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J Am Coll Cardiol, 2000; 36:900-907 © 2000 by the American College of Cardiology Foundation |
a Cardiovascular Imaging Center, Department of Cardiology, the Cleveland Clinic Foundation, Cleveland, OhioUSA
b Laboratory of Animal Surgery and Medicine, National Institutes of Health, Bethesda, MarylandUSA
c Cardiology Branch, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland, USA
Manuscript received September 30, 1999; revised manuscript received March 7, 2000, accepted April 14, 2000.
Reprint requests and correspondence: Dr. Takahiro Shiota, Department of Cardiology/Desk F 15, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, Ohio 44195
shiotat{at}ccf.org
| Abstract |
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To validate the accuracy of real-time three-dimensional echocardiography (RT3DE) for quantifying aneurysmal left ventricular (LV) volumes.
BACKGROUND
Conventional two-dimensional echocardiography (2DE) has limitations when applied for quantification of LV volumes in patients with LV aneurysms.
METHODS
Seven aneurysmal balloons, 15 sheep (5 with chronic LV aneurysms and 10 without LV aneurysms) during 60 different hemodynamic conditions and 29 patients (13 with chronic LV aneurysms and 16 with normal LV) underwent RT3DE and 2DE. Electromagnetic flow meters and magnetic resonance imaging (MRI) served as reference standards in the animals and in the patients, respectively. Rotated apical six-plane method with multiplanar Simpsons rule and apical biplane Simpsons rule were used to determine LV volumes by RT3DE and 2DE, respectively.
RESULTS
Both RT3DE and 2DE correlated well with actual volumes for aneurysmal balloons. However, a significantly smaller mean difference (MD) was found between RT3DE and actual volumes (7 ml for RT3DE vs. 22 ml for 2DE, p = 0.0002). Excellent correlation and agreement between RT3DE and electromagnetic flow meters for LV stroke volumes for animals with aneurysms were observed, while 2DE showed lesser correlation and agreement (r = 0.97, MD = 1.0 ml vs. r = 0.76, MD = 4.4 ml). In patients with LV aneurysms, better correlation and agreement between RT3DE and MRI for LV volumes were obtained (r = 0.99, MD = 28 ml) than between 2DE and MRI (r = 0.91, MD = 49 ml).
CONCLUSIONS
For geometrically asymmetric LVs associated with ventricular aneurysms, RT3DE can accurately quantify LV volumes.
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This study was designed to show that RT3DE is more accurate than 2DE for the quantification of volumes in aneurysmal LVs. To accomplish this goal, we: 1) validated the accuracy of RT3DE for quantifying modeled aneurysmal LV volumes in vitro; 2) determined the accuracy of RT3DE for measuring LV stroke volume (SV) compared with electromagnetic flow meter (EM) methods in a chronic LV aneurysm animal model; and 3) evaluated the feasibility and accuracy of RT3DE for measuring LV volumes compared with a reference standard of magnetic resonance imaging (MRI) in patients with and without ischemic aneurysmal heart disease.
| Methods |
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| Animal study |
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Experimental protocol. Four hemodynamic conditions were obtained for each sheep and included baseline conditions, after 500 ml of blood infusion and during intravenous nitroprusside (50 µg/ml) and angiotensin II (2 µg/ml) infusion. Real-time three-dimensional images were acquired from the epicardial LV apex under each condition. All RT3DE images were stored on optical discs for off-line analysis. After each RT3DE data set acquisition, 2DE images were obtained from apical four-chamber and two-chamber views and were recorded on one-half inch VHS videotape. Care was taken to include the largest LV cavity and aneurysm in the RT3DE pyramidal volumetric data set and 2DE image set for the entire cardiac cycle. Left ventricular SV obtained by integrating the instantaneous aortic forward flow rate measured during systole from the EM probe was compared to simultaneously recorded RT3DE and 2DE echocardiographic assessment of SVs.
| Patient study |
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Study protocol. After institutional review board committee approval, written informed consent was obtained from each patient. In each patient, cardiac MRI was performed to determine LV volume and EF and was then followed by RT3DE and 2DE studies, each performed within 1 h of the other.
RT3DE. Using the same RT3DE machine as in the phantom and animal study, image data set acquisition was performed from a transthoracic apical view with the patient in a left decubital supine position. The entire LV volumetric data set was acquired simultaneously including apical four-chamber views, apical two-chamber views and a series of short axis images and was stored digitally after the highest quality image was obtained. Care was taken to include the entire LV cavity in the real-time pyramidal volumetric data set during the entire cardiac cycle.
2DE. Conventional transthoracic 2DE images were obtained in the apical four-chamber view and apical two-chamber view with the same machine as in the phantom and animal studies. All of the 2DE studies were performed immediately after RT3DE examination.
MRI. Magnetic resonance imaging was employed as a reference standard for determining absolute LV volumes in patients. All images were obtained with a commercially available 1.5 Tesla whole-body scanner (Siemens Vision, Erlangen, Germany) with a phased-array coil. Electrocardiogram-gated localizing spin-echo sequences were used to identify the intrinsic long axis of the heart. For all patients, short-axis dynamic gradient-echo ("Cine") MRI images were started in the mitral valve plane with a slice thickness of 8 to 10 mm. These images were manually segmented using commercially available image analysis software (Argus, Siemens Medical Systems, Iselin, New Jersey) using Simpsons algorithm to determine total LV volumes, that is, end-diastolic volume (EDV) and end-systolic volume (ESV). Ejection fraction was also calculated. All measurements were performed by an investigator experienced in cardiac MRI analysis and without knowledge of the RT3DE measurements.
