative strain rate (shortening) and cyan to blue for positive strain rate (lengthening). Green represents area and time of no deformation.

Both methods measures the same physiological phenomena (deformation), and results can in principle be displayed the same way.

Curves

The most common way is by displaying curves of the strain and strain rate, typically the time course during one heart cycle. Each curve will then represent the deformation in one region of the myocardium, but acquisition of a full sector allows display of multiple curves simultaneously in the same image for comparison.[ citation needed ]

Colour display

Strain and strain rate values can be reduced to colour coded images, where strain or strain rate are shown as colours in semi-quantitative parametric imaging. This makes the method more robust, but numerical values are not available. On the other hand, this may result in a better spatial resolution. The displays most commonly used, are Bull's eye (reconstructed from multiple apical planes), which displays all parts of the left ventricle simultaneously, but only at one point in time. This is useful for either mid systolic strain rate or end systolic strain. Inhomogeneous strain rate or strain, representing regions with reduced contractility are often very evident visually.[ citation needed ]

Curved anatomical M-mode [22] from either one wall, or both walls simultaneously, gives a space-time diagram of the deformation, showing both spatial or temporal inhomogeneities in deformation. It is most useful when applied to strain rate, due to the rapid shifts in phase visible as seen by the figure. The strain rate values are reduced to semi quantitative visual display, but this mode allows measurement of timing, as well as depth, and is best suited to space-time relation measurement[ citation needed ]

Clinical use

It is a major point that strain rate imaging is just a part of an integrated echocardiographic examination. Like all other measures, deformation measurements have limited accuracy, and should be considered together with the rest of findings. Also, a knowledge of the pitfalls and artefacts of the specific methods, is an advantage. However, the methods offer unique ways of imaging regional dysfunction, that may strengthen the conclusion.[ citation needed ]

Regional function

Normal values for strain and strain rate has been established by the HUNT study. [23]

Myocardial infarction

In myocardial infarction, a limited region of the heart muscle has reduced or totally absent function. It has been shown to be at least as accurate as B-mode echocardiography. [10] [13] [24] Deformation imaging has also been shown to be useful in following recovery of an infarcted myocardial area, to ascertain the amount of Myocardial stunning vs. necrosis. [25] [26] [27]

Myocardial ischemia

In stress echocardiography (see Cardiac stress test), the regional dysfunction due to ischemia will become evident when the myocardial oxygen demand surpasses the Coronary flow reserve of a stenosed coronary artery. Strain rate imaging during stress has been shown to give incremental value over ordinary echocardiography, both diagnostic [28] [29] and prognostic. [30] In stress echo, the increased heart rate has speckle tracking at a disadvantage, due to the limited frame rate that affects tracking at higher heart rates.[ citation needed ]

Ventricular dyssynchrony

In Left bundle branch block (LBBB), the asynchronous activation of the left ventricle gives asynchronous contraction as well. This asynchrony can be visualised by ordinary echocardiography. [31] It can also be demonstrated by tissue velocities, but strain rate imaging will in addition demonstrate the distribution of the asynchrony, and the demonstration of the amount of inefficient work done by the asynchronous ventricle. Disappointingly, large scale studies have not been able to establish additional echo criteria for selection of Heart failure patients with LBBB who may respond to Cardiac resynchronization therapy, [32] although smaller studies are promising [33]

Global function

In later years, Global strain by speckle tracking has achieved popularity as the global functional measure. It has an advantage over Ejection fraction (EF), it shows reduced cardiac function also in hypertrophic hearts with small ventricles and normal ejection fraction (HFNEF), which is often seen in Hypertensive heart disease, Hypertrophic cardiomyopathy and Aortic stenosis. The EF is not a pure functional measure, as it is also dependent on wall thickness [34] It has also been shown to be more sensitive than EF. [35] [36] However, the incremental diagnostic and prognostic value of measuring LV shortening was already shown for the absolute measure [37] [38] [39] [40]

Global strain is basically LV shortening/LV end diastolic length, which means that this is a normalisation of LV shortening for LV heart size. It remains to be proven that this actually confers additional information.[ citation needed ]

Related Research Articles

<span class="mw-page-title-main">Medical ultrasound</span> Diagnostic and therapeutic technique

Medical ultrasound includes diagnostic techniques using ultrasound, as well as therapeutic applications of ultrasound. In diagnosis, it is used to create an image of internal body structures such as tendons, muscles, joints, blood vessels, and internal organs, to measure some characteristics or to generate an informative audible sound. The usage of ultrasound to produce visual images for medicine is called medical ultrasonography or simply sonography, or echography. The practice of examining pregnant women using ultrasound is called obstetric ultrasonography, and was an early development of clinical ultrasonography. The machine used is called an ultrasound machine, a sonograph or an echograph. The visual image formed using this technique is called an ultrasonogram, a sonogram or an echogram.

