 M-mode ultrasound scan

An M-mode ultrasound scan repeatedly samples one pulse-echo ultrasound beam and places successive depth profiles alongside one another in time. Depth is one display axis and acquisition time the other; echo strength controls brightness. This makes motion along the selected line visible. ^[ref]

A stationary interface traces a line at constant depth; an oscillating interface traces a varying depth. Converting echo delay to depth still assumes a sound speed and a return path. This display records one beam over time, whereas a B-mode ultrasound scan combines beam lines across space.