Author + information
- Received January 8, 2008
- Revision received January 25, 2008
- Accepted February 6, 2008
- Published online July 1, 2008.
- Victor Mor-Avi, PhD⁎,⁎ (, )
- Carly Jenkins, MS†,
- Harald P. Kühl, MD‡,
- Hans-Joachim Nesser, MD§,
- Thomas Marwick, MD†,
- Andreas Franke, MD‡,
- Christian Ebner, MD§,
- Benjamin H. Freed, MD⁎,
- Regina Steringer-Mascherbauer, MD§,
- Heidi Pollard, BS⁎,
- Lynn Weinert, BS⁎,
- Johannes Niel, MD§,
- Lissa Sugeng, MD⁎ and
- Roberto M. Lang, MD⁎
Reprint requests and correspondence:
Dr. Victor Mor-Avi, University of Chicago, MC5084, 5841 South Maryland Avenue, Chicago, Illinois 60637.
Objectives We sought to study: 1) the accuracy and reproducibility of real-time 3-dimensional echocardiographic (RT3DE) analysis of left ventricular (LV) volumes in a multicenter setting, 2) interinstitutional differences in relationship with the investigators' specific experience, and 3) potential sources of volume underestimation.
Background Reproducibility and accuracy of RT3DE evaluation of LV volumes has not been validated in multicenter studies, and LV volumes have been reported to be underestimated compared to cardiac magnetic resonance (CMR) standard.
Methods A total of 92 patients with a wide range of ejection fractions underwent CMR and RT3DE imaging at 4 different institutions. Images were analyzed to obtain LV end-systolic volume (ESV) and end-diastolic volume (EDV). Reproducibility was assessed using repeated analyses. The investigation of potential sources of error included: phantom imaging, intermodality analysis-related differences, and differences in LV boundary identification, such as inclusion of endocardial trabeculae and mitral valve plane in the LV volume.
Results The RT3DE-derived LV volumes correlated highly with CMR values (EDV: r = 0.91; ESV: r = 0.93), but were 26% and 29% lower consistently across institutions, with the magnitude of the bias being inversely related to the level of experience. The RT3DE measurements were less reproducible (4% to 13%) than CMR measurements (4% to 7%). Minimal changes in endocardial surface position (1 mm) resulted in significant differences in measured volumes (11%). Exclusion of trabeculae and mitral valve plane from the CMR reference eliminated the intermodality bias.
Conclusions The RT3DE-derived LV volumes are underestimated in most patients because RT3DE imaging cannot differentiate between the myocardium and trabeculae. To minimize this difference, tracing the endocardium to include trabeculae in the LV cavity is recommended. With the understanding of these intermodality differences, RT3DE quantification of LV volume is a reliable tool that provides clinically useful information.
Volumetric measurements using 3-dimensional echocardiography (3DE) avoid the need for geometric assumptions and the risk of underestimating volumes in foreshortened views. Consequently, the evaluation of left ventricular (LV) volumes and the ejection fraction (EF) has been shown to be more reproducible and accurate with 3DE than both 2-dimensional and M-mode based techniques, when compared with cardiac magnetic resonance (CMR) (1–6). The superiority of the 3DE imaging in terms of improved accuracy (7–12) and reproducibility (12,13) was recently also demonstrated for real-time 3-dimensional echocardiographic (RT3DE) imaging, which allows fast and largely automated volumetric analysis of LV volume and function based on endocardial surface detection (Fig. 1) (13,14). Although this methodology has been compared against CMR in single-center studies by several investigators, it has not been validated in a standardized protocol in a multicenter setting.
Importantly, several recent studies have reported that RT3DE underestimates LV volumes (13,15–17) to a variable extent, but no consensus has been reached regarding the factors contributing toward this error. We hypothesized that this volume underestimation may be due to differences in spatial and contrast resolution between RT3DE and CMR imaging that determine the level of detail with which the left ventricle is visualized. This is because the ability to visualize endocardial surface detail, including trabeculae and papillary muscles, and the mitral apparatus, is likely to affect the identification of the LV boundaries and determine to what extent these structures are included in the LV volume. Also, the intermodality discordance may be increased by analysis-related differences, such as different views used to identify the endocardial boundary as well as different algorithms used for volume calculations and their software implementations.
