1. Ultrasound imaging apparatus comprising: an ultrasound probe configured to transmit ultrasound signals and to receive ultrasound echo signals; an ultrasound system configured to process the ultrasound echo signals to yield a two-dimensional ultrasound image representing an image plane within a subject; a display connected to display the ultrasound image; a position-sensing system configured to monitor a position and orientation of the ultrasound probe in a coordinate system to yield probe position and orientation information; a processor configured to: process the probe position and orientation information to determine: a display location in the ultrasound image corresponding to a perpendicular projection of a predetermined location onto the image plane and a distance between the predetermined location and the image plane; and superimpose on the ultrasound image on the display location marking indicia at the display location and having one or more appearance characteristics determined at least in part by the distance between the predetermined location and the image plane.