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Huda > Chapter 9 Fluoroscopy > Flashcards

Chapter 9 Fluoroscopy Flashcards

1
Q

power and currents for fluoro

A

3 mA
< 0.5 kW

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2
Q

focal spot size

A

0.6 mm

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3
Q

beam quality

A

80 kV, 3 mm Al HVL
use 3 mm Al filtration
pediatric fluoro will have additional 0.1-0.2 mm Cu filtration to reduce pediatric dose

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4
Q

grid ratio

A

10:1

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5
Q

what do image intensifiers do?

A

conert incident x-rays into bright light images

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6
Q

parts of II

A

evacuated envelope made of glass or aluminum
-input phosphor
-photocathode
-electrostatic focusing electrodes
-output phosphor

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7
Q

what does input phosphor do

A

absorbs x-ray photons and re-emits part of them as light photons

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8
Q

what does photocathode do?

A

absorbs light photons from input phosphor and emits photo-electrons

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9
Q

what does the elctrostatic lens do

A

accelerates the electrons to high energies and focuses them onto output phosphor

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10
Q

size of input phosphor vs output phosphor

A

output = 2.5 cm
input = 25 cm

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11
Q

what does output phosphor do?

A

-absorb electrons and emit light photons

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12
Q

flux gain

A

of light photons emitted at output phosphor for each photon emitted at input phosphor

-usually ~ 50

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13
Q

what is minification gain

A

increase in image brightness that results from reduction in image size from input phosphor to output phosphor

-100 for input phosphor of 25 cm vs output phosphor of 2.5 cm (area is 100 X smaller)

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14
Q

what is brightness gain

A

product of flux gain and minification gain
~ 5000

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15
Q

what does reducing area of input phosphor do?

A

-reduces minification gain and brightness gain

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16
Q

how do TV systems build up images?

A

-series of horizontal lines (raster scanning)
-500 or 1000 lines

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17
Q

progressive vs interlaced raster scanning

A

progressive: each line is read sequentially
interlaced: odd lines read first, then even lines

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18
Q

standard TV display frequency

A

30 frames/s

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19
Q

how many TV lines does fluoro use

A

500

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20
Q

CCD vs TV in fluoro

A

CCD is cheaper
-similar levels of mottle because fluoro imaging is quantum mottle limited

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21
Q

how do we get digital fluoro?

A

analog voltage signal from TV or CCD is digitized using analog to digital converter

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22
Q

number of pixels in 500 line TV frame

A

1/4 pf a million pixels
-2 bytes/pixel
-0.5 MB/frame

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23
Q

storage space required for 5 minutes of fluoro at 30 frames/s

A

4.5 Gb
9000 images

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24
Q

last image hold

A

lets you look at last acquired image when the x-ray is switched off

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25
Q

what is temporal filtering

A

-frame averaging
-occurs in real time
-reduces random noise
-only available for digital fluoro
-can introduce lag of some objects

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26
Q

definition of fluoro

A

view dynamic images in real time

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27
Q

fluoro acquisition rate vs image display rate

A

acquire at 15 frames/s but display at 30 frames/s
display each frame twice to avoid a flicker

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28
Q

why are fluoro images not of diagnostic quality?

A

of photons used to create image is a hundred times lower than in radiographic imaging

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29
Q

uses of fluoro

A

GI study- see barium given to patients
-GU exams- see iodine given to patients

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30
Q

pros and cons of overhead x-ray tubes

A

-minimizes magnification of kidneys but gives more operator doses

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31
Q

fluoroscopy table FDA requirement

A

< 2 mm Al equivalence
-the table attenuates about 1/3 of x-ray beam

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32
Q

portable fluoro

A

C-arm devices

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33
Q

tube voltage when iodine contrast is used

A

70 kV, so that avg x-ray photon energy is close to iodine k-edge (33 kV)
-maximizes absorption of iodine so you can see the vasculature

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34
Q

tube voltage for GI studies (barrium)

A

high voltage > 100 kV is used to make sure there is some penetration of the barium in the GI
-collimation is used to reduce scatter, improve contrast, reduce patient dose

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35
Q

exposure times in fluoro

A

minutes, vs < 0.1 s in radiography

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36
Q

fluoro is _ whereas radiography is _

A

dynamic
static

37
Q

how many images does a fluoro test usually take?

