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Three-dimensional echocardiography: Limitations of apical biplane imaging for measurement of left ventricular volume
Three-dimensional Echocardiography: Limitations of Apical Biplane Imaging for Measurement of Left Ventricular Volume Peter M. Sapin, MD, Klaus M. Schr6eder, MD, Aasha S. Gopal, MD, Mikel D. Smith, MD, and Donald L. King, MD,
Lexington, Kentucky, and New York, New York
A new three-dimensional echocardiographic system creates a "line of intersection" display to allow precise and known positioning of echocardiographic images. Our purpose was to determine whether use of the line-of-intersection display will improve positioning of the apical four-chamber and apical two-chamber views and thereby improve the agreement between estimates of left ventricular volume by apical biplane echocardiography and cineventriculography. Unguided and line o f intersection-guided apical biplane views were ohmined in 31 patients immediately before cardiac catheterization and single-plane cineventriculography. I n 1 5 patients the line-of-intersection display was used to measure the position of the image plane in studies of unguided and guided methods. Linear regression and limits of agreement analysis were used to assess the agreement between cineventriculographic volumes and echocardiographic volumes determined from each set of images. The Wilcoxon test was used to compare guided and unguided image positioning. The line-ofintersection display improved four-chamber and twochamber view positioning closer to the center of the ventricle and rotation closer to orthogonal positioning. Guided-image positioning was not able to correct displacement of the ultrasound beam anterior to the
ventricular apex without deterioration of image quality in most patients. Despite improvements in image plane positioning, the agreement between echocardiographic and cineventriculographic volumes was unchanged. For end-diastole views, the unguided images had an r value = 0.84, standard error of the estimate of _+23.0 cc, and limits of agreement of +62.4 cc. Corresponding values for the guided images at end diastole were r = 0 . 8 5 , standard error of the estimate of +22.9 cc, and limits of agreement of_+60.8 cc. At end systole the unguided results were r = 0.94, standard error of the estimate of 14.5 cc, limits of agreement of +49.0 cc, and the guided results were r = 0.91, standard error o f the estimate of 16.8 cc, and limits of agreement of +52.2 cc. The line-of-intersection guiding of image plane positioning can improve apical image positioning but does not improve the agreement between apical biplane echocardiographic and cineventriculographic left ventricular volumes. The optimal apical imaging window is frequently occluded by the rib cage, resulting in a decrease in image quality. This reduction of image quality, combined with assumptions of left ventricular geometry, limit the accuracy of estimates of left ventricular volume from apical biplane echocardiography. (J AM Soc ECHOCARDIOGR1995;8:576-84.)
Kaineventriculography is often used to measure left ventricular volume, but its invasive nature makes it impractical for routine or serial measurements. Two-dimensional echocardiography is an alternative, widely used noninvasive technique capable o f measuring left ventricular volume. Although its results have
correlated well with those obtained by cineventriculography, diftkrences between two-dimensional echocardiographic and cineventriculographic volumes in individual patients can be considerable)7 One reason for this variability is malpositioning o f the echocardiographic images caused by rib occlusion of the apical acoustic window. In a significant fraction o f patients, occlusion o f the apical window by the underlying ribs causes displacement of the transducer above or below the apex, resulting in foreshortening o f the ventricle. 4,s It may also result in displacement o f the imaging plane from the center o f the ventricle and loss o f the desired 90-degree rotation between the two-chamber and four-chamber views. These image plane positioning errors can contribute to variability of two-dimensional echo-
From the Division of Cardiology,Universityof KentuckyMedical Center, and Columbia University College of Physicians and Surgeons. Reprint requests: Peter M. Sapin, MD, Division of Cardiology MN-670, University of Kentucky Medical Center, 800 Rose St., Lexington, KY40536. Copyright 01995 by the American Societyof Echocardiography. 0894-7317/95 $5.00 + 0 27/1/63017 576
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cardiographic left ventricular volumes calculated by this m e t h o d . 4'5"8'9 Recently a three-dimensional echocardiographic system has been developed that provides a means to assist the operator to obtain m o r e precise and reproducible image position and orientation. This means is an interactive line-of-intersection display. 1~ I n this display an image is first acquired to serve as a reference image. Subsequent real-time images are obtained in different planes and their lines o f intersection with the reference image are c o m p u t e d and displayed as a white line in the reference image. T h e line o f intersection moves as the real-time imaging plane is altered and is used as a guide for positioning the real-time image with respect to the refkrence image. It has previously been shown that the line-ofintersection display may be used to improve position and orientation o f the apical views. 9 We postulated that i m p r o v e d positioning w o u l d result in closer agreement between left ventricular volumes comp u t e d by the apical biplane m e t h o d and those obtained by cineventriculography. To test this hypothesis, we obtained standard u n g u i d e d apical biplane views, followed by guided apical biplane views in 31 patients u n d e r g o i n g cardiac catheterization. Volumes were calculated f r o m b o t h sets o f apical views and were c o m p a r e d with cineventriculographic volumes.
METHODS Study Population We performed echocardiography in 31 unselected patients undergoing elective cardiac catheterization who had an adequate cineventriculogram for volume computation. The 31 patients had a mean age of 48 +- 10 years (range 32 to 69 years), and 20 (63%) were men.
Instrumentation Unguided two-dimensional echocardiographic images were obtained with a 2.5 M H z ultrasound transducer and a commercial two-dimensional scanner (model 77020AC; Hewlctt-Packard Co., Andovcr, Mass.). Thc threedimensional echocardiographic system has been described previouslyY '~~ Briefly, the three-dimensional scanner consists of the above real-time two-dimensional scanner linked to an acoustic three-dimensional spatial locater (model GP 8-3D; Science Accessories Corp., Stratford, Conn.) that associates three-dimensional spatial coordinates with each set of images. The real-time images and spatial coordinates are transmitted to a personal computer (model 325D; Dell Computer Corp., Austin, Texas) that controls system operation and provides a means for subsequent three-
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dimensional analysis of acquired data. The threedimensional acoustic spatial locater consists of an array of three sound emitters rigidly attached to a 2.5 MHz ultrasound transducer and an array of four microphones suspended over the examination table, as well as electronic circuitry necessary for their operation. The sound emitters, energized in rapid sequence, produce 60 kHz sound waves that travel to each of the four overhead microphones. The time taken for sound to travel from each emitter to each microphone is measured, corrected for environmental conditions, and used to calculate a range between the two points. From these ranges the x, y, and z Cartesian coordinates of the transducer and subsequently its image are computed in a spatial coordinate system defined by the microphone array. The accuracy of the three-dimensional acoustic spatial locater is approximately 0.1% of the distance from the sound emitter to the microphone. The acoustic locater data are checked continuously, and data are rejected if computed lengths and angles fbrmed by the three sound emitters mounted on the transducer do not fall within specified limits of_+1 mm or _+1 degree. The digitized real-time images and their spatial coordinate data are combined in the computer video display to produce an interactive line-of-intersection display of the relative position and orientation of a reference image and a real-time image. A sequence of 16 frames, every other video frame gated to the electrocardiographic R wave, can be acquired and displayed as a cine loop.
