Ultrasound Flashcards
1
Q
Acoustic waves
A
- sound is mechanical energy transmitted by pressure waves
- a medium is required for propagation
2
Q
Acoustic wave cycle
A
frequency: number of cycles per second (Hz)
3
Q
Ultrasound Hz range
A
> 20,000Hz
4
Q
Medical ultrasounds
A
2-15 MHz - 2,000,000 - 15,000,000 Hz
5
Q
humans can detect sound within what frequency range?
A
20-20,000Hz
6
Q
Wavelength
A
distance between sam point on subsequent waves
7
Q
Amplitude
A
- height of crest or trough
- energy power of the wave
8
Q
Frequency and wavelength
A
inversely proportional
9
Q
what does increasing frequency do?
A
- decreases wavelength
- improves axial resolution
10
Q
Propagation
A
- when a source vibrates, it transfers energy to the surrounding particles causing them to vibrate
- requires an elastic medium
- different mediums propagate sound differently
11
Q
compression
A
- molecules are forced or pressed together
12
Q
rarefaction
A
- molecules are given extra space and allowed to expand
13
Q
compression and rarefaction
A
- as molecules compress, they pass kinetic energy to each other - travel outward from the source
- direction of oscillation is same same as wave travel
14
Q
What would be the wavelength of a 1Mhz wave in soft tissue?
A
Acoustic velocity is 1540m/s in most soft tissue
1,540,000mm/s
1MHz = 1,000,000Hz (cycles per second)
= 1540000mm/s/1,000,000Hz
= wavelength of 1.54 mm
15
Q
Acoustic Velocity (c)
A
the speed at which a wave propagates through a medium
c = f λ
16
Q
acoustic velocity determinants
A
- density
- compressibility
- rigidity/stiffness
17
Q
What would be the wavelengths in soft tissue of beams from a 5MHz transducer versus a 2.5 MHz transducer?
A
= 1540000mm/s/5,000,000Hz
= wavelength of 0.308 mm
= 1540000mm/s/2,500,000Hz
= wavelength of 0.616 mm
18
Q
density (ρ)
A
the greater the density, the more it impedes sound
19
Q
Compressibility (K)
A
- decrease in volume when pressure is applied
- easier to reduce the volume = greater compressibility
- greater compressibility = greater impedance
20
Q
Rigidity/Stiffness (bulk modulus, B)
A
the stiffer, the faster sound moves through it
21
Q
how acoustic velocity determinants affect the velocity?
A
↑ Density: ↓ c
↑ Compressibility (K): ↓ c
↑ Rigidity/Stiffness (Bulk modulus, B): ↑ c
22
Q
Acoustic velocity
A
- Sound travels faster in solids
- Density affects c less than compressibility or stiffness
- Faster in solids because they are much less compressible even though they are generally denser
23
Q
Echo range principle
A
- send out a pulse of sound
- listen for the echo to return to the transducer
- assume a constant c (1540 m/s)
24
Q
Echo range principle
A
- speed=distance/time
- distance of echo production calculated based on time to return
- pixel is plotted at depth of echo production
- echo strength determines pixel brightness
25
Pulsed wave output
- US waves are generated in pulses
- 2-3 pulses of the same frequency
- pulse repetition frequency (RPF)
26
what is pulse repetition frequency?
of pulses emitted per time
27
piezoelectric effect
- pressure wave is incident on the crystal, causing it to vibrate and creates an electronic effect
28
transmission gain
- modulates the voltage applied to the crystal and produces an acoustic pulse of varying intensity
29
what does increased transmission gain do to the transmitted signal?
makes it stronger
30
Reverse piezoelectric effect
- when charge is applied to a piezoelectric crystal, it vibrates creating a pressure mechanical wave
31
echo pulse principle
- sound is transmitted from the transducer to tissue
- signal is echoed back from tissue to the transducer
32
intensity
- rate at which energy is transmitted by the sound wave over a small area
- increased intensity causes increased oscillation of particles and maximum particle velocity increases
33
Acustic impedance (Z)
- How much resistance an ultrasound beam encounters as it passes through tissue
- How readily tissue particles move under the influence of the wave pressure
34
what does acoustic impedance depend on?
tissue density (ρ) and speed of sound wave (c)
Z = ρ x c
35
interaction of acoustic waves with tissue
- reflection
- scatter
- refraction
- absorption
36
acoustic impedance and reflection
- the ability of an ultrasound wave to move from one tissue to another (transmission) depends on the difference in Z
- if the difference is large, a portion of the sound is reflected
37
Reflection fraction
(Z2-Z1/Z2+Z1)^2
38
Percent reflectivity %R
(Z2-Z1/Z2+Z1)^2 *100
39
Percent transmission %T
100-%R
40
specular reflection
- occurs at large, smooth surfaces such as bone
- similar to light reflections off a mirror
- the angle of incidence = angle of reflection
- sound reflected in a single direction
41
challenge with specular reflection
- angle of reflection away from the transducer
- transducer need to be perpendicular to specular surface/interfaces
42
tissue boundary width and reflection
- if the tissue boundary width is less than the wavelength of the US wave, the wave will not be reflected
43
air and reflection
- air between transducer and skin causes 99.9% reflection
- requires use of gel
44
reflection and transmission
- beam encounters multiple interfaces on its path through the body
- at each one, a % of the incident beam intensity is reflected or transmitted
45
mirror image artifact
- the primary beam reflects from a highly reflective surface
- encounters another structure and is reflected back to the highly reflective surface, then reflects back toward the transducer
46
diffuse reflection (scatter)
- Most echoes from US are from scattering
- Soft tissue (non-smooth surfaces of organs, RBCs) is a diffuse reflector
- Sound waves scatter in various directions; scattered echoes have smaller amplitudes
- Gives tissues their unique echo texture
47
refraction
- change in direction when crossing a boundary between two media in which the speeds of sound are different
- no refraction if the angle of incidence is 90
- change in direction at all other angles
48
refraction and misregistration artifact
- US machine plots the echo as though it returned in a straight line, causing a misregistration artifact
49
absorption
- As US waves pass through tissue, orderly vibrational energy is converted into random vibrational heat energy; basis of therapeutic ultrasound
- Wave amplitude reduces with distance travelled
- Higher frequency wave = more rapidly attenuation; penetrance is inversely related to the frequency of the beam
- Absorption rate also depends on tissue involved
50
Time gain compensation
- Absorption increases with depth; lower intensity echoes than superficial ones
- Time Gain Compensation (TGC) electronically amplifies the returned signal as a function of depth (or echo time)
- Done with Time Compensation sliders
51
echogenicity
hypo: fat/blood
hyper: bone
Iso: liver, kidneys and muscle
An: fluid filled cysts
