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RA6007: Radiation Physics > Ultrasound > Flashcards

Ultrasound Flashcards

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1
Q

what is the velocity of the wave

A

Velocity of the wave = wave frequency x wavelength

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2
Q

what type of wave is ultrasound?

A

ultrasound is a longitudinal wave

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3
Q

ultrasound requires a medium. T/F?

A

True

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4
Q

what are the two components of the wave as it prorogates through tissue?

A

the wave undergoes a series of compressions and rarefractions.

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5
Q

is ultrasound higher or lower frequency than what the human ear can detect

A

it is higher than what we can hear (x1000)

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6
Q

what is the difference between doppler and normal ultrasound?

A

Doppler has much higher pressure than normal ultrasound

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7
Q

what is young’s modulus?

A

young’s modulus is a measure of the elasticity of a medium

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8
Q

what is young’s modulus equation?

A

c = ⎷E/⍴
⍴ = density of medium
E = youngs modulus

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9
Q

what needs to be matched for passing of ultrasound from one medium to another

A

you need to match the acoustic impedance

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10
Q

what is acoustic impedance

A

a measure of how “easy” it is for sound to pass through a medium

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11
Q

what is high power ultrasound used for?

A

lithotripsy

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12
Q

is the intensity of the ultrasound beam uniform?

A

no

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13
Q

write down equation used to compare intensities of the beam

A

dB = 10log10 I2 / I1

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14
Q

what unit is the loss of power of the US beam

A

1Db/cm/MHz

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15
Q

what is the effect when acoustic impedance difference is very big?

A

there is a lot of reflection when the acoustic impedance difference is large

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16
Q

what are the different fates of US beam

A

refraction, reflection, scatter

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17
Q

what is the angle of incidence?

A

the angle of incidence is the angle of reflection

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18
Q

what happens to the beam as it passes through one medium to another

A

the beam bends

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19
Q

what is diffraction?

A

the bending of the US beam into the shadow of a strong absorber

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20
Q

where does diffraction commonly occur?

A

absorber edge, ie: gallstone

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21
Q

how are images constructed?

A

images are constructed by computing the time is takes for the beam to travel from the transducer and return from a reflecting surface. this is our depth data

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22
Q

what dictates the brightness of the display

A

the magnitude of the echo

23
Q

what is a transducer?

A

a device that converts one form of energy into another form

24
Q

what happens to the piezoelectric crystal when an electric current is applied?

A

the crystal alters shape, this is because the molecular dipoles change direction

25
Q

name components of the transducer and their basic function

A

acoustic insulation
acoustic lens
backing block - absorbs US going backwards, attenuates stray beam
matching layer - allows 100% transmission of beam into tissues, minimising reflection due to traversing mediums (acoustic impedance)
piezoelectric crystals - converts electrical to mechanical energy and forces the ceramic plate to move

26
Q

what is the function of the crystal in receive mode?

A

the small voltage generated by deformation of the crystal is detected and amplified, crystal compresses and expands, wave sent into patient and backwards into the probe, bounces back to the crystal and registered as an impulse

27
Q

what does the matching layer improve?

A

improves sound transmission by compensating for large acoustic impedances

28
Q

what does the US gel do?

A

it eliminates the air layer

29
Q

what is continuous wave be used for

A

measuring femur length in utero

30
Q

are higher or lower frequency waves absorbed more?

A

Higher frequency waves are absorbed more

31
Q

when SPL changes, what is also effected?

A

the spatial resolution

32
Q

what is meant by the term dispersion

A

the filtering out of the higher frequencies

33
Q

when SPL is longer, what can happen to two adjacent objects

A

when the SPL is longer, two adjacent objects can be seen as overlapping objects

34
Q

what is the pulse repetition frequency?

A

the number of pulses per second

35
Q

image depth

A

the time between two pulses must be greater than the time for the “return trip” which is equivalent to twice the image depth

36
Q

what determines the imaging depth

A

the PRF

37
Q

what is the bandwidth

A

the number of frequencies within a pulse

38
Q

what effect does increasing spatial pulse length have on bandwidth

A

increasing SPL will decrease the bandwidth

39
Q

short pulses have greater or lesser mixtures of frequencies?

A

short pulses have a greater mixture of frequencies

40
Q

SPL in imaging vs doppler

A

SPL shorter in regular US, longer in doppler

41
Q

what is specular reflection

A

occurs when the boundaries of two structures are smooth, generates echoes that define organ boundaries like liver.

42
Q

what happens when sound waves encounter structures much smaller than their frequencies

A

they are scattered in all directions, happens with RBCs for example

43
Q

what do scattered waves produce on the image

A

a speckle appearance, does not correspond to anatomical detail

44
Q

what is the mirror artefact?

A

when reflecting surface meets the beam at a large angle of incidence, such as the diaphragm, part of the beam not reflected directly back to the transducer, produces a secondary reflection

45
Q

effect of higher frequency on near & far fields

A

higher frequency, longer near field

46
Q

what is lateral resolution?

A

spatial resolution in a plane perpendicular to the beam

47
Q

another term for the near field

A

fresnel zone

48
Q

another term for far field

A

Fraunhoffer zone

49
Q

in which field is lateral res preserved

A

lateral resolution is preserved in the near field/fresnel zone

50
Q

what happens to the beam after the fresnel zone

A

the beam diverges

51
Q

what optimises lateral resolution at depth

A

a wide transducer of high frequency as divergence decreases with wider transducer or increasing frequency

52
Q

what does the acoustic lens do

A

narrow the beam to improve lateral resolution

53
Q
A

RA6007: Radiation Physics (4 decks)

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