Real-time three-dimensional echocardiography using a matrix probe with live xPlane imaging of the interventricular septum

Authors

  • Y. Xiong,

    1. Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong
    2. Department of Ultrasound, Shenzhen People's Hospital, Jinan University, Shenzhen, PRC
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  • Y. M. I. Wah,

    1. Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong
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  • M. Chen,

    1. Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong
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  • T. Y. Leung,

    1. Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong
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  • Prof T. K. Lau

    Corresponding author
    1. Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Prince of Wales Hospital, The Chinese University of Hong Kong, Hong Kong
    • Fetal Medicine Unit, Department of Obstetrics and Gynaecology, Prince of Wales Hospital, The Chinese University of Hong Kong, Shatin, Hong Kong
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Abstract

Objective

To describe a technique to rapidly visualize the in-plane view of the fetal interventricular septum (IVS) to enable the identification of a ventricular septal defect (VSD).

Methods

One hundred and fifty-one women were invited to participate after their routine fetal morphology scan, including four suspected to have congenital cardiac defects which were confirmed postnatally. A standard examination protocol using real-time three-dimensional (3D) echocardiography with live xPlane imaging was developed. The ability of this new technology to examine the ventricular septum was investigated.

Results

The in-plane view of the fetal IVS was visualized successfully in 150 (99.3%) cases by real-time 3D echocardiography with live xPlane imaging, including 82 (54.3%) cases with the spine posterior and 68 (45.7%) cases with the spine anterior. The in-plane view of the IVS successfully visualized the VSDs in three fetuses with VSD and displayed the intact IVS in one fetus with transposition of the great arteries without VSD.

Conclusion

We describe live xPlane imaging, a simple method for the real-time assessment of the in-plane view of the IVS that has the potential to enhance the diagnostic accuracy of fetal cardiac examination. Copyright © 2009 ISUOG. Published by John Wiley & Sons, Ltd.

Introduction

Congenital heart defects (CHD) represent the most common form of congenital anatomical malformations1. With increased experience and improvement in ultrasound technology, prenatal diagnoses of many major structural fetal cardiac abnormalities are now possible, with the detection rate much higher in expert centers2. However, the detection rate among non-referral centers and in low-risk populations of CHD in general3, 4 and of the more subtle abnormalities, including ventricular septal defects (VSDs) in particular5, remains low; VSDs are among the CHDs most often missed at prenatal scanning6, 7.

At present, two-dimensional (2D) ultrasound is the main method for the screening and diagnosis of congenital malformations. In order to increase the detection rate of CHD, various views have been proposed, including the four-chamber, outflow tract and three vessels and trachea views8–10. However, none of these methods provides comprehensive visualization of the interventricular septum (IVS), and the four-chamber view provides only a cross-sectional image. The introduction of three-dimensional (3D) echocardiographic techniques using spatiotemporal image correlation (STIC) enabled the acquisition of a heart volume which could then be manipulated to demonstrate cardiac structures that are otherwise difficult to show, such as the IVS11, 12. However, the two major limitations of STIC technology are (1) that the image is not real-time and requires offline analysis, and (2) that the acquisition time is long, making fetal movement artifacts a major problem during the acquisition process.

Real-time 3D echocardiography is now possible with the development of 2D matrix-array probe technology. Unlike 3D echocardiography using STIC, which produces a reconstructed heart volume based on many cardiac cycles, 3D echocardiography using a matrix-array probe enables visualization and examination of the pulsating fetal heart in real time without the need for gating, just as in real-time gray-scale sonography13, 14. The use of real-time 3D echocardiography has been reported extensively in adult cardiology, and has been shown to be particularly useful in the assessment of valvular disease and cardiac function15, 16.

There are two major applications of matrix-array probe technology: live 3D imaging and live xPlane imaging. Live 3D imaging provides a real-time volumetric examination of the target organ, while ‘live xPlane imaging’, a term coined by the manufacturer, displays two high-resolution real-time views of the target organ. Unlike the multiplanar view of 3D volumes acquired by conventional mechanical 3D probes, in which only the primary plane is acquired with full resolution while the other two planes are reconstructed, both views acquired by live xPlane imaging are of high resolution displayed in real time, due to the use of the matrix-array probe. Images are displayed using a split-screen format, with the primary image plane as the reference plane, displayed on the left side of the screen. The secondary image plane can be selected by rotation through a full 360° around the reference plane, together with a lateral tilt of − 45° to + 45° around the primary image plane at a reference line which can be determined by the operator, or by tilting (elevation) from − 30° to + 30°.

