Quantitative Three-Dimensional Echocardiography: Image Analysis for Left Ventricular Volume Assessment
Marek Belohlavek M.D., Ph.D. August 1996
Assessment of left ventricular (LV) function is clinically important. Diastolic and systolic volumes and derived parameters, such as ejection fraction, are generally accepted indicators of LV function. Echocardiography is a widely available clinical method which allows measurement of these parameters. Precision and accuracy of echocardiographic measurements is typically compromised by user's subjectivity, limited windows of access for obtaining echocardiographic scans, and ultrasound signal attenuation and scattering resulting in image artifacts.
In the past decade, compared to single or biplane tomographic techniques that require geometrical assumptions about LV shape, three-dimensional (3D) echocardiography has been shown to provide more precise and accurate evaluation of cardiac volumes. This is particularly relevant in patients with irregular LV geometry, detected during an initial echocardiographic study through multiple tomographic projections. So far, however, the 3D technique has been limited to experimental use due to cumbersome image acquisition (i.e., restricted access windows, long acquisition times, custom probes and special probe holders), time consuming and subjective data processing (i.e., digitization from video tapes and manual delineation of boundaries in numerous serial tomograms), the use of special projections (i.e., parallel or rotational scans with specific incremental steps), and mapping of endo and epicardium to multiparameter (i.e., computationally intense and subjective for required adjustment of multiple features) models that allowed only limited objective utilization of a priori knowledge about LV geometry.
The objective of this research was to develop a clinically implementable method for reliable assessment of LV volumes by 3D echocardiography.
The hypothesis of this research is that 3D echocardiographic reconstruction associated with computer-driven, knowledge-based, and noise-resistant LV endocardial border recognition provides precise, accurate and reproducible measurements of LV diastolic and systolic volumes.
The methods presented in this thesis result in precise and accurate computation of LV volume. This is accomplished through a
- quick image acquisition using rotational (polar) scans with commercially available rotatable transducers
- analysis of echocardiographic image resolution (i.e., resolution cell size measurement)
- assessment of textural properties (i.e., determination of discriminating power of various texture features)
- analysis of random and impulsive noise (i.e., design of a noise resistant edge detector)
- computer-driven endocardial boundary recognition for LV volume measurement (i.e., development of a trainable, knowledge-based system for endocardial surface mapping)
The results indicate, that quick image acquisition of 6–9 spatially related tomograms is suitable for 3D reconstruction of endocardial surface. Determination of the endocardium in individual serial echotomograms can be accomplished best through im age classification using features related to varying pixel values (Mahalanobis distance > 4.0) and 3D anatomical relationships in the reconstructed volume. Combination of the quick acquisition and classification techniques with a trainable computer system for LV cavity boundary delineation, that assimilates expert knowledge about realistic LV shapes, provides clinically feasible (tests conducted during regular outpatient clinical studies), precise (R2 > 0.96, p < 0.001, standard deviation < ±5.75 ml), accurate (root mean squared error < 6.08 ml, negative bias < 2.4 ml), and reproducible (intraindividual variability < 6.46%) estimations of enddiastolic and endsystolic volumes.
In conclusion, we developed quick 3D echocardiographic image acquisition and knowledge-based recognition techniques (''a trainable computer expert'') for precise, accurate and reproducible assessment of LV volumes. Practical feasibility of these techniques was shown through tests in clinical environment using typical (i.e., noisy and artifact corrupted) diastolic and systolic ultrasound images of the heart. The potential clinical application to evaluation of LV function in patients with various congenital and acquired heart diseases is beyond the tested application. Utilization for other cardiac chambers, such as right ventricle, and in other areas of ultrasound imagery, such as intracardiac echocardiography, is anticipated.