Open Journal of Obstetrics and Gynecology, 2011, 1, 163-167
doi:10.4236/ojog.2011.14031 Published Online December 2011 (http://www.SciRP.org/journal/ojog/ OJOG ).
Published Online December 2011 in SciRes. http://www.scirp.org/journal/OJOG
Comparison of the reproducibility of 2D doppler and 3D
STIC in the measurement of fetal cardiac output
Rajeswari Parasuraman1, Clive Osmond2, David T. Howe1
1Wessex Maternal and Fetal Medicine Unit, Princess Anne Hospital, Southampton, UK;
2MRC Lifecourse Epidemiology unit, Southampton General Hospital, Southampton, UK.
Received 4 September 2011; revised 25 October 2011; accepted 30 October 2011.
ABSTRACT
Objectives: Two methods have been described to as-
sess fetal cardiac output (CO). It has usually been
calculated by using 2D ultrasound to measure the
diameter of outflow valves and Doppler ultrasound to
measure flow velocity through the valves. Recently
CO has been assessed using 3D spatio-temporal im-
age correlation (STIC) to measure stroke volume. We
aimed to compare the reproducibility of these tech-
niques. Methods: In 27 women with singleton preg-
nancies, examinations were performed in three gesta-
tional age groups: 13 - 15, 19 - 21 and >30 weeks of
gestation. Each mother was scanned once. Using 2D
pulsed wave Doppler the duration of flow and avera-
ge flow velocity in systole were measured through
aortic and pulmonary valves. We averaged values f-
rom three consecutive Doppler complexes. The outlet
valve diameters were measured and the cardiac out-
put was calculated for each valve. The measurements
were repeated to assess reproducibility. In the same
women, we acquired STIC volumes of the fetal heart.
The volume measurements were made using the 3D
Slice method by one observer. Using 2 mm slices the
circumference of the ventricles was traced at the end
of systole and diastole to calculate ventricular volume
before and after contractions to calculate stroke vo-
lume and hence cardiac output. The measurements
were repeated to assess reproducibility. Results: The
root mean square difference of log (CO) of repeat
measurements ranged between 0.12 and 0.21 using
Doppler compared to 0.7 to 1.47 using STIC. The
differences in reproducibility reached statistical sig-
nificance for both sides of the heart at all but one
gestation. Conclusions: We found that Doppler asse-
ssment of fetal cardiac output was more reproducible
than measurement using STIC.
Keywords: Cardiac Output; Fetal; STIC; 2D Doppler;
Ultrasound
1. INTRODUCTION
Fetal cardiac output has been measured by various tech-
niques using both 2D Doppler [1-3] and 3D ultrasono-
graphy [3-11]. Cardiac outflow has been estimated by
using Doppler ultrasound to measure the average flow
velocity across the aortic and pulmonary valves and 2D
ultrasound to measure the valve diameters [3].
After the advent of 3D ultrasonography, Hamill et al
suggested it might have advantages over 2D assessment
of cardiac output. Using a single acquired volume ana-
lysed with STIC (spatio-temporal image correlation) and
VOCAL (virtual organ computer aided analysis) they
assessed ventricular volume at end diastole and end-sys-
tole to calculate stroke volume. They felt that these volu-
me assessments would be more accurate than using 2D
methods such as multiplanar assessment and Simpson’s
rule. They felt it might also avoid the variability that
occurs with 2D measurements due to small changes in
measurement of valvular diameter having a large effect
on calculated cardiac output [6].
Uitttenbogaard et al [10] recently described calculation
of cardiac output using 3D spatio-temporal image corre-
lation (STIC) to provide cross-sectional slices of the
heart based on Simpson’s rule [12]. They provided refe-
rence ranges of cardiac output from 12 weeks until term
in a longitudinal study of 63 fetuses and argued that be-
cause of the potential for errors in conventional 2D Do-
ppler assessment of cardiac output calculations from
STIC volumes were likely to become the method of choi-
ce for assessment of fetal cardiac function. Their study
did not compare their method with Doppler assessment
in the same fetuses so we aimed to compare directly the
reproducibility of these techniques from 13 weeks until
term.
2. MATERIALS AND METHODS
This was a cross-sectional observation study of women
undergoing normal singleton pregnancy taking part in a

R. Parasuraman et al. / Open Journal of Obstetrics and Gynecology 1 (2011) 163-167 164
larger study of fetal cardiac function [13]. When women
attended the ultrasound department for routine dating or
anomaly scans they were invited to take part in the study
by coming for an additional research scan. In 27 women
with singleton pregnancies, examinations were perfor-
med in three gestational age groups: 13 to 15, 19 to 21
and more than 30 weeks of gestation. The scans were
arranged so that each woman attended for a single asse-
ssment of fetal cardiac function. The study was appro-
ved by the local research ethics committee.
