Ultrasound Physics Series · Propagation artefacts
Propagation-Speed Displacement Artefact
The system measures round-trip echo time and converts it to depth using an assumed sound speed. A slower path segment places a reflector too deep; a faster segment places it too shallow.
Approved education edition
TEACHING PRESETS:
Outgoing pulse marker
Returning echo
True target position
Apparent target position (displayed)
Illustrated path vs displayed reconstruction
Same reflector and measured echo time; different interpretation of depth.
On a narrow screen, swipe horizontally to compare the two panels.
The left panel shows a pulse traversing a selected layer before reaching a target at 80 millimetres. The right panel shows where the same echo is displayed when the system assumes a uniform sound speed of 1540 metres per second. The numerical values below provide the accessible result.
Layer speed (c)
1450 m/s
illustrative adipose-like value
True target depth
80 mm
fixed teaching target
Displayed depth
81.9 mm
reconstructed axial position
Displacement error
+1.9 mm deeper
+2.3% of true depth
What is happening → The system converts round-trip echo time to depth using an assumed sound speed of 1540 m/s. In this simplified perpendicular path, a slower segment adds travel time, so the reflector is displayed too deep. A faster segment removes travel time, so it is displayed too shallow.
Measurement takeaway — This is spatial misregistration, not a duplicated “ghost”. A substantial difference between actual path-average speed and the system assumption can bias axial depth or distance measurements. The size of the bias depends on the speed and length of every path segment, as well as beam geometry. When precision matters, optimise the acoustic window, compare alternative windows where appropriate, and apply the relevant measurement protocol. Do not use this teaching model as a correction calculator.
Equation used by this model → For a straight beam perpendicular to flat layers, the displayed depth is the sum of each one-way segment scaled by the assumed-to-actual speed ratio:
displayed depth = Σ [segment length × (1540 / segment speed)]
The animation exaggerates the visible speed difference for teaching; the millimetre result uses the displayed equation and the selected illustrative values.
displayed depth = Σ [segment length × (1540 / segment speed)]
The animation exaggerates the visible speed difference for teaching; the millimetre result uses the displayed equation and the selected illustrative values.
Procedural boundary → A needle and target seen through a shared path may undergo similar axial displacement, but that does not guarantee accurate tip identification or safe alignment. Oblique interfaces can add refraction, and needle appearance is also affected by beam width, side lobes, angle and reverberation. Do not derive needle length or trajectory corrections from this widget; use real-time confirmation and the procedure-specific safety protocol.