Refraction, Ultrasound Interaction with Matter | Ultrasound Physics | Radiology Physics Course #7

Understanding Ultrasound Interaction with Matter

Reflection in Ultrasound

• The initial interaction between ultrasound and matter is reflection, which can be categorized into perpendicular reflection, specular reflection, and non-specular (diffuse) reflection.

• In perpendicular reflection, differences in acoustic impedance values of various tissues help determine the amount of incident ultrasound pulse reflected back to the transducer versus transmitted through the tissue.

Specular Reflection and Refraction

• Specular reflection occurs when an ultrasound beam strikes a large flat surface at an angle, resulting in an echo that reflects at the same angle as incidence.

• Combining specular reflection with transmittance leads to refraction; this involves changes in angles when ultrasound beams interact with tissue boundaries.

Angle Changes and Speed of Sound

• When transmitting through different tissues, the transmittance angle may either decrease or increase compared to the incidence angle due to variations in sound speed.

• The change in angle is influenced by whether the speed of sound slows down or speeds up as it moves from one tissue type to another.

Calculating Transmittance Angles

• The relationship between angles can be calculated using a formula involving sine ratios: fracsin(textTransmittance Angle)sin(textIncidence Angle) = fractextSpeed of Sound in Second TissuetextSpeed of Sound in First Tissue.

• Key takeaway: It’s crucial to focus on speed differences rather than acoustic impedance values for determining angle changes during refraction.

Implications for Ultrasound Imaging

• Acoustic impedance differences affect energy transfer but are not directly calculated for imaging purposes since they do not significantly impact creating actual ultrasound images.

• Refraction only occurs at angles; if an ultrasound beam is perpendicular to a surface, it travels straight through without changing angles despite speed differences.

Understanding Wavelength Changes

• Frequency remains constant while wavelength adjusts according to changes in speed as ultrasound waves pass through different tissues.

• A faster speed results in longer wavelengths; conversely, slower speeds lead to shorter wavelengths. This adjustment affects how angles change upon entering new tissues.

Understanding Wavelength and Transmittance Angles

The Relationship Between Speed of Sound, Wavelength, and Angles

• To determine how the angle must change for wavelengths to align at a tissue boundary, one must consider that moving into a tissue with a faster speed of sound requires an increase in wavelength and transmittance angle.

• After calculating the transmittance angle using the appropriate formula, it is essential to verify if the result aligns with expectations based on the diagram: if the speed of sound increases, then both wavelength and transmittance angle should also increase.