Two-dimensional fetal echocardiography was perfor-
med using a Toshiba Aplio ultrasound machine. Exami-
nation of blood flow was made by Doppler assessment
of the duration and average flow velocity in systole th-
rough the aortic and pulmonary valves. In order to make
the measurements, the Doppler gate was reduced to its
minimum size (1 mm) and placed centrally just beyond
the outlet valve. The valves ‘clicks’ are evident in the
Doppler flow and help to defin e the opening and closing
of the valves. The ultrasound probe was placed so that
the beam was as near to parallel as possible with the
direction of blood flow and the offset was always less
than 30˚.. The machine settings were used to correct for
the angle of insonation if the beam was not truly parallel.
There is a single peak of flow through the aortic and
pulmonary valves, during cardiac systole and we mea-
sured the average velo city of systolic flow and th e length
of the ejection time (tsyst) as shown in Figure 1. For each
measurement we used the aver age values from three con-
secutive Doppler com plexes.
To measure the outlet valve diameter, the ultrasound
beam was placed perpendicular to the valve to define the
vessel borders clearly and the diameter measured three
times between the valve leaflets, with the average value
used for the calculations. The cross-sectional area of the
outlet valve was calculated from the average measured
diameter.
The stroke volume was calculated as the time avera-
ged velocity × output time × cross-sectional area of the
outlet valve, and the cardiac output as the stroke volume
× heart rate.
In the same women, we acquired STIC volumes of the
fetal heart using a Voluson 730 (GE Medical Systems)
ultrasound machine. The mean number of STIC volumes
for each fetus was 5 with a range from 3 to 7. The STIC
acquisition time was 7.5 seconds. A short acquisition ti-
me was used to limit degradation of the volumes due to
fetal movement during capture. The volumes were stored
and the clearest two analysed by one observer using the
3D Slice method described by Uittenbogaard with the
GE post processing software [10]. We used multiple 2
mm slices (Figure 2) and calculated the volume by a
sum of the areas traced each multiplied by the thickness
(2 mm). The circumference of the ventricles was traced
Figure 1. Doppler waveform of an outlet valve showing
measurement of the length of the e jection time t(syst).
Figure 2. Images of 2 mm slices obtained from STIC volume.
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at the end of systole (just before opening of the atrio-
ventricular valves) and the end of diastole (just after
closure of the atrio-ventricular valves) to calculate ven-
tricular volume before and after contractions to calculate
stroke volume and hence cardiac output. We followed
Uittenbogaard’s technique by tracing ventricular con-
tours on the echogenic side of the endocardial border
and considered papillary muscles to be part of the ven-
tricular cavity. The measurements were repeated on a se-
parately acquired STIC volume to assess reproducibility.
3. RESULTS
We included 10 women in the 13 week - 15 week gesta-
tional period, 8 women between 19 week - 21 weeks and
9 women after 30 weeks of gestation. All fetuses had
biometric measurements within the normal range for ge-
station at the time of the scan. For each gestation and te-
chnique.
We calculated the root means square of the differences
in repeat measurements and these are shown in Tabl e 1 .
Using 2D echocardiography, the root mean square dif-
ference (rms) of log (cardiac output) across the gesta-
tional age group 13 to 15 weeks was 0.18 on the left and
0.17 on the right side. The rms across the 19 to 21 weeks
group was 0.16 on the lef t and 0.21 on the right whereas
the respective values in the >30 weeks gestation group
were 0.12 and 0.14 using 2D echocardiography. Doppler
measurements were achieved in 26 of the 27 fetuses e-
xamined.
The rms for the STIC method at 13 to 15 weeks was
0.70 for the left ventricle and 1.07 for the right. on the
left and right sides in the 13 to 15 weeks gestation group,
At 19 to 21 weeks it was 1.47 and 1.39 and at >30 weeks
it was 0.62 and 0.9 for the left and right ventricles re-
spectively. Volume measurements were not possible with
STIC in 2 fetuses (8%), one in the 13 to 15 week gesta-
tional group and the other in the 19 to 21 week group as
the clarity of the views in the captured volumes were
suboptimal.
We found that Doppler assessment of fetal cardiac
output was more reproducible than measurement using
STIC as the rms values are much higher in the latter
method. We demonstrated that this reached statistical si-
gnificance (p < 0.05) in all but one of the gestation
groups on both left and right sides of the heart (Table 1).