Quantitative RT3DE and 2DE measurements. For all RT3DE and 2DE images acquired from the animals and patients, LV EDV, ESV, SV (EDV-ESV) and EF (SV/EDV) were determined. The RT3DE data sets were transferred to a computer system (3D EchoTech, Echo Tech America, Lafayette, Colorado) with a dedicated software program that was used to display the RT3DE images and to measure LV volumes. The computer automatically displayed the dynamic RT3DE images as six apical images with a 30° angle between each image. The LV volumes were calculated using a series of symmetrically rotated apical six planes as previously described (15). The cavity of the LV in each image plane was manually traced, and the LV volume was calculated using the multiplanar Simpsons method (Fig. 2). For 2DE, LV volumes were calculated using the biplane Simpsons method from the apical four-chamber and two-chamber views. For all measurements, LV trabeculations and papillary muscles were carefully excluded from the LV cavity contour.
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Statistical analysis. Values are expressed as mean ± SD for continuous variables. The volume correlations between RT3DE, 2DE and actual volume in the phantom study, between RT3DE, 2DE and EM in the animal study and between RT3DE, 2DE and MRI in the patient study were performed using simple linear regression analysis. Correlation coefficients were compared after Fisher z-transformation. Agreement between methods was evaluated using the Bland and Altman method (16). Paired Student t tests were used to compare the difference between methods, and a value of p < 0.05 was considered statistically significant.
| Results |
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Although there was no significant difference for LV volume measurements between RT3DE and 2DE in the animals without LV aneurysms, 2DE significantly overestimated LV EDV by RT3DE (p < 0.05) and also tended to overestimate LV ESV in the animals with LV aneurysms, Table 1.
Clinical studies. Results of LV EDV, ESV and EF measurements by RT3DE, 2DE and MRI are presented in Table 2. Compared with MRI, there were no significant differences for LV EF calculated by RT3DE and by 2DE both in the control and the LV aneurysm groups (p > 0.05). However, RT3DE and 2DE significantly underestimated LV EDV and LV ESV as determined by MRI in both groups.
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Left ventricular SV measured by RT3DE, including control and LV aneurysm patients, correlated and agreed with those measured by MRI (r = 0.92, y = 0.77x + 6.73, MD = 13 ± 14 ml, p < 0.0001), but the correlation and agreement between 2DE and MRI for LV SV were decreased (r = 0.82, y = 0.78x + 1.60, MD = 17 ± 21 ml, p < 0.001, Fig. 6).
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| Discussion |
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Buck et al. (20) demonstrated the accuracy of reconstructed three-dimensional echocardiography for determining LV volumes and function in 23 patients with chronic LV aneurysms. They obtained dynamic three-dimensional echocardiographic image data sets from a transthoracic apical view using a rotating probe with acquisition gated to control for electrocardiogram and respiration variation. In their study, LV volumes were calculated from the three-dimensional echocardiographic data sets by summating the volumes of multiple short axis parallel disks. When compared with MRI, their results showed excellent correlation and agreement for LV EDV (r = 0.97, SEE = 14.7 ml), for ESV (r = 0.97, SEE = 12.4 ml) and for EF (r = 0.74, SEE = 5.6%) and a better correlation and agreement than that obtained with two-dimensional techniques. However, these three-dimensional echocardiographic reconstruction methods are technically complicated and time-consuming (prolonged image acquisition and reconstruction time); therefore, clinical use has been limited.
Compared with the reconstruction techniques of previously described three-dimensional echocardiographic methods, RT3DE has retained many of the clinical advantages of 2DE. On-line adjustment of conventional echocardiographic planes can be performed to ensure adequate quality three-dimensional data sets. Once the entire LV was included in the RT3DE data sets, the apical aneurysm present could be displayed by six rotated apical long axis planes. The quantification of LV volumes can be performed quickly when using the apical six-plane method instead of using traditional short axis views (a series of C-scans) (15,24). In addition, since the RT3DE hand-held probe is similar to that of the transthoracic 2DE probe, sonographers do not need extraordinary expertise for acquiring RT3DE data sets, a major advantage over the previously reported reconstruction methods of three-dimensional echocardiography (711,25). Another advantage of RT3DE imaging is the elimination of the need for electrocardiogram or respiratory gating. These factors are particularly important for studying critically ill patients, for acquiring epicardial RT3DE images in the operation room and for imaging patients with arrhythmias (24).
Study limitations. The limited pyramidal angle and low frame rate of current RT3DE may be major factors for the volume underestimation in patients. In patients with dilated hearts, the 60° x 60° pyramidal angle often limits the ability to image the entire ventricle. We have shown in our initial study and in this study that RT3DE significantly underestimated LV volumes when compared with MRI in patients with severely dilated ventricles (24). Inadequate resolution of the LV endocardium because of the mitral valve apparatus and low lateral resolution of the RT3DE images may explain this underestimation. Also contributing to this is the MRI technique itself, by which endocardial tracing tends to include LV trabeculations, whereas they are excluded by 2DE and RT3DE.
The maximal achievable frame rate is only 20 frames per second at an image depth of 14 cm. Low frame rates may be problematic for capturing precise end-diastolic and end-systolic frames, especially in the presence of tachycardia. This imprecision is another reason for RT3DE underestimation of LV volumes, especially LV EDV under clinical conditions.
Conclusions. Real-time three-dimensional echocardiography showed excellent correlation and agreement with actual volumes in a phantom study, for LV SV versus EM in a chronic LV aneurysm animal model and for absolute LV volumes versus MRI in patients with known LV aneurysms. These correlations and agreements were better than those obtained by conventional 2DE. Our results demonstrate the feasibility and validate the accuracy of RT3DE for the clinical assessment of patients with asymmetric LVs.
| Acknowledgments |
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| Footnotes |
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| References |
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