Accordingly, this study was designed to: 1) validate volumetric analysis of the left ventricle from RT3DE datasets against the standard CMR reference technique in a multicenter setting; 2) compare the reproducibility of this analysis with that of CMR volume measurements; 3) study interinstitutional differences in accuracy and reproducibility of the RT3DE volume measurements in relationship with the level of the investigators' experience with the analysis software in each site; and 4) identify and evaluate the relative contributions of the potential sources of error.
Initially, aims #1 and #2, namely the accuracy and reproducibility of RT3DE volume measurements, were addressed by enrolling patients within a wide range of LVEF, referred for CMR evaluation of LV size and function in 4 institutions. In each patient, RT3DE and CMR imaging were performed on the same day. All images were analyzed to obtain LV end-systolic volume (ESV), LV end-diastolic volume (EDV), and EF, which were compared between the 2 modalities. We also used these data to compare in how many patients RT3DE and CMR differed in their classification in terms of EF being above or below 35%, a clinically important cutoff in patients with heart failure. Reproducibility of both RT3DE and CMR techniques was studied using repeated measurements.
To achieve aim #3, namely the experience-related interinstitutional differences, investigators in the participating institutions, all experienced echocardiography researchers, were provided with different levels of instruction and training with the prototype software tool for analysis of LV volume (QLAB, 3DQ-Advanced, Philips Medical Systems, Bothell, Washington). Among the 4 sites, the level of experience ranged from at least 1 year of frequent use for previous research projects to several hours of instruction. The investigators were not informed that the level of experience was a variable in the study design. Accuracy and reproducibility of the volumetric analysis were compared between institutions and correlated with the level of experience.
To achieve aim #4, the identification of the potential sources of error, we performed several additional protocols. First, we obtained a series of RT3DE datasets from phantoms, which were used to: 1) rule out a calibration error in the analysis software; 2) calculate how much a minimal change in a boundary position would affect the measured volume; and 3) trace the boundaries in different ways to determine which boundary position would yield correct volume measurements in agreement with the known true volumes.
In human hearts, a potential source of error that we investigated was the criteria for inclusion/exclusion of basal LV short-axis slices in the CMR reference technique. Over the past years, studies have used different criteria that ranged from: 1) including all slices below the LV outflow tract; 2) including all slices below the mitral annulus; to 3) the current convention used in this multicenter study that includes all slices in which at least 50% circumference of the LV cavity is surrounded by myocardial tissue (18). To determine how much the use of criterion 3 could have contributed to the intermodality discordance, CMR LV volumes obtained at 1 site were recalculated using criterion 2 above, that is, excluding slices in which mitral annulus was visualized, irrespective of myocardium surrounding the LV cavity (Fig. 2).
Additional potential errors related to CMR analysis include tracing endocardial boundaries in short-axis CMR slices (vs. initializing the endocardium in orthogonal long-axis planes extracted from RT3DE datasets) as well as different algorithms used for volume calculations. To determine the magnitude of these errors, CMR images obtained in a subgroup of patients were interpolated into a 3D format identical to that of the RT3DE datasets and analyzed using the same volumetric analysis software (QLAB, 3DQ-Advanced) (Fig. 3). The results of these measurements were compared with those obtained using the standard CMR analysis technique.
Finally, because the visualization of the endocardial trabeculae by RT3DE imaging is limited in many patients (Fig. 4), the trabeculae may be erroneously perceived as part of the myocardium. We thus hypothesized that this may also be an important source of error in the quantification of LV volumes. Accordingly, volumetric analysis of the reformatted CMR 3D datasets was repeated while excluding the trabeculae from the LV cavity (19). The results of these unconventional measurements were compared to the RT3DE values obtained in the same patients.