Between 13 weeks - 15 weeks the probability approa-
ched statistical significance for the right side of the heart
at the 5% level (p = 0.08) and achieved significance for
left heart measurements.
4. DISCUSSION
Rizzo et al. determined the stroke volume of normal fetu-
ses at 20 w eeks - 22 we eks and 28 w eek s - 32 we eks a nd
growth restricted fetuses at 26 weeks - 34 weeks of ges-
tation and reported that there is a good agreement be-
tween 2D Doppler and 4D STIC measurements. They
obtained the outlet valve measurements between the
open aortic and pulmonary valves. They calculated the
time velocity integral by placing the Doppler gate distal
to the outlet valve leaflets with an angle of insonation <
20 and used recordings from six consecutive waveforms.
They reported that the 4D STIC method was less ope-
rator dependent and was more time efficient than the 2D
Doppler but they also noted that fetal movement and
reduced amniotic fluid might affect the quality of the
cardiac volume acquisition [3].
Uittenbogaard et al. [14] reported fetal cardiac volu-
metry in both in-vivo an d in-vitro settings. They used the
volume datasets from fetuses between 16 and 30 weeks
of gestation and a miniature balloon model. Volume cal-
culations were performed using the 3D Slice method and
they reported that measurement errors in the fetus de-
creased with increasing operator experience and that the
reliability was better for stroke volume than for ejection
fraction. They noted that there was variation in measu-
rement of the end-systolic and end-diastolic volumes
which became more noticeable in calculating the ejec-
tion fraction as it included three volume measurements.
Measurement of the volume of the fetal cardiac ven-
tricle has been described by Messing et al. [7] using 4D
STIC combined with the inversion mode after 20 weeks
of gestation. They used this to calculate stroke volume
and ejection fraction and reported that their intra and
Table 1. Root mean square (rms) of repeat m easurements of cardiac output in right and left ventricles assessed by 2D Doppler or 3D STIC.
Right ventricle Left ventricle
Gestation 2D Doppler (rms) 3D STIC (rms)probability (p value)2D Doppler (rms) STIC (rms) probability (p value)
13-15 weeks (n = 10) 0.17 1.07 0.083 0.18 0.7 0.003
19-21 weeks (n = 8) 0.21 1.39 0.001 0.16 1.47 0.001
>30 weeks (n = 9) 0.14 0.9 0.000 0.12 0.62 0.000
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interobserver agreement reached 96%. Their method was
not suited to use in early gestation since it was not pos-
sible to use their method in fetal heart volumes under 20
weeks of gestation whereas fetuses >37 weeks of gesta-
tion posed different problems of unfavourable fetal posi-
tion and relative oligohydramnios.
Molina et al. [8] have established reference intervals
for stroke volume and cardiac output using the VOCAL
technique in pregnancies at 12 weeks - 34 weeks of ges-
tation. Their values for cardiac output were lower than
previous reports which used 2D Doppler techniques. They
also reported that the reproducibility of volume measu-
rements is poor in the first trimester and in late gestation
when the myocardium appears thicker. Similar tech-
niques using VOCAL have been described by Simioni et
al. [11] from 20 weeks - 34 weeks of gestation with the
intra- and interobserver agreement reaching 95%.
Ventricular volume, stroke volume, cardiac output and
ejection fraction have been evaluated from 19 to 42
weeks of gestation by Hamill et al. [6] with 4D ultra-
sound using both STIC and VOCAL techniques. They
commented on the limitations of using a computer gene-
rated dataset, the learning curve involved and the sig-
nificant amount of time needed to analyse the data.
When compared directly we found that cardiac output
calculation by 2D Doppler was more reproducible than
the 4D STIC slice method described by Uittenbogaard
[10]. In the latter method, we found the volumes were
difficult to acquire and analyse at early and late gesta-
tions for the same reason s mentio n ed in previou s stud ies.
Fetal movement artefacts affected measurements in early
pregnancy (1 3 weeks - 15 w eeks subgroup) and fetal po -
sition caused shadowing of the heart after 30 weeks.
Uittenbogaard et al. [10] were only able to acquire STIC
volumes in 71% of women attempted and they exclu-
ded fetuses examined after 30 weeks as they only had
three technically acceptable datasets. In addition, the
process was time consuming and accurate tracing of the
ventricular borders was difficult introducing errors into
calculation of stroke volume and hence cardiac output.
Our data do not support their suggestion that their tech-
nique will become the method of choice for assessing
fetal cardiac function.
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