Key words
fetal echocardiography - speckle tracking - cardiac function - wall motion tracking
Introduction
Initially fetal echocardiography was used for the identification of structural congenital
heart diseases [1]
[2]. Attempts to find improved methods of characterizing and risk stratifying fetuses
with cardiovascular compromise, e. g., twin-twin transfusion syndrome, fetal tumors,
hydrops fetalis, congenital heart disease, fetomaternal incompatibility, have turned
to measures of myocardial function [3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]. Quantification of fetal myocardial function is still challenging. To date, it has
been assessed by using conventional B-mode, spectral and color Doppler interrogation
or M-mode.
With the recent introduction of speckle tracking echocardiography (STE), a promising
tool has been found to evaluate global and regional myocardial function in the fetus.
This non-Doppler and angle-independent technique allows the quantification of myocardial
dynamics and deformation in a chosen myocardial region of interest (ROI) that is based
on post-processing 2D image frame-by-frame analysis. Speckles, caused by the interference
of energy from randomly distributed scatter echoes in the myocardium, create fine
false structures, called “speckle noise”. The speckles move with the tissue and can
be followed over sequential frames.
Different ultrasound systems and software solutions have been used for STE mostly
to establish normal values in healthy fetal [11]
[12] and pediatric populations [13]
[14]. A few studies have been focused on special fetal conditions like twin-to-twin-transfusion
syndrome [7] and intraamniotic infection [15], while other studies concentrated on cardiac anomalies [16]
[17].
Although there are limitations in the application of this tool to fetuses due to fetal
heart size, high heart rates and maternal and fetal movement during acquisition, several
studies have been performed and report good reproducibility and feasibility [18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]. However, published data in this field is very inconsistent with partially massively
differing strain and strain rate data [24]
[27]
[28]
[29]. Vendor-dependent speckle tracking imaging is usually based on grayscale B-mode
images of endocardial and/or epicardial borders. Wall motion tracking (WMT) technology
enables not only endo- and/or epicardial border tracking but tracking of the whole
myocardium. As far as we know, this technique has not been investigated yet in fetal
echocardiography.
It has been shown that deformation parameters assessed by different ultrasound systems
and software packages (e. g., Automated Function Imaging, GE Healthcare, Waukesha,
WI [12]
[19]
[26]
[27]; Velocity Vector Imaging (VVI), Siemens Healthcare, Erlangen, Germany [30]
[31]
[32]) are often not comparable [33]. Furthermore, the frame rates of the acquired video loops show a huge variation.
Archived Digital Imaging and Communications in Medicine (DICOM) data with 30 frames
per second (fps) was sometimes used for analysis [34]
[35]. Other groups worked with the original frame rate with a frequency from 44 fps up
to more than 150 fps [36]
[37]
[38]
[39]
[40].
The primary objective of this study was to determine the feasibility and reproducibility
of 2D speckle tracking imaging based on the WMT technique. The secondary objective
of this study was to compare left- and right-ventricular global and segmental longitudinal
peak strain values.
Methods
Study population
The study population of this prospective cross-sectional study included fetuses selected
from women who were referred for fetal echocardiography to the fetal heart program
at the Department of Fetal Diagnosis & Therapy, University Hospital Giessen and Marburg,
from April 2014 – September 2014. The institutional review board approved this study
and participants provided their written informed consent. Exclusion criteria were
structural or chromosomal anomalies, twin pregnancies and conditions with a possible
effect on fetal hemodynamics, e. g., evidence of fetal infection, maternal diabetes,
preeclampsia, preterm labor, endocrinological disorders (e. g., thyroid disease).
All patients underwent a full morphological examination of the fetal heart in order
to exclude any congenital heart defects. According to the study protocol, all patients
were examined once.
