(Circulation. 1996;93:1877-1885.)
© 1996 American Heart Association, Inc.
Articles |
Echocardiographic Quantification of Regional Left Ventricular Wall Motion With Color Kinesis
From the Noninvasive Cardiac Imaging Laboratories, Section of Cardiology, Department of Medicine, the University of Chicago (Ill) Medical Center.
Correspondence to Roberto Lang, MD, and Victor Mor-Avi, PhD, University of Chicago Medical Center, MC 5084, 5841 S Maryland Ave, Chicago, IL 60637. E-mail rlang@medicine.bsd.uchicago.edu.
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Abstract |
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Background Color kinesis is a new technology for the echocardiographic assessment of left ventricular wall motion based on acoustic quantification. This technique automatically detects endocardial motion in real time by using integrated backscatter data to identify pixel transitions from blood to tissue during systole on a frame-by-frame basis. In this study, we evaluated the feasibility and accuracy of quantitative segmental analysis of color kinesis images to provide objective evaluation of regional systolic endocardial motion.
Methods and Results Two-dimensional echocardiograms were obtained in the short-axis and apical four-chamber views in 20 normal subjects and 40 patients with regional wall motion abnormalities. End-systolic color overlays superimposed on the gray scale images were obtained with color kinesis to color encode left ventricular endocardial motion throughout systole on a frame-by-frame basis. These color-encoded images were divided into segments by use of custom software. In each segment, pixels of different colors were counted and displayed as stacked histograms reflecting the magnitude and timing of regional endocardial excursion. In normal subjects, histograms were found to be highly consistent and reproducible. The patterns of contraction obtained in normal subjects were used as a reference for the objective automated interpretation of regional wall motion abnormalities, defined as deviations from this pattern. The variability in the echocardiographic interpretation of wall motion between two experienced readers was similar to the diagnostic variability between the consensus of the two readers and the automated interpretation.
Conclusions Color kinesis is a promising new tool that may be used clinically to improve the qualitative and quantitative evaluation of spatial and temporal aspects of global and regional wall motion. In this initial study, segmental analysis of color kinesis images provided accurate, automated, and quantitative diagnosis of regional wall motion abnormalities.
Key Words: systole • endocardium • imaging • myocardial contraction • echocardiography
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Introduction |
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Two-dimensional (2D) echocardiography is widely used for the evaluation of regional left ventricular function because of its ability to depict endocardial excursion and wall thickening in real time. In view of the shortcomings of the qualitative methods currently used to assess the extent and severity of regional wall motion abnormalities, a variety of quantitative techniques have been developed, most based on manual off-line frame-by-frame tracing of the myocardial border.1 2 3 4 5 6 7 8 9 10 Because it often is difficult to precisely define the endocardial and epicardial boundaries, these time-consuming methods remain subjective and impractical for routine clinical use. More recently, computerized methods of edge detection have been proposed by several p investigators.8 11 12 13 14 15 Although automated to a great extent, these methods also require off-line processing and have been successful primarily in the short-axis view.13 14
Color kinesis, a new technique based on acoustic quantification,16 17 has been developed to facilitate the evaluation of regional wall motion. Color kinesis tracks the motion of the endocardium in real time throughout systole and results in color-encoded images reflecting the magnitude and timing of endocardial motion. The aims of this study were to (1) establish the feasibility of tracking endocardial motion with color kinesis in normal subjects and patients with regional wall motion abnormalities, (2) develop a method to quantify wall motion based on segmental analysis of color kinesis images, (3) describe the normal pattern of regional endocardial excursion with this segmental analysis, and (4) evaluate the ability of this analysis to facilitate the objective detection of regional wall motion abnormalities.
