Download Assessment of strain and strain rate by two

European Heart Journal – Cardiovascular Imaging (2013) 14, 765–773
doi:10.1093/ehjci/jes274
Assessment of strain and strain rate by twodimensional speckle tracking in mice: comparison
with tissue Doppler echocardiography and
conductance catheter measurements
V. Ferferieva 1*, A. Van den Bergh2, P. Claus 1, R. Jasaityte 1, A. La Gerche 1,
F. Rademakers 1, P. Herijgers 2, and J. D’hooge 1
1
Cardiovascular Imaging and Dynamics, Catholic University Leuven, Leuven, Belgium; and 2Department of Cardiovascular Sciences, Experimental Cardiac Surgery,
Catholic University Leuven, Leuven, Belgium
Received 6 July 2012; revised 26 October 2012; accepted after revision 7 November 2012; online publish-ahead-of-print 2 December 2012
Aims
This study was designed in order to compare the strain and strain rate deformation parameters assessed by speckle
tracking imaging (STI) with those of tissue Doppler imaging (TDI) and conductance catheter measurements in chronic
murine models of left ventricular (LV) dysfunction.
.....................................................................................................................................................................................
Methods
Twenty-four male C57BL/6J mice were assigned to wild-type (n ¼ 8), myocardial infarction (n ¼ 8) and transaortic
and results
constriction (n ¼ 8) groups. Echocardiographic and conductance measurements were simultaneously performed at
rest and during dobutamine infusion (5 mg/kg/min) in all animals 10 weeks post-surgery. The LV circumferential
strain (Scirc) and the strain rate (SRcirc) were derived from grey scale and tissue Doppler data at frame rates of
224 and 375 Hz, respectively. Scirc and SRcirc by TDI/STI correlated well with the preload recruitable stroke
work (PRSW) (r ¼ 20.64 and 20.71 for TDI; r ¼ 20.46 and 20.50 for STI, P , 0.05). Both modalities showed
a good agreement with respect to Scirc and SRcirc (r ¼ 0.60 and r ¼ 0.63, P , 0.05). During stress, however,
TDI-estimated Scirc and SRcirc values were predominantly higher than those measured by STI (P , 0.05). The similarity of Scirc and SRcirc measurements with respect to the STI/TDI data was examined by the Bland–Altman
analysis.
.....................................................................................................................................................................................
Conclusion
In mice, the STI- and TDI-derived strain and strain rate deformation parameters relate closely to intrinsic myocardial
function. At low heart rate-to-frame rate ratios (HR/FR), both STI and TDI are equally acceptable for assessing the LV
function non-invasively in these animals. At HR/FR (e.g. dobutamine challenge), however, these methods cannot be
used interchangeably as STI underestimates S and SR at high values.
----------------------------------------------------------------------------------------------------------------------------------------------------------Keywords
Strain † Strain rate † Tissue Doppler † Speckle tracking † Contractility
Introduction
Small animal models have provided valuable insights into cardiac
pathophysiology. However, most frequently, the cardiac measures
employed are fairly basic 1- or 2-dimensional assessments of
cardiac size.1,2 Such measures are insensitive to early changes in
the myocardial function and so there has been increased interest
in the use of non-invasive indices, such as strain and strain rate,
which have the potential to more accurately describe the intrinsic
myocardial contractile properties. Although the sensitive indices of
the left ventricular (LV) function, such as preload recruitable stroke
work (PRSW) and end-systolic elastance (Ees), are feasible in mice
using invasive haemodynamic measurements, these measures
usually necessitate the subsequent sacrifice of the animal, thus
* Corresponding author. Medical Imaging Research Center, U.Z. Gasthuisberg, Herenstraat 49, 3000 Leuven, Belgium. Tel: +32 16 34 90 73; fax: +32 16 34 34 67,
Email: [email protected]
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2012. For permissions please email: [email protected]
766
precluding longitudinal data collection.3,4 A second important advantage in assessing the non-invasive measures in small animal
models is that they provide a means by which the echocardiographic measures may be validated under significant physiological
stress with inference to their utility as accurate surrogates of
human myocardial contractility.
