Transthoracic three-dimensional echocardiographic reconstruction of left and right ventricles: In vitro validation and comparison with magnetic resonance imaging☆,☆☆
Section snippets
METHODS
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3D echocardiographic system
To perform the transthoracic 3D reconstruction of the heart by ultrasound, we developed an echocardiographic system based on a 3.5 MHz dynamically focused annular array transducer rotating 180 degrees around its central axis. The ultrasonic beam profile exhibited a regular shape along the entire depth used to visualize the heart, with a lateral resolution of 1.3 mm at −6 dB. This transducer allows the acquisition of 51 standard fan-shaped 2D echocardiograms at 3.6-degree increments of rotation.
Excised heart preparation
Fourteen freshly excised sheep hearts were prepared by sewing shut one of the two venae cavae and three of the four pulmonary veins, to leave only one open vessel in each atrium. Pulmonary artery and aorta were left open to purge air. The hearts were fixed in formalin for 48 hours with pressure expansion to avoid chamber collapse; a pressure of 25 to 30 mm Hg was used to expand the left and right cavities. To obtain the ventricular casts, the heart cavities were filled with a heated 5% solution
Echocardiographic image acquisition and volume calculation
Echocardiographic studies were performed with the excised hearts suspended and immersed in a fish tank filled with water. For each heart, two complete rotations of the imaging plane were performed to encompass the left and the right ventricles.
From the 3D volume images of the left and right ventricles reconstructed from the nonparallel images, up to 49 parallel tomographic planes were derived beginning parallel to the mitral or tricuspid valve plane, respectively, and extending to the
MRI acquisition and volume calculation
MRI was performed using a 0.5 Tesla commercially available system (Philips Gyroscan, Eindhoven, The Netherlands). The excised hearts were positioned inside the magnet and imaged with the head probe. We used a gradient echo imaging technique without ECG gating and with the following parameters: alpha = 60 degrees, TE = 12 msec, TR = 21 msec, slice thickness = 5 mm, 256 × 256 matrix, field of view = 190 mm, two measurements. In a preliminary study, these parameters were found to provide optimal
Anatomic volume calculation
After imaging of the excised hearts, the left and right atrial myocardium was incised and carefully peeled off. After the atrioventricular planes were identified, the atrial casts were separated from the corresponding ventricular casts, the reference toothpicks being included in the ventricular cavities. Then the ventricular myocardium was incised and carefully peeled off the ventricular casts. The actual volumes of the ventricular casts were measured by water displacement in a graduated
Statistical analysis
Values are expressed as mean ± SD. The accuracy of the ventricular volumes derived from the 3D echocardiographic reconstructions and MRI was examined by simple linear regression analysis or linear regression analysis with replication. Regression lines and line of identity were compared according to the method proposed by Zar. 23 The method of Bland and Altman 24 was used to determine differences between methods and to assess whether differences were systematically related to ventricular size.
3D echocardiography
The reconstruction of the left ventricular cavities derived from the 3D echocardiographic data sets accurately reproduced the shape of the actual casts. Left ventricular volumes derived from 3D reconstructions agreed well with actual volumes. The correlation between measured and true left ventricular volumes was also excellent for the individual observers (Table I) ; the slopes and intercepts of the three individual regression lines were statistically equal to the line of identity, with a
DISCUSSION
The present in vitro study demonstrated that our 3D echocardiographic system based on a transthoracic rotating transducer allows for excellent measurements of left and right ventricular volumes of excised hearts. Interestingly, no substantial overestimate or underestimate by 3D echocardiography was detected, even for the trabeculated, irregularly shaped right ventricle. Indeed, the SEE was similarly low for both left and right ventricles. Compared with MRI, 3D echocardiography exhibited a
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2010, American Journal of CardiologyCitation Excerpt :Simpson's method can provide accurate measurement of the RV volume if a large number of short-axis views are used for analysis. Pini et al13 traced up to 49 parallel planes from ultrasound images and found that the volumes of the summed slices agreed well with those from magnetic resonance imaging (r = 0.97, SEE = 2.55 ml, n = 14 in vitro hearts). However, the accuracy for in vivo analysis has not been consistent.
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Reprint requests: Riccardo Pini, MD, Institute of Gerontology, University of Florence, Via delle Oblate 4, 50141 Florence, Italy.
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