Echocardiography
In every patient a complete transthoracic fetal echocardiography was performed by
one experienced operator (C. E.) on a Toshiba Artida system (Toshiba Medical Systems
Corporation, Otawara, Tochigi, Japan). To obtain video loops of a high resolution,
zoomed B-mode of an apical 4-chamber view, a 1–5 MHz curved array probe (PVT 375 BT)
was used. To achieve high frame rates, the B-mode image depth was reduced and the
sector width was narrowed. For storage in raw data format, a concurrent 60 Hz dummy
electrocardiogram (ECG) signal (phantom 320, Mueller & Sebastiani Elektronik GmbH,
Ottobrunn, Germany) was necessary.
According to a strict protocol for every patient, at least 3 video loops of a 4-chamber
view, each with a duration of 2 s, were acquired for the left and the right ventricle.
To ensure high image quality, attention was taken with respect to clear delineation
of the right ventricular (RV) and left ventricular (LV) free wall as well as of the
interventricular septum (IVS). The cine loops were digitally stored with the acquisition
frame rate.
Wall motion tracking technology
Speckle tracking is an application of pattern matching technology to ultrasound cine
data. A template image is created using a local myocardial region in the starting
frame of the image data. In the next frame an algorithm searches for the local speckle
pattern that most closely matches the template. A movement vector is then created
using the location of the template and the matching pattern in the subsequent frame.
Multiple templates are used to observe movement of the entire myocardium. The process
is then repeated by creating new templates and observing their movement in the subsequent
frames until the entire cardiac cycle has been assessed. This method does not make
use of Doppler information, so there is no Doppler angle dependency [41].
Strain analysis
The offline analysis was performed by 2 operators on a workstation equipped with the
TestDriver software (Toshiba Medical Systems Corporation, Japan). Apical 4-chamber
views of good quality 2D B-mode cine loops were chosen, either displaying the right
(right free wall and IVS) or the left (left free wall and IVS) ventricle. The frame
rate for analysis was 60 fps according to DICOM standard. Without the possibility
of acquiring a fetal ECG, one fetal heart cycle was identified for analysis by the
movement of the atrioventricular (AV) valve. End-diastole was defined by complete
closure of the AV valves. The time cursor was set firstly with closure of the AV valves
and secondly just before the next AV valve closure. The fetal heart rate was calculated
on the basis of a heart cycle duration. Based on heart rate and fps the frames per
cycle (fpc) were assessed.
Strain measurements of the left and right ventricle were taken either from the same
clip or from another clip if the sector width had limited the display window to 1
ventricle only. In end-diastole, endocardial markers were set along the endocardium.
Beginning either just above the lateral or septal AV valve annulus, several markers
were set along the endocardium in a counterclockwise direction up to the apex and
back to the septal or lateral AV valve annulus. Automatically the endocardial border
of a ventricle was traced (inner line) and an outer line parallel to the inner line
delimitates the epicardium ([Fig. 1], left). The myocardial wall was detected with the possibility of manual adjustment
of the myocardial thickness in the case of mismatching. After selection of the markers
and myocardial tracing, the 2D WMT analysis of all patterns inside the user-defined
region of interest (ROI) was performed through the stored fetal cardiac cycle. Accuracy
of tracking was subjectively verified. Cases in which satisfactory tracking could
not be obtained after several attempts were classified as inadequate and excluded
from data analysis.
Fig. 1 (Left) traced myocardial wall of the LV and interventricular septum with 6 segments
(BL (basal lateral), ML (middle lateral), AL (apical lateral), AS (apical septal),
MS (middle septal), BS (basal septal)). (Right) longitudinal strain (%) curves of
the 6 segments and global strain (%) for one fetal heart cycle.
According to the software setup, the LV and RV myocardium were automatically divided
into 6 segments, 2 basal, 2 middle and 2 apical ones, in each ventricle with 3 lateral
free wall and 3 septal segments. The LV and RV each contained the lateral free wall
and the IVS.
Based on changes in speckle location in the data set, the longitudinal strain for
each segment was calculated. The Lagrangian strain, which is the difference in the
end-diastolic and end-systolic length of the inner ventricular contour, is assessed
with WMT technology. The results were displayed graphically as well as in numeric
format for all segmental data ([Fig. 1], right).
