Brainscape's Knowledge GenomeTM


Top Flashcards (Certified)
All Flashcards

Physics > Ultrasound > Flashcards

Ultrasound Flashcards

1
Q

What is range equation?

A

Speed (of sound in the medium) x Time / 2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is axial resolution?

A

Known as Depth Resolution

(3x better than lateral resolution)

The min distance between 2 reflectors along the beam direction that can be distinguished

High frequency = high axial resolution

Dependent on:

  • frequency
  • pulse length (fewer and shorter pulses)

Independent of beam width

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is lateral resolution?

(Also known as Azimuthal Resolution)

How do you increase lateral resolution?

A

Ability to differentiate two reflectors side by side at the same depth in the same scan plane

Determined by:

  • High frequency
  • Focusing
  • Transducer diameter
  • Distance from transducer
  1. Is equal to the beam diameter
  2. Gets worse at increasing distance from transducer due to divergence
  3. A smaller transducer can improve lateral resolution when it is near the transducer
  4. Anything that increases length of near zone will improve lateral resolution. Once in far zone, the beam diverges, decreasing resolution

Independent of:

  • Pulse length
  • Damping

Increased by:

  • Focusing the beam
  • High Frequency (increases length of near zone)
  • Increased number of scan lines
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is constructive interference?

What is destructive inteference?

A

Where two ultrasound waves meet in phase

Their amplitudes are then added together

Destructive

Where two out of phase waves meet

They are added together and their signal is nulled

For both constructive and destructive, waves have to be of same wavelength

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What happens if object is larger than the beam wavelength?

A

It wil be reflected or will change direction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What happens if the object is smaller than the beam wavelength?

A

It will scatter

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What measures increase near field and reduce the angle of beam divergence?

A
  • Increasing frequency
  • Increasing transducer diameter

Near field length is equal to the transducer diameter2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Transducer Build

A

Backing layer is matched to the impedence of the transducer (but not the same) so that waves can travel backwards and be scattered within the probe itself without relfection

Matching layer is 1/4 wavelength thick to reduce impedence

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the thickness of piezoelectric crystals?

Crystals also known as discs

A

They are half the desired wavelength thick

Usually 256 crystals

Are made by heating the crystals above curie temp and polarizing with external voltage which is maintained until temp falls below curie point

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the doppler frequency?

A

The difference between the Transmitted and Received frequency

Known as the doppler shift

The higher the frequency of the doppler shift the higher the velocity

Change in frequency is inversely proportional to the velocity of sound in the medium

Aliasing occurs when the doppler shift frequency exceeds half the PRF

Doppler shift frequency is directly proportional to the frequency of US Beam

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What factors does doppler frequency depend on?

A
  1. Speed of sound
  2. Frequency of US beam
  3. Cosine of Angle the wave strikes the object

The max doppler frequency that can be detected is equal to half the PRF

Sound velocity is NOT DEPENDENT on blood velocity (just dependent on density and compressibility

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Harmonic Imaging

A

As sound waves pass through tissue they become distorted

Distortion only occurs in the central high energy part of the beam

Distorted wave is made up of several harmonic frequencies which are multiples of first harmonic

E.g. If a 2MHz pulse is sent out then the returned harmonic frequencies are 4,6,8

Done by:

  1. Harmonic filter
  2. Pulse inversion
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How do we use harmonic imaging?

Harmonic Filter

A

Harmonics are produced in the RETURNING echo

Using a harmonic filter the fundamental harmonic (transducer frequency) is removed

Done by:

  1. Harmonic filter

Advantages

  • Second harmonic is one used
  • Better visualisation of low contrast lesions
  • Better visualisation of gallbladder and bladder (liquid filled cavities)
  • Improves lateral resolution
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What type of transducers needed for Harmonics?

A
  • Heavily damped
  • High frequency
  • Broad bandwidth
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Harmonic imaging

What happens in pulse inversion?

A

Odd harmonic frequencies (including first one) are removed

The remaining harmonic frequencies are doubled

  • Gives better axial resolution
  • Broad bandwidth and short pulses so no filtering required

Subject to motion artefact however as multiple pulses

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is speckle artefact?

A

Interference from many small structures

Causes textured appearance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is reverberation artefact?

A

Due to a strong reflector near the surface

Caused by multiple reflections to and fro between tranducer face

Produced a series of delayed echoes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What is acoustic shadowing?

A

Where a strongly attenuating structure causes shadowing behind them

  • Bowel gas
  • Lung
  • Bone
  • Gallstones
    *
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is acoustic enhancement?