A

tens of thousands

38
Q

mottle in radiography vs fluuro

A

10 X less mottle in radiography because tube currents are 100 times higher

39
Q

spot films

A

conventional radiographs obtained by introducing an overhead x-ray tube to expose a digital detector

40
Q

photospot images

A

diagnostic quality images that are obtained through the fluoroscopic imaging chain

-tube currents increase from a few mA to several hundred mA

41
Q

focal spot for photospot imaging

A

1.2 mm, to minimize exposure time

42
Q

TV lines for photospot imaging

A

1000 lines, double the resolution of fluoro image with 500 lines

43
Q

number of pixels in photospot

A

a million pixels
2 bytes/pixel
2 MB

44
Q

electronic magnification

A

irradiates smaller diameter of input phosphor but maintains same output size
-reduces minification gain

45
Q

automatic brightness control

A

-used in magnification mode
-increases radiation incident on input to maintain constant brightness at output

Mag I halves the exposed area, doubles Kair incident on II
-Kair quadruples in Mag II and increases by 8 in Mag III

46
Q

is kerma- area product affected with magnification?

A

No, because area is reduced but Kair is increased due to automatic brightness control

47
Q

what does electronic magnification do to peak skin dose?

A

-increases it (ex. doubles in Mag 1)

48
Q

patient stochastic risk with magnification mode

A

not increased, because total radiation incident on patient isn’t changed

49
Q

pulsed fluoro

A

-in normal fluoro, tube current flows continuously
-using pulsed reduces motion blur for same patient dose
-
-tube current is on for only a short time, but increased to maintain the same mAs for each frame as in continous fluoro

50
Q

does pulse fluoro reduce patient dose?

A

-only when acquiring frames at < 30 frames/s (ex. 15 frames/s)
-when number of acquired images is 15 frames/s, each image is displayed twice so there is no flicker

51
Q

how much does switching pulse fluoro from 30 fps to 15 fps reduce patient dose by?

A

35 % not 50%
because 15 fps uses a higher dose per frame to reduce the perceived level of random noise
-increases radiation per frame by ~ 30%

52
Q

modes of radiation intensity in fluoro

A

low, normal, high
Kair at low ~ 50% of normal, 50% higher than normal at high

use low mode when looking at high contrast region (mottle less important) and high mode when looking at low contrast lesions (mottle must be reduced)

53
Q

what happens to brightness with collimation?

A

it is reduced because areas outside of collimation will be dark
-collimation doesn’t activate ABC
-Kerma-area product is reduced

54
Q

why use collimation?

A

-reduces patient stochastic dose
-improves image quality because of reduced scatter
-does not affect patient peak skin dose

55
Q

what does AEC do

A

-AEC adjusts radiation to keep constant value of Kair at image receptor

56
Q

difference between AEC and ABC

A

AEC keeps Kair at image receptor constant irrespective of x-ray beam area whereas ABC alters Kair only when eletronic magnification is changed

57
Q

compare fluoro with electronic magnification vs collimation

A

electronic magnification:
-ABC is on
-Kair is increased
-kerma area product stays same
-resolution improves

collimation:
-ABC is off
-Kair is unchanged
-kerma-area product is reduced
-resolution is unchanged

58
Q

2 ways to get more radiation on a larger patient

A

increase mA:
-doesn’t affect HVL and maintains contrast
increase kV:
-reduce contrast

-patients get lower doses from increase in kV vs increase in mA

-systems have several AEC curves to help optimize dose and contrast for larger patients

-usually increase both mA and kV
-when mA is increased more than kV, patient dose and lesion contrast are higher than when kV is increased more than mA

