Free-hand imaging usually employs a spatial sensor device (including a transmitter and a receiver of acoustic or magnetic signals) to locate the spatial position of the imaging plane, while the ultrasound transducer can be moved manually to scan the heart.

Sequential imaging often requires a motor device fixed to the ultrasound transducer. The movement of the device is controlled by a computer in order to scan the heart in a sequential, step-wise manner such as parallel, rotational or fan-like scanning. Another way of sequential imaging is to employ a multiplane transducer (such as multiplane transesophageal transducer) that scans the heart rotationally without the need of a motor device.

Real-time imaging applies a volumetric ultrasound transducer that images the heart in 3 dimensions and acquires a 3DE data set in real-time.

The raw images acquired in the above manner are processed to produce volumetric data sets. These data sets can then be reconstructed and displayed in various formats including cross-sectional views and volume-rendered 3D images. Region of interest can be extracted and displayed individually. Area and volume of a given region can be measured from the 3DE data sets. The volume of a given region, such as a dysfunctional region of the left ventricle, is usually measured from a 3D data set in the following way. First, a short-axis view of the left ventricle is defined and multiple cutting planes parallel to the short-axis view are derived with equal intervals to divide the left ventricle into multiple slices; or, a long axis of the left ventricle is derived and multiple short-axis views are then derived perpendicular to the long-axis view. Second, each short-axis view of the left ventricle is played in a dynamic mode for the observer to recognize the dysfunctional region, which is then traced and a volumetric label is given. Third, after all the dysfunctional regions are traced in all slices, the volume of the dysfunctional myocardial mass on each slice and on all left ventricular slices is calculated automatically. The volume of the total myocardial mass of the left ventricle can be measured in a similar way, only that the total myocardial regional is traced on each slice. Therefore, the percentage of the left ventricle involved in dysfunction can also be derived. Similarly, the volume and ejection fraction of the left ventricle, the volume of abnormal perfusion defect mass can be assessed as well.

3D assessment of regional wall motion abnormality. CAD often manifests left or right ventricular regional wall motion abnormalities. The location, extent and severity of these abnormalities vary from case to case. 3D display and quantitative assessment of these abnormalities can be helpful in understanding the location of the coronary disease(s), extent of the myocardium involved in dysfunction and even the mechanism of the complications such as mitral valve regurgitation. In an experimental study of acute myocardial infarction induced by coronary artery occlusion in canine models, a good correlation was obtained between the dysfunctional mass and the anatomically determined infarct mass.¹ In a clinical study, dysfunctional mass from 3DE correlated well with that from magnetic resonance imaging.²

Not only can regional wall motion be demonstrated in multiple cross-sectional views of the 3DE data set, but also dynamic volume-rendered images of the ventricles can be reconstructed and displayed and the region of abnormal wall motion visualized. Using an automated regional ejection fraction analysis program, the quantitative process is expedited and facilitated. The 3D information of regional wall motion of the whole left ventricle can be displayed in a simple bulls-eye format. Tissue Doppler has emerged as a new modality of imaging the myocardial function. When 3D data is acquired in this mode, the regional function of the left ventricle can be displayed and evaluated in 3 dimensions in tissue Doppler mode.

3D assessment of myocardial perfusion. Contrast enhanced 2DE has proven to be useful in evaluating regional myocardial perfusion abnormalities in ischemic animal models and in patients with coronary artery disease. To obtain optimal myocardial contrast enhancement throughout the process of 3DE data acquisition, a steady status of the contrast enhancement is required. This can be achieved in the following ways: One is to maintain a steady concentration of contrast in the circulation by giving the intravenous contrast agent as a slow bolus or by continuous infusion of the contrast agent. Another one is to minimize destruction of the contrast by ultrasound and to prolong myocardial enhancement time by using triggered imaging or by lowering ultrasound out-put power, such as the newly developed real-time perfusion imaging. Last, 3DE data acquisition time can be reduced by using real-time 3DE or other fast data acquisition methods.

The contrast enhanced 3DE data can be used for display and measurement of myocardial perfusion abnormalities in ways similar to that used in conventional 3DE for regional wall motion analysis. The perfusion defect can be displayed and quantified in cross-sectional views (Figure 3). In addition, the defect can be displayed in 3D reconstruction (Figure 4) as well as in bulls-eye presentation (Figure 5).³

3D reconstruction of coronary arteries. 3DE data of coronary arteries can be obtained via transesophageal or transthoracic imaging. The coronary arteries can be reconstructed and presented in cross-sectional views, volume-rendered 3D images or in extracted cast-like display format (Figure 6).⁴ Our clinical study demonstrated the feasibility of 3D reconstruction and displaying coronary arteries and the ability of 3DE in recognizing stenotic lesions in the proximal coronary segments.

 

Figure 6. The coronary arteries can be reconstructed and presented in cross-sectional views, volume-rendered 3D images or in extracted cast-like display format. (Yao 2000)

The 3DE technique is being developed rapidly in all its facets, including data acquisition, data processing, image reconstruction and quantitative analysis. 3DE is getting faster and easier to perform. Many ultrasound machine manufacturers are incorporating 3DE ability into their products, so that 3DE data can be collected during 2DE examinations. Other imaging modalities including contrast enhanced imaging, tissue Doppler imaging, metabolic imaging, electrophysiologic imaging and so on are also being developed in 3D formats. An ideal technique in the future for the evaluation of CAD is one that provides comprehensive information of the heart in a multi-dimensional, multi-functional and multi-modality format.