Reverberation Artefact Refresh
An ultrasound machine assumes a single pulse of ultrasound enters the tissues, is reflected off a structure, and returns directly to the transducer for interpretation. When this does not occur ultrasound artefacts are created.
Not infrequently an ultrasound pulse encounters two parallel reflective surfaces lying perpendicular to its path. Some of the pulse becomes caught between the two surfaces, bouncing forwards and backwards before returning in increments, between each reverberation, to the transducer. This reverberation causes a repetitive artefact on the ultrasound image.
The appearance of the reverberation artefact depends on:
- The size of the two reflective surfaces
- The distance between the two reflective surfaces (long vs short path reverberation artefacts)
- How much ultrasound energy is lost – dissipated or attenuated, between each re-reflection.
Comet tail artefact
Comet tail artefact is a short path reverberation artefact that weakens with each reverberation, resulting in a vertical echogenic artefact that rapidly fades as it continues in to the ultrasound image.
Normal lung. Sliding is seen as the two pleural surfaces move against each other through the respiratory cycle. Originating at the pleural surface is an echogenic vertical comet tail artefact. The reverberation focus is a tiny area of interstitial fibrous tissue or fluid with non parallel surfaces.
How comet tail artefact is formed
The ultrasound pulse is trapped between reflective surfaces that are not parallel. Some of the reverberating pulse is reflected back to the transducer but much more is reflected off in other directions.
With each reverberation the ultrasound pulse returning back to the probe is much weaker and it quickly fades.
Alternatively the ultrasound pulse is trapped between two parallel surfaces in a tissue that rapidly attenuates the energy of the pulse. Each returning pulse is therefore significantly weaker and the reverberation artefact fades.
Short path reverberation artefacts explained
- Long path reverberation artefact (A-lines in lung)
- Short path reverberation artefact (B-lines in lung; ringdown anywhere else!)
- Fatiguing short path reverberation artefact (Comet tails)