Statistical analysis
To carry out comparisons between groups, either a general regression model or a random
intercept model was used. The test was performed using the SPSS procedure MIXED (random
intercept and random slopes model). Myocardial function parameters with independent
data were analyzed with a general linear regression model (ANCOVA). If the analysis
showed a dependency of data, the parameters were analyzed with the SPSS procedure
MIXED (random intercept model). For both the linear regression model and the random
intercept model, gestational age was considered as a covariate. In addition, a linear
regression analysis was performed in order to investigate a possible influence of
gestational age studied on mean global strain values of both ventricles.
The intraobserver and interobserver variability of the echocardiographic measurements
were assessed in a subset of 10 echocardiograms from randomly selected patients at
various times. 2 operators (C.E. and F.A.) analyzed the same images independently.
The intraclass correlation coefficient (ICC 2-way-random, absolute agreement, single
rater) was used for interobserver variability. The analysis of intraobserver variability
was performed with Cronbach’s alpha. Values of 0.7–0.8 for the intraclass correlation
coefficient or Cronbach’s alpha indicate good agreement and values>0.8 strong agreement
between measurements.
All values were considered significantly different at p<0.05.
Results
33 fetuses were included in the study. Every patient was examined once in pregnancy.
Speckle tracking analysis could be performed in 29 patients (88%). 4 patients, in
which speckle tracking measurements could not be successfully obtained, were excluded
from data analysis. The mean gestational age was 26.5±4.9 weeks (range 18.3–36.6 weeks),
the mean fpc was 26 (range 23 – 30 fpc). The global strain values for LV and RV were
−16.34% and −14.65%, respectively ([Table 1]). The difference between both ventricles was statistically significant (p<0.001).
The highest mean segmental strain values were found in the apical segment of the lateral
free wall of the left and right ventricle, whereas the basal segments presented with
the lowest values. Within the ventricular septum of both ventricles, higher mean segmental
strain values were detected at the apex compared to the basal part of the ventricular
septum. Segmental strain analysis revealed a basis to apex gradient with the lowest
strain values in the basal and the highest strain values in the apical segments.
Table 1 Mean segmental and global strain values and the 95% confidence interval for left
ventricular (LV) and right ventricular (RV) free wall and the interventricular septum,
analyzed either from the LV or the RV.
|
Segment
|
Strain (%)
|
95% CI
|
LV free wall
|
Basal
|
−14.79
|
−15.93 –(−13.66)
|
Middle
|
−16.12
|
−17.26 –(−14.99)
|
Apical
|
−23.53
|
−24.67 –(−22.40)
|
RV free wall
|
Basal
|
−13.22
|
−14.49 –(−11.96)
|
Middle
|
−15.10
|
−16.37 –(−13.84)
|
Apical
|
−19.51
|
−20.78 –(−18.25)
|
Septum LV
|
Basal
|
−12.06
|
−13.19 –(−10.93)
|
Middle
|
−14.73
|
−15.86 –(−13.60)
|
Apical
|
−23.10
|
−24.23 –(−21.97)
|
Septum RV
|
Basal
|
−12.85
|
−13.79 –(−11.91)
|
Middle
|
−13.97
|
−14.91 –(−13.03)
|
Apical
|
−19.04
|
−19.97 –(−18.10)
|
Global strain LV
|
Global
|
−16.34
|
−16.94 –(−15.75)
|
Global strain RV
|
Global
|
−14.65
|
−15.19 –(−14.12)
|
[Table 2] displays the results of a comparison of the basal, middle and apical strain values
among each other. Significant differences were depicted within all segments of LV,
RV and IVS (p<0.05). Linear regression analysis revealed no significant influence
of the gestational age studied on mean global strain values of both ventricles (p=0.44
for LV, p=0.44 for RV).
