(Å* or A*) A unit used to measure the wavelength of X-rays.
Definition: 1A* = 0.1 nanometer or 10^-10 meter.

Anisotropic is an area or structure with varying acoustic properties according to the direction in which they are imaged. For example, the characteristic ultrasound appearances of tendons are hyperechoic, but they appear hypoechoic when the sound beam is not at right angles to the tendon fibers.

Annular arrays are transducer assemblies with circular or ringlike elements, used to focus the beam. Annular arrays must be steered mechanically since they can only be fired in an outward-inward progression due to the rings. Annular arrays reduce section thickness artifacts.

Antegrade blood flow is the naturally direction of the flowing blood.

An imaging direction, frontwards relative to a short axis of the human body from the back to the front.