Physics Principles of Diagnostic Ultrasound Flashcards
Describe attenuation
As ultrasound waves travel through tissue it becomes progressively weaker
How do we calculate attenuation?
a=(db/cm)xd
As amplitude is decreased, what happens to intensity?
It decreases
If there is an increase in frequency, what happens to attenuation?
It increases
What is transverse?
Medium moves perpendicular to direction of the wave
What is longitudinal?
Medium moves parallel to the wave
What is compression?
Transducer moves towards the body and compresses tissue
*highest pressure of soundwave
What is rarefaction?
Transducer moves away from the body and decompresses tissue
*smallest point of pressure of soundwave
What is propagation speed?
The speed of the ultrasound waves through tissue
What speed does the machine think soft tissue is?
1540m/s
What is the frequency of sound waves?
Number of times per second
What is amplitude?
Peak pressure
What is intensity?
Power per unit in the wave
As frequency increases what happens to the wavelength?
It decreases
As the wavelength decreases what happens to the resolution?
It is enhanced
What is the wavelength?
The wavelength is the distance between one peak of the wave and the next peak
What is absorption?
Acoustic energy lost in the medium because it is converted into heat
What 3 things is absorption determined by?
1) viscosity of the medium
2) relaxation time of the medium
3) frequency of the sound
What is propagation speed?
Speed of sound affected by the density and elasticity of the medium it is travelling through
Is propagation speed higher in bone or air?
Bone
What is reflection?
Interaction of ultrasound with relatively large and smooth surfaces
What 3 things does reflection depend on?
1) acoustic properties of two tissues
2) angle of incidence
3) reflecting surface
What is specular reflector?
strong echoes
What is non-specular reflector?
Weak echoes
What is the reflection coefficient equation?
R=reflected intensity/incident intensity
What type of reflect occurs at a perpendicular incidence?
No reflection
What reflection occurs at tissues with a large impedance difference?
Full reflection
When does scattering occur?
At interaction of small structures such as RBC & capillaries
What is Rayleigh scatters?
Reflection of energy equally in all directions
If frequency is increased, what happens to scatter?
It increases
Describe acoustic impedance
Density of the medium and speed at which sound travels through the medium
What is a Rayls?
kg/m2s
The unit of specific acoustic impedance
What is the equation of acoustic impedance?
z=pxc
(tissue density x propagation speed)
Define amplitude?
Strength, peak pressure
What is the equation for amplitude reflection coefficient?
R=Z2-Z1 / Z2+Z1
(z1=acoustic impedance on proximal side of interface)
(z2=acoustic impedance on distal side of interface)
What is refraction?
Bending of the soundwave
What are the two conditions required for refraction?
1)incident sound wave must not be perpendicular
2)speed of sound must be different on the two sides of the interface
What is snell’s law equation and what does it predict?
predicts the direction of a transmitted sound beam when refraction occurs.
Sin01/c1=sin0t/C2
If there is a critical angle, what angle is the transmitted angle?
90 degrees
If the transmitted angle is bigger than the critical angle what happens?
Total reflection
What is diffraction?
Occurs when a wave passes through an opening or around a barrier
What is penetration depth?
At some depth the attenuation is so great the returning US echoes are too weak to be detected
What is the round trip attenuation equation?
(ax2pxf)
What is the equation for calculating the attenuation when there is a reduction in the intensity?
dB=10 log I/I0
What is the difference between speed and velocity?
Speed refers to an object’s change in distance with respect to time, whereas velocity refers to an object’s change in displacement with respect to time.
What is compression and rarefaction?
compressions are regions of high pressure due to particles being close together. rarefactions are regions of low pressure due to particles being spread further apart
What does the following equation show?
λ = c/f
the wavelength of the
ultrasound (λ) can be calculated by dividing the ultrasound propagation
speed (c) by the ultrasound frequency (f).
