Acoustic Impedance | Ultrasound Physics | Radiology Physics Course #5

Understanding Ultrasound Wave Interactions with Matter

Introduction to Pulse Echo Ultrasonography

• The discussion begins with a review of parameters used to describe sound waves and specific parameters for pulse echo ultrasonography, which generates images displayed on screens.

Acoustic Impedance and Its Importance

• To understand ultrasound interactions with tissues, the concept of acoustic impedance is introduced. This property is crucial for analyzing how ultrasound waves interact with different materials.

Types of Tissue Interactions

• Reflection occurs at tissue boundaries, defined as interfaces between two distinct tissues (e.g., muscle and fat).

• There are various types of reflection: partial reflection (some energy transmitted), complete reflection (all energy returns), specular reflection, and non-specular reflection.

• Refraction happens when an ultrasound wave hits a tissue boundary at an angle, leading to changes in direction due to varying speeds of sound in different tissues.

• Scattering occurs when ultrasound interacts with small structures within the tissue that are smaller than the wavelength, resulting in energy loss and dispersion into smaller waves.

Defining Acoustic Impedance

• Acoustic impedance (denoted as Z) is defined as a property specific to each tissue type. It can be calculated by multiplying tissue density by the speed of sound through that tissue.

• The SI units for acoustic impedance are kilograms per meter squared per second.

Factors Influencing Acoustic Impedance

• Tissue density (measured in kg/m³) and speed of sound (measured in m/s) contribute significantly to acoustic impedance values.

• The bulk modulus—reflecting stiffness or resistance to compression—plays a major role in determining acoustic impedance.

Interaction Outcomes Based on Acoustic Impedance Differences

• When an ultrasound pulse encounters a tissue boundary, the difference in acoustic impedance determines how much wave energy is transmitted versus reflected back towards the machine.

Variability Among Tissues

• A table presents various acoustic impedance values for different soft tissues relevant in diagnostic imaging; significant differences lead to greater reflections.

Reflection Dynamics at Tissue Boundaries

• If two tissues have similar acoustic impedances, most ultrasound will transmit through rather than reflect back. Conversely, large differences result in substantial reflections regardless of whether it’s from low to high or vice versa.

Conceptualizing Stiffness Through Analogies

Understanding Wave Behavior in Springs

Interaction of Springs and Energy Transfer

• The behavior of a tightly packed spring, when pulsed, demonstrates how energy is reflected back as a pulse echo towards the source. This analogy illustrates wave reflection in physical systems.

• When two springs with similar acoustic impedance values are connected, the generated wave continues to propagate with minimal reflection or echo, indicating efficient energy transfer between mediums.

• In contrast, if a spring is connected to a highly compressible tissue with low acoustic impedance, the energy reaching the end will not have an effective medium to transfer into. Consequently, this results in significant reflection as the energy returns to its original position.