INTEREACTIONS

reduction in the intensity of an ultrasound beam as it travels through a medium

Attenuation

Interactions that occur include:

Absoprtion, refraction, reflection, diffraction, scattering, interference

refers to the conversion of ultrasonic to thermal energy. also happens as sound travels through soft tissue, the particles that transmit the waves vibrate and cause friction and a loss of sound energy occurs and heat is produced

Absorption

Three factors determine the amount of absorpOon.

Viscocity, Relaxation time, frequency of the sound

relates to the cohesiveness or adhesivedness of the molecules

VISCOCITY

high VISCOCITY results in increased?

Absorption

is the Ome take for a molecule to return to its original position afteer it has been displaced Hence Tissue with longer relaxaOon absorbs more ultrasound energy

Relaxation time

viscocity and relaxation time is affected by the frequency

Frequency of the sound

High frequency will result into?

Increased absorption

Change in the direction due to change in medium is referred to as?

Refraction

The angle of refraction is determined by the change in the speed of sound that occurs at the boundary, and is related to the angle of incidence by?

Snell’s law

Incidence angle

Qi

Transmitted angle

Ot

Velocity of sound for incident medium

c1

Velocity of sound for transmitting medium

c2

When the reflecting surface is irregular in surface, and its dimensions are smaller than the diameter of ultrasound beam, the ultrasound energy dispersed in all directions.

Scattering

are much weaker than specularly reflected echoes he beam in different directions

Scattered echoes

These are caused by irregular surfaces

Non specular reflection

provide optimal returned signal todiagnostic imaging- called backscatter

Specular reflections

is the uniform spreading of an ultrasound beam as it propagates from the source

Diffraction

The smaller the source of the sound .

The higher the diffraction of the beam

more diffracted equals to?

More attenuated

the waves are in phase, i.e. one zone of compression meets another zone of compression

Constructive interference

the waves are out of phase, i.e. the zone of compression meets a zone of rarefaction.

Destructive interference

The most important single interaction process for purposes of generating an ultrasound image is

Reflection

Best reflection occurs from a smooth surface and is called

Specular reflection

The ratio of the pressure over an imaginary surface in a sound wave to the rate of particle flow across the surface

Acoustic impedance

The amount of reflecOon is determined by the angle between the sound beam and reflecOng surface

Angle of incidence

Incident beam is perpendicular to the tissue boundary

Perpendicular reflection

Incident angle is equal to the transmitted angle

Specular reflection

in an irregular surface

Non Specular Reflection

the factors that affect attenuation are:

Transducer frequency, acoustic impedcance, acoustic impedance mismatch