A

Occurs in fluid filled structures

Increase intensity of echoes behind

TGC makes acoustic enhancement worse

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What size are US contrast microbubbles and how are they destroyed?

A

They act by increasing the reflections from the tissue containing agent

Microbubbles <4µm (Microaggregated albumin)

Nanoparticles <1µm (Perflurorcarbons)

Destroyed by high energy US or within a few hours by the body

  • Usually have a gaesous core (MAA)
  • Reasonance frequency falls within diagnostic US range
  • Mainly accumulate in blood but can be uptake by endothelial cells in liver or spleen
  • Perfluorocarbon nanoparticles dont have gaseous core
    • can stay in circulation longer
    • have a low echogenicity
    • can only be imaged after accumulation
    • can potentially be used as multi-modality contrast agents
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Ultrasound Safety

What is Time averaged intensity limit?

A

Should never exceed:

100mW/cm2

  • average energy for an exam is 10mW/cm2

Total sound energy should never exceed 50J/cm2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What is Thermal index (TI) ?

A

Estimated temperature rise

Equal to: power output/power required to raise temp by 1 degrees

Are three types:

  1. Soft tissue TIs
  2. Bone TIb (scanning through soft tissue into bone)
  3. Cranial TIc (scanning through bone into soft tissue)

TI up to 1 is safe

Generally aim TI < 0.7

  • No restrictions on scanning TI <0.7
  • 60 min restriction on scanning 0.7 - 1.0
  • 30mins scanning in TI >1.0 in fetal scanning

Should never used TI >3 in fetal scanning

TI <1 in ophthalmology

  • TIs soft tissue should be monitored

Pulsed Doppler has greatest potential to increase temperature due to high PRF

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What should mechanical index be set at?

A

Risk of cavitation

Should be < 0.9 or 0.7

  • MI < 0.5 for fetal scanning
  • Above 0.7 should never be used for contrast US
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Acoustic Impedence

