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Four-Dimensional Fetal Echocardiography With Spatiotemporal Image Correlation (STIC): A Systematic Study of Standard Cardiac Views Assessed By Different Observers

Posted on: Saturday, 30 July 2005, 03:00 CDT

Abstract

Objective. To test the agreement between observers and reproducibility of a technique to display standard cardiac views of the left and right ventricular outflow tracts from four-dimensional volume datasets acquired with Spatiotemporal Image Correlation (STIC).

Methods. A technique was developed to obtain dynamic multiplanar images of the left ventricular outflow tract (LVOT) and right ventricular outflow tract (RVOT) from volume datasets acquired with STIC. Volume datasets were acquired from fetuses with normal cardiac anatomy. Twenty volume datasets of satisfactory quality were pre- selected by one investigator. The data was randomly assigned for a blinded review by two independent observers with previous experience in fetal echocardiography. Only one volume dataset was used for each fetus. After a training session, the observers obtained standardized cardiac views of the LVOT and RVOT, which were scored on a scale of 1 to 5, based on diagnostic value and image quality (1 = unacceptable, 2 = marginal, 3 = acceptable, 4 = good, and 5 = excellent). Median scores and interquartile range, as well as inter- and intraobserver agreement were calculated for each view.

Results. The mean menstrual age at the time of volume acquisition was 25.5 4.5 weeks. Median scores (interquartile range) for LVOT images, obtained by the first and second observers, were 3.5 (2.25- 5.00) and 4 (3.00-5.00), respectively. The median scores (interquartile range) for RVOT images obtained by the first and second observers were 3 (3.00-5.00) and 3 (2.00-4.00), respectively. The interobserver intraclass correlation coefficient for the LVOT was 0.693 (95% CI 0.3800.822), and 0.696 (95% CI 0.382-0.866) for the RVOT. For the intraobserver agreement analysis, observer 1 gave higher scores to the LVOT the second time the volumes were analyzed [LVOT: 3.50 (2.25-5.00) vs. 5.00 (4.00-5.00, p = 0.008)].

Conclusion. STIC can be reproducibly used to evaluate fetal cardiac outflow tracts by independent examiners. Slightly better image quality rating scores during the intraobserver variability trial suggests the presence of a learning curve for the manipulation and analysis of volume data obtained by STIC.

Keywords: Three-dimensional ultrasound, four-dimensional ultrasound, STIC, outflow tracts, aorta, pulmonary artery, 3D, 4D

Introduction

Congenital heart disease is the leading cause of infant mortality related to birth defects in the United States [1]. The mortality rate varies according to the type of defect, with hypoplastic left heart syndrome being responsible for the highest number of deaths [2]. Other congenital heart defects associated with a high mortality rate include transposition of the great arteries, ventricular septal defects, coarctation of the aorta, tetralogy of Fallot, truncus arteriosus, and atrioventricular septal defects [2].

Prenatal diagnosis of congenital heart disease is associated with improvement in neonatal morbidity and mortality for the following conditions: transposition of the great arteries [3], hypoplastic left heart syndrome [4,5] and coarctation of the aorta [6]. However, the diagnostic accuracy for prenatal diagnosis of congenital heart disease is limited [736]. Reasons for the low detection rates include: (1) inadequate examination; (2) anomalies not present at the time of routine ultrasound scan (e.g., valvular disorders may evolve during pregnancy); and (3) incomplete evaluation of the outflow tracts [9,10,3744].

Three-dimensional [45-51] and, more recently, four-dimensional ultrasonography of the fetal heart [52-58] have the potential for making the examination less operator-dependent. Once a volume dataset is acquired, the heart can be examined using multiple planes of section, some of which are unobtainable with two-dimensional ultrasound. In addition, volumes can be stored and sent digitally to other examiners for further diagnostic evaluation if necessary.

In a previous article [58], we proposed a technique to examine volume datasets acquired using fourdimensional ultrasound with Spatiotemporal Image Correlation (STIC) for standard views of the outflow tracts. The objective of the present study was to evaluate the reproducibility and agreement between observers in the use of this technique.