In order to assess strain and the strain rate in the mice, various
echocardiographic tools have been extensively applied during the
last years. Tissue Doppler imaging (TDI), which has been shown
to accurately reflect myocardial velocities and strain rates with a
high spatial resolution and rapid sampling, has been proposed
and validated as a novel non-invasive method for quantifying LV regional systolic function in small animals under different physiological/pharmacological conditions.5 – 7 Alternatively, speckle tracking
imaging (STI) technique based on the appearance of speckles
within the tissue on two-dimensional ultrasound imaging has
been recently used for the estimation of the radial and circumferential strain in small animal models of cardiac dysfunction.8,9
Despite the promising results of 2D strain application in rodents,
this approach has not been compared with invasive ‘gold standard’
techniques. Moreover, this technique typically works on lower
temporal resolution data, and hence, given the rapid heart rates
(HRs) of the mouse might not accurately resolve peak strain and
strain rate in these animals. This study was therefore performed
to compare the strain and strain rate deformation parameters
assessed by STI with those of tissue Doppler imaging and conductance catheter measurements in chronic murine models of LV
dysfunction.
V. Ferferieva et al.
Experimental protocol
Conductance and echocardiographic measurements were simultaneously performed in all animals 10 weeks post-surgery. Mice were
anaesthetized with a mixture of urethane (1200 mg/kg, i.p.) and
a-chloralose (50 mg/kg, i.p.) and were mechanically ventilated (140
strokes per minute, 7 mL/g tidal volume; Minivent 845; Hugo Sachs/
Harvard Apparatus). Dobutamine (5 mg/kg/min) was infused after
baseline acquisition via the right jugular vein until a HR increase of at
least 15% was obtained. After stabilization of the animals, a second
set of echocardiographic and haemodynamic measurements was
recorded. Blood conductance was determined at the end of each experiment using four cylindrical chambers with known volume loaded
with fresh heparinized blood from each individual.
Echocardiography
Transthoracic echocardiography was performed at rest and during
dobutamine challenge with a Vivid 7 ultrasound machine (GE
Vingmed Ultrasound, Horten, Norway) using a 14-MHz linear array
transducer (i13L). A parasternal long-axis view and short-axis views
at basal, mid-ventricular and apical levels were acquired using
B-mode and TDI-mode with temporal resolution of 224 and 375
frames per second, respectively, and a depth of 1 cm. The velocity
range for TDI was set as low as possible while still preventing aliasing.
LV dimensions were measured from three consecutive cardiac
cycles on the B-mode images. LV epicardial and endocardial volumes
were calculated by p*D2M*B/6, where DM indicates the epi/endo diameter of the ventricle in the mid-ventricular short-axis view and B is the
LV length on the parasternal long-axis image. Subsequently, LV myocardial volumes (LVMVs) were calculated as the difference of the epicardial and endocardial volumes. Likewise, the LV ejection fraction (EF)
was measured as LV end-diastolic volume 2 LV end-systolic volume/
LV end-diastolic volume, and expressed in %.
Methods
This investigation conforms to the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH
Publication No. 85-23, revised 1996) and was approved by a local
ethical committee (Ethische Commissie Dierproeven, KULeuven,
Leuven, Belgium).
Animal preparation
Twenty-four male C57BL/6J mice (23 weeks, body weight 32 + 3 g)
were used in this study. The animals were assigned to wild-type
(WT; n ¼ 8), myocardial infarction (MI; n ¼ 8) and transaortic constriction (TAC; n ¼ 8) groups. These surgically induced mouse
models of cardiovascular diseases are widely used to study the
chronic pressure overload and LV remodelling after MI.10
Surgical interventions
Mice were anaesthetized with pentobarbital sodium (40 – 70 mg/kg,
i.p.), intubated and mechanically ventilated (140 strokes per minute,
7 mL/g tidal volume; Minivent 845; Hugo Sachs/Harvard Apparatus).
