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European Journal of Echocardiography 2004 5(1):18-24; doi:10.1016/S1525-2167(03)00050-7
© 2004 by European Society of Cardiology
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Copyright © 2003, The European Society of Cardiology

Comparison of left atrial size by freehand scanning three-dimensional echocardiography and two-dimensional echocardiography

J Kawaia, K Tanabea,*, C.-L Wanga, T Tania, T Yagia, H Shiotanib and S Moriokaa

aDivision of Cardiology, Kobe General Hospital, 4-6 Minatojima-nakamachi, Chuo-ku, Kobe 650-0046, Japan
bHealth Science, Kobe University Graduate School of Medicine, Kobe, Japan

Received 5 February 2003; received in revised form 26 May 2003; accepted after revision 28 May 2003.

* Corresponding author. Tel.: +81-78-302-4321; fax: +81-78-302-2487. kazuaki_tanabe{at}medical.general.hp.city.kobe.jp


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 
Aims: As the left ventricular (LV) dimension is a poor indicator of LV volume, there are well-known limitations of left atrial (LA) antero-posterior dimensions as indicators of chamber size. LA volume has been shown to provide a more accurate assessment of LA size than LA dimension. To evaluate two-dimensional (2D)-derived LA volume in assessing LA size, we compared LA dimension and 2D LA volume with three-dimensional (3D)-derived LA volume.

Methods: We performed transthoracic freehand scanning 3D echocardiography (3D EchoTech, Germany) using magnetic fields and a harmonic imaging system in 32 patients. We collected a series of LA tomograms by slowly tilting the probe (fan-like scanning) in the parasternal position. The 3D LA volume was calculated by using the multiplanar Simpson's method. The 2D LA volume was measured by using the modified biplane Simpson's rule.

Results: LA antero-posterior dimensions and 2D volumes showed a significant positive correlation with 3D LA volumes. However, the correlation coefficient was significantly greater for the relationship between 2D LA volumes and 3D LA volumes than for that between LA dimensions and 3D LA volume.

Conclusions: The 2D LA volumes provide a more accurate measure of the true size of the LA and are more sensitive to changes in LA size.

Keywords: echocardiography; left atrium; volume


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 
Left atrial (LA) size is an important determinant of LA pressure, diastolic function and prognosis1–3 and is associated with the likelihood of developing atrial fibrillation.4,5 LA size is also significantly related to the risk of stroke.6 As the left ventricular (LV) dimension is a poor indicator of LV volume, there are well-known limitations of LA antero-posterior dimensions as indicators of chamber size. The inaccuracy of the M-mode method is more apparent when the LA enlarges, because enlargement is not uniform in each LA dimension and enlargement in the antero-posterior dimension is often constrained by the spine and sternum. While two-dimensional (2D) echocardiography-derived LA volume has been shown to provide a more accurate assessment of LA size than M-mode LA dimension,7–9 the problem of geometric assumptions still remains.

Three-dimensional (3D) echocardiography has demonstrated superior accuracy and precision comparable to conventional 2D echocardiography for measuring LV volume, because no geometric assumptions are necessary.10–12 The 3D echocardiographic reconstruction has been validated for LA volume quantitation.13–15 The introduction of a method for freehand dynamic echocardiographic acquisition and reconstruction using magnetic-field systems and raw digital ultrasound data provides an alternative to the preserve high temporal resolution in the 3D reconstruction.16,17 In addition, tissue harmonic imaging without contrast agents improves overall image quality with particular enhancement of LA endocardial borders.18,19 Therefore, freehand scanning 3D echocardiography in combination with tissue harmonic imaging would be an accurate and precise method for LA volume measurement. The purpose of this study was to compare LA antero-posterior dimension and 2D-derived LA volume with 3D-derived LA volume as the reference.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 5. Conclusions
 References
 
2.1 Study subjects
We performed a freehand 3D echocardiographic scanning in patients for whom a standard transthoracic echocardiographic examination was clinically indicated. Exclusion criteria were cardiac arrhythmias, including patients with pacemaker and implantable cardioverter defibrillator. Thirty-two consecutive patients (25 men and 7 women; mean age, 65.4 years, 31–85 years) entered the study. Twenty-one patients had previous myocardial infarction, one had severe mitral regurgitation, three had dilated cardiomyopathy and three had hypertrophic cardiomyopathy. In four patients, LV function was evaluated. Informed consent was obtained before the study from all patients.

2.2 Three-dimensional echocardiographic study
The data were acquired using a SONOS 5500 digital ultrasound scanner (Philips medical systems, Andover, MA) with tissue harmonic imaging. For 3D data processing, a PC based 3D freehand system (3D EchoTech, Munich, Germany) was used. The 3D technique has been described elsewhere.20 To test the accuracy of volumetry by transthoracic freehand scanning 3D echocardiography, eight variously shaped latex balloon phantoms filled with water and suspended in a water bath were used for in vitro studies. The actual measured volumes ranged from 30 to 200 ml. We collected a series of balloon phantom tomograms by slowly tilting the probe (fan-like scanning), covering the whole balloon phantoms (Fig. 1). The analysis was performed on a PC based 3D freehand system after transferring the raw digital ultrasound data directly from the scanner. The computer displayed rotating radial balloon phantom images around a center axis. The true long axis was defined in the reconstructed cross-section showing the maximal balloon phantom area. Using equally spaced coaxial four-plane images (i.e. 45° increments), the boundaries of balloon phantoms were manually traced (Fig. 2) and the volumes were calculated using the multiplanar Simpson's method.


Figure 1
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  		Figure 1 Left, schematic drawing of balloon phantom used for in vitro study. Right, short-axis view of the balloon phantom.

