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Physics > Ultrasound > Flashcards

Ultrasound Flashcards

1
Q

What is range equation?

A

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

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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

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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
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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

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5
Q

What happens if object is larger than the beam wavelength?

A

It wil be reflected or will change direction

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6
Q

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

A

It will scatter

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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

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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

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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

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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

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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

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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
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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
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14
Q

What type of transducers needed for Harmonics?

A
  • Heavily damped
  • High frequency
  • Broad bandwidth
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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

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16
Q

What is speckle artefact?

A

Interference from many small structures

Causes textured appearance

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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

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18
Q

What is acoustic shadowing?

A

Where a strongly attenuating structure causes shadowing behind them

  • Bowel gas
  • Lung
  • Bone
  • Gallstones
    *
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19
Q

What is acoustic enhancement?

A

Occurs in fluid filled structures

Increase intensity of echoes behind

TGC makes acoustic enhancement worse

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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
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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

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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

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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
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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

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25
Q

What is the attenuation of US in soft tissue and in air?

A

Soft tissue: loses 1Db per cm for every 1MHz

Air: loses 40Db per cm for every MHz

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26
Q

How is intesnity measured?

A

Amplitude2

Intensity is proportional to the acoustic impedence and to the amplitude2

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27
Q

What is damping?

A

High Q = low damping and longer pulse

Low Q = high damping and short pulse length

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28
Q

Pulse repition frequency equation

A

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

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29
Q

What is best doppler angle?

A

Less than 60 degrees

Doppler shift frequency is proportional to Cosine of angle

  • Side lobes cause artefact in doppler
    *
30
Q

What is Continuous Wave Doppler?

A

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
Q

How do we calculate length of near field?

A

Frequency x beam diameter

32
Q

How to estimate pressure in a stenosed vessel?

A

Bernoulli Formula = 4 x velocity2

33
Q

How to measure Echo intensity?

(or amount of reflected echoes)

A

Z2 - Z1 / Z2 + Z1

The difference in impedence of two tissues divided by the sum of impedence of two tissues

34
Q

Display of Echoes in A Mode and B Mode

A

A mode = spikes

B mode = dots

35
Q

What is Mechanical Index?

A

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
Q

How to measure power of an US probe?

A
  • Calorimeter (measures heat output)
  • Force balance
37
Q

Continuous wave ultrasound

A

Only a single frequency is emitted

Good depth of view

High Q is best (low damping)

38
Q

How to calculate Q value?

A

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
Q

Pulsed Ultrasound (Lower penetration depth)

A

A range of sound waves with different frequencies are emitted

Uses Wider bandwidth (low Q)

40
Q

Is there lateral resolution in A mode imaging?

A

No

A minimum of two pulses are required for lateral resolution

41
Q

Probe terminology

A

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
Q

Phased array (curvilinear) can alter

A
  • Beam direction
  • Axial resolution
  • Lateral resolution
  • PRF
43
Q

Ring Down artefact

(Also known as Comet Tail artefact)

A

Associated with gas bubbles

When US beam encounters a small fluid collection surrounded by gas bubbles

44
Q

Quality Assurance US

A

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
Q

Ultrasound Wavelength

A

Ranges from 0.1 - 1.5mm

46
Q

What is specular reflection?

A

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
Q

What is Snells law?

A

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
Q

What is the critical angle?

A

90 degrees

It is the angle of incidence, where the refracted beam travels parallel to boundary

49
Q

How is length of near field calculated?

A

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
Q

How to obtain a shorter focal distance?

A

Long delay between energising the outer vs inner elements

51
Q

What are grating lobes?

A

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

53
Q

Typical Number of scan lines?

A

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
Q

What is restrictive index?

A

RI = Peak systolic flow - end diastolic flow / peak systolic flow

Examples of low RI structures:

  • Renal artery
  • uterine artery
  • external carotid artery
55
Q

What are flash artefacts?

A

Bursts of signal due to motion

Seen in power doppler more than continuous wave

56
Q

Contrast US

A
  • 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
Q

What is string phantom used for?

A

Used for testing velocity in doppler mode

58
Q

What should probe temperature be?

A

Should be less than 43 degrees celsuis internal and external

59
Q

How to measure amount of wave reflection at an interface?

A

(Z1 - Z2)2 / (Z1 + Z2)2

60
Q

Coupling Gel purpose

A

Allow passage of beam between:

Probe - Air - Tissue

61
Q

Attenuation

A

A change of 3Db is approximately equal to a doubling or halving of power

62
Q

Do Matching layers reduce acoustic impedence?

A

NO

They only allow easier tranmission of the pulse to the patient

63
Q

Can an annular array focus in 2 dimensions?

64
Q

How to calculate PRF?

A

Lines per frame x frame rate

The max PRF is limited by depth

65
Q

What is reflection coefficient for soft tissues?

A

Always less than 5%

Muscle - soft tissue = 0.04%

Bone - soft tissue = 40% reflection

66
Q

What is considederd attenuation?

A
  • Absorption
  • Scatter
  • Reflection

Attenuation in bone is higher than soft tissue

67
Q

How many US pulses required to build up a scan line?

68
Q

Does increasing US intensity help visualise deep structures?

A

Yes it will increase echo amplitude at depth

69
Q

Processing of colour and pulse doppler

A

Pulsed doppler: Fourier transforms

Colour doppler: autocorrelation

70
Q

What results in Range ambiguity in doppler??

71
Q

What is acoustic streaming?

A

Corresponds US being absorbed in a medium and inducing fluid flow

This has the potential to cause Cellular damage

72
Q

What is comet tail artefact?

A

A type of reverberation artefact

Seen in adenomyosis of gallbladder

Physics (10 decks)

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