Body temperature was maintained with a heating pad and monitored
with a rectal probe. In one group of animals (n ¼ 8) myocardial infarction was induced by ligation of the left coronary artery as described
previously.11
In a second group of animals (n ¼ 8) transaortic constriction was
performed by ligation of the aorta between the innominate and left
common carotid arteries, with an overlying 27-gauge needle, followed
by removal of the needle.12
Strain and strain rate by TDI
TDI analysis was performed offline using dedicated software
SPEQLE.13 The sampling volumes were positioned along the lateral
free wall for the assessment of Scirc and SRcirc with a strain estimation
length of 0.8 mm. Strain rate was calculated as the spatial gradient in
myocardial velocities over the computation area. Lagrangian strain profiles were subsequently estimated by time integrating the strain rate
profile. Lateral averaging of three to five beams/pixels was performed
and the values for peak systolic velocity, strain, and strain rate were
averaged over a mean of 10 + 4 consecutive cycles. The software
allows tracking of the region of interest based on manual tracking of
the motion in two orthogonal anatomical m-modes. This was done
in order to ensure that the sample volume is maintained in the same
anatomical position throughout the cardiac cycle. To indicate end diastole and end systole the posterior myocardial velocity profiles were
used, as described previously.14 Finally, in order to obtain one representative value for the entire ventricle the strain and strain rate measurements, obtained from the basal, mid-ventricular and apical levels,
were averaged.
Strain and strain rate by 2D speckle tracking imaging
STI data analysis was performed on an EchoPAC workstation (GE
Vingmed Ultrasound, version 7.0.1, Horten, Norway), as described
previously.9 Briefly, the measurements of circumferential strain and
strain rate at basal, mid-ventricular and apical levels were performed
on the selected best-quality two-dimensional images and averaged
over the three levels. The endocardium was manually traced in an
optimal frame, from which a speckle tracking region of interest was
Strain and strain rate by two-dimensional STI in mice
automatically selected. The region of interest width was adjusted as
needed to fit the wall thickness from the endocardium to the epicardium. The software detects and tracks the speckle pattern subsistent
to the standard two-dimensional echocardiography after segmenting
the ventricular silhouette into six segments.15 The tracking quality
was then visually inspected, and, if satisfactorily for at least five segments, the tracing was accepted.
As registration of the electrocardiogram was not always feasible in
these animals, end systole and end diastole were therefore defined
as the minimum and maximum LV short-axis area, respectively.
Finally, the heart rate-to-frame rate ratio (HR/FR) was calculated for
each 2D image that was used for STI analysis.
Pressure– volume measurements
Conductance measurements were performed in all animals with the
use of 1.4F microtip pressure– volume catheter (Millar Instruments)
that was advanced into the left ventricle via the right carotid
artery.16 Analysis of the pressure– volume loop data sets was performed with PVAN 3.2 software (Millar Instruments). The maximal
rate of developed LV pressure (dP/dtmax) and PRSW were measured
at rest and during dobutamine infusion as previously described.17
Statistical analysis
The statistical analysis was done using STATISTICA 7.1 (StatSoft, Inc.,
Tulsa, OK, USA) and MedCalc, version 9.6.4. Values are expressed as
the mean + standard deviation (SD). The conductance and conventional echocardiographic measurements for each animal group were
compared with baseline by paired t-test. Differences between the
S/SR values with respect to the different groups and methods were
tested using the two-way ANOVA with an additional Tukey post hoc
test when appropriate. A value of P ,0.05 was considered statistically
significant.
Correlations between the echocardiographic and haemodynamic
variables were assessed by Pearson correlation coefficient using
mixed models accounting for repeated measures in the mice. The similarity of the LV strain and strain rate with respect to the STI/TDI data
were examined by a Bland –Altman analysis as well.18 The bias was
expressed as the mean difference between the two methods and
the limits of agreement as two SDs of the differences in the two
methods.
Results
Haemodynamics
A summary of the invasive haemodynamic measurements obtained
at baseline and during stress for the different groups is shown in
Table 1.
At rest, the HR was similar in all groups. A significant decrease in
dP/dtmax was observed in the MI group, whereas ESP was increased
in the banded mice as compared with the control group (P , 0.05
for both parameters). During stress, HR and dP/dtmax increased significantly in all groups. PRSW was substantially increased in the
WT and TAC mice, yet, no change in this parameter was observed
in the MI group.
Conventional echocardiography
At rest, the end-diastolic posterior wall thickness (PWd) and myocardial volumes were significantly increased in the TAC group,
767
whereas increased LV diameters were observed in the MI group
along with a decrease in EF and fractional shortening (FS) (Table 2).
Dobutamine infusion resulted in a substantial decrease in LV diameters and higher FS only in the WT and TAC groups (P , 0.05
vs. rest).
