Ultrasound artefacts

Below is a comprehensive table explaining:

1. What causes each artefact

2. When and where it might occur in real scans

3. Why it happens – the physics behind it

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### 🧠 Ultrasound Artefacts Explained

| Acoustic Enhancement | Low attenuation behind a fluid-filled structure. | Common behind cysts, bladder, gallbladder, or amniotic fluid. | Fluid transmits sound better than surrounding tissue, so deeper echoes appear brighter than expected. |

| Acoustic Shadowing | High attenuation or reflection from a dense object. | Behind bones, gallstones, calcifications, air, or bowel gas. | Sound is strongly reflected or absorbed, leaving little to reach beyond the structure, resulting in a dark shadow. |

| Reverberation | Repeated reflection of sound between two strong reflectors (like the probe and a metal object). | Often seen near the anterior bladder wall, bowel gas, or metallic objects. | Sound bounces back and forth, creating multiple equally spaced echoes deeper than the actual structure. |

| Speed of Sound Artefact | Occurs when tissue has a different sound speed than the assumed 1540 m/s. | E.g., behind fat, muscle, or cysts. | Structures may appear displaced or distorted in shape or depth due to incorrect timing of returning echoes. |

| Refraction | Bending of sound at tissue interfaces with different speeds and oblique angles. | Can occur near muscle-fat interfaces, e.g., rectus abdominis in abdominal scans. | Beam changes direction, leading to misplaced echoes or duplicated structures. |

| Edge Shadowing (Refraction Shadowing) | Refraction and beam divergence at the curved edge of a structure. | Seen at ovaries, kidneys, fetal skull, or gallbladder wall. | Beam is bent away at the curved edge, creating shadow-like areas to the side of the structure. |

| Mirror Image | Strong reflector causes the beam to bounce and return from a false second location. | Seen under diaphragm or pleura (e.g., liver/lung or liver/stomach interface). | Beam reflects off a strong interface (e.g., diaphragm) and bounces back from deeper tissue, creating a duplicated, deeper structure. |

| Aliasing | Doppler artefact when blood flow velocity exceeds the Nyquist limit (PRF ÷ 2). | Common in high-velocity arterial flow, e.g., carotid or uterine arteries. | Flow is incorrectly displayed with colour or spectral wraparound, giving false impression of reverse flow. |

| Comet Tail | Type of reverberation with very short path and strong reflectors. | Common with surgical clips, cholesterol crystals, air bubbles. | Closely spaced reverberations create a dense tapering tail of echoes. |

| Ring-Down | Similar to comet tail but caused by resonance of gas bubbles. | Seen in biliary tree gas, bowel gas, or infections producing gas (e.g., emphysematous cholecystitis). | Continuous sound wave emission from vibrating gas bubbles, producing a long, bright tail extending from gas. |