(Doppler look angle) The angle of incidence is the angle at which the ultrasound beam strikes an interface, or the angle at which the Doppler beam intersects the blood flow. Doppler signals are best obtained at angles of 60° or less.

See also Snells Law.

Doppler Shift is the change in the perceived frequency relative to the transmitted frequency. The Doppler shift is dependent on the insonating frequency, the velocity of moving blood, and the angle between the sound beam and direction of moving blood.
Doppler equation:
Doppler shift frequency: fD = fr - f0 = 2f0v/c
Where fD is the Doppler shift frequency = the difference between transmitted and received frequencies.
Ultrasound system use the following equation:
Doppler shift frequency with incident angle: fD = 2f0v/c cosØ
Where f is the transmitted frequency, v is the blood velocity, c is the speed of sound in tissue, cosØ is the Cosine of the blood flow to beam angle.
The Doppler angle (theta) is the angle of incidence of the beam upon the object. If the beam is parallel to the flowing blood, the angle theta is zero, and the determination of flow is most accurate. If the angle of incidence is greater, the results are less reliable. Doppler shift results with an angle greater than 20° should not be used for the calculation.

See also Doppler Interrogation Frequency, Zero Crossing Detector, Doppler Effect, Doppler Ultrasound and Motion Discrimination Detector.

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.

The Doppler angle (theta) is the angle of incidence between the ultrasound beam and the estimated flow direction (parallel to the long axis of the vessel). If the beam is parallel to the flowing blood, the Doppler angle is zero, and the determination of flow is most accurate.

See also Beam Vessel Angle, Doppler Effect and Doppler Ultrasound.

Reflection of the sound beam occurs when it hits a boundary between materials having different acoustic impedance. The reflection (echo) is the portion of a sound that is returned from the boundary. The reflection time (the time taken for the wave to return to the probe) can be used to determine the depth of the object.
The reflection within the body produces the ultrasound image, but should be minimized at an ultrasound couplant to skin boundary where the couplant acts as an acoustic window through which the image is seen. The amount of sound waves, which are reflected back at the interface between two tissues is depend on the angle of incidence and the difference between the acoustic impedance values of the two tissues.
If the difference is great, a large part of the sound waves will be reflected back. If too much sound is reflected back and not enough waves are remaining to be able to penetrate the tissue, the imaging will be poor.
If the difference is small, a small amount will be reflected back. Enough sound signal remains to continue with ultrasound imaging.
If the ultrasound beam meets a rough surface or small object, the beam is scattered in all directions and only a small amount will be received by the probe.

See also False Distance Artifact, Target Strength, and Snells Law.