Tissue Doppler-derived strain and strain
rate
At rest, Scirc and SRcirc were significantly decreased in the MI
group (P , 0.05 for both parameters) (Table 3). Dobutamine infusion induced an increase in Scirc and SRcirc in both the control and
banded mice (P , 0.05). Regarding the MI group, there was a trend
towards increased strain and strain rate values, however, this did
not reach statistical significance.
Mixed models of peak systolic Scirc and SRcirc showed a significant correlation with the invasive ‘gold-standard’ measure of contractility, PRSW (r ¼ 20.78 and 20.80, respectively; P , 0.0001)
(Figure 1).
Strain and strain rate by speckle tracking
imaging
Compared with the control group, both Scirc and SRcirc were significantly reduced in the MI group at rest (Table 3). During inotropic stimulation, no increase in the strain and strain rate values was
noted in the different groups, although there was a trend
towards higher values (P ¼ NS vs. rest).
STI-derived Scirc and SRcirc showed a significant correlation
with PRSW (r ¼ 20.50 and r ¼ 20.59, respectively; P , 0.05;
Figure 1)
Comparison of circumferential strain and
strain rate between STI and TDI
Under rest conditions, the Scirc and SRcirc values were comparable for both imaging modalities (P ¼ NS), although there was a
trend towards higher values measured by TDI (Table 3). During
stress, however, TDI-estimated deformation parameters were significantly higher than the STI-derived values in both the control
and banded mice (P , 0.05).
Both methods correlated well with respect to Scirc and SRcirc
(r ¼ 0.60 and r ¼ 0.63, respectively; P , 0.001; Figure 2). The
Bland– Altman plot is shown in Figure 3. At rest, the mean difference between both methodologies with respect to Scirc and
peak systolic SRcirc was 2.1% with limits of agreement (29.6;
13.8) and 2.3 1/s (P ¼ NS for both) with limits of agreement
(23.9; 8.5), respectively. During dobutamine challenge, however,
a significant bias of 5.4% for Scirc with limits of agreement
(26.1; 16.8) and 5.2 1/s for SRcirc with limits of agreement
(21.4; 11.7) was observed (P , 0.05 for both), with consistently
greater values when derived by TDI.
The impact of the HR/FR ratio on the difference in the strain/
strain rate values between both methodologies was tested by
simple linear regression models. The difference in the strain
values between TDI and STI did not show a significant correlation
with the HR/FR ratio, while SRcirc showed a significant correlation
with HR/FR (r ¼ 0.50; P , 0.05), indicating that the difference in
the SR measurements increases with higher HR/FR ratios (Figure 4).
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Table 1
V. Ferferieva et al.
Haemodynamic characteristics of all groups at rest and during stress conditions
Parameter
Rest
................................................................
WT
TAC
MI
Stress
.........................................................................
WT
TAC
MI
...............................................................................................................................................................................
Heart rate (b.p.m.)
LVESP (mmHg)
LVEDP (mmHg)
dP/dtmax (mmHg/s)
PRSW (mmHg)
560 + 40
545 + 41
75 + 19
99 + 23**
3+1
6795 + 1896
5+3
6095 + 1217
77 + 14
92 + 32**
518 + 35
666 + 40*
652 + 39*
624 + 57*
75 + 7
86 + 18
115 + 13**
82 + 15
4+3
4844 + 1364**
2+1
12 095 + 3363*
53 + 18
4+4
10 056 + 2596*
110 + 20*
3+4
8600 + 3893*,**
180 + 36*,**
64 + 30**
LVESP, left ventricular end-systolic pressure; LVEDP, left ventricular end-diastolic pressure; dP/dtmax, the maximum value of the first derivative of the LV pressure; PRSW, preload
recruitable stroke work. All values are expressed as the mean + SD.
*P , 0.05 vs. rest; **P , 0.05 between groups for the same stage.
Table 2
Conventional echocardiographic characteristics of the groups during rest and stress conditions
Parameter
Rest
...................................................................
WT
TAC
MI
Stress
...................................................................
WT
TAC
MI
...............................................................................................................................................................................