We studied 92 patients (age 57 ± 16 years; 69 men and 23 women) referred for CMR evaluation of LV size and function in 4 institutions. Patients were enrolled into 4 groups according to EF as determined by biplane 2-dimensional echocardiography: group 1 included 21 patients with EF ≤20%, group 2 included 23 patients with EF 21% to 40%, group 3 included 24 patients with EF 41% to 55%, and group 4 included 24 patients with EF >55%. Exclusion criteria were prior cardiac surgery and known contraindications for CMR imaging, including pacemaker or defibrillator implantation, atrial arrhythmia, claustrophobia, and dyspnea precluding a 10- to 15-s breath-hold. The protocol was approved by the Institutional Review Board of each participating institution. Written informed consent was obtained in each patient.
Magnetic resonance imaging and analysis
The CMR images were obtained using a 1.5-T scanner with a phased-array cardiac coil. Equipment manufacturers varied between institutions and included Philips (Intera Achievea, Best, the Netherlands), Siemens (MAGNETOM Sonata, Erlangen, Germany), and General Electric (Sigma Excite, Milwaukee, Wisconsin). In each patient, retrospective electrocardiogram-gated localizing spin-echo sequences were used to identify the long axis of the heart. Steady-state free precession dynamic gradient-echo cine loops were then obtained using retrospective electrocardiographic gating and parallel imaging techniques (sensitivity encoding for Philips, modified sensitivity encoding for Siemens, and array spatial sensitivity encoding technique for GE) during 10- to 15-s breath-holds with a temporal resolution of 30 frames per cardiac cycle. In all patients, cine loops of 8-mm thick short-axis slices with 2-mm gaps and 2.0 × 2.0-mm in-plane spatial resolution were obtained from just above the ventricular base to just below the apex.
The CMR images acquired at each site were analyzed at that site using commercial software supplied by the corresponding manufacturer (Philips: ViewForum; Siemens: Argus; GE: MASS Analysis). Analysis included slices from the first basal slice that showed at least 50% of the circumference of the LV cavity surrounded by myocardial tissue through the last apical slice that showed the LV cavity (18,19). The LV endocardial boundary was semiautomatically traced with the papillary muscles and trabeculae included in the LV cavity in every slice at end-diastole (first frame in the sequence) and end-systole (smallest LV cavity, as visually determined from 2 to 3 different slices) and manually adjusted when necessary. All tracings were performed by investigators experienced in CMR analysis of LV size and function using the above jointly agreed criteria. The investigators had no knowledge of the echocardiographic measurements. The ESV and EDV were calculated using the disk-area summation method (modified Simpson's rule). The EF was calculated from the ESV and EDV using the standard formula. These values were used as a reference for comparison with the RT3DE data.
Echocardiographic imaging and analysis
The RT3DE harmonic imaging was performed using the Philips iE33 imaging system and an X3-1 matrix array transducer with the patient in the left lateral decubitus position. A wide-angled acquisition “full-volume” mode, in which 5 wedge-shaped subvolumes are acquired over 5 consecutive cardiac cycles, was used during a single breath-hold. Special care was taken to include the entire LV cavity within the pyramidal 3D volume. Before each acquisition, images were optimized for endocardial visualization by modifying the gain, compress, and time gain compensation controls. Acquisition off all RT3DE datasets required 10 to 15 min.
Digital RT3DE images were analyzed at each site using prototype software (QLAB, 3DQ-Advanced, Philips) by an investigator blinded to the results of the CMR measurements. First, 2- and 4-chamber views with the largest long-axis dimensions were selected from the RT3DE pyramidal dataset as described previously (20) in the first time frame of the dataset (Fig. 1, top panels), that is, end-diastole. In these 2 planes, 5 points, including 4 points on the mitral annulus (2 in each plane) and the apex in either plane, were manually initialized to define the endocardial surface. Then, the initial endocardial surface was manually adjusted in multiple apical planes, while including the papillary muscles in the LV cavity, and its position was corrected as necessary in multiple arbitrary cut planes until the best match was visually verified (Fig. 1, bottom right). Then, the voxel count inside the endocardial surface was used to calculate the EDV without any geometric modeling. This analysis was then repeated for the end-systolic frame, which was identified as the frame that showed the smallest LV cavity, resulting in a measurement of ESV. The EF was calculated from the ESV and EDV using the standard formula. The adjustments in endocardial surface position, being the most time-consuming part of the analysis procedure, required approximately 5 min per dataset in most patients.
