Real-time, volumetric echocardiography: usefulness of volumetric scanning for the assessment of cardiac volume and function.

BACKGROUND: A novel imaging system has been introduced which uses a dedicated two-dimensional echo probe for rapid beam forming to scan a pyramidal volume in real time. Real-time volumetric echocardiography has the potential to determine accurate cardiovascular anatomy, volume and function in the beating heart without reconstructions. The results of animal and human studies using volumetric echocardiography are evaluated for the potential for clinical applications. IMAGING
METHODOLOGY: A new type of ultrasound imaging, high-speed volumetric scanning based on phased array principles permits real-time three-dimensional, volumetric echocardiography (real-time 3-DE). The system requires no off-line reconstruction techniques, thus enabling dynamic three-dimensional visualization and quantification of the heart in real time using a transthoracic approach. Real-time 3-DE uses a 2-D matrix phased array transducer. Image formation employs 16:1 parallel processing to scan a pyramidal volume composed of multiple steering directions in the azimuth dimension and in the elevation dimension. The finished transducer is mounted in a hand-held case with a circular aperture of 16 mm diameter. The array consists of approximately 1,600 elements, operating at 2.5 MHz. Real-time 3-DE permits simultaneous, multiple plane display of two sector arcs (B-scans) and C-scan (parallel to the transducer face or inclined) on a single monitor, conveying the three-dimensional nature of the ultrasound data. This system also allows these planes to be angled for extra diagnostic flexibility. The motion of all the structures during the cardiac cycle can be evaluated in dynamic mode.
METHODS: Real-time 3-DE was assessed for accuracy of volume measurement by measuring the volume of balloons of different size and shape, and the hearts of 15 closed chest dogs with myocardial contrast enhancement, and compared to the volumes measured by left ventricular angiography in the dogs. Real-time 3-DE was used to evaluate the endocardial border determination of the entire left ventricle by injecting contrast agent in 12 patients. The endocardial border determination of each segment was scored, and the endocardial border score index calculated. Both real-time 3-D images and cine magnetic resonance imaging (MRI) were performed in 16 patients to assess the accuracy of volume measurement of the left ventricle in humans. The endocardial border of the left ventricle was manually traced, and the volumes calculated by Simpson's rule.
RESULTS: The volumes measured by real-time 3-DE correlated well with the true volumes for different sizes of balloon and for asymmetric balloons. The end-diastolic volume and end-systolic volume linear correlation of real-time 3-DE versus angiography measurements using manual tracing in vivo also gave a good correlation (r = 0.97, p < 0.001; r = 0.92, p < 0.01). Fifty-eight of 192 segments were rated as good at baseline and 143 rated as good after Levovist injection. Endocardial border determination was improved by Levovist injection in 100 of 137 segments (74.6%). The endocardial border score index was significantly higher after Levovist administration than at baseline (p < 0.003). The end-diastolic volume and end-systolic volume of the left ventricle measured by real-time 3-DE in humans correlated well with those measured by MRI (end-diastolic volume: r = 0.97, p < 0.001; end-systolic volume: r = 0.96, p < 0.001).
CONCLUSIONS: Transthoracic real-time, volumetric echocardiography opens a new and exciting field of echocardiography. The results of these studies demonstrate that this system can accurately measure the ventricular volume and function without use of geometric assumptions. This volumetric mode or V-mode scanning is a new imaging modality that provides a practical methodology to investigate important clinical and research questions.