Table 2 Comparison of basal, middle and apical strain values among each other for left ventricular
(LV) and right ventricular (RV) free wall and the interventricular septum, analyzed
either from the LV or the RV.
|
Segments
|
Strain (%)
|
∆ mean values
|
p-value
|
LV
|
Basal vs. middle
|
−14.79 vs. −16.12
|
−1.33
|
0.023
|
Basal vs. apical
|
−14.79 vs. −23.53
|
−8.74
|
0.000
|
Middle vs. apical
|
−16.12 vs. −23.53
|
−7.41
|
0.000
|
RV
|
Basal vs. middle
|
−13.22 vs. −15.10
|
−1.88
|
0.005
|
Basal vs. apical
|
−13.22 vs. −19.51
|
−6.29
|
0.000
|
Middle vs. apical
|
−15.10 vs. −19.51
|
−4.41
|
0.000
|
Septum LV
|
Basal vs. middle
|
−12.06 vs. −14.73
|
−2.67
|
0.000
|
Basal vs. apical
|
−12.06 vs. −23.10
|
−11.04
|
0.000
|
Middle vs. apical
|
−14.73 vs. −23.10
|
−8.37
|
0.000
|
Septum RV
|
Basal vs. middle
|
−12.85 vs. −13.97
|
−1.13
|
0.047
|
Basal vs. apical
|
−12.85 vs. −19.04
|
−6.19
|
0.000
|
Middle vs. apical
|
−13.97 vs. −19.04
|
−5.06
|
0.000
|
Within 10 fetuses of the whole study population, the intra- and interobserver variability
was assessed. For both examiners, in all cases Cronbach’s alpha was >0.7 ([Table 3]). The interobserver variability showed a strong agreement in 50% of the segments
(ICC 0.71–0.90 in 6/12 segments). There was also a strong agreement of global RV and
LV peak strain values ([Table 4]).
Table 3 Analysis of intraobserver variability with Cronbach’s alpha.
Segments
|
Examiner 1
|
Examiner 2
|
Right ventricle (RV)
|
Basal free wall
|
0.955
|
0.943
|
Middle free wall
|
0.832
|
0.962
|
Apical free wall
|
0.949
|
0.938
|
Basal septal
|
0.924
|
0.993
|
Middle septal
|
0.754
|
0.984
|
Apical septal
|
0.887
|
0.944
|
Global RV
|
0.956
|
0.972
|
Left ventricle (LV)
|
Basal free wall
|
0.718
|
0.879
|
Middle free wall
|
0.921
|
0.974
|
Apical free wall
|
0.886
|
0.968
|
Basal septal
|
0.912
|
0.885
|
Middle septal
|
0.957
|
0.932
|
Apical septal
|
0.788
|
0.954
|
Global LV
|
0.966
|
0.982
|
Table 4 Analysis of interobserver variability with intraclass correlation coefficient (ICC).
Segments
|
ICC
|
Right ventricle (RV)
|
Basal free wall
|
0.886
|
Middle free wall
|
0.703
|
Apical free wall
|
0.905
|
Basal septal
|
0.886
|
Middle septal
|
0.692
|
Apical septal
|
0.774
|
Global RV
|
0.892
|
Left ventricle (LV)
|
Basal free wall
|
0.464
|
Middle free wall
|
0.700
|
Apical free wall
|
0.479
|
Basal septal
|
0.871
|
Middle septal
|
0.946
|
Apical septal
|
0.352
|
Global RV
|
0.890
|
Discussion
First, this study shows good reproducibility and feasibility of longitudinal strain
analysis with high intra- and interobserver correlations in fetal echocardiography.
The success rate of 88% that we achieved in our speckle tracking measurements is comparable
to already published data [20]
[22].
In recent years many different techniques, e. g., tissue Doppler imaging (TDI), have
been used to investigate fetal myocardial function. By the use of TDI, it was possible
to assess annular or myocardial velocities online as well as to assess strain, strain
rate and cardiac time intervals offline by post-processing analysis [9]
[10]
[42]. One of the major criticisms of TDI has been the directional bias of the technology
[43]
[44]
[45]
[46]
[47].