Material and methods

Subjects

Between 9 January and 14 April 2003, 69 fetuses were examined with four-dimensional ultrasonography using STIC (VOLUSON 730 EXPERT, GE Medical Systems. Kretztechnik GmbH, Zipf, Austria). A detailed morphological scan was performed prior to volume acquisition, and 51 fetuses (71.8%) with normal cardiac anatomy were included in the study. The mean gestational age at the time of volume acquisition was 25.5 4.5 weeks. All studies were conducted under protocols approved by the Institutional Review Boards of Wayne State University, William Beaumont Hospital and the National Institute of Child Health and Human Development (NICHD). Patients gave written informed consent prior to participation in the study.

Volume acquisition

Volumes were acquired with a motorized curvedarray transducer (2- 5 or 4-8 MHz). Once the fourchamber view of the fetal heart was visualized, acquisition was performed using automated transverse sweeps through the fetal chest and abdomen. The region of interest included the ventricular and atrial chambers, great vessels and venous return to the fetal heart. Volume acquisition time was between 7.5 and 12.5 seconds and, whenever possible, the procedure was performed in the absence of fetal movements or breathing. Patients were also asked to momentarily suspend breathing.

Volume postprocessing

Volume datasets were visualized using the multiplanar display method. Image postprocessing included color filtering and adjustment of brightness/contrast to optimize tissue contrast resolution. The multiplanar display method allows dynamic images of the fetal heart to be simultaneously visualized in three orthogonal planes (Figure 1).

A technique to systematically examine volume datasets and obtain multiplanar cine sequences of the left and right outflow tracts was developed, and its details have been published elsewhere [58]. The technique takes advantage of the almost orthogonal relationship of the great vessels as they leave the ventricular chambers. Briefly, when volumes are acquired using a transverse sweep and the fourchamber view as the starting point, this view is automatically displayed in the original plane of acquisition (Figure 1). The volume is then rotated around the z-axis until an apical four- chamber view is displayed. If necessary, further rotation around the yaxis is performed until the left side of the heart is displayed on the left side of the image. Once this initial step is completed, the volume is rotated approximately 30-45 degrees around the z-axis (Figure 2A). Then, the reference dot is moved to the center of the interventricular septum (Figure 2B). Next, the volume is rotated clockwise around the yaxis until visualization of the left ventricular outflow tract (LVOT) (Figure 2C). Finally, a reference dot is repositioned at the center of the LVOT, above the aortic valve, and a short axis view of the right ventricular outflow tract (RVOT) is simultaneously visualized in the right upper panel (Figure 2D).

Part of the diagnostic value of STIC relies on the simultaneous display of both cardiac outflow tracts. An examiner can create virtual planes that allow navigation through the volume data from viewing perspectives not possible by conventional sonography. Detailed views of moving fetal intracardiac structures are also feasible using three-dimensional surface and volume rendering. If desired, the same dataset can even be used to create a static twodimensional image or video clip.

Reproducibility and agreement study

All volume datasets were first reviewed for inclusion into this protocol by one of the investigators (LG). Volumes fulfilling all of the following criteria were selected to test reproducibility and agreement: (1) volume acquisition using a transverse sweep through the four-chamber view of the fetal heart; (2) fourchamber view adequately visualized in the original plane of acquisition; (3) minimum motion artifact on panel B; and (4) only one volume per fetus was selected (in cases where two or more volumes of acceptable quality were available, the one with the highest quality was selected).

Figure 1. Multiplanar display of a volume dataset acquired with a transverse sweep through the fetal chest. Panel A shows the apical four-chamber view, panel B is a sagittal orthogonal plane to panel A, and panel C is a sagittal coronal plane to panel A. The apical four-chamber view on panel A, with the left side of the heart on the left side of the image, and the right side of the heart on the right side of the image, is the first step towards a systematic approach to explore the volume and, at the end, visualize the left and right ventricular outflow tracts.

Figure 2. (A) The first step of the technique to visualize the left and right outflow tracts is, with panel A selected, to rotate the volume dataset about 30 to 45 degrees around the z-axis. (B) The second step consists of moving the reference dot to the middle of the interventricular se\ptum. (C) The third step is, with panel A selected, to rotate the volume around the y-axis until the ascending aorta is visualized. (D) Once the ascending aorta is visualized, the last step consists of moving the reference dot to the center of the ascending aorta. With this move, the short axis view of the pulmonary artery will appear on panel B.

