Answer #47

The correct answer is C: The percent reflection between two surfaces can be calculated using only their acoustic impedances.

Acoustic impedance is a property of a substance that is related to proportional to BOTH the density and velocity of sound propagation through it (B is therefore incorrect). The higher the density of the object the higher the acoustic impedance. Thus we would expect that blood would have a lower acoustic impedance than tissue (Choice D is therefore incorrect)

Acoustic impedance is commonly abbreviated by the letter Z and is measured in the units of Rayls. It is a key factor that helps us understand how much of an ultrasound source will be reflected back vs transmitted through an object. The larger the impedance mismatch between two surfaces at an interface, the LESS ultrasound is transmitted and the MORE is reflected back (choice A is therefore incorrect). See the figure below. [Insert percent reflection and acoustic impedance]
The equation to calculate the percent reflection at an interface is based solely on the acoustic impedances of the two surfaces is shown in the figure as well ( from surface Z1 to surface Z2) The concept is important because piezoelectric crystals have a high acoustic impedance compared to the human body. Hence matching layers are used in transducers to make the final interface of the probe to decrease this difference. This is also why we use ultrasound gel. It helps increase the amount of transmission by decreasing the difference in acoustic impedance at the surface. This is also how contrast bubbles work. They have a higher acoustic impedance than blood cells and act as many small tiny reflectors of ultrasound to help in visualization.

Hence choice C is correct.