Ultrasound Flashcards
What is ultrasound?
Is it a longitudinal or transverse wave?
mechanical wave that has higher frequency, than the upper limit of the audible range of the human ear
→ f > 20,000 Hz
- longitudinal in liquids / gases
- longitudinal + transverse in solids
In which direction do longitudinal and transversal waves propagate resp.?
longitudinal → same as direction of oscillation
transverse → perpendicular to direction of oscillation
What is damping of ultrasound?
It depends on … ?
Formula + unit.
⍺ = characterizes the attenuation of the ultrasound
⇒ proportional to frequency
(since μ is also proportional to f)
⍺ = 10 lg J0/J
⍺ = 10 μ x lg e
in [dB]
- J0 = incident intensity
- J = intensity after passing through the layer
- μ = absorption coefficient
- x = layer thickness
What is acoustic impedance?
Formula.
resistance type of quantity, characterizing the sound conducting medium
Z = c * ρ
- c = velocity of sound in the medium
- ρ = density of the medium
What is reflexitivity ?
Formula.
percentage of reflected intensity
⇒ R = 0 for no, and R = 1 for total reflection
R = Jrefl / J0 = ((Z1 - Z2) / (Z1 + Z2))2
- Z1, Z2 = acoustic impedances of the 2 media
Why is a gel used as a coupling medium for ultrasound examinations in practice?
the higher ΔZ → the higher Jrefl
⇒ on liquid-gas / solid-gas interface US is totally reflected
ΔZ btw coupling medium and transducer/skin very low, thus Jrefl is much smaller
What is the direct piezoelectric effect?
Why is it relevant?
when pressure/tension is applied on piezoelectric metal the centers of positive/negative charges are seperated
→ measurable voltage btw electrodes placed on the opposite sides of the crystal
⇒ mechanism of detection of US
(REMEMBER: <strong>D</strong>irect piezoelectric effect → <strong>D</strong>etection)
What is the inverse piezoelectric effect?
Why is it relevant?
when electric voltage is applied on the electrodes of the piezoelectric metal
→ becomes deformed due to seperation of positive/negative charges
⇒ mechanism of generation of US
What is a transducer?
device that converts electrical signal into mechanical and vice versa via periodical change btw transmission and reception
How is US generated?
- sine wave oscillator produces high frequency AC voltage → electrical pulse
- conducted to transducer = piezoelectric material
- electric E is converted into US pulse via inverse piezoelectric effect
How is US detected?
- transducer switches into receiver mode
- echo signals reflected
- reach transducer → deformation via direct piezoelectric effect
- electrical pulse generated
- conducted through amplifier
What is the pulse-echo method?
Formula to calculate the distance btw reflecting surface and transducer.
periodical change btw transmission and reception in transducer
→ time delay and amplitude decrease of the echo signal contains all information abt location of the examined organs
d = c*Δt / 2
- Δt = time btw emission + arrival US echo
- c = velocity of the sound in the medium
What are different kinds of US imaging methods?
- 1D A-image
- 1D, 2D B-image
- M-image
- reconstructed 3D, 4D image
What is a 1D A-image?
single US beam
→ time/distance vs. intensity of echo signal
⇒ histogram (lower part of picture)
(A corresponds to amplitude modulation)
What is a 1D, 2D B-image resp.?
1D
single US beam
→ brightness of pixels proportional to amplitude of US signal
⇒ thin line with different bright pixels (**very bottom of picture)
2D
fan shaped US beam array
→ series of 1D B-images at different angles
⇒ real image reconstructed
(B corresponds to brightness modulation)
What is a M-image?
sequence of 2D B-images
→ plotted vertically as a function of time
⇒ position of given surface as a function of time (left part of picture)
What is a reconstructed 3D, 4D image resp.?
3D
2D B-images of parallel scanned planes
→ processed by computer
⇒ image looks almost like photography (left part of picture)
4D
series of reconstructed 3D images
→ processed by computer
⇒ US movie
What is the Doppler effect?
Formula.
frequency (pitch) change of the wave as a result of the relative motion of the source and the observer
f = f0 (1 +/- 2 v/c)
- f = reflected frequency
- f0 = incident frequency
- v = motion in direction of US propagation
- c = velocity of US in the medium
- + = if observer/medium move towards each other
- - = if observer/medium move away from each other
What is the Doppler shift?
How is it observed?
difference btw reflected and incident frequency
(f - f0)
proportional to:
- v/c = relative velocity
- f0 = incident frequency
⇒ is observed by loudspeaker, thus frequency difference must fall in audible frequency range
What are different kinds of US imaging methods where Doppler effect is applied?
- Dopper time-velocity image, Color coded Doppler image
- Doppler flow meter
What is a Doppler time-velocity, and a color coded Doppler image resp.?
Doppler time-velocity image
selected section 2D B-image
→ velocity of observed surfaces plotted
⇒ time vs. function graph (right part of picture)
Color coded Doppler image
→ velocity of parts shown simultaneously in same image
⇒ 2D B-image with color coded velocity (left part of picture)
What is a Doppler flow meter?
Formula.
velocity of blood flow in large vessels measured
v = c(f-f0) / (2f0 * cos Θ)
- v = velocity of blood flow
- (f - f0) = Doppler shift
- Θ = angle btw US beam and axis of blood flow