59
Q

US limit on fluoro entrance Kair rate

A

100 mGy/min
-this may be inadequate image quality on very large patients
-high level control allows max rate to be increased to 200 mGy/min

-limit doesn’t apply to photospot or cardiac cine images

60
Q

contrast agents during fluoro

A

iodine
barium
air

air is negative constrast agent (appears black)

61
Q

what is veiling glare

A

light scatter in output phosphor
reduces contrast

62
Q

Kair at image receptor

A

0.01- 0.03 uGy/frame
-varies with magnification and changes in frame rate

-diagnostic imaging (spot, photospot) is 1-3 uGy/image

63
Q

what is used to assess fluoro performance

A

contrast detail phantoms
deep holes and shallow holes
deep holes are typically seen even when small, shallow holes are harder to see even when large

64
Q

lesion contrast in photospot image vs fluoro

A

IDENTICAL
because contrast is independent of mA

HOWEVER, CNR improves because there is less noise

65
Q

limiting resolution of II

A

5 lp/mm
-removing TV yields full resolution

66
Q

what determines II resolution?

A

input phosphor CsI thickness, which can be made of thin columns to limit the spread of light

67
Q

resolution of II when viewed through TV camera

A

reduced from 5 lp/mm to 1 lp/mm with 500 line TV and 2 lp/mm with 1000 line TV

68
Q

how to improve fluoro resolution

A

use electronic magnification
halving field of view doubles spatial resolution

69
Q

resolution in photospot imaging with II

A

2 lp/mm
1000 line TV image is digitized

70
Q

resolution in spot imaging (conventional radiography)

A

-dtermined by detector pixel size
-3 lp/mm

71
Q

why is the input window curved?

A

-permits window to be thin, minimizing absorption of incident x-rays

72
Q

geometric distortion

A

-input window is cruved
-projecting this surface onto a flat output phosphor results in geometric distortions

73
Q

pincushion distortion

A

straight lines appear curved
-less concern with small field size

74
Q

vignetting artifact

A

fall-off in brightness at periphery of II field
-less concern with small field size

75
Q

S-distortion

A

-due to electron paths being affected by earth’s magnetic fields- distortions vary as the II rotates

76
Q

Glare

A

-occurs when there is significant increase in x-ray transmission
-happens at tissue interfaces
-visibility in higher transmission area is reduces because of light scattering in the output phosphor (veiling glare)

77
Q

saturation

A

occurs when II has reached its peak value and appears as uniform white intensity in image

78
Q

lag

A

moving object appears smeared out

79
Q

entrance Kair in normal sized adult

A

10 mGy/min

80
Q

entrance Kair in normal sized adult for photospot or spot image

A

1-3 mGy

81
Q

how does increasing patient thickness by 3 cm affect entrance Kair rate

A

doubles it

-reducing patient thickness by 3 cm would halve entrance Kair rate when quality kept constant

82
Q

Kerma area product (Gy-cm^2) for exams in
eophagus
upper GI
Ba enema
cystogram

A

esophagus 10
upper GI 15
Ba enema 40
cystogram 15

83
Q

average kerma area product

A

20 Gy-cm^2, with all exams within factor of 2 of this benchmark

-fluoro studies use KAP 20X higher than correspondin g values in radiography

84
Q

typical patient effective dose in fluoro-guided exam

A

4 mSv
-Ba swallows, upper GI studies have lower doses
-Barium enemas have higher doses
-GI and urological studies are moderate dose

85
Q

size of one frame of digital fluoro

A

0.5 MB

86
Q

for fluoro units at the floor, what % of incident beam is attenuated by the table?

A

30%

87
Q

% of image intensifier phosphor area exposed for each mode

A

normal: 100%
Mag 1: 50%
Mag 2: 25%

88
Q

max allowed Kair in standard fluoro

A

100 mGy/min

89
Q

rule of thumb for dose from photospot images

A

1 min of fluoro (10 mGy/min) delivers same patient dose as 10 photospot images (1 mGy/image)

Huda (20 decks)

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