The objective of this study was to explore the potential use of live xPlane imaging in the visualization and assessment of the IVS, and in particular to describe the methodology.

Patients and Methods

One hundred and fifty-one women with singleton pregnancies who attended their routine mid-trimester ultrasound examination in the department of obstetrics and gynecology of a university teaching hospital were invited to participate in this observational study. Informed consent was obtained in all cases.

All patients had an additional 3D echocardiographic examination for the purposes of the study. Initially, the examination was arranged at 25–30 weeks of gestation. Towards the end of the study, the examination was arranged at 19–24 weeks of gestation. All examinations were performed using an IU-22 ultrasound machine (Philips Medical Systems, Bothell, WA, USA) equipped with a 7–2-MHz matrix-array probe. No special patient preparation was required. All 3D echocardiography examinations were performed by one operator (Y.X.).

The fetal heart examination was performed using the manufacturer's Fetal Echo preset. First, we acquired an apical four-chamber view of the fetal heart, with the apex either at the 12 o'clock position (if the fetal spine was posterior) or at the 6 o'clock position (fetal spine anterior). Next, the xPlane imaging function was activated. The primary image plane was the apical four-chamber view, displayed on the left side of the screen. By moving the reference line on the primary image plane, a secondary image plane cutting across the reference line was displayed on the right side of the screen. By default, the secondary plane was rotated + 90° with respect to the reference plane. For the purposes of this study, this rotation was satisfactory and required no adjustment. The reference line was adjusted within the reference plane to lie along the IVS in the four-chamber view, and the in-plane view of the IVS was displayed in the right window.

Results

A total of 151 cases were studied, including four cases with congenital cardiac defects. The mean ± SD maternal age was 32.0 ± 4.2 (range, 21–40) years and the mean ± SD maternal body mass index (BMI) was 21.2 ± 2.8 (range, 16.3 to 32.1). Fifty-five (36.4%) cases were examined between 19 and 24 weeks and 96 were examined between 25 and 30 weeks.

The in-plane view of the IVS was visualized successfully by real-time 3D echocardiography using live xPlane imaging in 150 (99.3%) cases. The cardiac apex was at the 12 o'clock position in 82 (54.3%) cases and at the 6 o'clock position in 68 (45.7%) cases (Figures 1 and 2). Once an apical four-chamber view was obtained, the use of live xPlane imaging enabled almost immediate acquisition and display of the in-plane view of the IVS. In the case in which visualization of the IVS failed, this was because we could not obtain a satisfactory apical four-chamber view after repeated attempts. The patient had a gestational age of 20 weeks, a normal BMI of 20.2 and a Cesarean section scar.

Figure 1.

In-plane view of a normal interventricular septum (arrows) at 27 gestational weeks displayed by live xPlane imaging. The fetal spine was posterior. The reference line shown in the primary image on the left side has been added because the original line disappears when the live image is frozen. AO, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle.

Figure 2.

In-plane view of a normal interventricular septum (arrows) at 25 gestational weeks displayed by live xPlane imaging. The fetal spine was anterior. AO, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle.

The four cases with CHD included one case of isolated VSD, one case of atrioventricular septal defect, one case of truncus arteriosus with VSD and one case of transposition of great arteries without VSD. The VSDs in the first two cases were clearly displayed in the in-plane view of the IVS using live xPlane imaging (Figures 3–5), as was the intact IVS in the last case. The abnormalities of all four cases were confirmed postnatally.

Figure 3.

In-plane view of a defective interventricular septum at 23 gestational weeks, showing the defect displayed by live xPlane imaging in a case of isolated ventricular septal defect (VSD). The fetal spine was posterior. ao, aorta; pa, pulmonary artery; RA, right atrium.

Figure 4.

The same case as Figure 3. In-plane view of a defective interventricular septum at 23 gestational weeks, showing the defect (arrow and dotted line) displayed by live xPlane imaging in a case of isolated ventricular septal defect (VSD). The fetal spine was anterior. ao, aorta; pa, pulmonary artery; RA, right atrium.

Figure 5.

