[3D echocardiography. Mathematical principles and technical realization].

Abstract

The ultimate goal of any imaging technique for the investigation of the anatomy of the beating heart is a 3D-display of the cardiac morphology throughout a complete heart cycle. The reason for this interest is quite clear: 3D-imaging has the potential for a better understanding of the individual morphology under normal and pathological conditions and especially, if complex therapeutic decisions have to been made. In the clinical practice, the echocardiographer attempts to obtain a spatial information by a mental reassembling of the 2D echocardiographic images, that are obtained from different imaging planes. This procedure, however, is very subjective and, thus, highly susceptible for errors. Therefore, the 3D-echocardiography has been developed to replace this mental process by an "objective" and reproducible computerized reconstruction. Prerequisite for such a 3D-surface reconstruction is a cubic, isotropic digital data set with cubic data volumes, so called "Voxels" (Figure 1). The term "isotropic" means, that the resolution is identical in all directions, and that the data density within the cube is homogeneous. Those cubes are the mathematical basis for any 3D-reconstruction. At the first step on the way to 3D-images, the data cubes have to be filled with 2D echo information. So far, three principal modalities of image acquisition are available for the clinical routine: parallel scanning from the esophagus (Figure 2), rotational scanning (transesophageal--Figure 3a--or transthoracic--Figure 3b). In all cases, the imaging planes are incremented by an external stepper motor using a dedicated computer logic for gated image acquisition. At the present time, despite geometrical shortcomings, the TEE omniplane probe with rotational scanning is the most widely used system. It can be applied for standard investigations as well as for "3D"-data acquisition after only minor modifications. The process of 3D-reconstruction is a sequence of repeated steps of image processing. The first step is the elimination of a problem, that is common to all image reconstruction techniques from tomographic information: the imaging planes are recorded at different time points, and mostly under varying conditions. Although several gating techniques are implemented into the image acquisition, some variability is unavoidable, simply because neither the heart nor the surroundings can be frozen during image acquisition.(ABSTRACT TRUNCATED AT 400 WORDS)