LVIDd (mm)
3.6 + 0.3
LVIDs (mm)
PWd (mm)
2.6 + 0.4
0.89 + 0.2
3.2 + 0.5
2.5 + 0.4
1.1 + 0.2**
5.3 + 1.6**
3 + 0.4*
2.6 + 0.3*
5.3 + 1.5**
4.6 + 1.8**
0.79 + 0.1
1.9 + 0.3*
1 + 0.2*
1.7 + 0.4*
1.3 + 0.2*
4.3 + 1.8**
0.98 + 0.1*
LVMV (mL)
54 + 8
73 + 18**
54 + 16
52 + 9
70.8 + 19*
EF (%)
50 + 11
45 + 12
25 + 11**
60 + 7
59 + 7
57.8 + 18
23 + 10**
LVFS (%)
32 + 9
23 + 8
16 + 12**
45 + 9*
32 + 6*
21 + 15**
LVIDd, indicates left ventricular end-diastolic diameter; LVIDs, left ventricular end-systolic diameter; PWd, end-diastolic posterior wall thickness; LVMV, left ventricular myocardial
volume; EF, ejection fraction; LVFS, left ventricular fractional shortening. All values are expressed as the mean + SD.
*P , 0.05 vs. rest; **P , 0.05 between groups for the same stage.
Table 3
Echocardiographic characteristics of the groups during rest and stress conditions
Parameter
Rest
Stress
.........................................................
................................................................
WT
TAC
MI
WT
TAC
MI
217.2 + 4.1
27.1 + 1.6
212.3 + 3.5
26.1 + 1.1
212.3 + 3.1
26.4 + 1.8
210.5 + 4.1
24.1 + 0.8
25.5 + 2.2*
24.1 + 1.7*
25.4 + 4.9*
22.9 + 1.9*
222.4 + 5.1**
213.7 + 2.2**
213.9 + 2.7***
26.4 + 1.4***
218.9 + 3.1**
212.9 + 2.2**
211.7 + 3.5***
26.1 + 2.3***
26.5 + 1.9
25.8 + 3.4
24.9 + 2.6
22.9 + 1.1
...............................................................................................................................................................................
Tissue Doppler
Speckle tracking
Scirc (%)
SRcirc (1/s)
Scirc, %
SRcirc, 1/s
Scirc, end-systolic circumferential strain; SRcirc, peak systolic circumferential strain rate. All values are expressed as the mean + SD.
*P , 0.05 vs. animal group, **P , 0.05 vs. stage, ***P , 0.05 vs. method.
Discussion
Echocardiographic analysis of the mouse phenotype has brought
important insights into the pathophysiological mechanisms underlying cardiovascular diseases, including hypertension, heart failure,
and remodelling.2,12,19 However, there is growing evidence that
changes in conventional echocardiographic systolic indices, such
as EF and FS, manifest late in the disease process and may not
be sensitive enough to unmask subtle changes in the LV structure
and global function.20 Accordingly, the scientific interest in adopting strain rate imaging into the standard murine echocardiography
has evolved over the past years. However, while it is relatively well
established in humans and large animals, performing strain rate
imaging in mice is challenged by the rapid HRs and the small
cardiac size. Initial studies, based on a TDI g approach, have
revealed the potential of strain and strain rate deformation
769
Strain and strain rate by two-dimensional STI in mice
Figure 1 Correlations between Scirc and SRcirc, estimated by tissue Doppler and speckle tracking imaging modalities, and the preload recruitable stroke work (PRSW).
parameters to accurately reflect regional myocardial contractility in
small animals and to enable longitudinal follow-up of function (e.g.
after cardioprotective therapeutic strategies).1,16,21,22 Notwithstanding these promising results, one of the major pitfalls of this
ultrasound technique is its angle dependency and the possibility
to measure strain/strain rate only in the anterior/posterior LV
wall for the radial component, and in the lateral free LV wall for
the circumferential component on a parasternal short-axis view.
As an alternative approach, STI, a technique based on the appearance of speckles within the tissue on two-dimensional ultrasound
imaging, overcomes these limitations. As such, several studies
have recently been conducted applying the speckle tracking approach in different small animal models of LV dysfunction.8,9
Although STI-derived strain measurements have been shown to
accurately reflect pathology and the time course of heart failure
development, no comparison was made with invasive indices of
myocardial contractility, and as such, the actual accuracy of this
methodology remains unknown.
This is the first study which compares the STI-derived strain
parameters directly to invasive ‘gold standard’ haemodynamic measurements of myocardial contractility and TDI in mouse models of
transaortic constriction and myocardial infarction during rest and
pharmacologically induced stress conditions.
