Two Extra Echoes That Should Not Be There
A pelvic scan is performed through a full bladder with a 3.5 MHz curved array. A bright linear reflector is seen at 2.8 cm, attributed to the anterior bladder wall and rectus sheath. Equally spaced echoes then appear at 5.6 cm and 8.4 cm with progressively lower amplitude, and the 5.6 cm echo looks like a 3 cm cystic structure with a bright wall inside the uterus. On a separate liver view, a rounded hypoechoic lesion at 9 cm appears again above the diaphragm at the same distance from the transducer. Harmonic imaging is available and a second transducer is on the cart.
1. Identify each artifact and explain its mechanism in terms the referring clinician would accept.
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First finding: reverberation. Sound bounces repeatedly between the transducer and the strong reflector at 2.8 cm. Each round trip adds the same path length, so the spurious echoes land at exact multiples of the reflector depth (2.8, 5.6, 8.4 cm) and decay in amplitude with each additional reflection. The key signature is arithmetic spacing tied to the depth of the real reflector, plus decreasing amplitude. It is not a cyst; it is the same interface echoed repeatedly. If the reflector is very small and the echoes merge into a bright tapering tail, the same phenomenon is called comet tail or ring-down.
Second finding: mirror image. The diaphragm is a large, smooth, highly reflective interface acting as an acoustic mirror. The lesion's echoes reflect off the diaphragm and return to the transducer by a longer path, so a duplicate is registered beyond the diaphragm at a distance equal to the lesion's distance from the mirror. It is always farther from the transducer than the reflector, which is why the duplicate sits where no liver exists.
2. What maneuvers would confirm each artifact, and what would you expect to see if the finding were real?
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Reverberation:
- Vary the depth or pressure on the probe. The spacing of the echoes must change with the reflector, and the multiples move together.
- Change the window or angulation. A true cystic structure remains at a fixed anatomical location; the reverberation track follows the reflector.
- Apply harmonic imaging, which suppresses reverberation clutter substantially. A real cyst does not fade.
Mirror image:
- Scan from a different approach so the diaphragm is not in the beam path. A true lesion persists; the mirror image disappears.
- Confirm the duplicate is deeper than the reflector and at the same distance behind it as the real structure is in front. A true second lesion is not geometrically constrained this way and would appear on both approaches.
If real: stable position across approaches, no dependence on reflector geometry, persistence with harmonics, and a corresponding appearance on orthogonal views. For the uterine finding, a real cyst would also be present when the bladder is partially emptied and the anterior wall is out of the near field.
3. Harmonic imaging is turned on. Predict the effect on each artifact and explain why the two differ.
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Reverberation: markedly reduced. Tissue harmonics are generated in tissue along the beam, so echoes from a round trip that includes an extra reflection off a strong near-field interface are weaker in the harmonic band than the fundamental. Harmonic imaging therefore suppresses this clutter and is a standard remedy.
Mirror image: little or no improvement. A mirror image is a specular reflection; the returning echoes have genuinely traveled the longer path through tissue and carry harmonic content generated along it. Harmonics do not discriminate the extra path from a true deep structure.
The general principle: harmonics help when the offending echoes are weak, low-amplitude clutter, such as reverberation, sidelobe, and slice-thickness artifact. They do not help when the artifact is a strong, geometrically faithful reflection, such as a mirror image. Judgement: do not accept a cleared image as proof the finding is real, and do not accept a persisting finding as proof it is not an artifact; use geometry and a second approach.