A

Acoustic impedence = density x velocity

Acoutic impedence increases proportionally with square root of density

Units: kg/sq metre/sec

INDEPENDENT OF FREQUENCY

High impedence = less able to pass through

Large difference in Z = more energy reflected

Small difference in Z = more energy transmitted

No difference = full transmission

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
What is the attenuation of US in soft tissue and in air?
Soft tissue: **loses 1Db per cm for every 1MHz** Air: **loses** **40Db per cm for every MHz**
26
How is intesnity measured?
Amplitude2 Intensity is proportional to the acoustic impedence and to the amplitude2
27
What is damping?
High Q = low damping and longer pulse Low Q = high damping and short pulse length
28
Pulse repition frequency equation
Number of pulses emitted by transducer per second **Frame rate x lines per frame** Lower PRF = higher depth of view Higher PRF = shallow depth of view
29
What is best doppler angle?
Less than 60 degrees Doppler shift frequency is proportional to **Cosine of angle** * **Side lobes cause artefact in doppler** *
30
What is Continuous Wave Doppler?
Hand held doppler * Transmission frequency 2-10MHz used * Uses 2 diferently angled transducers (one to transmit and one to receive) * Received freq subtracted from initial transmitted freq
31
How do we calculate length of near field?
Frequency x beam diameter
32
How to estimate pressure in a stenosed vessel?
Bernoulli Formula = 4 x velocity2
33
How to measure Echo intensity? (or amount of reflected echoes)
Z2 - Z1 / Z2 + Z1 The difference in impedence of two tissues divided by the sum of impedence of two tissues
34
Display of Echoes in A Mode and B Mode
A mode = spikes B mode = dots
35
What is Mechanical Index?
Can cause cavitation if \>0.7 * lungs are prone to cavitation * not fetal lungs however as they arent aerated * in neonates possibility of lung or intestine damage occurs with MI \>0.3 Peak rarefaction (negative) pressure/square root of frequency of US beam Should be less than 0.7 Lower frequencies cause higher MI
36
How to measure power of an US probe?
* Calorimeter (measures heat output) * Force balance
37
Continuous wave ultrasound
Only a single frequency is emitted Good depth of view High Q is best (low damping)
38
How to calculate Q value?
Q value = Mean frequency / Bandwidth Bandwidth = FWHM (spectrum of frequencies used) Short pulses and LOW Q(high damping) have a wide bandwidth Long pulses and high Q (low damping) have a narrow bandwidth
39
Pulsed Ultrasound (Lower penetration depth)
A range of sound waves with **different** frequencies are emitted Uses Wider bandwidth (low Q)
40
Is there lateral resolution in A mode imaging?
No A minimum of two pulses are required for lateral resolution
41
Probe terminology
Footprint = width of the probe In linear probes the field of view is the same width as the footprint Large nearfield in linear probes In curvilinear probes the field of view is wider than the footprint
42
Phased array (curvilinear) can alter
* Beam direction * Axial resolution * Lateral resolution * PRF
43
Ring Down artefact (Also known as Comet Tail artefact)
Associated with gas bubbles When US beam encounters a small fluid collection surrounded by gas bubbles
44
Quality Assurance US
Resolution: Perspex box with water inside and parallel wires **When using perspex, adjustments need to be made as sound travels faster in perspex** **Gelatine based phantons can also be used to mimic tissue** Dynamic range, Sensitivity and A scan calliper: persepex box with parallel vertical rods B mode Grey scale and Doppler: Tissue phantoms using gelatine Power output: force balance or calorimeter
45
Ultrasound Wavelength
Ranges from 0.1 - 1.5mm
46
What is specular reflection?
Happens when US encouters large smooth surfaces such as bone US waves are reflected back in a uniform direction The angle of specular reflection is equal to the angle of incidence **Enables visualisation of tissue boundaries**
47
What is Snells law?
Law of refraction The angle of refraction depends on the velocities of US in the media on both sides of the boundary In other words The ratio of sines of the angles of incidence and refraction is equal to the ratio of velocities in the two tissues forming the boundary
48
What is the critical angle?
90 degrees It is the angle of incidence, where the refracted beam travels parallel to boundary
49
How is length of near field calculated?
It is proportional to the **Transducer diameter2** or **Frequency x Diameter2** Inversely proportional to the wavelength Increasing frequency causes increased nearfield length and decreased far field divergence
50
How to obtain a shorter focal distance?
**Long delay between energising the outer vs inner elements**
51
What are grating lobes?
Are weak replicas of main US beam Happen when crystals are larger than half the wavelength **Unique to array transducers** **-not seen in annular transducers** **Most transducers used are ARRAY** Apodization is a technique used to reduce side lobes where less power is sent to the outer elements
52
53
Typical Number of scan lines?
Usually 100 is sufficient Scan lines = PRF / Frame rate e.g. 2kHz 2000/25 =80 scan lines **Frame Rate = Speed of tissue / 2 x depth of view x number of scan lines**
54
What is restrictive index?
RI = Peak systolic flow - end diastolic flow / peak systolic flow Examples of low RI structures: * Renal artery * uterine artery * external carotid artery
55
What are flash artefacts?
Bursts of signal due to motion Seen in power doppler more than continuous wave
56
Contrast US
* Microbubbles can be used for drug delivery to tissues * Low MI scanning is used to characterise liver lesions and look for washout in arterial/pv phase * Can be viewed in real time at low MI to prevent bursting Perfluorocarbons do NOT have a gaseous core
57
What is string phantom used for?
Used for testing velocity in doppler mode
58
What should probe temperature be?
Should be less than 43 degrees celsuis internal and external
59
How to measure amount of wave reflection at an interface?
(Z1 - Z2)2 / (Z1 + Z2)2
60
Coupling Gel purpose
Allow passage of beam between: **Probe - Air - Tissue**
61
Attenuation
A change of 3Db is approximately equal to a doubling or halving of power
62
Do Matching layers reduce acoustic impedence?
NO They only allow easier tranmission of the pulse to the patient
63
Can an annular array focus in 2 dimensions?
Yes
64
How to calculate PRF?
**Lines per frame x frame rate** ## Footnote **The max PRF is limited by depth**
65
What is reflection coefficient for soft tissues?
Always less than 5% Muscle - soft tissue = 0.04% Bone - soft tissue = 40% reflection
66
What is considederd attenuation?
* Absorption * Scatter * Reflection Attenuation in bone is higher than soft tissue
67
How many US pulses required to build up a scan line?
Just 1
68
Does increasing US intensity help visualise deep structures?
Yes it will increase echo amplitude at depth
69
Processing of colour and pulse doppler
Pulsed doppler: Fourier transforms Colour doppler: autocorrelation
70
What results in Range ambiguity in doppler??
High PRF
71
What is acoustic streaming?
Corresponds US being absorbed in a medium and inducing fluid flow This has the potential to cause **Cellular damage**
72
What is comet tail artefact?
A type of reverberation artefact Seen in adenomyosis of gallbladder

Physics (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2025 Bold Learning Solutions. Terms and Conditions