Three-dimensional echocardiography: new views from old windows

Let there be light! The meteoric rise of cardiac imaging

Imaging plays a central role in modern cardiovascular practice. It is a field characterised by exciting technological advances that have shaped our understanding of pathology and led to major improvements in patient diagnosis and care. The UK has played a key international role in the development of this subspecialty and is the current home to many of the leading global centres in multimodality cardiovascular imaging. In this short review, we will outline some of the key contributions of the British Cardiovascular Society and its members to this rapidly evolving field and look at how this relationship may continue to shape future cardiovascular practice.

A digital 3-dimensional method for computing great artery flows: in vitro validation studies

BACKGROUND

Conventional 2-dimensional Doppler large vessels are prone to inaccuracy. Three-dimensional (3D) volume imaging provides the opportunity to make cross-sectional flow calculations through digital spatiotemporal integration of flow velocity, area, and profile.

METHODS

A new digital 3D color Doppler reconstruction method was used to generate radially acquired flow data sets. Raw scanline data with digital velocity assignments, obtained by scanning parallel to flow, were transferred from a specially programmed but otherwise conventional ultrasonographic system, which controlled a multiplane transesophageal probe, to a computer workstation via an Ethernet link for assimilation into color 3D data sets. This configuration was used to study 20 pulsatile laminar flows (stroke volumes 30 to 70 mL and peak flow rates 65 to 205 mL/s) in a curved tube model with an oval cross-sectional geometry. After generation of the color 3D data set, flow velocity values from cross sections perpendicular to the tubes were analyzed to determine flow rate and stroke volume.

RESULTS

The flows from 3D digital velocity profiles showed close correlation with peak instantaneous flow rates (r = 0.99, y = 1.01x-0.9, standard error of estimate 4.1 mL/s). When interpreted with pulsed wave Doppler data obtained through the cardiac cycle, they also allowed computation of stroke volume (r = 0.98, y = 1.44x-2.5, standard error of estimate 3.8 mL).

CONCLUSION

The ability to compute laminar flows from 3D digital data sets obtained parallel to the direction of flow and without the need for geometric assumptions represents an important opportunity for and advantage of 3D color Doppler echocardiography.

Three-dimensional echocardiographic reconstruction of mitral valve color Doppler flow events

In vitro studies have suggested superior accuracy of 3-dimensional echocardiography over conventional methods for the characterization and quantitation of color Doppler flow events. Little in vivo work has been reported in this area; this study demonstrates the feasibility of 3-dimensional reconstruction of mitral valve flow events in an unselected group of adult patients and discusses optimal instrument settings for the acquisition of 3-dimensional datasets.

Developments in cardiovascular ultrasound. Part 3: Cardiac applications

Echocardiography is still the principal, non-invasive method of investigation for the evaluation of cardiac disorders. Using Doppler ultrasound, indices such as coronary flow reserve and cardiac output can be determined. The severity of valvular stenosis can be determined by the area of the valve, either directly from 2D echo, from pressure half-time calculations, from continuity equations or from the proximal isovelocity surface area method. Alternatively, the severity of regurgitation can be estimated by colour or pulsed ultrasound detection of the back-projection of the high-velocity jet into the chamber. Myocardial wall abnormalities can be assessed using 2D ultrasound, M-mode or analysis from the radio-frequency-ultrasound signal. Doppler tissue imaging can be used to quantify intra-myocardial wall velocities, and 3D reconstruction of cardiac images can provide visualisation of the complete cardiac anatomy from any orientation. The development of myocardial contrast agents and associated imaging techniques to enhance visualisation of these agents within the myocardium has aided qualitative assessment of myocardial perfusion abnormalities. However, quantitative myocardial perfusion has still to be realised.

Left ventricular volumes assessed by different new three-dimensional echocardiographic methods and ordinary biplane technique

Three-dimensional (3D) echocardiography may overcome the problems with inadequate accuracy and reproducibility of 2D volume measurements of the left ventricle.

AIMS

To establish the in vitro accuracy and reproducibility of two new methods for 3D echocardiographic volume determination as compared to biplane measurements.

METHODS

Validation of volume measurements by a multiplane 3D method was performed on asymmetric latex phantoms (n = 8, true volumes 45-304 ml) using rotational acquisition of 90 image planes. Porcine agarose-filled asymmetrical left ventricles (n = 7, true volumes 34-280 ml) were measured by the same multiplane 3D method based on images acquired by probe rotation axis perpendicular (A) and parallel (B) to the ventricular long axis. Ventricular volumes were also obtained by a simplified 3D system using only the three standard apical views (C) and by the ordinary biplane Simpson's method (D).

RESULTS

On latex phantoms systematic deviation from true volumes by multiplane 3D was less than 2%, and 95% variability of individual measurements from this mean was +/- 4.9%. For accuracy on left ventricles, systematic bias was small with all the methods (< 5%), but 95% variability of individual measurements was +/- 9.0%, 15.4%, 18.8% and 41.3% of true volumes for methods A-D respectively. Corresponding results in the same range were obtained for inter- and intraobserver variability.

CONCLUSION

Individual in vitro volume estimates of left ventricles are of similar quality using apical multiplane or apical triplane 3D echocardiography. Both methods were superior to the ordinary apical biplane method, but inferior to multiplane 3D method with the probe directed perpendicular to the ventricular long axis.