Original Paper
Published online: February 4, 2015

Fetal Diagn Ther
DOI: 10.1159/000371514

Assessing Fetal Cardiac Function by
Measuring Myocardial Radial Velocity
Using the Phased-Tracking Method
Katsusuke Ozawa a, b Jun Murotsuki a, b Susumu Miyashita a, b Jin Muromoto a, b
Hideyuki Hasegawa c, d Hiroshi Kanai c, d Nobuo Yaegashi eaDepartment of Maternal and Fetal Medicine, Miyagi Children’s Hospital, b Department of Advanced Fetal andDevelopment Medicine, Graduate School of Medicine, c Department of Biomedical Engineering, Graduate School of
Biomedical Engineering, d Department of Electronic Engineering, Graduate School of Engineering, ande
Department of Gynecology and Obstetrics, Tohoku University, Sendai, Japan

Abstract
Objective: This study aimed to assess the cardiac function ofhealthy and pathological fetuses by measuring radial velocity using phased tracking (PT). Based on phase differences,
PT allows the displacement of a specified point to be detected with improved spatial and temporal resolution. Methods:
PT was used to assess cardiac radial velocity in the basal freewall of the left and right ventricles in 134 healthy fetuses, 10second-trimester intrauterine growth-restricted (IUGR) fetuses, and 10 recipient twins with twin-to-twin transfusion
syndrome (TTTS). Maximum velocities were measured in systole and early diastole. Results: Maximum radial velocity was
successfully measured in 126 healthy fetuses (94%) at gestational ages of 16–40 weeks. Systolic and early diastolic maximum velocities increased with gestational age in both ventricles. As compared with controls, IUGR fetuses had significantly lower early diastolic maximum velocities in the rightventricle, and recipient twins with TTTS had significantly
lower systolic and early diastolic maximum velocities in both

© 2015 S. Karger AG, Basel
1015–3837/15/0000–0000$39.50/0
E-Mail karger@karger.com
www.karger.com/fdt

ventricles. Conclusions: PT demonstrated right ventricular
diastolic dysfunction in second-trimester IUGR fetuses aswell as systolic and diastolic dysfunctions in both ventricles
in recipient twins with TTTS. PT could be useful for evaluating fetal cardiac radial function.
© 2015 S. Karger AG, Basel

Introduction

In adults, cardiac function can be accurately – thoughinvasively – evaluated by measuring parameters such as
ventricular contractility (dP/dt), cardiac output, andpulmonary capillary wedge pressure through cardiac
catheterization [1]. Furthermore, invasive evaluation
techniques have been replaced by ultrasonography using
M-mode and Doppler methods. Tissue Doppler andspeckle-tracking techniques of measuring myocardial velocity have recently come into use, and it has been reported that myocardial velocity correlates well with systolicand diastolic performances that are assessed by cardiac
catheterization [2, 3].
Cardiac dysfunction occurs in fetuses with intrauterine growth restriction (IUGR) and twin-to-twin transfusion syndrome (TTTS), affecting life expectancy and neuKatsusuke Ozawa, MD
Department of Maternal and Fetal Medicine
Miyagi Children’s Hospital
4-3-17 Ochiai, Aoba-ku, Sendai, Miyagi 9893126 (Japan)
E-Mail ozawa2009 @ topaz.plala.or.jp

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Key Words
Phased-tracking method · Fetal cardiac function · Myocardial
velocity · Myocardial radial velocity ·Twin-to-twintransfusion syndrome · Intrauterine growth restriction

Methods
PT Method
Using ultrasonic diagnostic equipment (Aloka Prosound F75;
Aloka Co., Ltd., Tokyo, Japan), the direction of the ultrasonicbeam passing through the measurement points was selected in thefour-chamber view B-mode cross-sectional images so as to be almost perpendicular to the right (RV) and left ventricle (LV) during
measurement. Just after selection, the direction of the ultrasonicbeam was fixed and the beam-formed radio frequency (RF) signal
was converted from analog to digital (A/D) at a sampling frequency of 20 MHz during several heartbeats. The ultrasound system

