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Medical Imaging > Ultrasound > Flashcards

Ultrasound Flashcards

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USMLE® Step 1 flashcards
1
Q

rarefraction

A

low pressure and density

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2
Q

compression

A

high pressure and density

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3
Q

what is a wavelenght made up of

A

1 cycle = 1 compresion and 1 rarefraction

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4
Q

frequency

A

1 cycle per second = 1Hz

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5
Q

propagation

A

speed at which a wave moves through a medium
metres/second
in soft tissue - approx 1540 m/s

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6
Q

equation for propogation

A

propogation = frequency x wavelength

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7
Q

what does propgation depend on

A

density
stiffness

gases - lower propagation speed
solids - higher propagation speed

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8
Q

speed in imaging

bone

A

3000-5000m/s-1

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9
Q

speed in imaging

soft tissue

A

1440-1640m/s-1

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10
Q

speed in imaging

Fat

A

1440m/s-1

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11
Q

speed in imaging

air

A

330m/s-1

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12
Q

why can you not constantly put sound wave into tissue

A

will cause tissue to heat up

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13
Q

why does reflection occur

A

because of various densities and stiffness = impedence between tissues
similar impedance = weak echo
identical impedance = no echo
different impedance = stong echo

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14
Q

what is the range equation

A

t = 2d/c

t = echo time 
d = distance to reflective 
c = speed of sound
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15
Q

what is impedance

A

relates acoustic pressure and speed of particle vibration

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16
Q

impedance calculation

A

impedance = density of medium x propagation speed w

17
Q

what is intenisty transmission coefficient T

A

the fraction of the sound which is transmitted

18
Q

what is intensity reflection coefficient R

A

the fraction of the sound intensity which is reflected

19
Q

how are the sound waves generated

A 
the probe (transducer) converts electrical signal to sound waves and vice versa
uses piezoelectric crystals
20
Q

what material is used in ultrasound probes

A

man made ceramin material PLT

lead Zirconate titante

21
Q

how do piezoelectric crystals work

A

electrodes on opposites sides
dioples aligned obliqely
change in current create pressure changes around the crystals causing them to rotate to create compression and rarefraction

22
Q

what is a dipole

A

closely spaced pair of electric charges

23
Q

describe the process of the transmitter

A

electrical drive signal
apply an electrical field
crystals change in physical dimesions
generates an ossiciating acoustic pressure

24
Q

desribe the process of the reciever

A

osicilating acoustic pressure
change in physical dimesions of the crystal
generates an electrical field
electrical signal to display electrons

25
Q

connections in the transducer

A

coacial cables deliver voltage pulses which drive the crystals
two faces of the crystlas are coated with thin conducting films which contact the cable terminations
front electrode is earthed to prevent patient from electric shock

26
Q

what is the baking block

A

absorbs ultrasound from back face

limits crystal vibrations to allow short pulse duration

27
Q

what is the acoustic insulator

A

cork/rubber

prevents ultrasound propagating into the plastic housing and operators hand

28
Q

what is the role of gel

A

elimates layer of air between patient and probe to prevent reflection
prevents an impedance mismatch

29
Q

requirements of an image test

A 
define normal anantomy 
detect variations from normal 
charactise abnormaility 
stage of the disease 
guide interventions
monitor effects of treatment 
screening 
risk vs benefit
30
Q

requirements of an ultrasound

A

compact and portable
cheap
department based but can be moved
range of appliactions

31
Q

describe a low frequency curvilinar imaging probe

A

able to penetrate deep into the tissues of image internal organs, sector shaped image suitable for imaging large areas but still not enough to image whole organ such as liver
curved face

32
Q

describe a high frequency linear imaging probe

A

suitable for imaging surface structures only.
rectanglar shaped image so limited field of view
improves resolution but limits depth of penetration

33
Q

decribe a endo-cavaity/transaginal imaging probe

A

high frequency inserted into body cavity
close to area of interest so high fequency can improve resolution
invasive and uncomfortable

34
Q

doppler ultrasound

A

3 types
colourflow
power doppler
pulsed wave

35
Q

what does a colour flow detect

A

presence of flow
morphology of vasculature
identification of flow abnormalities, turbulance

36
Q

what does a pulsed wave detect

A

quatitive flow information, velocity, pulsatility index, resistance index etc

Medical Imaging (12 decks)

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