<span class="mw-page-title-main">Ventricle (heart)</span> Chamber of the heart

A ventricle is one of two large chambers located toward the bottom of the heart that collect and expel blood towards the peripheral beds within the body and lungs. The blood pumped by a ventricle is supplied by an atrium, an adjacent chamber in the upper heart that is smaller than a ventricle. Interventricular means between the ventricles, while intraventricular means within one ventricle.

<span class="mw-page-title-main">Echocardiography</span> Medical imaging technique of the heart

Echocardiography, also known as cardiac ultrasound, is the use of ultrasound to examine the heart. It is a type of medical imaging, using standard ultrasound or Doppler ultrasound. The visual image formed using this technique is called an echocardiogram, a cardiac echo, or simply an echo.

An ejection fraction (EF) is the volumetric fraction of fluid ejected from a chamber with each contraction. It can refer to the cardiac atrium, ventricle, gall bladder, or leg veins, although if unspecified it usually refers to the left ventricle of the heart. EF is widely used as a measure of the pumping efficiency of the heart and is used to classify heart failure types. It is also used as an indicator of the severity of heart failure, although it has recognized limitations.

A transthoracic echocardiogram (TTE) is the most common type of echocardiogram, which is a still or moving image of the internal parts of the heart using ultrasound. In this case, the probe is placed on the chest or abdomen of the subject to get various views of the heart. It is used as a non-invasive assessment of the overall health of the heart, including a patient's heart valves and degree of heart muscle contraction. The images are displayed on a monitor for real-time viewing and then recorded.

The Dor procedure is a medical technique used as part of heart surgery and originally introduced by the French cardiac surgeon Vincent Dor (b.1932). It is also known as endoventricular circular patch plasty (EVCPP).

Myocardial stunning or transient post-ischemic myocardial dysfunction is a state of mechanical cardiac dysfunction that can occur in a portion of myocardium without necrosis after a brief interruption in perfusion, despite the timely restoration of normal coronary blood flow. In this situation, even after ischemia has been relieved and myocardial blood flow (MBF) returns to normal, myocardial function is still depressed for a variable period of time, usually days to weeks. This reversible reduction of function of heart contraction after reperfusion is not accounted for by tissue damage or reduced blood flow, but rather, its thought to represent a perfusion-contraction "mismatch". Myocardial stunning was first described in laboratory canine experiments in the 1970s where LV wall abnormalities were observed following coronary artery occlusion and subsequent reperfusion.

<span class="mw-page-title-main">Doppler echocardiography</span> Medical imaging technique of the heart

Doppler echocardiography is a procedure that uses Doppler ultrasonography to examine the heart. An echocardiogram uses high frequency sound waves to create an image of the heart while the use of Doppler technology allows determination of the speed and direction of blood flow by utilizing the Doppler effect.

<span class="mw-page-title-main">Uhl anomaly</span> Medical condition

Uhl anomaly is a rare cardiac malformation that was first identified by Dr. Henry Uhl in 1952. It is characterized by the absence of the right ventricle (RV) myocardium, either entirely or partially, and the replacement of the RV myocardium by nonfunctional fibroelastic tissue that resembles parchment. As of 2010 less than 100 cases have been reported in literature.

<span class="mw-page-title-main">Noncompaction cardiomyopathy</span> Congenital disease of heart muscle

Noncompaction cardiomyopathy (NCC) is a rare congenital disease of heart muscle that affects both children and adults. It results from abnormal prenatal development of heart muscle.

<span class="mw-page-title-main">Cardiac magnetic resonance imaging</span> Biomedical imaging technology

Cardiac magnetic resonance imaging, also known as cardiovascular MRI, is a magnetic resonance imaging (MRI) technology used for non-invasive assessment of the function and structure of the cardiovascular system. Conditions in which it is performed include congenital heart disease, cardiomyopathies and valvular heart disease, diseases of the aorta such as dissection, aneurysm and coarctation, coronary heart disease. It can also be used to look at pulmonary veins.