To determine the reproducibility of LV volume measurements for each imaging modality, CMR and RT3DE image analysis was repeated by an additional investigator as well as by the same primary reader at least 1 week later. During these repeated analyses, the investigators were blinded to the results of all prior measurements.
The RT3DE-derived values of EDV, ESV, and EF were compared with the corresponding CMR reference values using linear regression with Pearson's correlation coefficients and Bland-Altman analyses to assess the bias and limits of agreement with the CMR reference. To verify the significance of the biases paired t test versus null values were applied. Any p values <0.05 were considered significant. Interobserver and intraobserver variabilities were calculated as the absolute difference of the corresponding pair of repeated measurements as a percentage of their mean in each patient and then averaged over the entire study group as well as for each site separately.
Phantom imaging and measurements
First, to rule out a calibration error, an egg-shaped phantom was immersed in a water bath and subjected to RT3DE imaging using the same matrix array transducer. Imaging settings were adjusted to optimize the visualization of the shell boundaries. A full-volume dataset of the phantom was acquired using simulated ECG gating and analyzed using QLAB 3DQ-Advanced software following the same steps as during analysis of LV volume. The measured volume was compared with the true volume specified by the manufacturer. Additionally, custom software was then used to expand the detected surface outward exactly 1 mm and measure the volume increment in milliliters as well as in percentage of the true volume.
In addition, to further explore the extent to which differences in surface tracing can affect volume determinations, 4 water-filled latex balloons with volumes comparable to human ventricles, were imaged using the same methodology. Digital datasets were analyzed using the same software to measure balloon volumes. Each balloon was traced 3 times by contouring along the inner interface, then along the outer interface, and finally in the middle of the latex layer. Volumes resulting from each tracing session were recorded and compared to the true volume of each balloon.
Comparisons with CMR
Figure 5 shows the results of the comparisons between the RT3DE measurements of EDV, ESV, and EF and the corresponding CMR values. Although the 2 techniques correlated highly, as reflected by r values of 0.91, 0.92, and 0.81, respectively, Bland-Altman analysis revealed negative biases of −67 ml (−29% of the mean CMR-derived EDV value, p = 5 × 10−17), −41 ml (−27% of the mean ESV value, p = 6 × 10−12), and −3% (p = 4 × 10−6). Importantly, the standard deviations of the intertechnique differences were quite wide, reflecting the inconsistent nature of volume underestimation by RT3DE technique in individual patients. Also, we found that RT3DE and CMR classifications in terms of the 35% EF cutoff were identical in 78 out of 92 patients (85%).
Table 1 shows the results of the regression and Bland-Altman analyses for each participating institution, which were arranged in the descending order of experience with the analysis software. Interestingly, measurements performed by the most experienced investigators (site A) showed biases that were roughly half of those noted in the entire study group. Despite the high correlations with the CMR reference values for all sites, the biases progressively increased with the decreasing level of experience, reaching maximum values for site D. Intersite comparisons of tracing methodology revealed that the investigators most experienced with this technique tended to trace endocardial boundaries as far outward as possible to include as much endocardial trabeculae as possible in the LV cavity. Conversely, less experienced users tended to trace endocardial boundaries along what appeared to be the blood-tissue interface, that is, the area of maximum intensity gradients.
Table 2 shows the results of the reproducibility analysis of LV volumes for CMR images and RT3DE datasets. For both EDV and ESV, both the interobserver and intraobserver variabilities were higher for RT3DE-derived volumes than for the CMR measurements. Not surprisingly, for both EDV and ESV measured by both techniques, the interobserver variability was higher than the intraobserver variability. Importantly, all variability values were within 10% with the exception of the interobserver variability of the RT3DE ESV measurements, which was 13%. It is worth noting, however, that in individual patients, variability levels of both imaging modalities far exceeded the acceptable 10% to 15% levels (Table 2). Of note also, there were no clear experience-related trends in the interobserver and intraobserver variabilities data.
