Reflection, Ultrasound Interaction with Matter | Ultrasound Physics | Radiology Physics Course #6

Understanding Acoustic Impedance and Reflection in Ultrasound

Introduction to Acoustic Impedance

• The acoustic impedance of a tissue is defined as the product of its density and the speed of sound traveling through it.

• It is largely determined by the bulk modulus, which reflects the stiffness or resistance to compression of that tissue.

Reflection in Ultrasound Imaging

• Reflection occurs when some ultrasound waves are echoed back towards the machine while others are transmitted through a tissue boundary.

• There are three main types of reflection: perpendicular reflection, specular reflection, and non-specular reflection.

Perpendicular Reflection

• This type occurs when an incident ultrasound beam strikes a large, smooth tissue boundary at a right angle.

• Complete reflection happens with significant differences in acoustic impedance values, such as between air (low impedance) and soft tissue (high impedance).

Specular Reflection

• Occurs when the ultrasound beam hits a flat surface at an angle; here, the incidence angle equals the reflection angle.

• Echoes reflected off at angles may not return to the ultrasound machine during listening time.

Non-Specular Reflection

• Involves irregular surfaces where incident beams scatter in multiple directions rather than reflecting uniformly.

• This can be likened to viewing reflections on rough mirror pieces—providing less clear images compared to smooth surfaces.

Impact of Acoustic Impedance Differences

• The difference in acoustic impedances determines how much energy is reflected versus transmitted through boundaries.

• Identical acoustic impedances lead to total transmission; larger differences result in increased reflections.

Calculating Reflectance Values

• A formula exists for calculating how much incident ultrasound will reflect back based on differences in acoustic impedances.

• The calculation involves subtracting one impedance from another, summing them up, squaring this sum, and deriving a percentage value for reflectance.

Energy Conservation Principle

Understanding Ultrasound Reflection and Transmission

Acoustic Impedance and Reflection Calculation

• The discussion begins with a common exam question regarding ultrasound beams reaching tissue boundaries, specifically between muscle (1.71 Rayls) and bone (7.8 Rayls).

• The formula for calculating the amount of echo reflected back towards the ultrasound transducer is introduced, emphasizing that these equations apply only to perpendicular reflections at smooth tissue boundaries.

• By substituting values into the formula, it is determined that 41% of incident ultrasound energy is reflected back to the transducer, while 59% is transmitted through the tissue boundary.

Understanding Energy Transmission Through Bone

• A surprising observation arises: despite 59% of energy being transmitted into bone, many users report minimal echoes when scanning over ribs. This raises questions about signal loss.

• It’s explained that although more than half of the ultrasound energy passes through bone, high attenuation due to scattering and heat production in dense bone contributes significantly to signal loss.

Tissue-Air Interface Reflections

• When examining a tissue-air interface, a large reflection occurs because air has an extremely low acoustic impedance value. Most ultrasound waves are reflected rather than transmitted.

• The high reflection at tissue-air boundaries leads to shadows in imaging; this phenomenon is attributed not only to differences in acoustic impedances but also to bone's high attenuation properties.

Types of Reflection

• Various types of reflection are discussed: perpendicular reflection (specular), non-specular (diffuse), and how differences in acoustic impedance influence returned ultrasound waves versus those transmitted through tissues.

Introduction to Refraction Concepts