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Methods |
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Color Kinesis: Principles of Operation
Acoustic quantification has recently been validated against a variety of techniques by multiple investigators.16 17 18 19 20 21 22 Color kinesis is an extension of this technology, which compares tissue backscatter values between successive acoustic frames as a means of automatically tracking and displaying endocardial motion in real time. This newly developed algorithm, which provides an integrated display of the timing and magnitude of endocardial wall motion in real time, was incorporated into a commercially available ultrasound system (SONOS 2500, Hewlett Packard). Color kinesis processes the ultrasound backscatter data and generates for each frame a binary blood-tissue mask image in which each pixel is classified as either blood or tissue. Pixel value transitions from blood to tissue in this mask image compared with the preceding frame are used to track systolic endocardial motion. For each video frame, a specific color hue is used to tag pixels that have changed from blood to tissue (Fig 1
). The colors accumulate frame by frame
throughout systole, resulting in a color overlay that is superimposed
on the 2D echocardiographic image. These color overlays
are generated in real time, and the display is updated continuously.
Thus, the time-motion history of the systolic endocardial
excursion is depicted in the end-systolic color-encoded
images on a beat-to-beat basis. Fig 2 shows four
short-axis images obtained from a single ejection cycle.
Examination of the end-systolic color overlay (Fig 2C)
provides a composite time-dependent picture of the wall motion
throughout systole.
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Study Population
Color kinesis data were obtained in two groups: 20 normal
subjects (13 women, 7 men with normal echocardiograms; mean age, 49±18
years) and 40 patients with regional wall motion abnormalities
diagnosed by 2D echocardiography (18 women, 22 men;
mean age, 66±13 years). Exclusion criteria were (1) inadequate 2D
image quality; (2) pericardial effusion; (3) the absence of normal
sinus rhythm; (4) abnormal interventricular septal
motion caused by previous sternotomy, right ventricular
pressure or volume overload, and/or left bundle branch block; and (5)
an inability to track wall motion with acoustic quantification in
>30% of the endocardial boundary. With these criteria, 3 normal
subjects and 9 patients with regional wall motion abnormalities were
excluded.
Data Acquisition
In all study subjects, ultrasound imaging was performed with a
2.5- or 3.5-MHz transducer. Midpapillary parasternal short-axis and
apical four-chamber views were obtained during end expiration in
the lateral decubitus position and recorded on videotape (model
AG-7350, Panasonic). After image quality was optimized, the acoustic
quantification system for endocardial boundary detection was
activated. Gain controls (total and lateral gain, time-gain
compensation) were adjusted to optimize tracking of the
blood-endocardial interface within a predefined region of
interest.23 Color kinesis then was activated for
on-line color encoding of endocardial excursion throughout systole.
Image sequences containing color kinesis data were obtained throughout
the cardiac cycle and stored in a digital format on optical disks for
off-line analysis.
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Data Analysis |
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Tracking of Blood-Endocardial Interface
The ability of color kinesis to accurately track the left ventricular endocardial boundary throughout systole was evaluated by reviewing the stored digital loops obtained in all normal subjects and patients with regional wall motion abnormalities. This analysis was based on the segmentation specified in the American Society of Echocardiography guidelines.24 Tracking was considered adequate when the visually assessed endocardial systolic excursion matched the color-encoded images.
Segmental Analysis of Regional Wall Motion
Digitized end-systolic left ventricular
color-encoded images were automatically divided into segments by
use of custom software (Fig 3). In the short-axis
view, the segmentation originated from the left ventricular
end-systolic cavity centroid defined by its
x1,2 coordinates as follows:
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). The left ventricle
was divided into six 60° wedge-shaped segments.
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In the apical four-chamber view, each image was initially divided
into two sections separated by a line (defined as the long axis)
connecting the manually determined distal apical endocardium with the
end-systolic left ventricular cavity centroid,
calculated as for the short-axis view. In each section, a
wedge-shaped sector was defined between the aforementioned long
axis and a line connecting the centroid with the manually determined
base of the mitral valve leaflet. This scheme excluded mitral valve
motion from the analysis. Each sector was then further divided
into three equiangled sectors. This procedure resulted in a total of
six sectors originating from the cavity centroid (Fig 3B
).