With 2D STE a relatively angle-independent technique was introduced with the possibility
of determining myocardial motion (translation and rotation) and deformation indices:
myocardial thickening and thinning, cardiac torsion [48]
[49]
[50] including apical twisting [51], radial motion and local thickening. Validation of 2D STE based on grayscale B-mode
images of endocardial and epicardial borders with sonomicrometry and magnetic resonance
imaging in adult and pediatric populations revealed 2D STE to be a reliable method
to assess myocardial function [18]
[52]
[53]. Langeland et al. could assess a good agreement between ultrasound and sonomicrometry
with an intraclass correlation coefficient of 0.80 for longitudinal components [54]. However, not only endo- and epicardial border tracking has been validated. Ishizu
et al. validated LV transmural strain measured by speckle tracking imaging against
sonomicrometry and reported good agreement between both techniques [55].
Our results are comparable with already published data, using different technologies,
reporting good reproducibility for the application of speckle tracking imaging in
the fetus. For their second trimester ultrasound-derived reference values, Kapusta
et al. revealed good or excellent ICCs for intra- and interobserver variance in most
cases [26]. Ta-Shma et al., who analyzed segmental and global fetal myocardial function by
automatic functional imaging, achieved an interobserver and intraobserver variability
that showed only a small bias among the observers with narrow 95% confidence intervals
[19]. Longitudinal deformation measurement seems to be more reproducible than others,
e. g., radial strain. Koopman et al. compared different speckle tracking and color
Doppler techniques to measure global and regional myocardial deformation in children.
For LV longitudinal strain measurements, they revealed the highest reproducibility,
smallest bias, and most narrow limits of agreement for the different techniques [33].
Second, our study demonstrates a significant gradient of deformation from base to
apex for longitudinal strain in both ventricles. All 3 segments (basal, middle and
apical) of both lateral free wall and IVS were significantly different from each other.
The apical segments of LV and RV free wall as well as of the IVS, independent of acquisition
from left or right, showed significantly higher strain values. Previously published
data about segmental strain differences is inconsistent. Several authors reported
an apex to base gradient with significantly greater strain values either in basal
segments of both ventricles [37] or in the RV only [26]
[29]
[38]. Other groups did not reveal significant differences between the segments [32]. Our findings are in agreement with those of Marcus et al. [56]. They established reference values for myocardial 2D strain echocardiography in
a healthy pediatric and young adult cohort. Not only in their youngest patient group
(<1 year) but also in the other groups they revealed the highest longitudinal peak
systolic strain values in the apical segments. The results are also comparable to
those of Bussadori et al., who also demonstrated a base to apex gradient in their
pediatric and adult population [57]. In their fetal second and third trimester population, Kim et al. also demonstrated
greater strain values in the LV and the IVS [38]. Depending on different imaging techniques, variable results have been published
according to left ventricular strain. In tagged MRI and 2D STE studies, higher apical
than basal or middle segmental strain values have been reported, whereas tissue Doppler
studies did not show a significant variation from base to apex [58]
[59]
[60]
[61]. This evidence was independent of the ultrasound scanner and software packages.
Beside the technical impact of 2D STE using WMT technology, it might be speculated
that there is a physiologic substrate in which the base-to-apex gradient is the result
of the torsional mechanism of the left ventricular system and the direction of contraction
of the descending fibers in the internal loop of the highly structured 3-dimensional
(3D) network of myocardial cells [56]. The recent introduction of 3D STE might offer more accurate and reproducible tracking
to confirm these findings. Like in 2D STE, the 3D technique was developed for strain
measurement in the adult heart. Enzensberger et al. presented preliminary results
of their first application of 3D WMT in fetal echocardiography. Unfortunately, due
to the lack of high-frequency transducers and algorithms designed especially for fetal
echocardiography, this is currently only possible in individual fetuses [62].
