Final answer:
Comet-tail artifacts appear as bright, curved lines in hypoechoic areas on ultrasound, resulting from echoed signals trapped between closely spaced reflectors.
Step-by-step explanation:
The type of artifact that appears as bright, curved lines in hypoechoic or echo-free structures on an ultrasound is the comet-tail artifact. This phenomenon occurs when a series of echoed signals are trapped between two closely spaced reflectors and are displayed as a series of discrete echoes on the ultrasound image. These artifacts are often seen trailing behind a strong reflector, suggesting the tail of a comet, hence the name comet-tail artifact.