12 Chapter 1 general principles of echocardiography (a) (b) (c) Fig. 1.12 Reverberation and shadowing artefact. (a) Reverberation and shadowing artefact: behind a highly reflective structure, the normal structures can be obscured not only by reverberations, but also by shadowing—absent image due to weak ultrasound transmission. In this example, in transoesophageal echocardiography (TOE), there are reverberations from a prosthetic mitral valve and also shadowing behind the valve ring. (b) Shadowing artefact: The example shows shadowing artefact on TOE, produced by the ring of tissue aortic valve prosthesis. (c) Shadowing artefact: the example shows an agitated saline study for interatrial septum assessment. There is a shadow in the right lower part of the image, produced by small calcium chunk in the interatrial septum and by the aortic valve which could be confused with bubbles wash-out by left-to-right flow through small interatrial communication. (a) (b) (c) (d) Fig. 1.13 Reverberation and side lobe artefact. (a) Side lobe artefact: the side lobe (grating lobe) artefact is an image created by a weak accessory beam. It can produce a misleading image of a highly reflective structure. In the example, the prosthetic aortic valve ring seems to be continued by an ascending aorta graft, which is not true. The image of the prosthetic ring is made look longer by the combination of true image and side lobe artefact. The image was ‘optimized’ to accentuate artefact for illustration purposes. (b) Comet tail: the example shows a small comet tail produced by a calcified plaque in the aortic root, giving the appearance of a line in the aortic root perpendicular to the anterior wall of the aorta, which could be misdiagnosed as dissection. (c) Reverberations and side lobe artefact: the example shows reverberations produced by prosthetic aortic valve and prolongation of the image of the anterior aspect of the valve ring by side lobe artefact. (d) Enhancement and reverberations artefact: enhancement artefact is the accentuation of the brightness of a reflector behind a structure with low attenuation. It usually creates increased brightness of the distal aspect of structures which have a cavity. In the example, the distal aspect the right atrial wall appears brighter. There are also reverberations like multiple comet tails.
iological effects of ultrasound and safety 13 (a) (b) Fig. 1.14 Stitching and drop-out artefact. (a) Stitching artefact. If volume 3D is acquired from four cardiac cycles stitched together, stitching artefact may appear, if the cardiac cycles are unequal (atrial fibrillation or ectopic arrhythmia), or because of motion due to breathing or unstable operator hand. The example shows stitching artefact in 3D pyramidal volume. On the 2D images, the separate volumes delineation is very clear (artefact enhanced for illustration purposes). (b) Drop-out artefact: the 3D images have to be acquired at higher gain than 2D images, to allow for a more extensive post-processing (which can reveal information hidden by exaggerate gain but cannot display an image if there are gaps of information). In the example, there is apparent tissue gap in the anterior wall, due to both low gain acquisition, and post-processing (cropping and reducing the gain to visualize the highly reflective stenosed mitral valve). (a) (b) (c) (d) Fig. 1.15 Performance measurement. (a) and (b) Tissue equivalent phantom imaging: lateral and axial resolution, contrast resolution, and penetration can be assessed. In the example, (a) represents fundamental imaging and (b) represents harmonic imaging. We can appreciate from the dimensions of the bright echoes the poor lateral resolution of fundamental imaging (long echo) compared with harmonic imaging (short echo). This can be appreciated also from the images of the cystic structures (hazy lateral walls in fundamental imaging). There is drop in axial resolution (the echoes are thick for their length in harmonic imaging). The improved contrast resolution with harmonic imaging can be appreciated from the image of the far lateral left cystic structure. (c) and (d) Penetration assessment: (c) is an image obtained with lower frequency–high penetration throughout the image field though low resolution suggested by the coarse appearance of speckles; (d) is an image obtained with higher frequency–low penetration, about two-thirds of the image field though high resolution suggested by the fine appearance of speckles.