- Reflection occurs when waves encounter an interface or boundary between materials with different acoustic impedances
- The difference in impedance between mediums is known as the reflection coefficient
- The greater the reflection coefficient, the greater the size of the wave (and percentage of energy) that is reflected at the interface
- For example, a large difference in impedance is present between air and tissue:
- Therefore, almost all of the energy is reflected back towards the probe
- Almost no energy is transmitted through the air to allow image generation for deeper structures
- This explains why it is vital to use ultrasound gel when scanning to eliminate air tissue interfaces
- The angle of reflection of a wave equals the angle of incidence:
- When a wave strikes a tissue interface 90 degrees a very strong reflection travels back toward the ultrasound probe generating a bright image
- Waves striking tissue at other angles are reflected away from the probe leading to loss of wave energy
