Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Huda > Chapter 3 Image Quality > Flashcards

Chapter 3 Image Quality Flashcards

1
Q

why can lesions be seen?

A

they absorb a different number of x-rays compared with background tissues

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

subject contrast

A

difference in x-ray intensity transmitted through a lesion in comparison to adjacent tissues

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what affects subject contrast

A

difference in Z (most important)
difference in density

(for x-ray imaging)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

positive vs negative contrast

A

Positive absorbs more, negative absorbs less

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

does subject contrast guarantee image contrast?

A

No, underexposed film looks all white and displays no image contrast even when there is subject contrast

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what does scattered radiation do to contrast?

A

reduces it

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

image contrast in screen-film radiography

A

difference in film density of a lesion compared to film density of adjacent tissues

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what affects film contrast in screen-film radiography?

A

-film density
-slope (gradient) of characteristic curve

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

what is image contrast in digital radiography?

A

difference in image brightness of a lesion in comparison to image brightness of adjacent tissues

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what is digital contrast proportional to?

A

intrinsic subject contrast of lesion
-also influence by image display that is controlled by the operator
-width of the display window

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

does a wider window show more or less contrast?

A

wide window reduces contrast between different types of tissue

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

how does subject contrast depend on photon energy?

A

low photon energy = high subject contrast

for example, calcified nodules will absorb much more than soft tissues when x-ray energy is low

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

how do you lower photon energy?

A

-reduce kV
-remove filters

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

issue with reducing photon energy

A

higher patient dose
can potentially be impractical due to reduced patient penetration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what is latitude

A

range of radiation intensity (Kair) that result in a satisfactory image contrast (i.e. Kair(max) to Kair(min))

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is dynamic range

A

ratio Kair(max):Kair(min)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

dynamic range of digital detectors vs film

A

digital is 10,000:1
film is 40:1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

how are latitude and contrast related?

A

they are inverse
wide latitude image has low contrast

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

image contrast of a CT lesion in a water background as a function of x-ray photon energy

A

50 keV, soft tissue = 100, iodinated vessle = 100
60 keV, 93, 68
70 keV, 88, 48
80 keV, 84, 37

Z for iodine is 53 vs 7.5 for soft tissue, so increasing the energy has a bigger effect on the contrast

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

what are contrast agents

A

barium, iodine, gases
-improve subject contrast

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

where is barium used?

A

-to see GI tract
k-edge 37 keV

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

how are iodinated contrast agents administered?

A

-intravenously
-arterially

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

ideal tube voltage of angiography

A

70 kV (so average photon energy is around k edge of iodine- 33 kV)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

what kind of contrast agent is air?

A

negative contrast agent. increases subject contrast because it is less attenuating than tissue
-CO2 also sometimes used as contrast agent in angiography

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

what is noise

A

-any content of an image that limits ability to see lesions or pathology
-can be fixed or random

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

example of fixed noise

A

anatomical structures that can inhibit visibility of lesions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

random variations in image intensity are called?

A

mottle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

what is white noise

A

random variations that have a Gaussian-like distribution
-has standard deviation expressed as percentage of the mean value (average pixel values) in a uniformly exposed image area

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

what introduces structure (texture) to noise?

A

blur
image reconstruction

standard deviation doesn’t fully characterize noise when texture is present

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

what creates film granularity

A

in film, the number of silver grains in a given area varies randomly

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

what creates mottle in flat panel detectors

A

scintillator thickness can exhibit random fluctuations, with higher x-ray absorption when thickness increases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

what is digitization noise?

A

when analog signals are digitized, similar signals can be allocated with different digital values

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

when does electronic noise contribute to mottle?

A

when detected signals are low

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

what is quantum mottle?

A

in a uniform x-ray image, adjacent pixels detect a different number of pixels in a random manner
-defined as percentage fluctuations about the mean value
-depends on number of x-ray photons used to produce an image

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

what is dominant source of random noise in most x-ray imaging?

A

quantum mottle
i.e. quantum mottle limited imaging
-includes film, digital radiograpy, analog and digital mammo, fluoroscopy, CT (unless imaging large patients in which case electronic noise may become an issue)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

will increasing scintillator light output reduce noise in a quantum mottle limited detector?

A

No

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

what is the only way to reduce quantum mottle?

