Attenuation, Reflection, and Refraction Flashcards

A sonographer is imaging a patient with a high-frequency linear transducer. They notice that the far field of the image is significantly darker than the near field. Which of the following is the primary reason for this observation?

Answer: The attenuation coefficient is directly proportional to frequency.

Attenuation, the weakening of the sound beam as it travels through tissue, is the primary reason for the far field appearing darker. The attenuation coefficient in soft tissue is approximately 0.5 dB/cm for every 1 MHz of frequency. Therefore, higher frequency transducers experience more attenuation, causing echoes from deeper structures (far field) to be weaker and appear darker.

An ultrasound wave travels from a medium with an acoustic impedance of 1.5 MRayls to a medium with an acoustic impedance of 1.7 MRayls. What will occur at the boundary between these two media?

Answer: A portion of the sound will be reflected back to the transducer.

Reflection of an ultrasound wave occurs at the boundary between two media with different acoustic impedances. The difference in acoustic impedance, known as impedance mismatch, determines the amount of reflection. Since the two media have different acoustic impedances (1.5 and 1.7 MRayls), a portion of the sound energy will be reflected at the interface.

For refraction of an ultrasound beam to occur at an interface between two tissues, which two conditions must be met?

Answer: Different propagation speeds and an oblique angle of incidence.

Refraction is the bending of the sound beam as it crosses a boundary between two media. This phenomenon requires two conditions: 1) a difference in the propagation speeds of the two media, and 2) an oblique angle of incidence (any angle other than 90 degrees). Snell's Law describes this relationship.

While performing a Doppler study of the red blood cells, a sonographer notes that the strength of the returning echoes increases significantly when they switch from a 3 MHz transducer to a 6 MHz transducer. This is best explained by:

Answer: Rayleigh scattering.

Rayleigh scattering occurs when the ultrasound wave interacts with structures that are much smaller than the wavelength, such as red blood cells. The intensity of Rayleigh scattering is proportional to the frequency raised to the fourth power. Therefore, doubling the frequency from 3 MHz to 6 MHz results in a 16-fold increase in scattering intensity, producing much stronger echoes.

A sonographer observes a strong, bright reflection from the diaphragm-lung interface, with a complete loss of signal (acoustic shadow) posterior to it. This is primarily due to:

Answer: A large mismatch in acoustic impedance.

The interface between soft tissue (diaphragm) and air (lung) represents a very large acoustic impedance mismatch. Because of this significant difference, nearly all (over 99%) of the sound is reflected at this boundary. This strong reflection creates a bright echo, and the lack of transmitted sound results in an acoustic shadow posterior to the interface.

Which of the following is a primary component of attenuation?

Answer: Conversion of sound to heat

Attenuation is the overall weakening of the sound beam and is composed of three main factors: absorption, reflection, and scattering. Absorption, which is the conversion of acoustic energy into heat, is the dominant component of attenuation in soft tissues.