In each segment, pixels of each color and pixels marked as blood were
counted. The number of pixels of each color represents the
incremental area change that occurred during the time frame
corresponding to that specific color (33-ms period). The
end-diastolic area of each individual segment is
represented by the total pixel count, ie, all colored
pixels and those marked as blood. Normalization of the incremental area
change by the end-diastolic area of the corresponding
segment results in a regional fractional area change (in percent of
end-diastolic area of that specific segment).
Incremental fractional area changes in all segments were displayed as a
stacked color histogram in which each time frame is
represented by a specific color identical to that used in
color kinesis images (Fig 4A).
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In addition, for each segment, the mean time of contraction
representing the average time required for a pixel to
change from blood to tissue was computed as
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).
In each patient, the mean time of contraction was averaged separately
for all normal and abnormal segments. The ratio of the abnormal to
normal segments was then calculated and averaged for the entire group
of patients.
Normal Patterns of Endocardial Excursion
Histograms obtained from normal subjects were averaged to obtain
the normal pattern of left ventricular systolic
endocardial excursion. These average histograms were then used to
evaluate the intersegmental variability of systolic endocardial
excursion and as a normal reference for comparison with those obtained
from patients with suspected regional wall motion abnormalities.
Detection of Regional Wall Motion Abnormalities
Individual histograms obtained from patients with regional wall
motion abnormalities diagnosed with 2D
echocardiography were compared with the averaged
reference histograms obtained from normal subjects. To facilitate
objective detection of regional wall motion abnormalities, individual
histograms were superimposed on the normal reference, defined as 1 SD
around the mean of the normal control group. Regional wall motion
abnormalities were diagnosed when the regional fractional area change
in at least one segment deviated from this normal reference.
Feasibility of Automated Detection of Regional Wall Motion
Abnormalities
To evaluate the feasibility of automated detection of regional
wall motion abnormalities with segmental analysis of color
kinesis data, the following protocol was conducted. Initially,
videotapes of 2D echocardiographic short-axis and
apical four-chamber views obtained from 40 patients with regional
wall motion abnormalities were independently interpreted by two
experienced readers using the conventional left ventricular
segmentation.24 The diagnostic variability
between readers was calculated as the number of discordant
interpretations divided by the total number of segments defined as
abnormal by at least one of the two observers. Subsequently, discordant
segments were reviewed by the two readers jointly to reach a consensus.
To obtain the intertechnique variability, this consensus was then
compared with the results of the above-described automated
detection of regional wall motion abnormalities on a
region-by-region basis.
Statistical Analysis
Intersubject variability was evaluated by averaging histograms
obtained from all normal subjects and calculating for each segment the
ratio between the SD and the mean of the total fractional area change.
Reproducibility (intrasubject variability) of the segmental
analysis was evaluated in a subgroup of nine randomly selected
normal subjects by acquiring and analyzing three nonconsecutive
end-systolic color-encoded images. Reproducibility was
quantified by averaging the histograms of these three repeated
measurements and calculating for each segment the SD divided by the
mean of the total fractional area change.
The significance of differences in the interobserver interpretation of
segmental systolic wall motion with 2D
echocardiography and the intertechnique variability
were determined with a 
2 test. This test was
designed to provide an estimate of how much the observed frequencies of
the interobserver and intertechnique disagreements differ from those
expected if no relationship existed between the method of
analysis (conventional visual examination of echocardiograms
versus automated detection based on segmental analysis of
color-encoded images) and the outcome.
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Results |
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As Fig 5A
shows, color kinesis tracked endocardial
motion accurately in >91% of left ventricular segments
(range, 69% to 100%). The apical-lateral segment in the apical
four-chamber view had the least adequate
tracking.
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In normal subjects, the pattern of regional left
ventricular systolic endocardial excursion was
found to be consistent and reproducible. The pattern was more
symmetrical in the short-axis view compared with the apical
four-chamber view, in which reduced motion was demonstrated in the
apical-lateral segment (segment 3, Figs 4A and 6A),
probably as a result of poor visualizing and tracking of the
endocardial border. The normal regional mean time of contraction ranged
between 85 and 204 ms and was found to be consistent with
intersegment variations (Fig 6B).