Imaging Protocol Patients were in a fhsting state, were given their usual medications, and underwent two echocardiographic studies 1/2 to 3 hours befbre transportation to the cardiac catheterization laboratory. In the first study, designated "unguided," apical four-chamber and two-chamber views were obtained in the standard fashion, with reference to anatomic landmarks and the appearance of the cardiac chambers. Views were recorded on 1/2-inch VHS videotape. The patient then underwent a second study, designated "guided," in which the same ultrasonographer reobtained the apical biplane views using the line-of-intersection display to guide positioning of the image planes according to predetermined criteria. The criteria were (1) four-chamber image plane passing through the center of the ventricle and interventricular septum at the mitral level, (2) twochamber image plane passing through the center of the ventricle at 90 degrees to the four-chamber image plane at the mitral level, and (3) both image planes bisecting the ventricle at the level of the apex. Locating the apical images was performed with the lineof-intersection display. First, reference short-axis images at the level of the body of the mitral valve leaflets and the tip of the left ventricular apex (defined by an end-diastolic dimension estimated at 1 to 2 cm) were obtained to guide positioning. The computer keyboard was used to retrieve each of these images for viewing, with the line of intersection of the real-time image displayed to indicate the relationship of the plane of the apical image to the reference
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image (Figure 1), When the line of intersection was positioned as close as possible to the above criteria, with at least half of the endocardial boundary visible, the apical images were acquired on videotape, and the reference images and lines of intersection were saved in the computer for later analysis. The additional time required to perform a guided echocardiographic study for calculation of left ventricular volume was 10 to 15 minutes. For guided and unguided studies, left ventricular endocardial outlines were traced and volumes were computed by commercial software (Nova Microsonics, Allendale, N.J.) with a summation-of-disks algorithm with 32 disks.* Left vcntricular volume was calculated by one observer, averaging wvo to three beats, with blinding to the cineventriculographic results.
Cineventriculography Patients underwent diagnostic catheterization by the femoral approach. A General Electric Advantx L U C / L P DXC Highline angiographic system (General Electric Co., Cleveland, Ohio) was used for imaging. A focal spot size of 0.9 mm, a 15 cm field of view, and a nominal x-ray exposure of 25 gR/frame were used. After coronary arteriography, a 6F pigtail catheter was inserted into the body of the left ventricle. Cineventriculograms were filmed in the right anterior oblique projection at 30 frames per second during the power injection of 35 to 50 ml iohexol at 15 ml/sec. The magnification factor in each projection was determined by filming a radiopaque grid or metal sphere of known dimensions at the estimated level of the midventricle. Cine films were viewed on a projector and the outlines o f the left ventricular cavity at visually estimated end diastole and end systole, along with the calibrating device, were traced on transparent acetate overlying the projector screen. Only sinus beats not following a premature beat were used for volume calculations. The outline of the papillary muscle was included in the left ventricular cavity. A digitizing pad and commercial software were used to calculate left ventricular volumc by a single-plane summation-of-disks method. These values were then adjusted for overestimation with the regression equations developed by Wynne et al. H
Analysis o f Image Plane Positioning In the last 15 patients, the line-of-intersection display was created and saved during the initial unguided study but was not referred to by the ultrasonographcr as it was during the guided studies. The short-axis cross section at the apical and mitral levels, with the computed lines of intersection, were then recalled and displayed on the computer monitor. The endocardial outline (excluding papillary muscles) and the lines of intersection were traced on transparent acetate overlying the monitor, and the following angular measurements were performed (Figure 2): (1) the angle between four-chamber and two-chamber views and (2) the angle between the obtained four-chamber view and the "ideal" fbur-chambcr view. Linear measurements
o f the distance of four-chambcr and two-chamber views from the center of the left ventricle were also performed (Figure 2, C through E). A diameter through the ventricle was constructed at right angles to the obtained image plane. The location of the image plane from one side of the ventricle to the other was expressed as a percent of the diameter, with 50% representing the image plane at the center. Thus for the four-chamber view (Figure 2, C), less than 50% represents inferomedial linear displacement and greater than 50% represents antcrolateral linear displacement. For the two-chamber view (Figure 2, D), less than 50% represents posterolateral linear displacement and greater than 50% represents anteromedial linear displacement. At the apical level, the measurement recorded was the distance of the four-chamber view from the center of the left ventricle, according to the convention described above in Figure 2, C.
Statistical Analysis Values are expressed as mcan _+ i SD. Least squares linear regression was used to compare the results o f each echocardiographic method with the results of cineventriculography. The differences between echocardiographic cineventriculography were compared with the mean values obtained by echocardiography and cineventriculography by the method of Altman and Bland, 12 with the limits of agreement defined as + 2 SD of the difference between the two methods. The Wilcoxon test was used to compare the diffkrence between ideal and obtained image position by guided and unguided methods. A p value <0.05 was considcrcd significant.
RESULTS Patient Characteristics A t cardiac catheterization, 17 patients h a d n o r m a l left vcntricular c o n t r a c t i o n , t w o h a d s e g m e n t a l hypoldncsis, five h a d diffuse hypokincsis, six h a d dyskinctic s e g m e n t s , and one h a d an akinetic s e g m e n t . T h e m e a n cinevcntficulographic end-diastolic volu m e was 127 _+ 54 cc (53 to 301 cc), and the m e a n cineventriculographic e n d systolic v o l u m e was 59 + 54 cc (10 to 248 cc).
Guided and Unguided Image Plane Positioning F i g u r c 3 shows the data for g u i d e d a n d u n g u i d e d image plane positions in the f o r m o f a b o x plot. F o r four o f the five p a r a m e t e r s o f i m a g e plane p o s i t i o n (angle b e t w e e n f o u r - c h a m b e r a n d t w o - c h a m b e r views, angle b e t w e e n o b t a i n e d a n d ideal fourc h a m b e r planes, a n d d i s p l a c e m e n t o f f o u r - c h a m b c r and t w o - c h a m b e r views f r o m the center o f the left ventricle at the mitral le'eel), the m a g n i t u d e o f the
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Figure 1 Reference images and lines of intersection. Left pond illustrates unguided image position at level of mitral leaflets. Right panel illustrates how line-of-intersection display can be used to improve position of image planes. During study, operator views both short-axis reference image (shown) and real-time imagc. White line represents intersection of real-time apical image and reference image and moves as operator alters apical image position.
malposition with the unguided imagcs was significantly greater than the magnitude of the malposition recorded when line-of-interscction guiding was used. However, in reference to the apical short-axis view, all o f the unguided four-chamber images were positioned anterior to the center of the left ventricle. In almost all cases, moving the transducer so that thc line o f intersection of the four-chamber view passed through the center of the apical cross section resulted in a major deterioration in image quality. This fbrced the operator to move the transducer again cephalad, to obtain an image with an acceptable visual representation o f the endocardial surface.