Haemodynamic and conventional echo
characteristics of the mouse models
As expected, 10 weeks of myocardial infarction induced ventricular
dilation and global LV dysfunction.23 Aortic banding mice showed
increased end-systolic pressures along with an increased wall thickness and myocardial volumes resulting in significant LV hypertrophy in these animals (Table 1). This is in agreement with
various experimental and clinical studies demonstrating that the
LV mass progressively increases after a period of increased afterload along with a prominent decrease in the EF.9,24
Comparison of strain and strain rate
between STI and TDI
At rest, similar S and SR values were found for both imaging modalities; however, there was a trend towards higher values measured by TDI (Table 3; Figure 3). The reason for these
discrepancies in strain and strain rate measurements with TDI/
770
V. Ferferieva et al.
Figure 2 Correlation between both imaging modalities with respect to Scirc (A) and SRcirc (B). TDI, tissue Doppler imaging; STI, speckle
tracking imaging.
Figure 3 Bland – Altman analysis for the estimation of the accuracy of Scirc and SRcirc measurements by the two methods at rest and during
dobutamine challenge. Solid line indicates the mean and dotted lines 2 SD limits of agreement. TDI, tissue Doppler imaging; STI, speckle tracking
imaging.
STI might be related to the different transmural sampling applied
by both approaches. Several studies have demonstrated a prominent difference in the magnitude of myocardial deformation
between the endocardial and epicardial layers with a continuous
decrease in the strain and strain rate from subendocardial to midmyocardial and subepicardial layers.25 This reduction was observed
Strain and strain rate by two-dimensional STI in mice
771
Figure 4 Relation between the heart rate-to-frame rate ratio (HR/FR) and the difference in the strain/strain rate values between both methodologies using simple linear regression models.
for all LV segments irrespective of their location within the LV.26
TDI deformation parameters are measured only at one point
within the ventricular wall. The region of interest is typically
placed in the mid-myocardium but its position may vary (e.g.
closer to the endocardium or epicardium) depending on the
image quality. STI, on the other hand, incorporates tracking of
speckle motions transmurally from subendocardial to subepicardial
layers and subsequently displays averaged values for all myocardial
layers. This different transmural sampling between both methods
might result in variant strain and strain rate measurements in
view of the existing transmural gradient in strain.
A second reason for these discrepancies might be related to the
intrinsic differences in the principles of both methodologies. TDI
analysis is based on measuring the instantaneous velocities of a
point within the myocardium in each acquired frame. Accordingly,
the displacement of the point between two successive frames is
then obtained as the product of the instantaneous velocity and
the acquisition time from one frame to another. STI, on the
other hand, relies on measuring the displacement of speckles
within the myocardium. The mean velocity of speckles between
two consecutive frames is then calculated from the ratio of the
measured displacement and the time between both acquisitions.
Assuming constant velocities, both methodologies should theoretically measure comparable strain/strain rate values. Nevertheless,
in reality, myocardial motion displays a considerable acceleration –deceleration pattern during the cardiac cycle resulting in intrinsic differences between the two methods.
In concordance with previous clinical and experimental studies,
dobutamine stress resulted in an increase in the TDI-derived strain
and strain rate parameters,5,27,28 whereas the STI values were unchanged when compared with baseline. Additionally, although both
methods seem to correlate well with respect to strain and strain
rate, the Bland–Altman analysis showed that they cannot be
used interchangeably (Figure 3). These findings suggest that STI, a
technique that typically operates at lower temporal resolution
data than TDI, might fail in the accurate detection of myocardial
deformation parameters at high values.
It is well known that the frame-to-frame tracking accuracy
depends on the HR/FR ratio.29 Therefore, one would anticipate
that accurate STI-derived SR measurements rely on the relation
between HR/FR. This was indeed demonstrated in the current
study (Figure 4). As a result, the error in the SR measurements is
lower for lower HR/FR ratios and increases with higher ratios
(e.g. post-dobutamine). As such, lower HR/FR ratios will result in
smaller relative dimensional error over a set of accumulated
motion estimates (e.g. myocardial strain). For higher HR’s, the
HR/FR ratio will increase and this will lead to prominent
decorrelation-related inaccuracy and higher cumulative error of
SR measurements using STI. This would be not true for Doppler
imaging, which makes use of pulse strains at high pulse repetition
frequencies to estimate myocardial velocities and as such, these
velocity estimates are relatively frame rate independent.
