What is the primary factor in determining the amount of reflection at a tissue interface?

What is the primary factor in determining the amount of reflection at a tissue interface?

• A. Acoustic impedance mismatch
• B. Frequency
• C. Angle of incidence
• D. Medium temperature

Answer: (A)

The key idea is that the amount of ultrasound energy reflected at a tissue boundary is determined by the acoustic impedance difference between the two tissues. Acoustic impedance Z is the product of tissue density and the speed of sound in that tissue (Z = ρc). When the impedances are similar, most of the energy keeps moving forward and only a small portion is reflected. When there’s a large mismatch, a big fraction is reflected back to the transducer. The mathematical expression for normal incidence, R = ((Z2 − Z1)/(Z2 + Z1))^2, shows that the reflection increases with the difference in impedance. Frequency, while it affects wavelength and how deeply the wave penetrates (and how tissues attenuate the signal), does not set how much is reflected at the boundary. Angle of incidence can alter the distribution of reflected versus transmitted energy at oblique angles, but the underlying driver of the reflection strength remains the impedance difference. Temperature has only a minor effect by slightly changing speed and density, so its impact on reflection is negligible compared to impedance mismatch.

The key idea is that the amount of ultrasound energy reflected at a tissue boundary is determined by the acoustic impedance difference between the two tissues. Acoustic impedance Z is the product of tissue density and the speed of sound in that tissue (Z = ρc). When the impedances are similar, most of the energy keeps moving forward and only a small portion is reflected. When there’s a large mismatch, a big fraction is reflected back to the transducer. The mathematical expression for normal incidence, R = ((Z2 − Z1)/(Z2 + Z1))^2, shows that the reflection increases with the difference in impedance.

Frequency, while it affects wavelength and how deeply the wave penetrates (and how tissues attenuate the signal), does not set how much is reflected at the boundary. Angle of incidence can alter the distribution of reflected versus transmitted energy at oblique angles, but the underlying driver of the reflection strength remains the impedance difference. Temperature has only a minor effect by slightly changing speed and density, so its impact on reflection is negligible compared to impedance mismatch.