Bias and Limits of Agreement Both cchocardiographic methods tended to underestimate the results o f cineventriculography (Figurns 4 and 5). The mean value for the difference (cineventriculographic volume minus echocardiographic volume) was i8.5 _+ 30.4 cc for guided enddiastolic volume and 23.0 _+ 31.2 cc for unguided end-diastolic volume ( p = 0.20). For end-systolic volumes, the mean underestimation for the guided and unguided methods, respectively, was 2.5 +_+_26.1 cc and 5.6 + 24.5 cc (p = 0.32).
Linear Regression The results of linear regression are shown in Figurcs 4 and 5. The relationships were all highly significant (p < 0.001). At end diastole, guided echocardiography had an r-- 0.85 and standard error o f
C D
Figure 2 Measurements made to quantitate image plane position. A, Angle between four-chamber and t~vochamber images. B, Angle between actual (heavy line) and ideal (dotted line) four-chamber image plane. C, D, and E, Dotted lines represent diameters at 90 degrees to obtained image plane. Location of image plane is expressed as fraction of short-axis diameter, from one to opposite endocardial surface. Thus value of 0.50 indicates image position at center of short-axis view, and values of 0 and 1.00 indicate positions tangential to endocardium at opposite sides of ventricle. Values greater than 1.00 denote positions outside of endocardium. C, Position of four-chamber view is measured at 0.25. E, Position of four-chamber view is 1.80.
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Mitral level
Mitral level A = Angle between 4- and 2-chamber views (degrees)
B = Angle between obtained and ideal 4-chamber views (degrees)
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Figure 3 Guided and unguided image plane positioning displayed in box plot. Box indicates 25th to 75th percentiles of data, with cross bar at 50th percentile. Whiskers indicate 10th and 90th percentile ranges, and filled circles are remaining outliers. Vertical dimension of display is therefore indicator of variability of image position for each echocardiographic technique. Differences between ideal and obtained image position for unguided and guided techniques were compared by Wilcoxon test. * p < 0.05; ** p < 0.01. Letters (A through E) and legends above each plot refer to definitions of image plane position illustrated in Figure 2. (LV, Left ventricular.)
the estimate = 22.9 cc, and unguided cchocardiography had r = 0.84 and standard error o f the estimate = 23.0 cc. At end systole, guided echocardiography had r = 0.91 and standard error o f the estimate = 16.8 cc, and unguided echocardiography had r = 0.94 and standard error o f the estimate = 14.5 cc. No formal comparison between the results for patients with normal and abnormal ventricles was performed, given the relatively small number in the latter subgroup. However, the data points for the 10 patients with segmental aldnesis (n = 1), dyskinesis ( n = 6), and severe global hypokinesis ( n = 3) are denoted in Figures 4 and 5.
DISCUSSION The standard two-dimensional echocardiographic method for determining left vcntricular volume, the apical biplane summation-of-disks algorithm, relies on two apical views that are assumed to bisect the ventricle and be at right angles to each other. 8 These images are obtained by use o f visualized anatomic landmarks and the proprioceptive sense o f the operator. However, previous studies o f conventionally ob-
tained images' and recent studics with our threedimensional echocardiographic system suggest that the actual position o f the image plane often differs significantly from what is assumed by the algorithm. 9 This study was designed to investigate whether use o f the line-of intersection display of our threedimensional echocardiographic scanner to guide positioning o f the apical views would improve the agreement between estimates o f biplane echocardiographic and single-plane cineventriculographic left ventricular volume. The results o f the study show that improvements in image plane positioning were obtained by line-of-intersection guided imaging, but these improvements did not result in better agreement between echocardiographic and cineventriculographic volumes.
ReasonsForLackof Improvementin VolumeEstimates It was found that all of the unguided apical fourchamber images passed through the ventricle anterior to the apex, resulting in foreshortening o f the ventricle in the image. This occurred because the rib cage at that level partially occludes the ideal apical imaging window and the operator moved to the
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Figure 4 Linear regression and mean versus difference plots for end-diastolic volumes. Results of linear regression of end-diastolic volume estimates by cineventriculography on y axis against echocardiographic methods on x axis. A, Results for guided echocardiography; B, results for unguided echocardiography. A, Open squares indicate patients with normal or hypokinctic mild global or segmental wall motion; filled squares indicate 10 patients with segmental akinesis, dyskinesis, or severe global hypokinesis. The same two groups are denoted in B by open and semifilled circles, respectively. C, Mean against difference plots, with two methods combined (open boxe~, guided; filled circles, unguided). The two wall motion groups are not identified in plot. Vertical columns on right show limits of agreement above and below mean value (center bar). (SEE, Standard error of estimate.) adjacent rib interspace to optimize image quality and border recognition. In some patients, transducer motion to optimize the position o f the four-chamber and two-chamber views at the mitral level and angular relationship o f the four-chamber and twochamber views also resulted in some deterioration in image quality. Thus the obtained guided images represent a compromise between image plane position and image quality, and the improvements in image position that were seen were not sufficient to overcome other errors.
Discrepancy Between Echocardiography and Cineventriculography In addition to the errors in image plane positioning already discussed, several other factors help to explain the similar discrepancies between unguided and
guided echocardiographic and cineventriculographic volumes. Recognition o f the endocardial border is a well-known difficulty common to both imaging methods. In the echocardiographic apical views, the endocardium close to the apex is often not well visualized. In addition, the two techniques use different conventions for tracing endocardial borders, with echocardiography tending to exclude and cineventriculography include the blood in the trabeculae carneae. 4 Also, apical biplane echocardiography excludes the left ventricular outflow tract, which is included in the cineventriculographic volume. Finally, both methods make assumptions about the shape o f the left ventricle that may not be valid in abnormally shaped ventricles. For example, for each disk in the apical biplane summation-of-disks method, four points are used to define an elliptic
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cross section of the ventricle. In single-planc cinevcntriculography, only two points arc obtained and define a circular cross section. These few points may be inadequate for representing the shape of the left ventricle, particularly in the presence o f distorted ventricular geometry.