The E/A ratio is a marker of the function of the left ventricle of the heart. It represents the ratio of peak velocity blood flow from left ventricular relaxation in early diastole to peak velocity flow in late diastole caused by atrial contraction. It is calculated using Doppler echocardiography, an ultrasound-based cardiac imaging modality. Abnormalities in the E/A ratio suggest that the left ventricle, which pumps blood into the systemic circulation, cannot fill with blood properly in the period between contractions. This phenomenon is referred to as diastolic dysfunction and can eventually lead to the symptoms of heart failure.

<span class="mw-page-title-main">Speckle tracking echocardiography</span>

In the fields of cardiology and medical imaging, speckle tracking echocardiography (STE) is an echocardiographic imaging technique. It analyzes the motion of tissues in the heart by using the naturally occurring speckle pattern in the myocardium.

<span class="mw-page-title-main">Heart failure with preserved ejection fraction</span> Medical condition

Heart failure with preserved ejection fraction (HFpEF) is a form of heart failure in which the ejection fraction – the percentage of the volume of blood ejected from the left ventricle with each heartbeat divided by the volume of blood when the left ventricle is maximally filled – is normal, defined as greater than 50%; this may be measured by echocardiography or cardiac catheterization. Approximately half of people with heart failure have preserved ejection fraction, while the other half have a reduction in ejection fraction, called heart failure with reduced ejection fraction (HFrEF).

Tissue Doppler echocardiography (TDE) is a medical ultrasound technology, specifically a form of echocardiography that measures the velocity of the heart muscle (myocardium) through the phases of one or more heartbeats by the Doppler effect of the reflected ultrasound. The technique is the same as for flow Doppler echocardiography measuring flow velocities. Tissue signals, however, have higher amplitude and lower velocities, and the signals are extracted by using different filter and gain settings. The terms tissue Doppler imaging (TDI) and tissue velocity imaging (TVI) are usually synonymous with TDE because echocardiography is the main use of tissue Doppler.

Harmonic phase (HARP) algorithm is a medical image analysis technique capable of extracting and processing motion information from tagged magnetic resonance image (MRI) sequences. It was initially developed by N. F. Osman and J. L. Prince at the Image Analysis and Communications Laboratory at Johns Hopkins University. The method uses spectral peaks in the Fourier domain of tagged MRI, calculating the phase images of their inverse Fourier transforms, which are called harmonic phase (HARP) images. The motion of material points through time is then tracked, under the assumption that the HARP value of a fixed material point is time-invariant. The method is fast and accurate, and has been accepted as one of the most popular tagged MRI analysis methods in medical image processing.

<span class="mw-page-title-main">Left ventricular thrombus</span> Medical condition

Left ventricular thrombus is a blood clot (thrombus) in the left ventricle of the heart. LVT is a common complication of acute myocardial infarction (AMI). Typically the clot is a mural thrombus, meaning it is on the wall of the ventricle. The primary risk of LVT is the occurrence of cardiac embolism, in which the thrombus detaches from the ventricular wall and travels through the circulation and blocks blood vessels. Blockage can be especially damaging in the heart or brain (stroke).

The assessment of the regional function of the heart is a good tool for early detection of deterioration in certain parts of the heart wall before a cardiac arrest is diagnosed. One of the most accurate measures of changes in regional function is the use of strain as a measure of the regional function of cardiac muscle.

Numerical manipulation of Doppler parameters obtain during routine Echocardiography has been extensively utilized to non-invasively estimate intra-cardiac pressures, in many cases removing the need for invasive cardiac catheterization.

In clinical cardiology the term "diastolic function" is most commonly referred as how the heart fills. Parallel to "diastolic function", the term "systolic function" is usually referenced in terms of the left ventricular ejection fraction (LVEF), which is the ratio of stroke volume and end-diastolic volume. Due to the epidemic of heart failure, particularly the cases determined as diastolic heart failure, it is increasingly urgent and crucial to understand the meaning of “diastolic function”. Unlike "systolic function", which can be simply evaluated by LVEF, there are no established dimensionless parameters for "diastolic function" assessment. Hence to further study "diastolic function" the complicated and speculative physiology must be taken into consideration.

References

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