Volumes selected for the reproducibility and agreement study had patient identifiers removed. Two copies of each volume were saved on separate CD-ROMs with different label numbers. After a brief session explaining the technique of how to examine the volumes and visualize the outflow tracts, CD-ROMs were given to two independent observers with previous experience in fetal echocardiography (WL and JE). Each observer was asked to rate the LVOT and RVOT views separately, according to a scoring system based on subjective assessment of diagnostic value and quality of the images obtained: 1 = unacceptable, 2 = marginal, 3 = acceptable, 4 = good, and 5 = excellent.

Statistical analysis

Median scores and interquartile range, as well as inter- and intraobserver agreement were calculated for each view. The Wilcoxon signed-rank test was used to test for differences in median scores. Interand intraobserver agreement for image scores were calculated using intraclass correlation coefficient (ICC), which was interpreted according to the guidelines proposed by Landis and Koch [59]: 0 = chance agreement, 0.01-0.19 = poor agreement, 0.20-0.39- fair agreement, 0.40-0.59 = moderate agreement, 0.60-0.79 = substantial agreement, and 0.80-1.00 = almost perfect agreement.

Results

Interobserver study

Median scores and interquartile range for images of the left and right ventricular outflow tracts obtained by each observer are shown in Table I. The median scores ranged between good and acceptable for all views, with no significant difference between observers. The interobserver intraclass correlation coefficient was 0.693 for views of the LVOT (95% CI 0.380-0.822) and 0.696 for views of the RVOT (95% CI 0.382-0.866).

Table I. Median scores and interquartile range for images of the left and right ventricular outflow tracts for each observer.

Intraobserver study

A comparison of median scores and interquartile range for images of the left and right ventricular outflow tracts obtained by observers 1 and 2 are shown in Tables II and III, respectively. No significant differences in median scores were observed for the RVOT. The first observer scored images of the LVOT significantly higher the second time the volumes were analyzed (LVOT: 3.55 1.27 vs. 4.30 1.03, p = 0.007).

Discussion

Prenatal diagnosis of conotruncal anomalies, such as transposition of the great arteries, tetralogy of Fallot and truncus arteriosus, requires visualization of the outflow tracts [42,60- 62]. Unfortunately, most population studies regarding the use of ultrasonography to detect congenital heart disease have reported low sensitivities for abnormalities of the great arteries [10,12,13,28,41,42,63,64]. Studies documenting higher detection rates emphasized routine visualization of the outflow tracts in addition to the usual four-chamber view [8,22,24,25,27,32,36,37,65- 67].

For a basic fetal cardiac evaluation, the American Institute of Ultrasound in Medicine (AIUM) currently recommends the examination of the four-chamber view in detail and, when technically feasible, to include outflow tract views [68]. Carvalho et al. [67] recently reported that, in order to add outflow tract views to their routine screening program and improve the prenatal detection rates for congenital cardiac anomalies, it was important to have an institutional infrastructure committed to continuous in-house training of obstetric ultrasonographers with concomitant feedback from fetal cardiologists. STIC may offer important benefits for house staff training and interactive review of suspected fetal heart anomalies. For example, the examiner's ability to analyze cardiac anatomy with an infinite number of arbitrary viewing planes is likely to offer additional diagnostic advantages for the accurate characterization of complex anomalies.

Table II. Median score and interquartile range for images of the left and right ventricular outflow tracts for observer 1.

Table III. Median score and interquartile range for images of the left and right ventricular outflow tracts for observer 2.

In a previous study, a technique was developed to systematically visualize the outflow tracts using four-dimensional ultrasound with STIC [58]. In the current study, we specifically investigated volume postprocessing, which is the ability to obtain desired planes of section using stored volume datasets. The results of this investigation show that both outflow tracts can be reproducibly visualized with acceptable to good image quality for clinical diagnostic purposes (Figure 3). A limitation of the technique described in this study, which utilized a transverse sweep for volume dataset acquisition, is that it does not allow imaging of the aortic and ductal arches. Imaging of the arch vessels is best accomplished when the STIC volume datasets are acquired with a sagittal sweep through the fetal chest [58], using a technique originally described by Bega et al. [49] for static three- dimensional volume datasets.

