Ultrasound

Question 1

What is ultrasound?

Is it a longitudinal or transverse wave?

Answer 1

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

Question 2

In which direction do longitudinal and transversal waves propagate resp.?

Answer 2

longitudinal → same as direction of oscillation

transverse → perpendicular to direction of oscillation

Question 3

What is damping of ultrasound?

It depends on … ?

Formula + unit.

Answer 3

⍺ = 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

Question 4

What is acoustic impedance?

Formula.

Answer 4

resistance type of quantity, characterizing the sound conducting medium

Z = c * ρ

• c = velocity of sound in the medium
• ρ = density of the medium

Question 5

What is reflexitivity ?

Formula.

Answer 5

percentage of reflected intensity
⇒ R = 0 for no, and R = 1 for total reflection

R = J_refl / J₀ = ((Z₁ - Z₂) / (Z₁ + Z₂))²​

• Z₁, Z₂ = acoustic impedances of the 2 media

Question 6

Why is a gel used as a coupling medium for ultrasound examinations in practice?

Answer 6

the higher ΔZ → the higher J_refl
⇒ on liquid-gas / solid-gas interface US is totally reflected

_{ΔZ btw coupling medium and transducer/skin very low, thus Jrefl is much smaller}

Question 7

What is the direct piezoelectric effect?

Why is it relevant?

Answer 7

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)}

Question 8

What is the inverse piezoelectric effect?

Why is it relevant?

Answer 8

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

Question 9

What is a transducer?

Answer 9

device that converts electrical signal into mechanical and vice versa via periodical change btw transmission and reception

Question 10

How is US generated?

Answer 10

1. sine wave oscillator produces high frequency AC voltage → electrical pulse
2. conducted to transducer = piezoelectric material
3. electric E is converted into US pulse via inverse piezoelectric effect

Question 11

How is US detected?

Answer 11

1. transducer switches into receiver mode
2. echo signals reflected
3. reach transducer → deformation via direct piezoelectric effect
4. electrical pulse generated
5. conducted through amplifier

Question 12

What is the pulse-echo method?

Formula to calculate the distance btw reflecting surface and transducer.

Answer 12

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

Question 13

What are different kinds of US imaging methods?

Answer 13

• 1D A-image
• 1D, 2D B-image
• M-image
• reconstructed 3D, 4D image

Question 14

What is a 1D A-image?

Answer 14

single US beam
→ time/distance vs. intensity of echo signal

⇒ histogram (lower part of picture)

(A corresponds to amplitude modulation)

Question 15

What is a 1D, 2D B-image resp.?

Answer 15

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)}

Question 16

What is a M-image?

Answer 16

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)

Question 17

What is a reconstructed 3D, 4D image resp.?

Answer 17

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

Question 18

What is the Doppler effect?

Formula.

Answer 18

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

Question 19

What is the Doppler shift?

How is it observed?

Answer 19

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

Question 20

What are different kinds of US imaging methods where Doppler effect is applied?

Answer 20

• Dopper time-velocity image, Color coded Doppler image
• Doppler flow meter

Question 21

What is a Doppler time-velocity, and a color coded Doppler image resp.?

Answer 21

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)

Question 22

What is a Doppler flow meter?

Formula.

Answer 22

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