Previous Studies That Use the Apical Biplane Method The use of paired apical four-chamber and twochamber views and a summation-of-disks algorithm is recommended by the American Society for Echocardiography for calculating left vcntricular volume. 8 One study that used this technique found the correlation coefficient between echocardiographic and single-plane cineventriculographic volumes in 70 patients to bc 0.80 for end-diastolic volume and 0.88 for end-systolic volume, with standard errors of the estimate for the respective regression cquations being _+34 and _+27 cc. ~ The results in a subset of 30 patients who underwent biplane cincvcntriculography wcre nearly identical. Another group performed a similar study in 30 patients undergoing biplane cincventriculography and found the correlations and standard errors of the estimate for end-diastolic and
end-systolic volumes to be 0.80 and +15 c c / m 2 and 0.90 and +8.5 c c / m 2, respectively.7 These studies in adult patients found echocardiography to underestimate cineventriculographic volumes, in agreement with studies that use a variety of other twodimensional multiplane and single-plane echocardiographic m e t h o d s ) 7
Implications For Quantitative Echocardiography Application o f the line-of-intersection display. Bccause thc linc-of-intcrscction display allows the operator to observe the truc relation o f a given image plane to nonvisualized anatomic landmarks in the heart, imagc plane positions and thc measurements derived from them can bc made morc reproducible. Thus the line-of-intersection display has been shown to improve the rcproducibility o f left vcntricular and left atrial diameter measurements, ~3 measurements o f left vcntricular volume, 14 and apical biplane mcasurements o f left atrial volume. 1~ Three-dimensional echocardiographic volu m e computation. Although guided apical image plane positioning did not improve estimates o f left ventricular volume with a biplane summation-of-
Journal of the AmericanSocietyof Echocardiography Volume 8 Number 5, Part 1
disks algorithm, the line-of-intersection display can be used to guide the positioning o f multiple shortaxis images. These images can be employed in a n e w algorithm for volume c o m p u t a t i o n based on polyhcdral surface reconstruction ~6 to improve the agreem e n t between estimatcs o f cineventriculographic and echocardiographic v o l u m e ) 7"I8 O n e advantage o f this technique is that polyhedral surface reconstruction does n o t require parallel or evenly spaced images and does n o t make assumptions a b o u t left ventricular shape, thus allowing images to be adjusted for better endocardial definition w i t h o u t affecting volume calculations. O t h e r investigators have used the spark-gap acoustic spatial locater and a different surface reconstruction algorithm to perform three-dimensional reconstruction, with excellent results. 19'2~This alternative system also allows r a n d o m image acquisition, with opcrator recall o f highquality images for b o u n d a r y tracing. O t h e r threcdimcnsional echocardiographic systems restrict transducer m o t i o n to graded rotation about a point, lincar m o t i o n , or pivoting/flexion. 2~ 2~ These systems have been studied mainly in in vitro or animal models, with little validation in patients. Given the p r o b l e m o f limited acoustic access to the heart from outside the body, it m i g h t be expected that transthoracic restricted transducer m o t i o n systems m i g h t have difficulty obtaining the requisite n u m b e r o f high-quality images for accurate three-dimensional reconstruction. O n the other hand, a newly develo p c d transesophageal system m i g h t be able to circ u m v e n t this difficulty. 2s
Study Limitations I n this study echocardiography and cinevcntriculography wcrc p e r f o r m e d nonsimultancously, and cincvcntriculography was p e r f o r m e d after c o r o n a r y artcriography. These factors may have resulted in changes in vcntricular volume that introduced further variability into the results, a l t h o u g h this w o u l d n o t be expected to alter the comparsion o f the two
echocardiographic methods. Conclusions Line o f intersection-guided positioning o f apical four-chamber and t w o - c h a m b e r views does n o t improve the agreement between left ventricular volumes obtained t?om these views and singlc-plane cineventriculography. Apical views are c o m m o n l y displaced anteriorly because o f occlusion o f the optimal imaging w i n d o w by ovcrlying ribs, with a consequent foreshortening o f the ventricular image. Imp r o v e d image position by use o f the line-ofintersection display was accompanied by a decrease o f image quality that impaired border recognition. These o p p o s i n g factors may a c c o u n t for the lack o f
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i m p r o v e m e n t fbund in this study. The inherent anatomic limitation o f the apical window, rib interference, limits the accuracy o f computations o f apical biplane ventricular volume that require assumptions about image plane positioning and left ventricular gcomctry. We thank Rosctta Hall, RDCS, and Annette Smith, RDCS, for their technical assistance.
REFERENCES 1. Tortoledo FA, Quinones MA, Fernandez GC, Waggoner AD, Winters WL. Quantification of left ventricular volumes by two-dimensional echocardiography: a simplified and accurate approach. Circulation 1982;67:579-84. 2. Parisi AF, Moynihan PF, Feldman CL, Folland ED. Approaches to determination of left ventricular volume and ejection fraction by reabtime two-dimensional echocardiography. Clin Cardiol 1979;2:257-63, 3. Schiller NB, Aquatella H, Ports TA, et al. Left ventricular volume ftom paired biplane two-dimensional echocardiography. Circulation 1979;60:547-55. 4. Schnittgcr I, Fitzgerald PJ, Daughters GT, et al. Limitations of comparing left ventricular volumes by two-dimensional echocardiography, myocardial markers and cineangiography. Am J Cardiol 1982;50:512-9. 5. Erbel R, Schweizer P, Lambertz H, et al. Echoventriculography: a simultaneous analysis of two-dimensional echocardiography and cineventriculography. Circulation 1983;67:205-15. 6. Folland ED, Parisi AF, Moynihan PF, Jones DR, Feldman CL, Tow DE. Assessment of left ventricular ejection fraction and volumes by real-time, two-dimensional echocardiogra phy. Circulation 1979;60:760 6. 7. Starling MR, Crawford MH, Sorensen SG, Levi B, Richards KL, O'Rourke RA. Comparative accuracy of apical biplane cross-sectional echocardiography and gated equilibrium radionuclide angiography for estimating left ventricular size and performance. Circulation 1981;63:1075-84. 8. Schiller NB, Shah PM, Crawford M, et al. Recommendations for quantitation of the left ventricle by two-dimensional echocardiography. J A,u Soc ECHOCAe,DIOGR1989;2:358-67. 9. King DL, Harrison MR, King DL Jr, Gopal AS, I4wan OL, DeMaria AN. Ultrasound beam orientation during standard two-dimensional imaging: assessment by three-dimensional echocardiography. J AM Soc ECHOCARDIOGR1992;6:569-76. 10. King DL, King DL Jr, Shao MY-C. Three-dimensional spatial registration and interactive display of position and orientation of real time ultrasound images. J Ultrasound Med 1990;9: 525-32. 11. Wynne J, Green LH, Mann T, Levin D, Grossman W. Estimation of left ventricular volumes in man from biplane cineangiograms filmed in oblique projections. Am J Cardiol 1978;41:726-32. 12. Altman DG, Bland JM. Measurement in medicine: the analysis of method comparison studies. Statistician 1983;32:30717. 13. King DL, Harrison MR, King DL Jr, Gopal AS, Martin RP, DeMaria AN. Improved reproducibility of left atrial and left ventricular measurements by guided three-dimensional echo cardiography. J Am Coil Cardiol 1992;20:1238 45. 14. Schr6eder I(M, Sapin PM, King DL, Smith MD, Kwan OL, DeMaria AN. Improved reproducibility of left ventricular volume measurements using guided two-dimensional and
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three-dimensional echocardiography [Abstract]. Circulation 1992;86:I-271. Schrteder KM, Sapin PM, Xie G, et al. Three-dimensional guiding of two-dimensional echocardiography improves the reproducibility of serial left atrial volume measurements [Abstract]. J AM Soc EC~OCAe,DIOCR1993;6:S22. Cook LT, Cook PN, Lee KR, et al. An algorithm for volume estimation based on polyhedral approximation. IEEE Trans Biomed Eng 1980;27:493-500. SapinPM, Schr6der KM, Smith MD, DeMariaAN, ICingDL. Three-dimensional echocardiographic measurement of left ventricular volume in vitro: comparison with twodimensional echocardiography and cineventriculography. J Am Coil Cardiol 1993;22:1530-7. Sapin PM, Schr6der KM, Gopal AS, Smith MD, DeMaria AN, King DL. Comparison of three-dimensional echocardiography and two-dimensional echocardiography to cineventriculography for measurement of left ventricular volume in patients. J Am Coll Cardiol 1994;24:1054-63. Sin SC, Rivera M, Guerrero JL, et al. Three-dimensional echocardiography: in vivo validation for left ventricular volume and function. Circulation 1993;88:1715-23. Handschumacher MD, Lethor J-P, Siu SC, et al. A new
21.