Figures 6A to 6C show a long-axis cut plane of the egg-shaped phantom extracted from a RT3DE dataset with the traced boundary superimposed (Fig. 6A). Volume measurements performed in the phantom yielded 68.7 ml. Expanding the surface only 1 mm outward (Fig. 6B) resulted in volume of 76.1 ml. Of note, a barely visible difference of 1 mm in the surface position (Fig. 6C) resulted in a volume difference of 7.4 ml or 11% of the true volume of 73.29 ml as per manufacturer's specifications.
Figures 6D and 6E show images of a water-filled balloon with the boundaries traced in 3 different ways. Table 3 summarizes the results of volume measurements obtained with such tracings in 4 different balloons. The most accurate measurements (within 1% error) were obtained by tracing through the center of the latex layer, while the other 2 tracings resulted in considerable errors of 12% to 23%.
Modifications to CMR reference
Two investigators, who jointly reviewed CMR images obtained in 20 randomly selected patients enrolled at 1 institution, determined that exclusion of basal slices depicting the mitral annulus could be justified in 13 out of 20 patients. Excluding these basal slices resulted in smaller CMR reference values and thus reduced the biases in LV volumes by approximately 20% (Table 4).
Interpolation of stacks of CMR short-axis slices resulted in 3D datasets suitable for volumetric analysis of LV size in 19 of a group of 23 patients enrolled at 1 of the 4 sites (Fig. 3). In the remaining 4 patients, misregistration of the left ventricle in the short-axis slices resulted in “stitch” artifacts in the long-axis views that did not allow the generation of smooth endocardial surfaces. The EDV and ESV measurements obtained from 3D datasets were similar to those measured using the conventional CMR technique based on the method of disk approximation (Fig. 7), as reflected by correlation coefficients of r = 0.997 for both volumes and small biases of −7 ± 15 ml and −5 ± 15 ml, respectively (−4 ± 4%, p < 0.05; and −2 ± 6%, p = 0.07).
In these 19 patients, exclusion of endocardial trabeculae from the LV cavity during volumetric analysis of interpolated 3D CMR datasets (Figs. 8A and 8B) improved the agreement between the RT3DE-derived LV volumes and the CMR reference values. Figures 8C and 8D summarize the results of linear regression and Bland-Altman analyses in this group of patients for both sets of CMR reference values, those obtained with and without the trabeculae included in the LV cavity. Exclusion of trabeculae resulted in clear improvement in the intermodality agreement, as reflected by regression slopes closer to 1.0 and smaller intercepts, higher correlation values, and a decrease in the magnitude of the biases from −14% and −9% (p < 0.05 for both) to −1% and 2% (NS for both).
Although most previous published reports have endorsed RT3DE evaluation of LV volumes for clinical use, their conclusions were based on single-center studies, in which data were acquired and measured by highly trained personnel. The rationale behind our study design was to simulate as closely as possible the conditions under which this methodology would ultimately be used clinically. In this setting, the levels of training and experience with RT3DE evaluation of LV volumes vary widely. The main question we sought to answer was whether or to what extent average end users of RT3DE equipment and volumetric analysis software could expect their LV volume measurements to be interchangeable with those performed with the current standard reference technique, namely CMR imaging. This question is of particular practical importance because these tools are becoming widely available and are anticipated by some to provide a quick, relatively inexpensive and portable alternative to CMR imaging (13,16).
Our results confirmed that although RT3DE and CMR measurements resulted in identical classification with respect to the 35% EF cutoff in the majority of patients, these 2 techniques do not yield identical LV volumes and EF. First, RT3DE-derived volumes are underestimated compared with CMR reference for a variety of reasons, some of which are experience-dependent and can be addressed by adequate training, but others are inherent to the technique and need to be taken into account when measurement results are interpreted. We found that the major source of error is that in most patients the spatial resolution of RT3DE imaging is insufficient to provide clear definition of endocardial trabeculae, which are, as a result, lumped together with the myocardium rather than being included in the LV cavity, as during analysis of CMR images. As our results show (Table 1, site A), this error can be minimized by learning how to identify the true endocardial boundaries beyond the blood-trabeculae interface. Contrast enhancement may potentially help with visualizing the trabeculae and may specifically allow separating them from the myocardium. This hypothesis remains to be tested in future studies.