2

Fetal Diagn Ther
DOI: 10.1159/000371514

used in the present study (Aloka F75) was modified by the manufacturer to allow the acquisition of RF signals. The acquired RF
signals were processed offline to obtain the M-mode image, velocity, and other parameters. The principles of the PT method, including theoretical and in vivo evaluations of the myocardial layerthickening rate, have been previously described in detail [9, 10].
In brief, RF pulses with an angular frequency of ω0 = 2 π f0
(f0 = 5 MHz) are transmitted at time intervals of ΔT from an ultrasonic transducer on the maternal abdominal wall. The phasedifference, Δθ(x; t), between the phase θ(x; t) of the quadraturedemodulated signal of the received signal, y(x; t), and the phaseθ(x; t + ΔT) of the quadrature-demodulated signal z(x; t +ΔT) of
the subsequently received signal, y(x; t + ΔT), is given byΔθ(x; t) = θ(x; t + ΔT) – θ(x; t) = – (2ω0)/c0 · Δx(t),(1)

where Δx(t) = x(t + ΔT) – x(t) is the movement of the object during the short period ΔT around time t, and c0 is the acoustic velocity in the soft tissue. In the present study, the period ΔT was set at
238 μs. That is, the frame rate was 421 Hz, so that the measurable
velocity was set at a fair limit of 0.32 m/s (at f0 = 5 MHz). By dividing the movement Δx by the period ΔT, the average velocity v(t +ΔT/2) of the object during the period ΔT is given byv(t + ΔT/2) = (Δx(t)) / ΔT = c0 (2ω0) · Δθ(x; t)/ΔT.

(2)

The phase difference Δθ(x; t) is accurately determined by theconstrained least squares approach, based on the complex crosscorrelation between z(x; t) and z(x; t + ΔT), under the condition
that the signal waveforms only change in terms of phase values
during the period ΔT. It is impossible to accurately determine thetrue lag value from the complex correlation function, which is derived from the standard normalized mean squared difference between the quadrature-modulated signals of the successively received signals [13]. However, the introduction of the constraint iseffective for determining the lag between these complex signals. By
multiplying the resultant velocity v(t + ΔT/2) by the period ΔT, thenext depth x(t + ΔT) of the object is estimated by
Δx(t + ΔT) = x(t) + v(t + ΔT/2) · ΔT.

(3)

Using the resultant signal x(t + ΔT), the displacement of theobject (the position of the region of interest) in the heart wall is
successfully tracked, and the velocity signal v|Ax(t) of the large
motion due to the heartbeat is accurately measured.
In conventional ultrasonographic techniques, deviations of apoint of interest that are smaller than the wavelength cannot be
detected because they are measured based on the amplitude of thereflected waves. For this reason, the precision of distance measurements in the transmitted beam direction is 0.5–1 mm at a frequency of 3–5 MHz, which is used in fetal measurements. However, themeasurement principle of the PT method differs from that of conventional methods. In the PT approach, an ultrasonic pulse istransmitted from the ultrasound probe and reflected by the cardiac wall with slight movement. By the time it returns to the probe,a phase delay has occurred in the ultrasound pulse due to propagation length. By detecting the phase difference between two ultrasonic pulses that are continuously transmitted and received, it ispossible to estimate displacements of a point of interest that aresmaller than the wavelength with a velocity measurement precision of 0.1 mm/s and a distance measurement precision of 0.5 μm[10]. In this study, a high-speed A/D converter was used to acquire
beam-formed RF signals, which were received by a 5-MHz convex-

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Hasegawa/Kanai/Yaegashi

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rological prognosis. Accurate methods of assessing fetal
cardiac function are required in cases of IUGR and TTTS.
Since invasive examination methods cannot be performed in fetuses, noninvasive methods have been used
to evaluate cardiac function [4–6], such as fractional
shortening and myocardial performance index (MPI, or
Tei index) assessment using ultrasound M-mode andDoppler methods. In addition, tissue Doppler and speckle-tracking techniques have also started to be used tomeasure myocardial longitudinal velocity for cardiac
function evaluations in fetuses [7, 8]. However, the changes to myocardial radial velocity with gestational age areunknown, as are the differences in myocardial radial velocity between normal fetuses, fetuses with IUGR, andrecipient fetuses with TTTS. The movement of the myocardium in the radial direction efficiently changes theventricular volume; therefore, the radial velocity of the
fetal myocardium is an important indicator of cardiac
function. However, because the radial velocity is smaller
than the longitudinal velocity, high spatial and temporal
resolutions are required to measure the radial velocity.
Phased tracking (PT) has been reported to achieve
greater precision than when performing measurementswith conventional ultrasonographic techniques [9, 10].
Further, it has been reported to allow highly precise assessments of doxorubicin-induced myocardial dysfunction in adults [11]. Increases in the myocardial thickening
rate have also been reported in healthy fetuses, indicatingthat such increases occur with gestational age [12]. The
first objective of this study was to measure the myocardial maximum velocity using PT and thereby evaluate
gestational age-related changes in systolic and diastolic
performances in healthy fetuses. The second objective
was to clarify the differences in cardiac function (classified as systolic and diastolic performances) between
healthy fetuses, pathological fetuses with IUGR, and recipient fetuses with TTTS by measuring myocardial maximum velocity using PT.