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integrated systern for three-dimensional echocardiographic reconstruction: development and validation for ventricular volume with application in human subjects. J Am Coil Cardiol 1993;21:743-53. Nixon IV, Saffer SI, Lipscomb K, Blomqvist CG. Threedimensional echoventriculography. Am Heart J 1983;106: 435-43. Ghosh A, Nanda NC, Maurer G. Three-dimensional reconstruction of echocardiographic images using the rotation method. Ultrasound Med Biol 1982;8:655-61. Kuroda T, Kinter TM, Seward JB, Yanagi H, Greenleaf JF. Accuracy of three-dimensional volume measurement using biplane transesophageal echocardiographic probe: in vitro experiment. J A~ Soc EcHocaamioG• 1991;4:475-84. Martin KW, Bashein G. Measurement of stroke volume with three-dimensional transesophagealultrasonic scanning.Anesthesiology 1989;70:470-6. Pandian NG, Nanda NC, Schwartz SL, et al. Threedimensional and four-dimensional transesophageal echocardiographic imaging of the heart and aorta in humans using a computed tomographic imaging probe. Echocardiography 1992;6:677-87.
B o u n d volumes available to subscribers Bound volumes of the JOURNAL OF THE AMERICAN SOCIETY OF ECHOCARDIOGRAPHY are available to subscribers (only) for the 1995 issues from the Publisher at a cost of $43.50 for domestic, $50.50 Canadian, and $50.50 for international subscribers for Vol. 8 (JanuaryDecember). Shipping charges are included. Each bound volume contains a subject and author index, and all advertising is removed. The binding is durable buckram with the JOURNALname, volume number, and year stamped in gold on the spine. Payment must accompany all orders. Contact Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63146-3318, USA; telephone (800)325-4177, ext. 4351, or (314)453-4351. Subscriptions m u s t be in force to qualify. B o u n d volumes are n o t available in place o f a regular JOURNAL subscription.
Lexington, Kentucky, and New York, New York
A new three-dimensional echocardiographic system creates a "line of intersection" display to allow precise and known positioning of echocardiographic images. Our purpose was to determine whether use of the line-of-intersection display will improve positioning of the apical four-chamber and apical two-chamber views and thereby improve the agreement between estimates of left ventricular volume by apical biplane echocardiography and cineventriculography. Unguided and line o f intersection-guided apical biplane views were ohmined in 31 patients immediately before cardiac catheterization and single-plane cineventriculography. I n 1 5 patients the line-of-intersection display was used to measure the position of the image plane in studies of unguided and guided methods. Linear regression and limits of agreement analysis were used to assess the agreement between cineventriculographic volumes and echocardiographic volumes determined from each set of images. The Wilcoxon test was used to compare guided and unguided image positioning. The line-ofintersection display improved four-chamber and twochamber view positioning closer to the center of the ventricle and rotation closer to orthogonal positioning. Guided-image positioning was not able to correct displacement of the ultrasound beam anterior to the
ventricular apex without deterioration of image quality in most patients. Despite improvements in image plane positioning, the agreement between echocardiographic and cineventriculographic volumes was unchanged. For end-diastole views, the unguided images had an r value = 0.84, standard error of the estimate of _+23.0 cc, and limits of agreement of +62.4 cc. Corresponding values for the guided images at end diastole were r = 0 . 8 5 , standard error of the estimate of +22.9 cc, and limits of agreement of_+60.8 cc. At end systole the unguided results were r = 0.94, standard error of the estimate of 14.5 cc, limits of agreement of +49.0 cc, and the guided results were r = 0.91, standard error o f the estimate of 16.8 cc, and limits of agreement of +52.2 cc. The line-of-intersection guiding of image plane positioning can improve apical image positioning but does not improve the agreement between apical biplane echocardiographic and cineventriculographic left ventricular volumes. The optimal apical imaging window is frequently occluded by the rib cage, resulting in a decrease in image quality. This reduction of image quality, combined with assumptions of left ventricular geometry, limit the accuracy of estimates of left ventricular volume from apical biplane echocardiography. (J AM Soc ECHOCARDIOGR1995;8:576-84.)
Kaineventriculography is often used to measure left ventricular volume, but its invasive nature makes it impractical for routine or serial measurements. Two-dimensional echocardiography is an alternative, widely used noninvasive technique capable o f measuring left ventricular volume. Although its results have
correlated well with those obtained by cineventriculography, diftkrences between two-dimensional echocardiographic and cineventriculographic volumes in individual patients can be considerable)7 One reason for this variability is malpositioning o f the echocardiographic images caused by rib occlusion of the apical acoustic window. In a significant fraction o f patients, occlusion o f the apical window by the underlying ribs causes displacement of the transducer above or below the apex, resulting in foreshortening o f the ventricle. 4,s It may also result in displacement o f the imaging plane from the center o f the ventricle and loss o f the desired 90-degree rotation between the two-chamber and four-chamber views. These image plane positioning errors can contribute to variability of two-dimensional echo-
From the Division of Cardiology,Universityof KentuckyMedical Center, and Columbia University College of Physicians and Surgeons. Reprint requests: Peter M. Sapin, MD, Division of Cardiology MN-670, University of Kentucky Medical Center, 800 Rose St., Lexington, KY40536. Copyright 01995 by the American Societyof Echocardiography. 0894-7317/95 $5.00 + 0 27/1/63017 576
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cardiographic left ventricular volumes calculated by this m e t h o d . 4'5"8'9 Recently a three-dimensional echocardiographic system has been developed that provides a means to assist the operator to obtain m o r e precise and reproducible image position and orientation. This means is an interactive line-of-intersection display. 1~ I n this display an image is first acquired to serve as a reference image. Subsequent real-time images are obtained in different planes and their lines o f intersection with the reference image are c o m p u t e d and displayed as a white line in the reference image. T h e line o f intersection moves as the real-time imaging plane is altered and is used as a guide for positioning the real-time image with respect to the refkrence image. It has previously been shown that the line-ofintersection display may be used to improve position and orientation o f the apical views. 9 We postulated that i m p r o v e d positioning w o u l d result in closer agreement between left ventricular volumes comp u t e d by the apical biplane m e t h o d and those obtained by cineventriculography. To test this hypothesis, we obtained standard u n g u i d e d apical biplane views, followed by guided apical biplane views in 31 patients u n d e r g o i n g cardiac catheterization. Volumes were calculated f r o m b o t h sets o f apical views and were c o m p a r e d with cineventriculographic volumes.
