Ultrasound Tissue Interactions: Reflection and Acoustic Impedance

Ultrasound Waves and Tissue Interactions

• When two adjacent tissues have identical acoustic impedance, there is no reflection of the ultrasound wave, and it passes through the tissue without interruption.
• An impedance mismatch between two tissues causes the ultrasound wave to reflect back, with a greater mismatch resulting in a larger amount of reflection.
• The angle of reflection (Ɵr) is equal to the angle of incidence (Ɵi) in ultrasound reflection, illustrating the concept of specular reflection.

Ultrasound Scattering

• The process of scattering refers to the random redirection of ultrasound waves by small particles or irregularities in the tissue, resulting in a loss of energy and image resolution.

Ultrasound Beam Propagation

• The property used to describe the resistance encountered by an ultrasound beam as it passes through a tissue is acoustic impedance.
• The tissue interface that results in nearly all of the sound wave being reflected back (approx. 99.9%) is the air-lung interface.

Effects of Impedance Mismatch

• A large difference in impedance between two tissues affects the transmission and reflection of an ultrasound wave, with more energy being reflected back and less energy being transmitted through the tissue.
• As the magnitude of impedance mismatch increases, the percentage of reflected sound at tissue interfaces also increases.

Attenuation of Ultrasound Waves

• The acoustic energy of the beam decreases as it passes through the body due to absorption and scattering of the sound wave by the tissue.
• Bone tissue would likely attenuate more ultrasound waves due to its higher density and acoustic impedance.

Ultrasound Refraction

• Refraction in the context of ultrasound refers to the bending of the ultrasound wave as it passes through a tissue with a different acoustic impedance.

Ocular Ultrasound

• The main function of a Brightness Scan (B-Scan) in ocular ultrasound is to produce a 2D image of the eye and its structures, allowing for visualization and diagnosis of ocular disorders.

Explore how ultrasound interacts with tissues in terms of reflection and acoustic impedance. Learn about the outcomes when two adjacent tissues have identical or different acoustic impedance levels.