A

-increase number of photons used to generate the image
-using more photons (increasing mAs, i.e. Kair) reduces quantum mottle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

issue with increasing mAs to reduce quantum mottle

A

increases patient dose

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

if average number of photons per pixel is 100, what is mottle

A

mottle = standard deviation = 10 % (i.e. square root of 100)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

how do we halve the quantum mottle?

A

quadruple the number of photons used to generate x-ray image

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

how does image receptor detection efficiency affect the amount of noise in an image?

A

-detectors that absorb 50% of the incident photons require twice the Kair at the image receptor to achieve the same level of mottle as detectors that absorb 100 %

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

how can image processing be used to reduce quantum mottle?

A

-average for adjacent pixels in fluoroscopy (binning) will reduce interpixel fluctuations (noise) but also the spatial resolution
-In CT, reconstruction algorithms can reduce image mottle at the price of reduced spatial resolution

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

is contrast affected by changes in mAs?

A

NO

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

why is mottle not usually visible in conventional photos

A

-convetional photos require 10^9 photons/mm2- large number of photons means mottle is very little

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

number of photons/mm2 in digital radiograpy

A

10^5/mm2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

number of photons/mm2 in cardiac cine imaging

A

10^4/mm2

47
Q

number of photons/mm2 in fluoroscopy

A

10^3/mm2

48
Q

number of photons/mm2 in nuc med

A

10/mm2

49
Q

air kerma at image receptor (uGy) and relative image receptor Kair for fluoro, cardiac imaging, photospot and radiography, and mammo

A

fluoro, 0.02, 1
cardiac, 0.2, 10
photospot and radiography 1-3, 50-150
mammo, 100, 5000

50
Q

what is resolution?

A

ability of an imaging system to display 2 adjacent objects as discrete entities
aka spatial resolution, high contrast resolution, detail visibility, sharpness, blur

51
Q

how is resolution quantified?

A

use spatial frequencies in lp/mm or cycles/mm

use parrallel line bar phantoms
-strips of lead absorb the x-rays and appear white whereas gaps transmit the x-rays and appear black

52
Q

if a bar phantom has 0.5 mm lead bars separated by 0.5 mm of material, what is its resolution?

A

1 lp/mm

53
Q

what is limiting spatial resolution?

A

max # of lp/mm that can be recorded by the imaging system

54
Q

what resolution can human eye resolve

A

5 lp/mm

55
Q

what affects spatial resolution in x-ray imaging?

A

focal spot size, patient motion, detector blur

56
Q

what does finite size of focal spot create?

A

blur
-blurred margin is penumbra

57
Q

what are focal spot penumbra

A

produced by x-rays arriving from slightly different places in the focal spot

58
Q

increasing focal spot size increases blur?

A

yes

59
Q

why use a larger focal spot?

A

-permit higher power loadings and thus reduce exposure time, at expense of increased blur

60
Q

how does focal spot blur change with magnification?

A

-it increases with increasing magnification
-thus, in magnification readiography, it is vital to use smaller focal spots

61
Q

is there focal spot blue in contact radiography?

A

No, because no geometric magnification
-also, focal spot blur is minimal in extremity radiography because geometric magnification is very small

62
Q

how does patient motion cause blur?

A

smears out object in the image
-gross movement
-involuntary organ movement

63
Q

how can patient motion be reduced?

A

-immobilization devices like compression paddle
-increase mA to reduce exposure time and minimize motion
-increase kV to reduce exposure if there is no alternative, but this will reduce contrast and increase scatter radiation

64
Q

does motion blur depend on magnification?

A

NO

65
Q

how does scintillator thickness affect spatial resolution?

A

Light spreads out prior to detection with the spreading increasing with thickness

66
Q

how do pixels affect resolution?

A

using discrete pixels introduces sampling
-large pixels= blurred edges

67
Q

what is sampling frequency

A

number of pixels in each mm

68
Q

what is pixel size for 500 matrix along a 250 mm diameter?

A

0.5 mm

69
Q

how to calculate sampling frequency?

A

1/ pixel size

70
Q

what is limiting spatial resolution

A

half the sampling frequency (Nyquist frequency)

71
Q

what is the limiting resolution for a sampling frequency of 2 pixels/mm?

A

1 lp/mm (1 white and 1 black pixel)

72
Q

highest spatial frequency in an object that can be faithfully reproduced

A

Nyquist frequency

73
Q

what happens if there are higher spatial frequencies than the Nyquist frequency

A

aliasing
arises from insufficient sampling

74
Q

limiting resolution of digital mammo

A

7 lp/mm

75
Q

limiting resolution of extremity radiography

A

5 lp/mm

76
Q

limiting resolution of chest radiography

A

3 lp/mm

77
Q

digital photospot limiting resolution

A

2 lp/mm

78
Q

limiting resolution of fluoroscopy

A

1 lp/mm

79
Q

limiting resolution CT

A

0.7 lp/mm

80
Q

what is line spread function?