METHODS Study Population We performed echocardiography in 31 unselected patients undergoing elective cardiac catheterization who had an adequate cineventriculogram for volume computation. The 31 patients had a mean age of 48 +- 10 years (range 32 to 69 years), and 20 (63%) were men.
Instrumentation Unguided two-dimensional echocardiographic images were obtained with a 2.5 M H z ultrasound transducer and a commercial two-dimensional scanner (model 77020AC; Hewlctt-Packard Co., Andovcr, Mass.). Thc threedimensional echocardiographic system has been described previouslyY '~~ Briefly, the three-dimensional scanner consists of the above real-time two-dimensional scanner linked to an acoustic three-dimensional spatial locater (model GP 8-3D; Science Accessories Corp., Stratford, Conn.) that associates three-dimensional spatial coordinates with each set of images. The real-time images and spatial coordinates are transmitted to a personal computer (model 325D; Dell Computer Corp., Austin, Texas) that controls system operation and provides a means for subsequent three-
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dimensional analysis of acquired data. The threedimensional acoustic spatial locater consists of an array of three sound emitters rigidly attached to a 2.5 MHz ultrasound transducer and an array of four microphones suspended over the examination table, as well as electronic circuitry necessary for their operation. The sound emitters, energized in rapid sequence, produce 60 kHz sound waves that travel to each of the four overhead microphones. The time taken for sound to travel from each emitter to each microphone is measured, corrected for environmental conditions, and used to calculate a range between the two points. From these ranges the x, y, and z Cartesian coordinates of the transducer and subsequently its image are computed in a spatial coordinate system defined by the microphone array. The accuracy of the three-dimensional acoustic spatial locater is approximately 0.1% of the distance from the sound emitter to the microphone. The acoustic locater data are checked continuously, and data are rejected if computed lengths and angles fbrmed by the three sound emitters mounted on the transducer do not fall within specified limits of_+1 mm or _+1 degree. The digitized real-time images and their spatial coordinate data are combined in the computer video display to produce an interactive line-of-intersection display of the relative position and orientation of a reference image and a real-time image. A sequence of 16 frames, every other video frame gated to the electrocardiographic R wave, can be acquired and displayed as a cine loop.
Imaging Protocol Patients were in a fhsting state, were given their usual medications, and underwent two echocardiographic studies 1/2 to 3 hours befbre transportation to the cardiac catheterization laboratory. In the first study, designated "unguided," apical four-chamber and two-chamber views were obtained in the standard fashion, with reference to anatomic landmarks and the appearance of the cardiac chambers. Views were recorded on 1/2-inch VHS videotape. The patient then underwent a second study, designated "guided," in which the same ultrasonographer reobtained the apical biplane views using the line-of-intersection display to guide positioning of the image planes according to predetermined criteria. The criteria were (1) four-chamber image plane passing through the center of the ventricle and interventricular septum at the mitral level, (2) twochamber image plane passing through the center of the ventricle at 90 degrees to the four-chamber image plane at the mitral level, and (3) both image planes bisecting the ventricle at the level of the apex. Locating the apical images was performed with the lineof-intersection display. First, reference short-axis images at the level of the body of the mitral valve leaflets and the tip of the left ventricular apex (defined by an end-diastolic dimension estimated at 1 to 2 cm) were obtained to guide positioning. The computer keyboard was used to retrieve each of these images for viewing, with the line of intersection of the real-time image displayed to indicate the relationship of the plane of the apical image to the reference
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image (Figure 1), When the line of intersection was positioned as close as possible to the above criteria, with at least half of the endocardial boundary visible, the apical images were acquired on videotape, and the reference images and lines of intersection were saved in the computer for later analysis. The additional time required to perform a guided echocardiographic study for calculation of left ventricular volume was 10 to 15 minutes. For guided and unguided studies, left ventricular endocardial outlines were traced and volumes were computed by commercial software (Nova Microsonics, Allendale, N.J.) with a summation-of-disks algorithm with 32 disks.* Left vcntricular volume was calculated by one observer, averaging wvo to three beats, with blinding to the cineventriculographic results.
Cineventriculography Patients underwent diagnostic catheterization by the femoral approach. A General Electric Advantx L U C / L P DXC Highline angiographic system (General Electric Co., Cleveland, Ohio) was used for imaging. A focal spot size of 0.9 mm, a 15 cm field of view, and a nominal x-ray exposure of 25 gR/frame were used. After coronary arteriography, a 6F pigtail catheter was inserted into the body of the left ventricle. Cineventriculograms were filmed in the right anterior oblique projection at 30 frames per second during the power injection of 35 to 50 ml iohexol at 15 ml/sec. The magnification factor in each projection was determined by filming a radiopaque grid or metal sphere of known dimensions at the estimated level of the midventricle. Cine films were viewed on a projector and the outlines o f the left ventricular cavity at visually estimated end diastole and end systole, along with the calibrating device, were traced on transparent acetate overlying the projector screen. Only sinus beats not following a premature beat were used for volume calculations. The outline of the papillary muscle was included in the left ventricular cavity. A digitizing pad and commercial software were used to calculate left ventricular volumc by a single-plane summation-of-disks method. These values were then adjusted for overestimation with the regression equations developed by Wynne et al. H
Analysis o f Image Plane Positioning In the last 15 patients, the line-of-intersection display was created and saved during the initial unguided study but was not referred to by the ultrasonographcr as it was during the guided studies. The short-axis cross section at the apical and mitral levels, with the computed lines of intersection, were then recalled and displayed on the computer monitor. The endocardial outline (excluding papillary muscles) and the lines of intersection were traced on transparent acetate overlying the monitor, and the following angular measurements were performed (Figure 2): (1) the angle between four-chamber and two-chamber views and (2) the angle between the obtained four-chamber view and the "ideal" fbur-chambcr view. Linear measurements
o f the distance of four-chambcr and two-chamber views from the center of the left ventricle were also performed (Figure 2, C through E). A diameter through the ventricle was constructed at right angles to the obtained image plane. The location of the image plane from one side of the ventricle to the other was expressed as a percent of the diameter, with 50% representing the image plane at the center. Thus for the four-chamber view (Figure 2, C), less than 50% represents inferomedial linear displacement and greater than 50% represents antcrolateral linear displacement. For the two-chamber view (Figure 2, D), less than 50% represents posterolateral linear displacement and greater than 50% represents anteromedial linear displacement. At the apical level, the measurement recorded was the distance of the four-chamber view from the center of the left ventricle, according to the convention described above in Figure 2, C.
