Which of the following factors can cause non-symmetry in a propagated ultrasonic beam when using focused transducers?

• A. Lens centering or misalignment
• B. All of the above
• C. Porosity in lenses
• D. Backing material variations

Answer: (B)

Symmetry of a focused ultrasonic beam depends on a clean, centered focus and a uniform transmission path. If the lens is not perfectly centered or aligned, the wavefront becomes tilted and the energy distribution around the focus becomes asymmetric. Porosity in the lens introduces local variations in acoustic properties and scattering, which disturb the transmitted wave and distort the beam shape. Backing material variations affect damping and the energy distribution behind the lens; nonuniform backing can cause uneven attenuation and phase shifts across the aperture, also leading to an asymmetric beam. Each of these issues can distort the beam, and together they can markedly amplify the asymmetry. Therefore, all of the above factors can cause non-symmetry in a propagated ultrasonic beam.