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Fig 5B shows the intersubject variability in regional endocardial
excursion. Mean values were 46% and 53% in the short-axis and
apical four-chamber views, respectively. Fig 7 shows
the intrasubject variability data for each segment (mean and range).
Repeated segmental analyses of nonconsecutive color kinesis
images were found to be reproducible within 11±4% in the
short-axis view and 12±2% in the apical four-chamber
view.
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Figs 8 and 9 show examples of data
obtained from patients with regional wall motion abnormalities. These
figures present the end-systolic color-encoded
images with the corresponding individual histograms superimposed on the
averaged normal reference to allow objective detection of regional wall
motion abnormalities. In the example shown in Fig 8, the gaps between
the normal reference and the individual patient's data in segments 2
and 3 in the short-axis view and segments 4 and 5 in the apical
four-chamber view demonstrate localized areas of hypokinesis in the
midanteroseptal and midinferoseptal segments and the apical and
midseptal segments, respectively. Fig 9 demonstrates hypokinesis in the
anterior, septal, and inferior walls (segments 1 through 4)
in the short-axis view and in the apical-lateral region and
entire interventricular septum (segments 3 through 6)
in the apical four-chamber view.
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In most segments identified as abnormal, the mean time of contraction was found to be shorter compared with normal segments. The mean time of contraction ratio between the abnormal and normal segments was 0.78±0.21.
The interobserver variability in the interpretation of regional
systolic wall motion based on 2D echocardiograms was 13.5%
(divergence in 31 of 230 segments diagnosed as abnormal by at least one
observer). The intertechnique variability between the conventional
interpretation of 2D echocardiograms and the above-described
automated detection was found to be 17.0% (divergence in 35 of 206
segments diagnosed as abnormal by the consensus of the two readers).
This intertechnique variability was not significantly different from
the interobserver variability of the conventional interpretation
(2=1.04).
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Discussion |
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Color kinesis is a new echocardiographic method based on real-time analysis of tissue acoustic properties that tracks left ventricular endocardial motion. In this study, we have shown that quantitative segmental analysis of end-systolic color kinesis images provides easily accessible objective information on the spatial and temporal aspects of regional left ventricular endocardial excursion.
Feasibility of Color Kinesis
Color kinesis constitutes an extension of the acoustic
quantification technology. In both modalities, accurate tracking of the
endocardial border is dependent on adequate settings of transmitted
power and lateral gain and time-gain
compensation.16 17 18 19 20 21 22 23 Increased heart rate did not appear to
impair color encoding of endocardial motion during systole.
Accordingly, before color kinesis was activated, it was
necessary to adjust the overall gain and near-field time-gain
compensation to ensure that the border automatically detected with
acoustic quantification matched the visualized endocardial border in
the 2D echocardiographic images. By use of this color
kinesis data acquisition strategy in our selected population, it was
possible to obtain excellent endocardial tracking in all segments,
except for less accurate tracking of the lateral apical segment (Fig 5A). The limited ability to track the apical lateral boundary is common
in 2D echocardiographic imaging25 and is
not unique to either acoustic quantification or color kinesis. It has
been previously attributed to lung interference, myocardial anisotropy,
and/or the angle between muscle fibers and ultrasonic
beam.25 26 27
Quantitative Approach to Color Kinesis
Color kinesis provides on-line information on endocardial
excursion, which, together with wall thickening, constitutes the
primary descriptors of regional left ventricular
function.9 10 Because the endocardium is usually well
defined, automated methods used to assess regional left
ventricular function based on endocardial excursion are
easier to develop compared with methods that rely on automated
recognition of both endocardium and epicardium to evaluate wall
thickening.9
To obtain quantitative information on regional wall motion, we used
end-systolic frames for quantitative analysis
because they contain the entire spatial and temporal histories of
systolic endocardial excursion. To facilitate the comparison
between the results of analysis of color kinesis data and the
conventional segmentation recommended by the American Society of
Echocardiography,24 color-encoded
images obtained from the short-axis and apical four-chamber
views were automatically divided into wedge-shaped segments with
custom-designed software (Fig 3B).
Segmentation of color-encoded images in both views originated from the end-systolic left ventricular cavity centroid. We calculated the centroid from the left ventricular end-systolic cavity area rather than the centroid of the endocardial border line because the former approach has been shown to provide more reproducible data.28 29 Segmentation of the short-axis view was based on a zero line connecting the centroid with the junction of the right ventricular posterior wall endocardium and the interventricular septum. This latter anatomic landmark was chosen because it previously was shown to be minimally affected by cardiac rotation and translation at rest.30 31 In the apical four-chamber view, the anatomic landmarks used were two points at the mitral annulus and one point at the apical endocardium. These segmentation schemes were used to minimize the variability caused by individual differences in left ventricular orientation at end systole and therefore facilitate intersubject comparisons. Regional wall motion was then assessed by computing the pixel counts in each left ventricular segment normalized by the corresponding regional end-diastolic area to take into account intersegmental geometrical differences and quantify regional fractional area change for each consecutive time frame.
Normal Patterns of Regional Left Ventricular
Contraction
Although normal left ventricular contraction appears
to be homogeneous and symmetrical when assessed by 2D
echocardiography, off-line quantitative studies
have demonstrated intersegmental differences in endocardial
excursion.3 5 6 12 For example, Pandian et
al5 observed a variability in both the magnitude and
timing of contraction between adjacent segments in the parasternal
short-axis view, in agreement with the results of the present
study (Fig 6). Similar results were reported by Haendchen et
al,3 who analyzed segmental fractional area
changes at the midpapillary level and attributed this finding to the
presence of systolic protrusion of the papillary muscles.
Schnittger et al12 demonstrated differences in wall motion
in different echocardiographic views and showed that
the heterogeneity may also be due to the specific
methods of analysis used.
The patterns of regional endocardial excursion obtained with color
kinesis were highly consistent in normal subjects. The
intersubject variability of our segmental analysis (Fig 5B)
reflected individual differences in left ventricular
chamber geometry and function. With repeated data acquisitions and
analyses, the reproducibility of this technique proved to be
similar to that of other techniques based on manual tracing of
endocardial border.4 5
Detection of Regional Wall Motion Abnormalities
The ability of color kinesis to improve the qualitative evaluation
of regional wall motion was recently described by Schwartz et
al,32 who compared color kinesis images with 2D
echocardiograms. Automated detection of regional wall motion
abnormalities based on segmental analysis of color kinesis data
initially required the establishment of normal patterns of segmental
wall motion. Abnormal histograms were then compared on a
segment-by-segment basis to this normal reference as previously
described.7 12 We found that in this selected group of
patients, analysis of color kinesis images allowed automated
detection of regional wall motion abnormalities that was as accurate as
that provided by experts interpreting 2D echocardiograms.
Therefore, color kinesis may become a useful aid for the less experienced readers of echocardiograms because it can potentially direct the physician's attention toward specific segments. This technique may also become helpful in conveying echocardiographic findings to referring physicians in a single end-systolic color image that contains the entire picture of systolic contraction and has the advantage of easy digital storage and retrieval. Furthermore, quantitative segmental analysis of color kinesis images can provide an automated objective detection of regional wall motion abnormalities.
In addition to the abnormal amplitude of endocardial motion, we found that in segments with severely reduced motion, the mean time of contraction was shorter compared with normal segments in the same patient. This finding indicates that most of the residual motion in the abnormal segments occurs early in systole.
Study Limitations
This study constitutes, to the best of our knowledge, the first
report describing the methodology and potential clinical applications
of color kinesis. The main goal of this study was to describe the
technique and determine the feasibility of automated detection of
regional wall motion abnormalities. Consequently, the number of normal
subjects studied is not large enough to establish true CIs for a normal
population, which would require acquisition and analysis of
data from large numbers of subjects over a wide age range. These CIs
could be obtained as ±2 SD of large samples of the normal population
in a future multicenter study.