Statistical Analysis Values are expressed as mcan _+ i SD. Least squares linear regression was used to compare the results o f each echocardiographic method with the results of cineventriculography. The differences between echocardiographic cineventriculography were compared with the mean values obtained by echocardiography and cineventriculography by the method of Altman and Bland, 12 with the limits of agreement defined as + 2 SD of the difference between the two methods. The Wilcoxon test was used to compare the diffkrence between ideal and obtained image position by guided and unguided methods. A p value <0.05 was considcrcd significant.
RESULTS Patient Characteristics A t cardiac catheterization, 17 patients h a d n o r m a l left vcntricular c o n t r a c t i o n , t w o h a d s e g m e n t a l hypoldncsis, five h a d diffuse hypokincsis, six h a d dyskinctic s e g m e n t s , and one h a d an akinetic s e g m e n t . T h e m e a n cinevcntficulographic end-diastolic volu m e was 127 _+ 54 cc (53 to 301 cc), and the m e a n cineventriculographic e n d systolic v o l u m e was 59 + 54 cc (10 to 248 cc).
Guided and Unguided Image Plane Positioning F i g u r c 3 shows the data for g u i d e d a n d u n g u i d e d image plane positions in the f o r m o f a b o x plot. F o r four o f the five p a r a m e t e r s o f i m a g e plane p o s i t i o n (angle b e t w e e n f o u r - c h a m b e r a n d t w o - c h a m b e r views, angle b e t w e e n o b t a i n e d a n d ideal fourc h a m b e r planes, a n d d i s p l a c e m e n t o f f o u r - c h a m b c r and t w o - c h a m b e r views f r o m the center o f the left ventricle at the mitral le'eel), the m a g n i t u d e o f the
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Figure 1 Reference images and lines of intersection. Left pond illustrates unguided image position at level of mitral leaflets. Right panel illustrates how line-of-intersection display can be used to improve position of image planes. During study, operator views both short-axis reference image (shown) and real-time imagc. White line represents intersection of real-time apical image and reference image and moves as operator alters apical image position.
malposition with the unguided imagcs was significantly greater than the magnitude of the malposition recorded when line-of-interscction guiding was used. However, in reference to the apical short-axis view, all o f the unguided four-chamber images were positioned anterior to the center of the left ventricle. In almost all cases, moving the transducer so that thc line o f intersection of the four-chamber view passed through the center of the apical cross section resulted in a major deterioration in image quality. This fbrced the operator to move the transducer again cephalad, to obtain an image with an acceptable visual representation o f the endocardial surface.
Bias and Limits of Agreement Both cchocardiographic methods tended to underestimate the results o f cineventriculography (Figurns 4 and 5). The mean value for the difference (cineventriculographic volume minus echocardiographic volume) was i8.5 _+ 30.4 cc for guided enddiastolic volume and 23.0 _+ 31.2 cc for unguided end-diastolic volume ( p = 0.20). For end-systolic volumes, the mean underestimation for the guided and unguided methods, respectively, was 2.5 +_+_26.1 cc and 5.6 + 24.5 cc (p = 0.32).
Linear Regression The results of linear regression are shown in Figurcs 4 and 5. The relationships were all highly significant (p < 0.001). At end diastole, guided echocardiography had an r-- 0.85 and standard error o f
C D
Figure 2 Measurements made to quantitate image plane position. A, Angle between four-chamber and t~vochamber images. B, Angle between actual (heavy line) and ideal (dotted line) four-chamber image plane. C, D, and E, Dotted lines represent diameters at 90 degrees to obtained image plane. Location of image plane is expressed as fraction of short-axis diameter, from one to opposite endocardial surface. Thus value of 0.50 indicates image position at center of short-axis view, and values of 0 and 1.00 indicate positions tangential to endocardium at opposite sides of ventricle. Values greater than 1.00 denote positions outside of endocardium. C, Position of four-chamber view is measured at 0.25. E, Position of four-chamber view is 1.80.
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Mitral level
Mitral level A = Angle between 4- and 2-chamber views (degrees)
B = Angle between obtained and ideal 4-chamber views (degrees)
Mitral level C = 4- chamber location (fraction of LV diameter)
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Figure 3 Guided and unguided image plane positioning displayed in box plot. Box indicates 25th to 75th percentiles of data, with cross bar at 50th percentile. Whiskers indicate 10th and 90th percentile ranges, and filled circles are remaining outliers. Vertical dimension of display is therefore indicator of variability of image position for each echocardiographic technique. Differences between ideal and obtained image position for unguided and guided techniques were compared by Wilcoxon test. * p < 0.05; ** p < 0.01. Letters (A through E) and legends above each plot refer to definitions of image plane position illustrated in Figure 2. (LV, Left ventricular.)
the estimate = 22.9 cc, and unguided cchocardiography had r = 0.84 and standard error o f the estimate = 23.0 cc. At end systole, guided echocardiography had r = 0.91 and standard error o f the estimate = 16.8 cc, and unguided echocardiography had r = 0.94 and standard error o f the estimate = 14.5 cc. No formal comparison between the results for patients with normal and abnormal ventricles was performed, given the relatively small number in the latter subgroup. However, the data points for the 10 patients with segmental aldnesis (n = 1), dyskinesis ( n = 6), and severe global hypokinesis ( n = 3) are denoted in Figures 4 and 5.
DISCUSSION The standard two-dimensional echocardiographic method for determining left vcntricular volume, the apical biplane summation-of-disks algorithm, relies on two apical views that are assumed to bisect the ventricle and be at right angles to each other. 8 These images are obtained by use o f visualized anatomic landmarks and the proprioceptive sense o f the operator. However, previous studies o f conventionally ob-
tained images' and recent studics with our threedimensional echocardiographic system suggest that the actual position o f the image plane often differs significantly from what is assumed by the algorithm. 9 This study was designed to investigate whether use o f the line-of intersection display of our threedimensional echocardiographic scanner to guide positioning o f the apical views would improve the agreement between estimates o f biplane echocardiographic and single-plane cineventriculographic left ventricular volume. The results o f the study show that improvements in image plane positioning were obtained by line-of-intersection guided imaging, but these improvements did not result in better agreement between echocardiographic and cineventriculographic volumes.