A

image of a narrow line function
width is a measure of blur or resolution

81
Q

where do you measure width of line spread function?

A

FWHM
larger width = worse resolution

82
Q

what can be used to measure narrow LSF (< 1 mm)

A

bar phantom

83
Q

where are wide LSFs used?

A

nuclear medicine

84
Q

how do you calculate limiting spatial resolution (lp/mm) from FWHM?

A

1/(2FWHM)

85
Q

what does modulation transfer function describe?

A

resolution capability of any imaging system
ratio of output to input modulation (signal amplitude) at each spatial frequency

86
Q

why is output modulation less than input modulation?

A

because of blue introduced by focal spot, motion, receptor size

87
Q

how is blur impacted by spatial frequency?

A

blur becomes more important as spatial frequency increases (objects of interest get smaller)

88
Q

MTF of low and high spatial frequencies

A

-at low spatial frequency, MTF is about 1 (good visibility of large features)
-at high spatial frequency, MTF goes to 0 (poor visibility of small features)

89
Q

what is meant by image quality being task dependent?

A

in the absence of any defined imaging task, it is not possible to determine image quality

90
Q

what determines the visibility of a lesion?

A

amount of lesion contrast
amount of image mottle
-if lesion contrast < image mottle, cannot detect lesion

91
Q

contrast to noise ratio

A

quantifies relative visibility of any lesion

92
Q

does mAs impact CNR?

A

yes because increasing mAs = less noise and same amount of contrast so CNR increases

93
Q

what does raising kV do?

A

-increases x-ray output and patient penetration, reducing mottle
-reduces lesion contrast

-since both mottle and contrast are reduced, the effect on CNR is indeterminate
-most likely to reduce CNR of high Z lesions but leave CNR of low Z lesions unchanged

94
Q

what features of the lesion does SNR account for?

A

contrast and size
-also image blur, noise, and response of the human visual system

95
Q

likelihood that a lesion will be detected by a human observer is characterized by?

A

SNR

96
Q

difference between SNR and CNR

A

SNR is absolute measure of visibility of a lesion
CNR is relative measure that only takes into account contrast and noise

SNR > 5 means lesion will be detected
CNR has no meaning, increases or decreases just mean relative improvement or degradation

97
Q

true positives

A

positive test results in patients who have the disease

98
Q

true negative

A

negative test results in patients who do not have the disease

99
Q

false positives

A

positive test results in patients who do not have the disease

100
Q

false negatives

A

negative test results in patients who have the disease

101
Q

sensitivity (true positive fraction)

A

TP/(TP+FN)
A sensitive test has a low false negative rate

102
Q

specificity (true negative fraction)

A

TN/(TN+FP)
A specific test has a low false positive rate

103
Q

accuracy (fraction of correct disgnosis)

A

(TP+TN)/(TP+FP+TN+FN)

104
Q

positive predictive value

A

probability you have the disease given a positive result
TP/(TP+FP)

105
Q

negative predictive value

A

probability not having the disease given a negative result
TN/(TN+FN)

106
Q

Diagnostic performance

A

depends on disease prevalence:
(TP+FN)/(TP+FP+TN+FN)

107
Q

ROC

A

receiver operator characteristic curve
-plot of sensitivity vs false positive fraction (1-specificity) as the treshold criterion is relaxed
-“strictest” criterion has sensitivity and false positive fraction at 0, “lax” criterion has both metrics at 1
-have to increase sensitivity while minimizing false positives

108
Q

“under-readers”

A

low sensitivity and low false positives

109
Q

“over-readers”

A

High? specificity and high false positives

110
Q

AUC

A

area under ROC curve
measures overall imaging performance

111
Q

ROC curve for random guessing

A

straight line through 0,0 and 1,1, AUC = 0.5
-as performance improves, AUC increases with maximum of 100 %
i.e. want high sensitivity for low false positives (fill the upper left part of curve!)

112
Q

focal spot blur is best minimized by increasing the…?

A

SID

113
Q

best indication that a lesion can be seen is given by SNR or CNR?

A

SNR

Huda (20 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions