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Huda > Chapter 18 Ultrasound II > Flashcards

Chapter 18 Ultrasound II Flashcards

1
Q

lines of sight

A

Each US pulse provides info for a single line of sight
Images are built up by generating a large number of lines of sight that are sequentially directed to cover the ROI in a patient

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2
Q

pulse repetition freqency

A

number of separate pulses (i.e. lines of sight) sent out every second

product of frame rate and lines per image

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3
Q

common pulse repetition frequency

A

4000 pulses/s

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4
Q

image frame rates

A

~ 30 frames/s

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5
Q

line density

A

of lines per image/ FOV

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6
Q

what does increasing line density do?

A

improves lateral resolution

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7
Q

how can line density be increased?

A

reduce frame rate
reduce FOV (but limits region of patient that can be seen)
increase pulse repetition frequency

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8
Q

what does reducing frame rate do?

A

will increase line density: improve lateral resolution but reduce temporal resolution

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9
Q

what does increasing pulse repetition frequency do?

A

increase line density which improves lateral resaolution, but also reduces listening intervals and thus decreases imaging depth

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10
Q

duration of each pulse

A

~ 1 us

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11
Q

listening interval

A

interval between pulses

transducer acts as a receiver

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12
Q

listening interval for 4 kHz pulse repetition frequency

A

250 us

increasing the frequency is a reduced listening interval fpr echo detection and vice versa

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13
Q

how is depth of interface producing the echo dtermined?

A

by the time interval between the emitted pulse and the returning echo

for v=1540 m/s a return time of 13 us is a depth of 1 cm (return trip of 2 cm)

return time of 26 us is depth of 2 cm
etc

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14
Q

different echo listening times and penetration depths for 4 kHz, 6 kHz, and 8 kHz PRF

A

4 kHz- 250 us- 20 cm
6 kHz - 167 us- 13 cm
8 kHz- 125 us- 10 cm

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15
Q

what would uncorrected echo data do?

A

show distant echoes as being much weaker than superficial echoes due to attenuation

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16
Q

how to US scanners compensate for increased attenuation with depth?

A

increase signal gain as the echo return time increases

```
depth gain compensation
time gain compensation
time varied gain
swept gain
all mean the same thing
~~~

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17
Q

what does the intensity of returning echoes along a line provide info about

A

differences in acoustic impedances between tissues

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18
Q

A mode imaging

A

depth on horizontal axis
echo intensity on vertical axis

ophthalmology uses A mode imaging

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19
Q

T-M mode imaging

A

time-motion
time on horizontal axis and depth on vertical axis

-displays time- dependent motion, valuable for studying rapid movement (cardiac)

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20
Q

B-mode

A

echo intensity is displayed as brightness value (B) along each line of sight

used for M -mode and 2D gray scale imaging

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21
Q

what do scan converters do?

A

compute 2D images from echo data from distinct beam directions which are subsequently displayed on a monitor

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22
Q

US image data and storage

A

512x512 matrix
1 byte per pixel

-each frame contains 0.25 MB of info

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23
Q

what is used for abdo imaging

A

convex arrays at ~ 4 MHz

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24
Q

what is used for superficial imaging

A

linear arrays at ~ 10 MHz

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25
Q

what is used for gyne and pelvic imaging

A

endo-array prove

transrectal

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26
Q

what is used for ped US

A

smaller footprint transducer

> 7 MHz

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27
Q

what is used for transcranial imaging?

A
  • through acoustic windows through the skull such as temples or eyes
  • 2 MHz
  • phased arrays
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28
Q

1.5 D arrays

A
  • lots of transducers in scan plane, small number in slice thickness direction
  • focusing the small number transducer elements can be used to reduce the slice thickness and improve elevational resolution
  • comparable lateral and elevational resolution
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29
Q

2D arrays

A
  • can do volume averaging
  • instead of sound waves being sent straight down and reflected back, they are sent at different angles
  • returning echoes are processed and yield a 3D volume image
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30
Q