Similar to acoustic quantification and other echo-based techniques,
the success of color kinesis for improved visualization of regional
wall motion abnormalities is dependent on the quality of 2D
echocardiographic images.23 32 Therefore,
to determine the clinical value of color kinesis in routine
echocardiographic practice, a large number of
consecutive patients needs to be studied with this method, similar to
acoustic quantification.25 In our experience, it is
possible to acquire clinically useful color kinesis images in 
80%
to 85% of consecutive patients referred to our laboratory. Although we
have performed quantitative analysis of data obtained only from
short-axis and apical four-chamber views, the feasibility and
accuracy of this approach in other standard
echocardiographic views (parasternal long-axis and
apical two-chamber views) must be determined.
Similar to other quantitative methods that assess endocardial excursion, color kinesis is affected by cardiac translation and/or rotation.7 10 In its current format, color kinesis does not allow correction for translation and rotation. In agreement with the experience of Bates et al,30 we found translation to be only a minor confounding factor in the interpretation of regional systolic wall motion based on echocardiograms obtained at rest. The need for correction during stress testing has yet to be established. In addition, the analysis of endocardial motion described here would be confounded if applied to images obtained from patients with abnormal septal motion (left bundle branch block, diastolic flattening secondary to right ventricular volume/pressure overload). Also, color kinesis was developed to assess endocardial motion rather than wall thickening. The impact of this limitation on the ability of this technique to assess myocardial viability has yet to be determined.
Future Directions
Quantitative assessment of temporal heterogeneity
in regional wall motion is extremely difficult with standard
echocardiographic methods.33 34 35 Color
kinesis provides the opportunity to directly quantify the temporal
patterns of regional myocardial contraction and expansion. A possible
quantitative index of the timing of endocardial motion is the mean time
of contraction (Figs 4B and 6B). In our normal subjects, the mean time
of contraction was similar in all segments in both views. As our
measurements of the mean time of contraction in normal versus abnormal
segments suggest, further studies are required to determine the
clinical utility of this parameter.
Color kinesis also can be activated during
diastole. In this case, pixel value transitions from
myocardial tissue to blood are used to determine whether endocardial
expansion has occurred in a given pixel area (Fig 10,
right). This feature of color kinesis is of particular relevance
because abnormalities in regional left ventricular filling
and relaxation constitute early signs of myocardial
ischemia.36 37
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Acquisition of color kinesis data during the entire cardiac cycle could
provide regional wall motion curves similar to the global left
ventricular area versus time waveforms obtained with
acoustic quantification (Fig 10). Also, segmental endocardial
dyskinetic motion can be detected by use of a distinct color code to
tag paradoxical transitions from tissue to blood during systole. This
dyskinetic motion can be analyzed to quantify the outward
systolic expansion in the dyskinetic segments.
Dobutamine stress echocardiograms are usually interpreted subjectively rather than analyzed quantitatively. As a consequence, the interpretation is influenced by the reader's experience.38 Recently, computer-aided approaches for the quantification of regional left ventricular function, including color coding of digitized echocardiograms, have been attempted to standardize the interpretation of echocardiographic images.13 30 Color kinesis may constitute a useful on-line tool to objectively detect and quantify regional wall motion abnormalities under stress.39 We recently demonstrated the feasibility of acquisition and analysis of systolic color kinesis images obtained under various inotropic conditions and increased heart rates.40
Conclusions
In conclusion, color kinesis is a promising new tool that may be
used clinically to improve the qualitative and quantitative evaluation
of the spatial and temporal aspects of global and regional wall motion
in both short-axis and apical four-chamber imaging planes. In
this initial study, segmental analysis of color kinesis images
appeared to be as accurate as expert visual diagnosis of regional wall
motion abnormalities but with the advantage of being automated and
quantitative.
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Acknowledgments |
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Dr Philippe Vignon is a recipient of a grant of the French Ministère des Affaires Etrangères (Foundation Lavoisier) and is supported by the Société de Réanimation de Langue Française. We gratefully acknowledge the advice and support of John Davidson, Susan Floer, and Tony Vallance from Hewlett Packard Co.
Received July 17, 1995; revision received October 26, 1995; accepted November 7, 1995.
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