ReasonsForLackof Improvementin VolumeEstimates It was found that all of the unguided apical fourchamber images passed through the ventricle anterior to the apex, resulting in foreshortening o f the ventricle in the image. This occurred because the rib cage at that level partially occludes the ideal apical imaging window and the operator moved to the
Journal o f the American Society o f Echocardiography Volume 8 N u m b e r 5, Part 1
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Figure 4 Linear regression and mean versus difference plots for end-diastolic volumes. Results of linear regression of end-diastolic volume estimates by cineventriculography on y axis against echocardiographic methods on x axis. A, Results for guided echocardiography; B, results for unguided echocardiography. A, Open squares indicate patients with normal or hypokinctic mild global or segmental wall motion; filled squares indicate 10 patients with segmental akinesis, dyskinesis, or severe global hypokinesis. The same two groups are denoted in B by open and semifilled circles, respectively. C, Mean against difference plots, with two methods combined (open boxe~, guided; filled circles, unguided). The two wall motion groups are not identified in plot. Vertical columns on right show limits of agreement above and below mean value (center bar). (SEE, Standard error of estimate.) adjacent rib interspace to optimize image quality and border recognition. In some patients, transducer motion to optimize the position o f the four-chamber and two-chamber views at the mitral level and angular relationship o f the four-chamber and twochamber views also resulted in some deterioration in image quality. Thus the obtained guided images represent a compromise between image plane position and image quality, and the improvements in image position that were seen were not sufficient to overcome other errors.
Discrepancy Between Echocardiography and Cineventriculography In addition to the errors in image plane positioning already discussed, several other factors help to explain the similar discrepancies between unguided and
guided echocardiographic and cineventriculographic volumes. Recognition o f the endocardial border is a well-known difficulty common to both imaging methods. In the echocardiographic apical views, the endocardium close to the apex is often not well visualized. In addition, the two techniques use different conventions for tracing endocardial borders, with echocardiography tending to exclude and cineventriculography include the blood in the trabeculae carneae. 4 Also, apical biplane echocardiography excludes the left ventricular outflow tract, which is included in the cineventriculographic volume. Finally, both methods make assumptions about the shape o f the left ventricle that may not be valid in abnormally shaped ventricles. For example, for each disk in the apical biplane summation-of-disks method, four points are used to define an elliptic
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cross section of the ventricle. In single-planc cinevcntriculography, only two points arc obtained and define a circular cross section. These few points may be inadequate for representing the shape of the left ventricle, particularly in the presence o f distorted ventricular geometry.
Previous Studies That Use the Apical Biplane Method The use of paired apical four-chamber and twochamber views and a summation-of-disks algorithm is recommended by the American Society for Echocardiography for calculating left vcntricular volume. 8 One study that used this technique found the correlation coefficient between echocardiographic and single-plane cineventriculographic volumes in 70 patients to bc 0.80 for end-diastolic volume and 0.88 for end-systolic volume, with standard errors of the estimate for the respective regression cquations being _+34 and _+27 cc. ~ The results in a subset of 30 patients who underwent biplane cincvcntriculography wcre nearly identical. Another group performed a similar study in 30 patients undergoing biplane cincventriculography and found the correlations and standard errors of the estimate for end-diastolic and
end-systolic volumes to be 0.80 and +15 c c / m 2 and 0.90 and +8.5 c c / m 2, respectively.7 These studies in adult patients found echocardiography to underestimate cineventriculographic volumes, in agreement with studies that use a variety of other twodimensional multiplane and single-plane echocardiographic m e t h o d s ) 7
Implications For Quantitative Echocardiography Application o f the line-of-intersection display. Bccause thc linc-of-intcrscction display allows the operator to observe the truc relation o f a given image plane to nonvisualized anatomic landmarks in the heart, imagc plane positions and thc measurements derived from them can bc made morc reproducible. Thus the line-of-intersection display has been shown to improve the rcproducibility o f left vcntricular and left atrial diameter measurements, ~3 measurements o f left vcntricular volume, 14 and apical biplane mcasurements o f left atrial volume. 1~ Three-dimensional echocardiographic volu m e computation. Although guided apical image plane positioning did not improve estimates o f left ventricular volume with a biplane summation-of-
Journal of the AmericanSocietyof Echocardiography Volume 8 Number 5, Part 1
disks algorithm, the line-of-intersection display can be used to guide the positioning o f multiple shortaxis images. These images can be employed in a n e w algorithm for volume c o m p u t a t i o n based on polyhcdral surface reconstruction ~6 to improve the agreem e n t between estimatcs o f cineventriculographic and echocardiographic v o l u m e ) 7"I8 O n e advantage o f this technique is that polyhedral surface reconstruction does n o t require parallel or evenly spaced images and does n o t make assumptions a b o u t left ventricular shape, thus allowing images to be adjusted for better endocardial definition w i t h o u t affecting volume calculations. O t h e r investigators have used the spark-gap acoustic spatial locater and a different surface reconstruction algorithm to perform three-dimensional reconstruction, with excellent results. 19'2~This alternative system also allows r a n d o m image acquisition, with opcrator recall o f highquality images for b o u n d a r y tracing. O t h e r threcdimcnsional echocardiographic systems restrict transducer m o t i o n to graded rotation about a point, lincar m o t i o n , or pivoting/flexion. 2~ 2~ These systems have been studied mainly in in vitro or animal models, with little validation in patients. Given the p r o b l e m o f limited acoustic access to the heart from outside the body, it m i g h t be expected that transthoracic restricted transducer m o t i o n systems m i g h t have difficulty obtaining the requisite n u m b e r o f high-quality images for accurate three-dimensional reconstruction. O n the other hand, a newly develo p c d transesophageal system m i g h t be able to circ u m v e n t this difficulty. 2s
Study Limitations I n this study echocardiography and cinevcntriculography wcrc p e r f o r m e d nonsimultancously, and cincvcntriculography was p e r f o r m e d after c o r o n a r y artcriography. These factors may have resulted in changes in vcntricular volume that introduced further variability into the results, a l t h o u g h this w o u l d n o t be expected to alter the comparsion o f the two
echocardiographic methods. Conclusions Line o f intersection-guided positioning o f apical four-chamber and t w o - c h a m b e r views does n o t improve the agreement between left ventricular volumes obtained t?om these views and singlc-plane cineventriculography. Apical views are c o m m o n l y displaced anteriorly because o f occlusion o f the optimal imaging w i n d o w by ovcrlying ribs, with a consequent foreshortening o f the ventricular image. Imp r o v e d image position by use o f the line-ofintersection display was accompanied by a decrease o f image quality that impaired border recognition. These o p p o s i n g factors may a c c o u n t for the lack o f
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i m p r o v e m e n t fbund in this study. The inherent anatomic limitation o f the apical window, rib interference, limits the accuracy o f computations o f apical biplane ventricular volume that require assumptions about image plane positioning and left ventricular gcomctry. We thank Rosctta Hall, RDCS, and Annette Smith, RDCS, for their technical assistance.
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