©Walter Rasmussen, R.D.C.S.
3-D Trans-Throacic Mitral Valve view from the left atrium, diastole and systole.
Three-dimensional ultrasound images are produced by a probe with a rectangular shaped array containing multiple rows of hundreds of individually controlled sonic elements. This is an arrangement similar to image sensors in modern digital cameras, except that the elements are pressure-sensitive rather than light sensitive. The ultrasound machine activates the crystals in order to produce a scanning volume rather than just a slice as in two-dimensional ultrasound. The three dimensions are called horizontal, vertical and elevational.
Two main types of 3-D echo can be recorded. In “Live 3-D”, a small volume is acquired in a single cardiac cycle. In “Full Volume” mode, a larger area is recorded and two to three successive cardiac cycles are combined in order to create an image with adequate spatial and temporal resolution. It is usually necessary to have the patient suspend their breathing so that the heart is not drifting in position during the multi-beat acquisition process.
Above: A full-volume image on right with reference two-dimensional images on left. View: Apical.
When any 3-D image is saved, it can later be recalled and edited, and by the use of Cropping Tools, multiple perspectives and sections of anatomy can be visualized and re-saved as individual clips.
When there is good penetration, three-dimensional images can be quite impressive due to the software’s ability to convey perspective, depth, shading and detail. Detailed analysis of three-dimensional images can also be used for such purposes of calculating volumes and ejection fraction from a single cardiac cycle.
Three-dimensional imaging requires a very steady hand and it is often necessary to have the patient suspend breathing while the multi-beat volume is acquired. If the heart shifts position during three-dimensional multi-beat acquisition changes its position, “stitching artifact” is produced; resulting in areas of distortion and a lack of uniformity of the three-dimensional image. The presence of stitching artifact requires that the clip be discarded.
While three-dimensional ultrasound systems can produce stunning views of internal cardiac anatomy from the chest wall, the images usually look somewhat out of focus and lack sufficient resolution to make them useful for many diagnostic purposes.
The 3-D probe requires a wider slice than a two-dimensional probe and it cannot fit as well between the narrow rib spaces and encroaching lung tissue. When the goal is to image an individual valve, this is less of a limiting factor however; when an image of the entire left ventricle is desired, it is often the case that significant portions cannot be included in the view.
Trans-Esophageal 3-D transducers are capable of producing far superior images of higher resolution when compared to the trans-thoracic route because the probe can be positioned within a few centimeters of the heart and the esophagus bypasses the bone and other tissue encountered from the trans-thoracic route.
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