A potential solution to the above problem could be increasing
the frame rate. However, frame rate is typically gained at the
cost of lateral resolution which in turn will lead to reduced
lateral definition of speckles, resulting in poorer tracking. A more
narrow sector scan may partially overcome this but limits the
field of view and thus overrules one of the advantages of STI
over TDI. This has relevance to imaging in humans. Although
HRs are lower, so too are the achievable frame rates given the
requirements for larger imaging sectors. Thus, the same principles
are likely to apply whereby TDI-derived techniques will be more
able to resolve true peaks in SR, particularly when HRs are high.
There is support for this premise when the two techniques
have been compared with measure cardiac deformation during
strenuous exercise.30
772
Conductance measurements remain the gold standard for assessing LV contractility. This technique has been previously applied to
mice in order to obtain sensitive measures of contractility, such as
Ees and PRSW.7,31 It has been recently demonstrated that PRSW is
an optimal measure of contractility in mice as it is independent of
pre- and afterload.17 Both TDI- and STI-estimated deformation
parameters correlated well with the invasive haemodynamic
measure of myocardial contractility, the PRSW. The finding that
the TDI-derived strain and strain rate closely reflect intrinsic myocardial contractility is not new. Several experimental studies have
demonstrated the potential of S and SR as sensitive indices for
quantifying LV regional systolic function in small animals under different physiological/pharmacological conditions.5,7,21 However, to
date, this is the first study to report a close correlation between
STI-derived deformation parameters and invasive indices of contractility, implying that the strain and strain rate by STI are sensitive
indices of myocardial contractility in mice.
Finally, the preferable choice of imaging modality for the assessment of murine cardiac function can be adapted by the experimental goals. The utility of STI alone may suffice if the overall goal of
the analysis is to detect the presence of myocardial disease and
if the HR/FR ratio is acceptable. However, the assessment of deformation parameters at higher HR/FR ratios by STI is more challenging and hence, TDI might be the preferable imaging tool when
performing echocardiographic studies in mice.
Limitations
STI- and TDI-derived strain and strain rate parameters were measured only in the circumferential direction. Although radial strain
components could be theoretically assessed form the acquired
parasternal short-axis images, several studies have demonstrated
that radial strain estimation by STI is less accurate and reproducible
which may be related to the smaller area and hence the total
number of speckles available for analysis.32 – 34 Moreover, the anatomical position of the murine heart makes acquiring longitudinal
function from an apical view impractical. Although parasternal
long-axis views can be used for assessing longitudinal deformation
parameters,35 regional strain analysis is not possible in all LV segments. Moreover, the additional value of measuring longitudinal
function in mice may be questioned as it has been previously
demonstrated that the long-axis contribution to heart function is
relatively small in these animals.36
It should be recognized as well that some haemodynamic parameters (such as HR, LVESP and dP/dtmax) were slightly lower than
those previously reported in the literature,5,7 which might be
related to the use of anaesthesia in these animals. Although measurements in awake mice are possible, it requires long conditioning
sessions to train them to endure physical restraint and to avoid
autonomic responses such as bradycardia due to vagal stimulation,
or tachycardia due to sympathetic stimulation. However, this
would not change the main conclusion of the study that acceptable
HR/FR ratios should be achieved for the accurate assessment of
S/SR deformation parameters by STI in mice.
Finally, it should be stressed that assessing myocardial deformation parameters by STI might be already compromised in mice
studied under physiological conditions (awake/lightly sedated) as
there will be a high HR/FR ratio. As such, using TDI might be
V. Ferferieva et al.
more advantageous and accurate when performing echocardiographic studies in conscious mice.
Conclusions
In mice, both STI- and TDI-derived strain and strain rate deformation parameters relate closely to the intrinsic myocardial function.
At low HR/FR ratios, both STI and TDI are equally acceptable for
assessing LV function non-invasively in these animals. At high HR/
FR ratios (e.g. dobutamine challenge), however, these methods
cannot be used interchangeably as STI underestimates S and SR
at high values.
Conflict of interest: None declared.
Funding
This work was supported by grant G.0684.08 of the Research Foundation Flanders (FWO-Vlaanderen)
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