4D fetal US

A

3D picture in real time

don’t have the lag associated with computer constructed image

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31
Q

spatial compounding imaging

A
  • multibeam imaging
  • combines multiple lines of sight to form a single composite image
  • echoes from the different directions are averaged together into a single composite image
  • reduces angle-dependent artifacts and clutter, improves contrast and margin definition
  • corners receive a subset of views compared to the center, which reduces image quality
  • used for breast, peripheral blood vessels, musculoskeletal injuries
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32
Q

extended FOV US

A

uses static B-mode to permit large subject area to be viwed on single static image

  • as images are acquired, they are stitched together
  • results in single slice image covering the whole area of interest

-useful when you need to see a large patient area and there is limited motion

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33
Q

harmonic imaging

A
  • requires broadband transducers
  • receives signals at twice the transmit frequency
  • reduces artifacts and clutter
  • good for patients with thick and complicated body wall structure
  • first harmonic (twice the frequency) is used- higher harmonics have too much attenuation
  • high frequencies arise from non-linear interactions of US with tissues
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34
Q

cardiac frequencies for harmonic imaging

A

transmit at 1.5 to 2 MHz, receive at twice that

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35
Q

US contrast agents

A
  • vascular and perfusion imaging
  • encapsulated microbubbles
  • micorbubbles produce harmonic frequencies
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36
Q

pulse inversion harmonic imaging

A
  • uses 2 pulses, standard and phase reversed
  • 2 pulses cancel out for normal tissue but not for microbubbles (contrast agents)

-in echocardiography, harmonic imaging has reduced imaging artifacts due to reverberations

37
Q

reverberation

A

large number of reflected waves, which can be perceived as continuous sound

38
Q

what is doppler effect?

A

changes in frequency resulting from a moving sound source

-objects moving toward detector reflect sound at higher frequencies and vice versa

39
Q

what is the shift in frequency in doppler effect proportional to?

A

cos(theta)
theta is angle between US beam and moving object
max is for 0 or 180 degrees
at 90 degrees there is no doppler shift

also proprtional to reflector velocity

40
Q

does doppler measure reflector velocity?

A

no, just change in frequency

41
Q

frequency shift for moving blood for 2 MHz transducer

A

260 Hz for 10 cm/s
780 Hz for 30 cm/s
2600 Hz for 100 cm/s

42
Q

frequency shift for moving blood for 5 MHz transducer

A

650 Hz for 10 cm/s
2000 Hz for 30 cm/s
6500 Hz for 100 cm/s

43
Q

blunt flow

A
  • in blood vessels

- constant flow across most of the cross-sectional area and reduced flow near the vessel walls

44
Q

what happens to velocity of blood as a blood vessel narrows?

A

velocities increase

45
Q

when can turbulent flow occur in blood vessel

A

when vessel is disrupted by plaque and stenoses

46
Q

how is doppler US used to evaluate blood flow in vessels?

A

based on backscatter of blood cells

47
Q

what does pulsed doppler provide?

A
  • frequency shift

- also depth information

48
Q

PRF values in doppler US

A

8 kHz

49
Q

duplex scanning

A

combines real time imaging with doppler detection

50
Q

B-mode images vs doppler

A

-Bmode images give info on stationary reflectors

Doppler shifts give info on flow present

51
Q

spectral analysis

A

frequency shift as a function of time

clinical conditions have distinct spectral waveforms

52
Q

brightness value

A

intensity at a given frequency shift at any moment in time

53
Q

what PRF must be used to avoid aliasing in doppler spectral analysis?

A

2X highest doppler frequency shift

can also avoid aliasing by adjusting spectral baseline

54
Q

color doppler

A

2d display of moving blood with frequency shifts encoded as colors
-flow info is provided by taking the average value of a number of samples obtained from each pixel
-gives info on direction and magnitude of flow over a selected ROI
-red = towards transucer
blue = away from transducer
-turbulent flow is green or yellow

55
Q

where is color doppler info displayed?