Color version available online

type ultrasonic transducer connected to modified ultrasonic diagnostic equipment. The use of a converter with large-scale memory
allowed us to collect data for several cardiac cycles.
The lateral four-chamber view is a fetal cardiac screening crosssection that is in wide clinical use. In this study, we rendered a lateral four-chamber view of the fetal heart in the B-mode of the ultrasound diagnostic equipment and transmitted a beam perpendicular to the basal portion of the ventricular free wall at the levelof the tip of the atrioventricular valves (fig. 1). We tracked the radial velocity of the subendocardial myocardium in the LV and RV
for 2 s and determined the average maximum systolic velocity andmaximum early diastolic velocity for 3 or 4 heartbeats in which
there was no fetal movement and the myocardial velocity was reproducible (fig. 2).
Subjects
The study design was approved by the institutional review
board, and each woman provided informed consent before participating in the study.
The study subjects were healthy single fetuses who had an estimated body weight within ±1.5 standard deviations (SDs) of thestandard values of ultrasonographic fetal biometry of the Japan
Society of Ultrasonics in Medicine [14]. Fetuses with structural or
chromosomal abnormalities were excluded. Using PT, maximum
systolic and maximum early diastolic velocities were measured in
134 subjects at 16–40 weeks of pregnancy (mean ± SD, 26.4 ± 6.1
weeks). The parameters were measured in both the LV and RV,
and the correlation with gestational age was examined. Furthermore, we used PT measurements to examine differences between
the LV and RV in terms of maximum systolic velocities and maximum early diastolic velocities with adjustments for gestational age.
Intraobserver agreement was evaluated using the intraclass correlation coefficient (ICC) in 10 subjects at 17–37 weeks of pregnancy.
Interobserver agreement was evaluated for two observers using the
ICC of 10 subjects at 22–33 weeks of pregnancy. The coefficient ofvariation was analyzed in systole and early diastole.
The subjects for the chronic reduced cardiac function model
were second-trimester IUGR fetuses. The IUGR group consistedof 10 fetuses with gestational ages of 18–28 weeks and an estimated body weight of less than –1.5 SD of the Standard Values of Ultrasonographic Fetal Biometry of the Japan Society of Ultrasonics
in Medicine [14]. Twenty fetuses with gestational ages of 18–28
weeks and an estimated body weight within ±1.5 SD of the Standard Values of Ultrasonographic Fetal Biometry of the Japan Society of Ultrasonics in Medicine [14] were randomly selected as thecontrol group. Fetuses with structural or chromosomal abnormalities were excluded. The IUGR and control groups were comparedin regard to gestational age, estimated body weight, the absence ofend-diastolic velocity in the umbilical artery and the ductus venosus, and the presence of a cerebroplacental ratio of ≤1.0. In addition, the two groups were compared in terms of MPI, as assessedby the Doppler blood flow method, and fractional shortening, asassessed by the M-mode technique. MPI and fractional shortening
are conventional indicators of fetal cardiac function. Maximum
systolic velocity and maximum early diastolic velocity in the twogroups were then measured by PT. The parameters were measured
in both the LV and RV.
The subjects for the acute reduced cardiac function model were
10 TTTS recipients with gestational ages of 18–22 weeks who had
not yet undergone fetoscopic laser photocoagulation of placental

mode cross-sectional imaging. The beam is transmitted perpendicular to the basal portion of the ventricular free wall (a). Basedon the acquired data, the starting point of the sites to be measured
is set offline with an M-mode image as a guide, and the program
tracks the velocity of the specified part using the PT method (b).

