What artifact might you see with an array transducer due to energy that propagates from the transducer in a direction different from the primary beam?

• A. Mirror image
• B. Reverberation
• C. Range ambiguity
• D. Grating lobe

Answer: (D)

In an array transducer, beams are created by delaying signals to the individual elements to steer and focus the ultrasound. If the element spacing is not small compared to the wavelength, the pattern develops extra directions with substantial energy, called grating lobes. Energy radiating along these off-axis directions can reflect from structures and return echoes to the transducer, but the system interprets them as coming from the main beam direction. The result is a ghost image of a reflector appearing at an off-axis location. This off-axis energy producing a secondary image is the grating-lobe artifact. Reverberation involves multiple internal reflections, range ambiguity comes from depth misassignment due to timing, and mirror-image artifacts come from reflections across a strong boundary; none describe off-axis energy relative to the primary beam in the same way grating lobes do.