Answer #28

The correct answer is D: Destructive interference is ideal at the surface of the matching layer so that the reflection of ultrasound that occurs at the interface of the match layer does not become part of image generated.

For this to occur the matching layer should be 1/4 a wavelength since the trip through the layer would be 1/4 a wavelength or 90 degrees and the trip back would be another 90 degrees for a total of 180 degrees. Thus the wave back to the crystal will be 180 out of phase with wave being sent out from the crystal, causing destructive interference and cancelling out these reflected waves from the matching layer as shown in the figure below.

The matching layer should have an impedance that is intermediate between that of the crystal and tissue to allow for more transmission and less reflection. Remember the higher the acoustic impedance the more the reflection. The whole point of the matching layer is to increase transmission to the tissue.