An ultrasound image obtained with a linear array transducer that uses phasing to steer the beam shows arrows pointing to which artifact?

• A. Reverberation
• B. Slice thickness
• C. Grating lobes
• D. Mirror image

Answer: (C)

Grating lobes. When a linear array is steered with phasing, the emitted field isn’t a perfect single beam because the array is made of discrete elements. Steering creates additional off-axis energy lobes—grating lobes—especially if the element spacing is large relative to the wavelength. These secondary beams generate echoes from structures that aren’t in the main line of sight, producing extra, spurious echoes in the image. That off-axis energy is what the arrows indicate. Other artifacts like reverberation (repeated reflections between interfaces), slice thickness (partial volume effect), and mirror image (duplication across a boundary) aren’t specifically tied to beam steering with phased arrays, so they don’t explain the arrows in this scenario.