Which of the following is a valid coupling method for ultrasound?

Which of the following is a valid coupling method for ultrasound?

• A. Gel
• B. Air gap
• C. Solid metal interface
• D. No interface

Answer: (B)

The key idea is that ultrasound energy must travel from the transducer into the body through a medium that transmits sound well and minimizes reflections at interfaces. A gel serves as that medium, filling any microscopic gaps between the transducer surface and the skin and providing an acoustic impedance closer to soft tissue. This keeps most of the energy moving forward into the body rather than bouncing back at the surface. An air gap disrupts this transfer because air has a very different acoustic impedance from tissue, causing a large portion of the energy to reflect at the boundary and little to enter the body. A solid metal interface would also reflect the waves rather than transmit them, and having no interface means there’s nothing to carry the sound into the tissue at all. So, using a gel is the valid coupling method because it creates a continuous, well-matched path for ultrasound to travel from the transducer into the body.

The key idea is that ultrasound energy must travel from the transducer into the body through a medium that transmits sound well and minimizes reflections at interfaces. A gel serves as that medium, filling any microscopic gaps between the transducer surface and the skin and providing an acoustic impedance closer to soft tissue. This keeps most of the energy moving forward into the body rather than bouncing back at the surface.

An air gap disrupts this transfer because air has a very different acoustic impedance from tissue, causing a large portion of the energy to reflect at the boundary and little to enter the body. A solid metal interface would also reflect the waves rather than transmit them, and having no interface means there’s nothing to carry the sound into the tissue at all.

So, using a gel is the valid coupling method because it creates a continuous, well-matched path for ultrasound to travel from the transducer into the body.