Assessing Fetal Cardiac Function Using
the Phased-Tracking Method

Fetal Diagn Ther
DOI: 10.1159/000371514

a

b

3

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Fig. 1. Lateral four-chamber view of a fetal heart displayed in B-

Outside
InsideIntraventricular
septum

30
20
10
0

–20
–30

A

–40

Statistical analysis
Data were analyzed using JMP 10.0.2 statistical software (SASInstitute, Inc., Cary, N.C., USA). Results are expressed as mean ±SD values or proportions. Fisher’s exact test, one-way analysis ofvariance (ANOVA), and two-way ANOVA were used for statistical comparisons, and p < 0.05 was considered significant.

Results

First, the correlation between myocardial maximum
velocity and gestational age was examined in the healthy
fetuses. Measurement was possible in 126 (94%) of the
134 subjects. In the other subjects, measurement was notpossible due to fetal presentation. The intra-observer
agreement using the ICC of maximum systolic velocity
Fetal Diagn Ther
DOI: 10.1159/000371514

Time (ms)
500

–10 0

communicating vessels for TTTS. According to the Quintero stage
classification [15], 5 subjects were stage I, 2 subjects were stage II,
2 subjects were stage IIIc, and 1 subject was stage IV. Twenty fetuses with gestational ages of 16–22 weeks and an estimated bodyweight within ±1.5 SD of the Standard Values of Ultrasonographic Fetal Biometry of the Japan Society of Ultrasonics in Medicine[14] were randomly selected as the control group. The TTTS recipient and control groups were compared in regard to gestationalage, estimated body weight, absent or reverse end-diastolic velocity of the ductus venosus, tricuspid regurgitation, mitral regurgitation, monophasic tricuspid valve inflow, and monophasic mitralvalve inflow. In addition, the two groups were compared in regard
to MPI, as assessed by the Doppler blood flow method, and fractional shortening, as assessed by the M-mode technique. Maximum systolic velocity and maximum early diastolic velocity in thetwo groups were then measured by PT. The parameters were measured in both the LV and RV.

4

B

40

SystoleDiastole

was 0.926 [95% confidence interval (CI), 0.746–0.981] for
the LV and 0.708 (95% CI, 0.214–0.917) for the RV. The
ICC of maximum early diastolic velocity was 0.870 (95%CI, 0.584–0.966) in the LV and 0.966 (95% CI, 0.878–
0.991) in the RV. The inter-observer agreement using the
ICC of maximum systolic velocity was 0.924 (95% CI,
0.740–0.980) for the LV and 0.987 (95% CI, 0.951–0.957)
for the RV. The ICC of maximum early diastolic velocity
was 0.984 (95% CI, 0.941–0.996) for the LV and 0.985(95% CI, 0.944–0.996) for the RV. The coefficients ofvariation of maximum myocardial velocities were 17.9%
in systole and 12.4% in early diastole. Maximum systolic
and maximum early diastolic velocity correlated positively with gestational age in both the LV and RV (LV maximum systolic velocity, r = 0.84, p < 0.01; LV maximum
early diastolic velocity, r = 0.69, p < 0.01; RV maximum
systolic velocity, r = 0.52, p < 0.01; RV maximum early
diastolic velocity, r = 0.56, p < 0.01; fig. 3). When comparing the myocardial maximum velocities of the LV and RV
adjusted for gestational age using two-way ANOVA, nosignificant difference in the maximum systolic velocity
was observed between the two ventricles, but the maximum early diastolic velocity was higher in the RV than in
the LV (p < 0.01).
Second, the myocardial maximum velocities werecompared between the second-trimester IUGR and control groups. The gestational age at the time of examination was 23.4 ± 3.8 weeks in the IUGR group and 23.3 ±3.0 weeks in the control group (table 1). The estimatedbody weight at the time of examination was 458 ± 296 g
Ozawa/Murotsuki/Miyashita/Muromoto/
Hasegawa/Kanai/Yaegashi

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Fig. 2. Change over time in the myocardial
velocity inside the ventricular free wall, as
measured using PT. The measurementsites are indicated by dots. A = Maximum
systolic velocity; B = maximum early diastolic velocity.

