Circulation. 2001;103:2882-2884
doi: 10.1161/hc2401.092234
(Circulation. 2001;103:2882.)
© 2001 American Heart Association, Inc.
Brief Rapid Communications |
Rapid Three-Dimensional Echocardiography
Clinically Feasible Alternative for Precise and Accurate Measurement of Left Ventricular Volumes
Marek Belohlavek, MD, PhD;
Kazuaki Tanabe, MD, PhD;
Decho Jakrapanichakul, MD;
Jerome F. Breen, MD;
James B. Seward, MD
From the Division of Cardiovascular Diseases and Internal Medicine (M.B.,
K.T., D.J., J.B.S.) and the Department of Diagnostic Radiology (J.F.B.), Mayo
Clinic, Rochester, Minn.
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Abstract
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BackgroundClinical
applicability of conventional ultrasonographic
systems using mechanical
adapters for 3D echocardiographic imaging
has been
limited by long acquisition and processing times. We
developed a rapid
(6-s) acquisition technique that collects
apical tomograms using a
continuously internally rotating transthoracic
transducer.
This study was performed to examine the clinical
feasibility of
rapid-acquisition 3D echocardiography to estimate
left
ventricular end-diastolic and
end-systolic volumes using electron-beam
computed tomography as
the reference standard.
Methods and ResultsWe
collected a series of 6 to 11 apical echocardiographic
tomograms, depending on heart rate, in 11 patients. There was good
correlation, low variability, and low bias between rapid 3D
echocardiography and electron-beam computed
tomography for measuring left ventricular
end-diastolic volume
(r=0.96; standard error of the
estimate, 21.34 mL; bias, -4.93 mL) and left ventricular
end-systolic volume
(r=0.96; standard error of the
estimate, 14.78 mL; bias, -6.97 mL).
ConclusionsThe
rapid-acquisition 3D echocardiography extends the
use of a multiplane, internally rotating handheld transducer so that it
becomes a precise and clinically feasible tool for assessing left
ventricular volumes and function. A rapid-image acquisition
time of 6 s would allow repeated image collection during the
course of a clinical echocardiographic examination.
Additional work must address rapid and automated data processing.
Key Words: echocardiography ventricles tomography
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Introduction
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Three-dimensional
echocardiography has demonstrated superior
accuracy
and reproducibility over conventional 2D
echocardiography
for measuring left
ventricular (LV) volume, because no geometric
assumptions
are necessary about LV shape.
1 Real-time volumetric
3D
echocardiography
2
with a very fast, internally rotating crystal
array for real-time 3D
imaging,
3 various mechanical
adapters
for rotation or translation with conventional
transthoracic
echocardiography
transducers,
4 or free-hand 3D
image acquisition
with magnetic
5 or
spark-gap
6 spatial
positioning systems were
investigated in an effort to use the 3D
approach in a clinical
setting. However, the clinical application of
these techniques
is not widespread because of compromised image
quality, challenging
technical design, or slow acquisition times. The
initial experience
with a prototype handheld transducer with an
internally rotating
crystal array for multiplane image acquisition was
reported
recently.
7 The
transthoracic echocardiography
transducer is
comparable in size to conventional probes and is stable
during
multitomographic data acquisition, because the rotating crystal
array
is not in direct contact with the body surface.
Using the 180° continuous-rotation mode of the multiplane
transducer, we developed a rapid 3D image-acquisition technique that
addresses the issue of clinical practicality of 3D
echocardiography.
The purpose of this study was to assess the clinical
feasibility and the precision and accuracy of the new rapid, 3D
echocardiographic acquisition technique for measuring
LV end-diastolic (LVEDV) and end-systolic (LVESV)
volumes. Electron-beam computed tomography (EBCT) was used as the
reference standard.
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Methods
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Study Population
We performed a rapid 3D apical scan in patients for
whom a standard
transthoracic
