The widening of the ultrasound beam in the far field is called which term?

• A. Angle of Divergence
• B. Angle of Incidence
• C. Angle of Refraction
• D. Beamwidth

Answer: (A)

The widening of the ultrasound beam in the far field is described by the angle of divergence. As sound travels, diffraction causes the beam to spread beyond the focus, and this angular spread—the divergence angle—quantifies how quickly the beam widens with distance. A smaller divergence angle means the beam stays narrow longer, improving lateral resolution at greater depths; a larger divergence angle means it spreads more quickly, reducing resolution sooner. This spreading is governed by the transducer aperture and the wavelength: a larger aperture or shorter wavelength reduces divergence, while a smaller aperture or longer wavelength increases it. Beamwidth, by contrast, refers to how wide the beam is at a specific depth, not the intrinsic angular spread of the beam.