Myocardial velocity (mm/s)

50Color version available online

Ventricular free wall

40
30
20
10
0

15

20

25

LV maximum early diastolic velocity(mm/s)

a

b

30

35

40

70

r = 0.69

60

p < 0.01

50
40
30
20
10
0

15

20

25

30

35

40

Weeks

d

p < 0.01

20

25

50
40
30
20
10
0

c

Weeks

r = 0.52

15

30

35

40

30

35

40

Weeks
70

r = 0.56

60

p < 0.01

50
40
30
20
10
0

15

20

25
Weeks

Fig. 3. Correlations of the following measurements with gestational age: maximum systolic velocity (a) and maximum early diastolic velocity in the LV (b) as well as maximum systolic velocity (c)and maximum early diastolic velocity in the RV (d). Two-way

ANOVA was used to compare the LV and RV; the maximum early diastolic velocity was greater in the RV than in the LV (p < 0.01).
r = Correlation coefficient.

in the IUGR group and 661 ± 324 g in the control group.
In the IUGR group, absent end-diastolic velocity of theumbilical artery was observed in 5 subjects and a cerebroplacental ratio of ≤1.0 was obtained for 7 subjects. The
MPI was 0.44 ± 0.12 in the LV and 0.48 ± 0.14 in the RV
in the IUGR group, and no significant difference was observed when comparing the IUGR group with the control
group. Similarly, the fractional shortening was 29.2 ±
6.6% in the LV and 25.7 ± 7.9% in the RV in the IUGR
group, and no significant difference was observed in comparison with the control group. In the PT measurements,
the maximum early diastolic velocity of the RV was significantly lower in the IUGR group than in the control
group (24.5 ± 6.2 vs. 32.1 ± 7.0 mm/s, p < 0.01).
Third, the myocardial maximum velocities were compared between the TTTS recipient and control groups.
The gestational age at the time of examination was 19.0 ±1.4 weeks in the TTTS recipient group and 19.4 ± 1.7
weeks in the control group (table 2). The estimated bodyweight at the time of examination was 265 ± 112 g in the
TTTS recipient group and 291 ± 104 g in the control
group. In the TTTS recipient group, absent or reverse
end-diastolic velocity of the ductus venosus was observedin 2 subjects, tricuspid regurgitation was observed in 4
subjects, and monophasic tricuspid valve inflow was observed in 1 subject. Since 1 fetus did not have antegrade
blood flow in the pulmonary artery, it was not possible tomeasure the MPI in the RV. In the TTTS recipient group,
the MPI was 0.60 ± 0.15 in the LV and 0.58 ± 0.11 in the
RV; both values were significantly higher than those observed in the control group (LV, p < 0.01; RV, p = 0.04).
Similarly, fractional shortening was 26.6 ± 9.6% in the LV
and 20.4 ± 7.9% in the RV in the TTTS recipient group.
RV fractional shortening was significantly lower in theAssessing Fetal Cardiac Function Using
the Phased-Tracking Method

Fetal Diagn Ther
DOI: 10.1159/000371514

5

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5060

Color version available online

60

70

p < 0.01

RV maximum systolic velocity(mm/s)

r = 0.84

RV maximum early diastolic velocity(mm/s)

LV maximum systolic velocity(mm/s)

70

Table 1. Comparison of background factors, MPI, fractional shortening, and myocardial maximum velocity
measurements using PT in the IUGR and control groups

Gestational age, weeks
Estimated fetal body weight, gAbsent umbilical artery end-diastolic flow
Cerebroplacental ratio <1.0
Ductus venosus absent atrial contraction wave
MPI
LV
RV
Fractional shortening, %LV
RVMaximum systolic velocity, mm/s
LV
RV
Maximum early diastolic velocity, mm/s
LV
RV

IUGR group(n = 10)Control group(n = 20)p

23.4 ± 3.8
458 ± 2965
7
023.3 ± 3.0
661 ± 324
0
1
0

0.93
0.10
<0.01*<0.01*
1.00

0.44 ± 0.12
0.48 ± 0.14

0.44 ± 0.08
0.48 ± 0.13

0.98
0.91

29.2 ± 6.6
25.7 ± 7.9

32.6 ± 7.1
30.4 ± 7.6

0.22
0.13

15.2 ± 5.5
17.5 ± 6.6

18.0 ± 3.9
21.1 ± 5.0

0.12
0.10

25.6 ± 9.6
24.5 ± 6.2

27.8 ± 8.5
32.1 ± 7.0

0.52<0.01*

Data are expressed as means ± SD or numbers. Calculated by ANOVA and Fisher’s exact test. * p < 0.05.