echocardiographic examination was clinically indicated.
Patients
with a variety of cardiac pathologies were considered in an
attempt
to cover a large range of LV volumes. Exclusion criteria were
cardiac
arrhythmias, sinus bradycardia, and any clinical
condition preventing
6 s of suspended respiration. Thirteen
patients entered the
study; echocardiographic data from
2 patients were excluded
because the image quality from remote
myocardial regions was
not adequate for determining the endocardial
border. In the
other 11 patients (8 men; mean age, 54 years; range, 36
to 68
years), the clinical transthoracic
echocardiographic study was
requested for
ischemic heart disease (n=2), mitral
regurgitation
(n=2), aortic
regurgitation (n=2), cardiac amyloidosis (n=1),
constrictive
pericarditis (n=1), pulmonary fibrosis (n=1), and
pulmonary
hypertension (n=1). In the final patient, LV function
was evaluated
before liver transplantation. The study protocol was
approved
by the Institutional Review Board of the Mayo Foundation.
Informed
consent was obtained before the study from all patients.
3D Echocardiographic Study
We used a HP 2500 system
(Hewlett-Packard) fitted with a prototype
5-MHz multiplane transthoracic transducer. An internal
crystal array continuously rotates and collects images over 180°
about its imaging axis. The desired diastolic and
systolic tomograms are selected during subsequent off-line data
processing on the basis of a synchronously acquired
electrocardiographic signal. The number of tomograms obtained from the
6-s 180° rotation cycle depends on the patients heart rate. For
example, with a rate of 80 beats/min, 8 tomograms are obtained. The
end-diastolic (temporally related to the R wave on the ECG)
and end-systolic (smallest chamber volume during the cardiac
cycle) phases were identified at each cardiac cycle. Images were
digitized, and LV endocardial borders were traced manually, including
the LV outflow tract up to the aortic valve. The mitral valve plane was
traced as the straight line between the boundaries of the mitral
annulus. Two independent observers (K.T. and D.J.), who were blinded to
the LV volumes obtained with the reference EBCT measurements, performed
endocardial diastolic and systolic tracings, which
took about 20 minutes per left ventricle. The LVEDV and LVESV were
measured from computer-generated LV cavity casts, which were
reconstructed from the interactive tracings using the Sun
SPARC station 20 and a custom software algorithm; this took
another 15 minutes.
EBCT Study
Each EBCT (Imatron C-100) study was performed within
48 hours of the echocardiographic examination. Each
subject was positioned in the scanner to obtain parallel tomographic
images in the short axis (transverse cardiac) from the LV apex to the
base of the right ventricular outflow tract. During
imaging, an intravenous infusion of nonionic contrast
medium was delivered at 3 mL/s for 20 s. Eight to 12 parallel
tomographic scans were obtained from the LV apex through the base,
depending on the overall long-axis dimensions of the ventricle. The
endocardial borders were traced manually, and the LVEDV and LVESV were
obtained using a disk summation method.
Statistical Evaluation
To assess precision, LV volumes estimated from rapid
3D echocardiography by 2 independent observers were
averaged and compared with those measured with EBCT by linear
regression. Interobserver variability was expressed as the coefficient
of variation between the 2 observers. To determine whether the
difference in the values between the 2 methods was statistically
significant, a paired t test
was performed; the level of significance was set to
P<0.05. The accuracy of the
rapid 3D echocardiography with respect to the EBCT
measurements was examined by a limits-of-agreement analysis.
The bias was expressed as the mean difference between the 2 methods,
and the limits of agreement as 2 SDs of the difference of the 2
methods.
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Results
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The mean±SD of the testing 3D
echocardiographic and reference
EBCT measurements were
165.1±68.0 mL and 160.2±67.6
mL, respectively, for LVEDV and
70.4±47.9 mL and 63.5±54.0
mL for LVESV. A regression
analysis and a Bland and Altman plot
for LVEDV measurements are
shown in Figures 1A