The 3-dimensional architecture of the myocardium and its complex motion during the
cardiac cycle lead to difficulties in tracking within a 2D plane because of the speckles
moving out of the 2D plane into the next frame. Consequently, only part of the real
myocardial motion can be detected up to impossible tracking [45]
[63]. Therefore, different authors report the requirement of high frame rates to ensure
adequate tracking. D’hooge recommends>80 frames per second (fps) in healthy adult
hearts with a normal heart rate for an adequate assessment of motion and deformation
parameters [64]. In fetal 2D STE low frame rates coupled with high heart rates have been a major
problem. Many studies used frame rates of 30 fps for deformation analysis [20]
[21]
[24]
[30]
[65]. It could be demonstrated that higher frame rates revealed both an increased success
rate for attaining adequate tracking and analysis and an increase in absolute strain
and strain rate parameters [22]. In the present study all echocardiographic images were stored automatically at
a frame rate of 60 Hz and the tracking quality was visually acceptable. The frame
rate per heart cycle (fpc) seems to be more important than the absolute frame rate.
As Roesner et al. could demonstrate in simulated data, accurate strain estimates could
be achieved at>30 fpc for longitudinal and circumferential strains [66]. Analysis of synthetic imaging data revealed that an insufficient fpc leads to systematic
underestimation of strain values. Although the global peak systolic longitudinal strain
acquired with 15–25 fpc in patient, which is below our mean fpc, data was not significantly
different from a reference group acquired with 46–65 fpc [66].
Our global and segmental peak systolic strain values assessed by WMT technology were
lower compared to other published data. One reason can be a higher frame rate used
by acquisition compared to the 60 Hz we used in our study. Roesner et al. revealed
a 2-fold higher longitudinal strain in the endocardium compared with an epicardial
ROI [66]. Kim et al., who also stored their images at 60 Hz, achieved slightly higher strain
values [38]. Other authors assessed their strain analysis at 30 Hz, showing LV strain values
which are comparable to our data [24]
[34]
[35]. To avoid artifacts due to fetal movement or change of cardiac cycle, we used only
one cardiac cycle for analysis instead of averaging over multiple cycles. This might
also be a reason for the lower peak systolic strain values.
In the present study, we revealed statistically significant differences in global
longitudinal peak systolic strain between LV and RV, with higher strain values in
LV. This is in contrast to some previously published data. While some authors report
higher strain values in the RV [25]
[38], other authors did not find any difference among both ventricles [26]
[28]. The different techniques of acquisition might be responsible for this inconsistency
in the published data.
For our study population regression analysis revealed stable RV and LV global peak
strain values throughout pregnancy. These results are similar to already published
data assessed with VVI [21]
[24]
[30]
[38]. Another recent study using the automated function imaging technique also demonstrated
stable global strain values [19]. Other authors reported a statistically significant decrease in the mean global
longitudinal strain of the RV, while the global strain of the LV remained constant
[25]
[27]. As many studies have already shown, cardiac growth mainly occurs by myocyte enlargement
rather than its proliferation in mid and late gestation [67]
[68]. Therefore, stable systolic strain values in the course of pregnancy might be explained
by the fact that the number of myocytes per ventricular wall volume remains unchanged
with advancing gestational age [38].
Our study was limited by the small number of patients.
To the best of our knowledge, this is the first report on the feasibility of the application
of 2D STE using the WMT technology in fetal echocardiography. The aim of this study
was not to establish reference values.
Originally developed for strain measurement in the adult heart, further development
of STE is necessary to improve its application in fetal echocardiography and to make
data more comparable between different ultrasound scanners and software packages.
A first step forward was made by the cooperation of the European Association of Cardiovascular
Imaging (EACVI), the American Society of Echocardiography (ASE) and a task force of
interested vendors to reduce the inter-vendor variability of strain measurement by
initiating standardized deformation imaging. A first consensus paper was recently
published [69].
With further progress in research and technical development, STE might be a helpful
tool in the clinical management of pathologies with the focus on strain analysis like
intrauterine growth restriction or congenital heart diseases.