An interesting observation was provided by data from the intraobserver study. One of the observers scored images of the LVOT significantly higher the second time the data was analyzed. An example of this is shown in Figure 4. In this image, the left and right ventricular outflow tracts are depicted clearer during the second exam. Although the use of a different color filter may have improved image quality in this example, becoming skilled at performing the technique may take time.

Figure 3. This image illustrates close agreement between the observers for the images obtained from a high quality volume dataset. Note that, despite differences in image magnification and post-processing, the views of the left ventricular outflow tract (LVOT) and right ventricular outflow tract (RVOT) received the highest score from both observers.

Figure 4. This image illustrates differences in image quality for the same volume dataset when it was examined for the second time by observer 1.

A potential criticism of this study is the preselection of the volume datasets. Our purpose in choosing this approach was to limit our study only to the postprocessing part of the examination. As volume acquisition can be affected by many factors such as operator skills, gestational age at examination, maternal body habitus, fetal position, fetal movement, fetal breathing, and amniotic fluid volume [54,55,58,69], it would be difficult to control for these factors when analyzing the data. Volume acquisition has been previously studied by Vinals et al. [55], who examined volume datasets of 100 consecutive fetuses between 18 and 37 weeks. A general obstetrician with limited knowledge of fetal echocardiography acquired the volumes using the four-chamber view as the starting point. These were subsequently stored for analysis by a fetal echocardiologist, who computed the success rate in the visualization of a comprehensive list of 20 structures/views. The crux of the heart, moderator band, atrioventricular valve insertion, ventricular proportions, axis, and size were visualized in 99-100% of the volumes. Visualization of the LVOT, RVOT and crossing of the great arteries was possible in 95%, 95% and 97% of the cases, respectively.

We do not envision the use of STIC as a replacement for two- dimensional ultrasonography. As more imaging systems become equipped with three- and four-dimensional ultrasound, however, we believe that validation of a systematic approach to volume postprocessing may help to improve diagnostic accuracy. Nonetheless, the practical application of four-dimensional cardiac ultrasonography and STIC warrants further investigation. Another potential source of criticism is the subjective nature of the criteria utilized to evaluate the diagnostic value and quality of the images obtained.

Video clips

Video clip 1. This video clip illustrates the steps to consistently visualize the left and right ventricular outflow tracts departing from the apical four-chamber view: (1) rotate the volume over the z-axis (30 to 45 degrees); (2) move the reference dot to the center of the interventricular septum; (3) rotate the volume over the y-axis until the long axis view of the left ventricular outflow tract is visualized; (4) move the reference dot to the center of the ascending aorta - the short axis view of the right ventricular outflow tract is simultaneously displayed on the opposite panel (panel B).

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LUS F. GONALVES1,2, JIMMY ESPINOZA1,2, ROBERTO ROMERO1, WESLEY LEE3, MARJORIE C. TREADWELL2, RAYWIN HUANG3, GREGGORY DEVORE4, TINNAKORN CHAIWORAPONGSA1,2, MARY LOU SCHOEN1,2, & BETSY BEYER1,2

1 Perinatology Research Branch, NICHD, NIH, DHHS, Bethesda, Maryland, 2 Department of Obstetrics and Gynecology, Wayne State University, Detroit, Michigan, 3 Division of Fetal Imaging, William Beaumont Hospital, Royal Oak, Michigan, and Fetal Diagnostic Center, Pasadena, California, USA

Correspondence: Roberto Rornero, MD, Perinatology Research Branch, NICHD, NIH, DHHS, Autzel Women's Hospital, 3990 John R, 4th Floor, Detroit, MI 48201, USA. Tel: + 1 313 993 2700. Fax: + 1 313 993 2694. E-mail: warfiela@mail.nih.gov

Presented as an Oral Communication at the 13th World Congress of Ultrasound in Obstetrics and Gynecology, Paris, France, 2003.

Copyright CRC Press May 2005

Source: Journal of Maternal - Fetal & Neonatal Medicine

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