and 1B

, respectively.
Linear regression indicated
a high correlation (
r=0.96;
P<0.0001)
between the
reference and the testing methods, with a standard
error of estimate of
21.34 mL. The limits-of-agreement analysis
demonstrated a
minimal mean difference (bias, -4.93±20.44
mL). The
t test indicated no significant
mean differences between
the 2 methods. The results of LVESV regression
and agreement
analysis, which are shown in
Figures 1C

and 1D

, also demonstrated
high precision and
accuracy (
r=0.96;
P<0.001; standard error
of
estimate, 14.78 mL; bias, -6.97±16.22 mL) and no significant
mean
differences between the 2 methods. Systolic volumes were
not
distributed as proportionally along the correlation line
as the
diastolic volumes, and the results were influenced by
a
large LVESV in one patient with ischemic heart
disease.

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Figure 1. A, Linear regression analysis for comparison of LVEDV estimated by 3D echocardiography (3D echo) and those estimated by EBCT, with the correlation and identity lines depicted as solid and dashed lines, respectively. Dotted lines delimit the 95% confidence band. B, Limits of agreement analysis. Difference of LVEDV between EBCT and 3D echocardiography is plotted against average values of the 2 methods. Solid line indicates mean; dashed line, zero line; and dotted lines, 2-SD limits. C, Linear regression analysis for comparison of LVESV estimated by 3D echocardiography and those estimated by EBCT. D, Limits of agreement analysis. Difference of LVEDV between EBCT and 3D echocardiography is plotted against average values of the 2 methods.
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The linear regression and agreement plots of corresponding
LVEDV and LVESV values estimated from the 3D
echocardiographic data by 2 independent observers are
shown in Figure 2
. These results showed excellent correlation and
only a small bias. In the estimation of the LVEDV and LVESV values,
interobserver variability was 6.11% and 9.14%,
respectively.

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Figure 2. A, Linear regression analysis for comparison of LVEDV estimated by 2 independent observers. B, Limits of agreement analysis of the 2 LVEDV observer estimates. C, Linear regression analysis for comparison of LVESV estimated by 2 independent observers. D, Limits of agreement analysis of LVESV estimates from the 2 observers. Solid, dashed, and dotted lines as in Figure 1 .
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Discussion
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We report a new rapid (6-s) 3D
echocardiographic image acquisition
method that uses a
prototype handheld transducer with an internally
rotating crystal
array. This approach represents a practical
solution to a
clinically feasible acquisition of 3D data and
provides precise and
accurate diastolic and systolic volumes
for a
functional assessment of the left ventricle. In the present
implementation,
diastolic and systolic frames,
which are required for 3D reconstruction
of volumetric data, are
selected a posteriori during off-line
processing, although on-line
processing is conceivable.
Advantages of Rapid 3D Echocardiography
The technique allows the rapid collection of data
during 6 s of suspended respiration, which makes the technique
feasible in most clinical scenarios. The prototype transducer is
capable of all current Doppler and harmonic imaging modalities.
Rotational geometry has been applied successfully to 3D
reconstruction,8 9 10
and the approach discussed herein represents a practical
extension of these efforts. An important characteristic of this
geometry is that the images are equispaced and particularly useful for
reconstructing the whole left ventricle. In addition, this system could
sample sufficient 4D data for a dynamic analysis of LV function
and shape.
Limitations
The manual delineation and off-line data processing
were time-consuming; however, this was not an issue from the viewpoint
of our experimental objectives and did not prolong the time needed to
examine a patient. The present implementation of the method is
limited to patients with a heart rate
60 beats/min to collect a
minimum of 6 tomograms. Multiplane transducers equipped with an
adjustable rate of rotation would overcome this limitation.
The 5-MHz frequency of the transducer was not optimal for
adult echocardiography but was dictated by the
initial prototype design using transesophageal
transducer mechanics. Consequently, limited signal penetration led to
the exclusion of 2 patient data sets from analysis. A
production system would certainly use a rotating transducer
with lower frequency for transthoracic clinical
applications.
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Conclusions
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Rapidly acquired 3D data sets of 6 to 11 sequential
tomograms
provide precise and accurate measurements of LV volumes and
ejection
fractions in humans. Because of the short duration of
acquisition
(6 s), the technique is clinically feasible, and it allows,
if
necessary, repeated collection of 3D data during the course
of a
clinical examination, further enhancing the results. Additional
work
must concentrate on rapid and automated data processing
before this
technique can be used widely.
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Acknowledgments
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This study was supported in part by grant HL 52494 from the
National Institutes of Health.
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Footnotes
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Reprint requests to Marek Belohlavek, MD, PhD, Division of Cardiovascular
Diseases and Internal Medicine, Mayo Clinic, 200 First Street
SW, Rochester, MN 55905.
Received March 16, 2001;
revision received April 23, 2001;
accepted April 30, 2001.
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