A

on top of B-mode image

56
Q

twinkle artifacts

A

in color doppler

occur behind a strong attenuator and show fuclutating reds and blues in tissues which have no flow

57
Q

flash artifact

A

in color doppler

fill color box with sudden burst of color due to sudden movement of patient or transducer

58
Q

power doppler

A
  • uses same info as color doppler
  • where color doppler would see 45 negative and 45 positive shifts as net 0 (no movement), power doppler would see it as 90 frequency shifts (i.e. magnitude)
  • power doppler color doesn’t vary with direction of flow
  • power doppler doesn;t have aliasing artifacts
  • power doppler is more sensitive than color doppler and is used to detect slow blood flow
59
Q

axial resolution

A

ability to seperate 2 objects lying along the axis of the beam

  • determined by pulse length
  • indepedent of depth

-~ 1/2 of pulse length, which is the US wavelength in tissue

60
Q

how to improve axial resolution?

A

increase transducer frequency to reduce pulse length

61
Q

axial resolution at 1.5 and 15 MHz

A

1 mm

0.1 mm

62
Q

what two quality items are always trade-offs in US imaging?

A

spatial resolution and imaging depth

63
Q

lateral resolution

A

ability to resolve 2 separate adjacent oibjects

64
Q

what determines lateral resolution in US?

A

US beam width
narrow beam = better resolution

increasing number of lines per frame also improves lateral resolution

65
Q

lateral resolution compared to axial resolution

A

lateral is 4X worse than axial

-lateral usually becomes worse at greater distance from transducer

66
Q

elevational resolution

A

plane perpendicular to image plane

  • aka slice thickness
  • depends on height of transducer elements
  • can be improves using 1.5D arrays
67
Q

assumptions in US (that can lead to artifacts)

A

echo depth is proportional to echo time
sound travels in straight lines
attenuation is uniform

68
Q

echoes reflected from side lobes and grating lobes

A

create artifacts

69
Q

mirror image artifact

A

sound is relfected off of a large interface, causing parts of the image to be in the wrong location

70
Q

speed displacement artifacts

A

caused by variability of speed of sound in different tissues

71
Q

reverberation echoes

A

multiple reflections occuring from 2 adjacent surfaces

72
Q

comet artifacts

A

occur when gap between reverberation echoes becomes very small, and reflections form one comet structure

73
Q

ring-down artifacts

A

show a streak like appearance from misture of gas (air) and fluids

74
Q

acoustic shadowing

A

reduced echo intensity behind a highly attenuating or reflecting object, creating a shadow

75
Q

acoustic enhancement

A

increased echo intensity behind a minimally attenuating object

76
Q

when can shadowing and enhancement be helpful?

A

provide clues to identify anatomical features like cysts and gallstones

77
Q

tissue mimicking phantoms

A

used to QA US
identify axial, lateral, elevational resolution
uniform phantoms can identify faulty transducers, nonuniformities, extraneous sources of noise

-can help differentiate issues with image formation vs image display

78
Q

ACR accreditation program

A

for US and breast US

  • QA results must be documented
  • most common issues relate to display on maladjusted monitors

-includes tests on uniformity, physical and mechanical inspection, geometric accuracy, US display, diagnostic display

79
Q

spatial peak intensity

A

highest in beam

80
Q

temporal average intensity

A

averaged over pulse and listening time

81
Q

intensities in B mode, M mode, Doppler

A

B-mode: 10 mW/cm2
M-mode: 4 times higher
doppler: 50 times higher

82
Q

what is cavitation

A

creation and collapse of microscopic bubbles

-can be caused by US

83
Q

mechanical index

A
  • estimates change of inducing cavitation

- gas-containing structures and contrast agents are more susceptible to effects of acoustic cavitation

84
Q

thermal index

A

predicts rise in tissue temperature

-TI of 1 increases tissue temperature by 1 degree celcius

85
Q

how many lines of sight make up an US image?

A

~ 100

86
Q

transcucer frequency for breast imaging

A

10 MHz

offers good axial resolution and can penetrate a typical compressed breast thickness (6 cm)

87
Q

where is extended FOV imaging most likely used?

A

MSK

88
Q

is temporal resolution independent of pulse length?

A

yes

-affected by changes in PRF, line density, and FOV

89
Q

US intensity refers to?

A

spatial peak intensity and temporal average intensity

Huda (20 decks)

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