 


Figure 2
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  		Figure 2 Balloon phantom volume measurement using equally spaced long-axis images (i.e. 45° increments).

 
In the clinical study, we collected a series of LA tomograms by slowly tilting the probe (fan-like scanning) in the parasternal position, covering the whole LA. The 3D echocardiographic data were recorded with normal respiration, and lasted typically for 10–20 cardiac cycles. After 3D data sets were transferred to the computer system, the computer displayed rotating radial LA images around a center axis. The true long axis was defined in the reconstructed cross-section showing the maximal LA area. Using equally spaced coaxial four-plane images (45° increments), LA endocardial borders were manually traced (Fig. 3) and the volumes were calculated.


Figure 3
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  		Figure 3 Coaxial sections of the LA (i.e. 45° increments) for volume measurement.

 
2.3 Two-dimensional echocardiographic study
All patients had a complete 2D echocardiographic and Doppler study. To determine LA antero-posterior dimension, the maximal dimension was measured between the leading edge of the posterior aortic wall and the leading edge of the posterior wall of the LA at end-systole using 2D parasternal long-axis view. Care was taken to maximize the LA dimension and prevent oblique or foreshortened view. Maximum LA volume obtained at LV end-systole just before opening of the mitral valve was measured by using the modified biplane Simpson's rule with the apical four-chamber and long-axis views21 (Fig. 4).


Figure 4
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  		Figure 4 LA volume measurement by biplane modified Simpson's method using apical four-chamber view (left) and apical long-axis view (right).

 
2.4 Statistical analysis
All values of the LA dimension and volume measurements were expressed as mean±SD. To determine whether the difference in the values between the two methods was statistically significant, a paired t-test was performed; the level of significance was set to P<0.05. The bias was expressed as the mean difference between the two methods, and the limits of agreement as ±2SD of the difference of the two methods.22


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 5. Conclusions
 References
 
3.1 In vitro three-dimensional echocardiography study
The time required for data recording was 10–20 s and the image analysis was performed within 2 min after transferring the raw digital ultrasound data from the scanner. There was a high correlation and good agreement (r = 0.998, P<0.0001, y = 0.98x+2.6, SEE = 4.0 ml, bias = 0.4 ml) between the true volume and the measured phantom volume.

3.2 Clinical study
LA antero-posterior dimension and 2D- and 3D-derived LA volumes were as follows: LA dimension ranged from 2.5 to 5.5 cm (3.9±0.7 cm), 2D-derived LA volume ranged from 28.3 to 119.5 ml (49.2±20.6 ml) and 3D-derived LA volume ranged from 30.0 to 102.0 ml (54.2±19.6 ml).

There was a fair correlation between LA antero-posterior dimension and 3D-derived LA volume (r = 0.70, P<0.001, y = 0.024x+2.6, SEE = 14.2 ml) (Fig. 5). As expected, there was a good correlation between LA volume by 2D and 3D methods (r = 0.81, P<0.001, y = 0.85x+3.1, SEE = 11.7 ml). The bias and limits of agreement between 3D-derived and 2D-derived LA volume were –5.0 and ±24.8 ml, respectively (Fig. 6).


Figure 5
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  		Figure 5 Linear regression analysis for LA antero-posterior dimension and LA volume by 3D echocardiography.

 


Figure 6
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  		Figure 6 Linear regression analysis (left) and limits of agreement analysis (right) for LA volume by 2D echocardiography and by 3D echocardiography.

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
5. Conclusions
 References
 
In this study, LA antero-posterior dimension and 3D-derived LA volume showed a significant positive correlation. However, as the LA enlarges, small increments in the dimension are associated with large changes in volume. The lack of a constant relationship between the major axes of the LA and LA volume is even more pronounced when the atria are enlarged. Despite the fact that 2D LA volume systematically underestimates 3D LA volume, the present study showed a close correlation of 2D LA volume with 3D LA volume.

There are potential sources of error in 2D planimetry of the LA. Apical imaging places the atria in the far field of the ultrasound beam, resulting in a loss of lateral resolution with limited visualization of the LA endocardium. In addition, planimetry requires visual estimation of the posterior and lateral wall locations to exclude the confluence of the pulmonary veins and the atrial appendage. The inaccuracies of 2D LA volume can be limited by maximizing LA size during scanning.

The estimation of LA volume by any method makes geometric assumptions that have not been extensively validated. The 2D LA volume derived by the biplane method of disks was chosen in this study. The application of the modified biplane Simpson's rule method for determination of the LA volume has shown a higher correlation with angiography7 and cine computed tomography.8 The use of transthoracic freehand scanning 3D echocardiography as a method of volumetric analysis has been validated for ventricular volumes.18 We demonstrated that with a magnetic position sensor integrated with the harmonic imaging system, 3D acquisition and reconstruction can be performed in a considerably shorter time than previously described. In addition, tissue harmonic imaging has been introduced and overall image quality is improved with particular enhancement of endocardial borders.16,17 Khankirawatana et al.15 reported that 3D reconstruction using three views can be applied in the clinical setting for LA volumetry. Therefore, we used 3D echocardiography as the reference in this study. For patients with severe arrhythmias, freehand scanning 3D echocardiographic measurement of LA volume described will result in reconstruction errors. We did not determine the minimal number of planes needed to achieve an accurate measure of LA volume. Further validation of this method will be needed in the clinical study.


   5. Conclusions
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusions
References
 
The 2D LA volumes provide a more accurate measure of the LA size and are more sensitive to changes in LA size. Estimation of LA volume by 2D echocardiography has the advantage of being repeatable and easily performed.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 5. Conclusions
 References
 

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