An imaging direction, downwards relative to the long axis of the human body.

The mirror artifact is similar to the reverberation artifact. Mirror image artifacts (mirroring) can occur if the acoustical impedances of the tissue is too much different and the ultrasound is reflected multiple times on tissue layers. The echo detected does not come from the shortest sound path, the sound is reflected off an angle to another interface so that like a real mirror, the artifact shows up as the virtual object.
An empyema or lung abscess can be simulated by a mirror image artifact of a hepatic cyst. This liver lesion can appear like a lesion within the lung because the wave is reflected off the diaphragm back into the liver. The angle of reflection is equal to the angle of incidence. The sound pulse hits the interfaces within the liver lesion and is reflected back to the diaphragm once again with an angle of reflection equal to the angle of incidence and then back to the transducer.
Also by a pelvic ultrasound scan the sound can be reflected off the rectal air at an angle so that the deep wall of an artifactual cyst represents the mirror image of the inferior and anterior walls of the bladder. Mirror image artifacts can cause other strange appearances such as invasion of a transitional cell carcinoma through the bladder wall.
Also called Cross Talk.

If available, some graphic aids can be helpful to show image orientations.
1) A graphic icon of the labeled primary axes (A, L, H) with relative lengths given by direction sines and orientation as if viewed from the normal to the image plane can help orient the viewer, both to identify image plane orientation and to indicate possible in plane rotation.
2) Ingraphic prescription of obliques from other images, a sample original image with an overlaid line or set of lines indicating the intersection of the original and oblique image planes can help orient the viewer.


In all cases the scanning surface is assigned to the top of the image. The orientation of single oblique slices can be specified by rotating a slice in one of the basic orientations toward one of the other two basic orthogonal planes about an axis defined by the intersection of the 2 planes.

See also Histogram.

The transforaminal or sub-occipital acoustic window is found in the space between the atlas and the base of the skull (through the foramen magnum insonated from the top of the neck below the occiput). Practiced in the prone position (or sitting).
This acoustic window allows the insonation of the vertebral arteries, basilar artery and some of the other branches of the posterior circulation (e.g. posterior inferior cerebellar artery).

See also Transcranial Doppler.

An imaging plane, parallel to the ground, perpendicular (rotated 90°) to the long axis of the human body; the axial plane separates the superior from the inferior part (the head from the feet). Also called axial, transaxial or transversal plane.