The sound beam can be focused by all of the following except:

• A. A Acoustic mirror
• B. B Acoustic lens
• C. C Curved crystal
• D. D Matching layer

Answer: (D)

Focusing the ultrasound beam is about shaping the wave so that its energy converges at a point. This can be done by altering the path of the waves: a curved transducer surface acts like a built-in lens, bending the waves so they meet at a focus; an acoustic lens placed in front of the crystal refracts the waves to create a focal point; and an acoustic mirror uses a curved reflective surface to redirect energy toward a focus.

The matching layer, on the other hand, sits between the crystal and the tissue to improve energy transfer by matching acoustic impedances and broadening bandwidth. It does not steer or bend the beam to a focal point, so it does not produce focusing. That’s why it cannot focus the sound beam by itself.