In this study we compared for the first time the LA volume measured by a commercially available 3-dimensional echocardiography system that utilizes semiautomated border detection, to an MRI gold standard and demonstrated good correlation. This technique has furthermore proven to have favorable intraobserver and interobserver variability compared to MRI and ALM.
Measurement of LA volume by 3D echocardiography in adult patients has been attempted in the past by Keller and Rodevand in 2 separate studies [8, 9]. The correlation with an MRI gold standard was excellent. The image acquisition, however, was obtained on a standard 2D transducer, with 3D reconstruction occurring off-line by specialized software. The off-line image processing took 6–10 minutes for LA volume measurement, making this a clinically impractical approach. Jenkins and co-workers applied real-time 3D echocardiography for measurement of the LA volume in 106 subjects . The study compared different 2D methods of volume estimation to the 3D technique, but did not include an MRI gold standard. Furthermore, the study by Jenkins, et al  used a different software algorithm (4D analysis, Tomtec Gmbh, Unterschlessheim, Germany) for LA volume estimation, requiring almost 8 times longer processing time (430 ± 15 s vs 56 ± 8 s in the present study). Similar to the present study, they found significant correlation between the volumes obtained by the ALM and the 3D technique.
The accuracy of MRI measurement of left atrial volume has been validated against water displacement of cadaveric atrial casts by Järvinen and coworkers . Our findings show that echocardiography systematically underestimates the LA volumes as compared to MRI. This phenomenon has been described from previous comparisons of echocardiography versus MRI and gated cardiac computer tomography for assessment of left atrial and ventricular volumes [9, 14, 15]. A likely explanation is the difference in spatial image resolution between imaging techniques. In both 2D and 3D echocardiography, the apical window places the left atrium at the far field of the ultrasound beam, resulting in loss of lateral image resolution. In contrast to MRI, planimetry of 2D and 3D ultrasound images may not distinguish the volumes within the intratrabecular areas . As illustrated in figures 1 and 2, the greater image resolution of MRI permits more accurate border detection of the left atrium as compared to 3DE that might explain an average of 15–20 ml difference in volume observed between modalities in the present study and in the literature. The underestimation of atrial diastolic and systolic volumes by 3DE resulted in mild overestimation of atrial ejection fraction compared to MRI (Fig. 3). Left atrial ejection fraction has been associated with left ventricular systolic and diastolic function . Routine use of left atrial ejection fraction is currently not part of the daily clinical practice likely due to the additional time required to obtain this parameter. With 3DE, atrial volume data at multiple time points per cardiac cycle and LA function are automatically calculated by the software in a time interval that is comparable or shorter than conventional ALM. Larger studies are however required to correlate the atrial ejection fraction assessed by 3DE to systolic and diastolic dysfunction.
In this study we demonstrated that time to obtain LAVI was significantly shorter using 3DE even with manual correction as compared to ALM measurements when measured off-line. Measurements using both techniques were performed off-line to replicate best clinical-practice guidelines. Although many modern ultrasound systems permit on-line ALM calculation of LAVI, a critical shortcoming is the inability to adjust sonographers' left atrial area tracings by the interpreting physician off-line. In our multi-vendor clinical laboratory, we have overcome this limitation by performing all measurements off-line on images already transferred to the digital reporting system.
Our findings suggest that in laboratories equipped with 3D matrix-array transducers and an off-line quantification application, the 3DE is the most time-efficient method of LA volume quantification. This finding, along with good MRI correlation and reproducibility, implies this may be the preferred method of LAVI measurement except in cases when poor image quality prevents automated border detection. Given the prognostic implication of LA size, the ease of 3D derived volume and ejection fraction calculation will allow more routine acquisition of these parameters. In a busy echocardiography imaging laboratory the time saved by the sonographers and readers to calculate such parameters might have significant impact on the efficiency of the lab as well as better patient care.