Table 2. Comparison of background factors, MPI, fractional shortening, and myocardial maximum velocity
measurements using PT in the TTTS recipient and control groups

Gestational age, weeks
Estimated fetal body weight, g
Ductus venosus absent or reverse atrial contraction wave
Mitral regurgitation
Tricuspid regurgitation
Monophasic mitral valve inflow
Monophasic tricuspid valve inflow
MPI
LV
RV
Fractional shortening, %LV
RVMaximum systolic velocity, mm/s
LV
RV
Maximum early diastolic velocity, mm/s
LV
RV

TTTS recipient group(n = 10)Control group(n = 20)p19.0 ± 1.4
265 ± 112
2
0
4
0
1

19.4 ± 1.7
291 ± 104
0
0
1
0
0

0.53
0.53
0.10
1.00
0.03*
1.00
0.33

0.60 ± 0.15
0.58 ± 0.11

0.45 ± 0.10
0.47 ± 0.13

<0.01*
0.04*

26.6 ± 9.6
20.4 ± 7.9

33.0 ± 8.3
34.3 ± 6.4

0.06<0.01*

10.1 ± 3.6
11.8 ± 4.3

13.8 ± 3.2
18.2 ± 4.6<0.01*<0.01*

16.5 ± 4.6
17.3 ± 8.5

22.5 ± 6.6
24.9 ± 5.8

0.01*<0.01*

6

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DOI: 10.1159/000371514

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Hasegawa/Kanai/Yaegashi

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Data are expressed as mean ± SD or numbers. Calculated by ANOVA and Fisher’s exact test. * p < 0.05.

Maximum systolic and maximum early diastolic velocities exhibited positive correlations with gestationalage in both ventricles. It has been reported that the maximum systolic velocity of the myocardium correlates wellwith dP/dt, a systolic performance indicator [2], and thatthe maximum early diastolic velocity correlates well withthe catacrotic time constant τ of the ventricular pressure
curve, a diastolic performance indicator [3]. The resultsof this study suggest that the systolic and diastolic performances of both ventricles increase with gestational age.
Although we observed no difference between the maximum systolic velocities of the LV and RV, the maximum
early diastolic velocity was higher in the RV than in the
LV. We believe this is because the RV is larger in size andhas a higher cardiac output than the LV in the fetal heart.
In reports published thus far, longitudinal velocity isgreater in the fetal RV than in the fetal LV, as measured
by tissue Doppler imaging [16, 17]. However, no reports
have been published on radial velocity. The myocardium
is a complex three-dimensional network of myocytes in amatrix of fibrous tissue in both ventricles. However, there
is a predominant longitudinal orientation to the myocytes, forming the myofibers in the RV. Although the longitudinal function of the RV is predominant prenatally,
the radial systolic function may have no difference in both
ventricles. The present study has clearly shown that fetuses have greater early diastolic radial velocity in the RV
than in the LV.
The RV maximum early diastolic velocity was lower insecond-trimester IUGR subjects than in healthy fetuses.
The decreased RV diastolic performance in the IUGR
subjects suggests the possibility that the afterload and theend-diastolic pressure of the ventricles increase in IUGR,

owing to the increasing vascular resistance of the placenta [18, 19]. Furthermore, the RV has a larger preload thanthe LV because it ejects more blood flow. Moreover, as
compared with the LV, the RV may have an anatomical
structure that is more easily affected by the afterload. The
LV mainly ejects blood flow to the brain, and the RVejects to the placenta through the ductus arteriosus. Although increased resistance of placenta effects afterload
is found in both ventricles, brain sparing leads to a decreased afterload in the LV. In our IUGR group, a cerebroplacental ratio of ≤1 was obtained for 7 subjects, anda ratio close to 1.0 was obtained for the others. In reports
published to date, systolic and diastolic longitudinal velocities in the fetal heart are lower in fetuses with IUGR
than in normal fetuses, as measured by tissue Doppler
imaging [20]. In this study, no difference in the MPI or
fractional shortening was seen between the IUGR andcontrol groups. This finding is explained by the many
mild cases that were included in our IUGR group. It has
previously been reported that early stages of IUGR more
typically affect the longitudinal function than the radialfunction of the fetal heart [4]. However, the results of thepresent study suggest that radial diastolic dysfunction in
the RV may occur in fetuses with early stages of IUGR.
As compared with the healthy fetuses, the TTTS recipient fetuses exhibited low maximum systolic and low
maximum early diastolic velocities of both the LV and
RV. Further, the two groups were observed to have different MPIs and fractional shortenings. In previous reports,
increased MPI and decreased fractional shortening havealso been noted in both ventricles of recipient fetuses with
TTTS [21]. Moreover, a study has found that myocardial
diastolic function measured by tissue Doppler imaging islower in recipient fetuses with TTTS than in healthy fetuses [22]. However, no report has included measurements of myocardial radial velocity in recipient fetuseswith TTTS. The present study has clearly shown that the
myocardial radial velocities of both the LV and RV are
lower in recipient fetuses with TTTS than in normal fetuses.
This study investigated the radial velocity of the ventricular free walls. Ultrasound-based measurements ofcardiac velocities can be divided into the following three
directions: longitudinal, circumferential, and radial. Tissue Doppler imaging has been used to measure the longitudinal velocity of the fetal myocardium [23]. However,very few reports have considered the radial velocity of the
fetal myocardium [8]. Fractional shortening is an indicator of radial-direction systolic performance, and the
movement of the myocardium in the radial direction ef-