An additional source of intertechnique discordance includes the CMR criteria for inclusion of basal LV slices, which can significantly affect the reference values. This problem does not exist for the RT3DE technique that uses mostly long-axis views for endocardial surface determination. Thus, this issue should not be regarded as an error of the RT3DE analysis but rather as its strength. Nevertheless, the users need to be aware of these intermodality differences when interpreting results of LV volume measurements.
The reproducibility of RT3DE measurements of LV volumes was lower than that of CMR analysis, most likely because endocardial definition of the latter images is in most cases better than that of RT3DE datasets. Nevertheless, in this study, both interobserver and intraobserver variabilities of RT3DE measurements were within what is widely considered as the clinically acceptable range of up to 10% to 15%. Notwithstanding, it is important to remember that these numbers refer to group averages, and differences between repeated measurements in individual patients can be significantly larger, as indeed was the case for both RT3DE and CMR measurements in this study. One of the limitations of this study is that the reproducibility of repeated acquisitions was not studied.
Importantly, all sites participating in this study used commercial RT3DE analysis software offered by one vendor (Philips). Therefore, the conclusions of this study can directly apply only to this specific software, because alternative software programs were not tested. Nevertheless, it is likely that such alternative analysis would have resulted in similar findings, given the experience-dependent differences in endocardial boundary tracing noted in this study.
Our investigation of the sources of error led us to first rule out the possibility of calibration error, either in the imaging system or in the analysis software. Because the degree of volume underestimation varied widely between patients, we did not anticipate finding such an error, and indeed, it was ruled out by the phantom measurements. Our phantom studies also demonstrated how crucial the exact boundary position is for accurate volume measurements, because a barely visible 1-mm difference in surface position resulted in considerable differences in the calculated volumes. Our measurements in water-filled balloons demonstrated that even for volumes as large human ventricles, minimal differences in boundary tracing can result in biased volume measurements, which would be the case if one systematically traced endocardial boundaries slightly more inward. These findings also explain in a quantitative manner the intermeasurement variability of RT3DE volume measurements of human ventricles where endocardial boundaries are never as well defined as the latex-water interface of the balloons, even in areas without prominent trabeculae.
The use of interpolated 3D CMR datasets allowed us to prove 2 important points. First, the differences between analysis techniques normally used for CMR images and RT3DE datasets could not have biased the measurements to an extent even close to what we found in our patients, because analysis of the same CMR images using the 2 techniques resulted in virtually the same volume values. Second, repeated volumetric analysis of these datasets while excluding endocardial trabeculae from the LV cavity (19) produced results very similar to those measured using the same analysis technique in RT3DE datasets. This finding allowed us to extrapolate our interpretation to state that, conversely, if trabeculae could be visualized on RT3DE images as well as they are visualized on CMR images and thus could be included in the LV cavity, one would expect RT3DE measurements to be very similar to the standard CMR reference.
In summary, this is the first study to test and validate volumetric quantification of LV volumes from RT3DE datasets against CMR standard reference in a multicenter setting wherein RT3DE data were analyzed by observers with variable levels of specific experience. Although in our patients RT3DE-derived LV volumes were underestimated compared to CMR reference values, this study clarified the role of different potential sources of error and provides guidelines for future users on how to minimize these errors as well as how to interpret their findings.
The authors thank Olivier Gerard, Pascal Allain, and Stephane Husson of Philips Medical Systems for their help with software development as well as Paul Kalman, Kerry Short, Lynette Ward, and others for their enthusiastic support.
Each of the 4 participating sites received a research grant from Philips Medical Systems, Andover, Massachusetts. Anthony DeMaria, MD, MACC, served as Guest Editor for this paper.
- Abbreviations and Acronyms
- 3-dimensional echocardiography
- cardiac magnetic resonance
- end-diastolic left ventricular volume
- ejection fraction
- end-systolic left ventricular volume
- left ventricular
- real-time three-dimensional echocardiographic
- Received January 8, 2008.
- Revision received January 25, 2008.
- Accepted February 6, 2008.
- American College of Cardiology Foundation
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