In-plane view of a defective interventricular septum at 25 gestational weeks, showing the defect (arrows) displayed by live xPlane imaging in a case of atrioventricular septal defect. AO, aorta; LA, left atrium; LV, left ventricle; PA, pulmonary artery; RA, right atrium; RV, right ventricle.

Discussion

Isolated VSD accounts for 30% of all pediatric cardiac defects and it is commonly found associated with other cardiac anomalies17, yet it is the type of CHD most commonly missed on prenatal screening6, 7. The main reason for such poor detection is probably the fact that there is no simple way to examine the IVS in its entirety; the four-chamber view provides only a limited assessment in a cross-sectional image. The ability to visualize the entire IVS in one single image could potentially improve the detection rate of VSD.

Paladini et al.18 first reported the examination of the entire IVS in the so-called ‘in-plane’ view using 2D sonographic techniques. By careful manipulation and tilting of the ultrasound probe, starting from the long-axis view of the left ventricle, the operator attempted to orientate the IVS in the same plane as the ultrasound beam, thus displaying the whole IVS, or the ‘in-plane’ view. However, image acquisition could be difficult and was strongly influenced by fetal position. The overall success rate was 65.8%, this rate being 100%, 36% and 0% when the fetal spine was posterior, lateral and anterior, respectively. Similar findings were reported by Lau19, who had an overall success rate of 46%.

Another way of visualizing the IVS is by using the rendered view on 3D echocardiographic volumes acquired using STIC technology; using this approach, prenatal identification of VSDs and atrial septal defects has been reported11, 20. More recently, Myers et al.5 described another method to rapidly access the in-plane view of the IVS on STIC volumes using multiplanar views instead of the rendered view. However, there are several limitations of these STIC-based approaches. First, the acquisition of good STIC volumes is not easy and is still in the hands of the experts only. Second, generation of the rendered and multiplanar views is performed as post-hoc volume analysis. Third, the rendered view is a reconstructed ‘surface’ view and therefore any ‘defect’ created by overlying shadowing or movement artifacts may not be easily recognized. This last limitation can be partly overcome by directing the STIC acquisition to the plane of the IVS such that the enface view of the septum is in the acquired plane of the STIC acquisition.

The live xPlane imaging method reported here allowed simultaneous visualization of two different imaging planes of the fetal heart in real time: the four-chamber view and the in-plane view of the IVS. This was achieved without moving the transducer, simply by placing the sample line along the interventricular septum in the four-chamber view. There are three obvious advantages of this approach. First, acquisition is simple and quick because both reference and secondary image planes are real-time. Second, movement artifacts are less of a problem than they are using 3D STIC technology because the aquisition and freezing of an image is similar to that in 2D scanning. Third, the IVS plane is an ordinary ultrasound image rather than a reconstruction and therefore any structure that causes acoustic shadowing will be shown on the same image, making assessment simple and straightforward.

Live xPlane imaging (or ‘biplane imaging’) of the fetal heart has been reported by Acar et al.13, who successfully used the technique to simultaneously visualize the four-chamber view of the fetal heart and the great vessels in 60 cases. However, although they commented that visualization of the IVS was possible, they provided no further details.

In this study we have shown how the in-plane IVS view can be visualized easily and successfully using live xPlane imaging. The procedure is simple; all that is required is a good four-chamber view for the reference image. The in-plane IVS view can then be obtained simply by activating the live xPlane function and, while maintaining the probe's position, placing the reference line along the IVS on the reference image. We have shown that the in-plane IVS view can be obtained successfully even if the fetal spine is anterior.

The main limitation is that obtaining the in-plane view using live xPlane imaging would not be possible if the axis of the IVS was not parallel to the ultrasound beam. However, this was not a major problem in our experience because fetal movement usually brought the fetal heart into a favorable position for imaging.

There are several limitations of this study. First, this was only a feasibility study, and establishing a definitive role for this new technology requires further comparative studies with alternative methods of examining the fetal IVS. Second, color Doppler was not used because the color setting of matrix probes remains to be optimized for fetal cardiac examination. Nonetheless, we have demonstrated that examination of the fetal IVS by the live xPlane imaging is both feasible and easy.

In conclusion, live xPlane imaging is an easy method for real-time imaging of the IVS, and potentially may be a useful tool for the assessment and diagnosis of VSD. However, further studies are required to evaluate the sensitivity and reproducibility of this technique in a large population.

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