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An A-mode ultrasound scan plots the returning echo amplitude along one pulse-echo ultrasound beam. With assumed sound speed $c$, echo delay $\tau$ is converted to depth by $z=c\tau /2$; the factor two accounts for outward and return travel. Peaks mark reflecting interfaces along that line. ^[ref]

The horizontal coordinate is depth inferred from travel time, while the vertical coordinate is signal amplitude. An A-mode ultrasound scan therefore provides a one-dimensional depth profile rather than the spatial cross-section assembled by a B-mode ultrasound scan.

A B-mode ultrasound scan maps echo strength to displayed brightness and assembles multiple pulse-echo ultrasound lines into a spatial image. Each echo's location is inferred from beam direction and return time using an assumed sound speed. ^[ref]

A bright point represents a strong received echo, not a direct measurement of tissue density. Acoustic impedance contrasts, propagation losses and the instrument's display mapping all affect brightness. A moving image consists of repeated spatial frames; in an M-mode ultrasound scan, the second display axis instead tracks time along one beam.

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.