Assessing Fetal Cardiac Function Using
the Phased-Tracking Method

Fetal Diagn Ther
DOI: 10.1159/000371514

Discussion

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TTTS recipient group than in the control group (p <0.01), and LV fractional shortening also appeared to belower in the TTTS recipient group, but the difference was
not statistically significant (p = 0.06). In the PT measurements, the maximum systolic velocities and maximum
early diastolic velocities of the RV and LV were significantly lower in the TTTS recipient group than in the control group (LV maximum systolic velocity, 10.1 ± 3.6 vs.
13.8 ± 3.2 mm/s, p < 0.01; LV maximum early diastolicvelocity, 16.5 ± 4.6 vs. 22.5 ± 6.6 mm/s, p = 0.01; RV maximum systolic velocity, 11.8 ± 4.3 vs. 18.2 ± 4.6 mm/s,
p < 0.01; RV maximum early diastolic velocity, 17.3 ± 8.5
vs. 24.9 ± 5.8 mm/s, p < 0.01).

ficiently changes the ventricular volume [24]. We therefore think that the radial velocity of the fetal myocardium
is one of the most important indicators for evaluating cardiac function. However, because the radial velocity issmaller than the longitudinal velocity, it requires more
precise measurements. In the present study, we used the
PT method to obtain precise measurements of radial velocity.
PT has many advantages for the measurement of myocardial velocity in fetuses. Because of the very small sizeand fast beating rate of the fetal heart, excellent spatial
and temporal resolutions are required to measure fetal
myocardial velocity with high precision. Currently, fetal
myocardial velocity is measured using tissue Doppler andspeckle tracking methods. Studies using tissue Doppler
and speckle tracking have evaluated cardiac function in
IUGR and TTTS recipient fetuses [25, 26]. Although
speckle tracking has less angular dependence, it may
nonetheless be difficult to obtain a sufficient frame rate
when examining fetuses [8]. PT has much greater spatial
and temporal resolutions than those offered by conventional ultrasonographic techniques [9]. In conventional
ultrasonographic techniques, deviations of a point of interest cannot be detected if they are smaller than the
wavelength. However, the PT method can be used to detect displacements of a point of interest that are smaller
than the wavelength, based on an evaluation of the phasedifference between two ultrasonic pulses. Accordingly,
PT provides improved spatial and temporal resolutions.

In water tank testing, the velocity measurement precisionof PT was found to be 0.1 mm/s [10]. PT can measure
myocardial velocity with higher precision than tissue
Doppler or speckle-tracking methods.
However, PT measurements are dependent on angle.
Since the heart moves with three-dimensional distortion,
the measurement of myocardial velocity in one direction
may not capture the systolic and diastolic performancesof the entire heart. It is also possible that the measurementof radial velocity at a single point on the endocardium
side of the ventricular free wall does not reflect the movement of the entire ventricle. In addition, myocardial velocity is affected by the pre- and afterload of the ventricles. Furthermore, the number of pathological fetal subjects in this study was small, and it must be increased toachieve more precise findings.
In this study, the evaluation of cardiac function involved precise measurements of myocardial radial velocity, which were performed separately for the contractionand expansion of the two ventricles in fetuses. We demonstrated that, in healthy fetuses, ventricular systolic anddiastolic radial performances increase with gestational
age. By measuring myocardial radial velocity using PT,
we found diastolic dysfunction of the RV in fetuses withIUGR as well as systolic and diastolic dysfunctions ofboth the LV and RV in TTTS recipients. PT may provide
a useful method of measuring the maximum radial velocity in the fetal myocardium, and thereby assessing systolic and diastolic performances in fetuses.

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