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hobson contrast > contrast grid > Flashcards

contrast grid Flashcards

1
Q

contrast

A
  • 2nd photographic factors that allows detail to be visible (seen)
  • Difference between adjacent densities (attenuating pattern)
  • most difficult to evaluate
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2
Q

Visibility of detail

A

image is visible to the human eye only because sufficient contrast (and IR exposure/density) exists to permit the structural details to be seen

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3
Q

Contras in CR and DR

digital

A
  • dynamic range

- adjusted by changing window width

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4
Q

image contrast

A

Difference between adjacent densities/image receptor exposure levels.
Total amount of contrast from the IR and the anatomic part

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5
Q

dynamic range

A
  • range of brightness as display in the monitor

- describes the concept of contrast for digital images

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6
Q

terms to describe High contrast

A

Short scale

Short/narrow dynamic range

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7
Q

term used to describe Low contrast

digital

A

Long scale

Large/wide dynamic range

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8
Q

High Contrast differences

A 
Few Shades of gray
More
Increased
Short Scale
Low kVp*(depends on exam)
Short(narrow) dynamic range
Narrow window width
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9
Q

Low Contrast (differences)

A 
Many shades of gray
Less
Decreased
Long Scale
High kVp* ( depends on exam)
Large (wide) dynamic range
Wide window width
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10
Q

good contrast

A
  • fulfil the purpose of the procedure

- should demonstrate all the structural differences that the body part has

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11
Q

scale of contrast

A

Number of useful, visible IR exposure values/density levels or shades of gray

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12
Q

Short scale

A

maximal differences between IR exposure values. Minimum number shades of gray
(whale)

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13
Q

Long scale

A

minimal differences between IR exposure values. Maximum number of shades of gray
(dolphin)

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14
Q

manipulating contrast (film)

A

Change in D log E curve of film

Adjustment to kVp (film*)

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15
Q

manipulating contrast (CR /DR)

A

Adjustment of window width

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16
Q

Controlling factors (DR)

A
  • Window Width
  • Histogram
  • Look-up-Table (LUT)
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17
Q

Image receptor (film) factors

A 
Range of densities film can record
 4 factors:
 Use Intensifying screen
 Film density (overall blackening)
 Slope of D log E curve (density curve)
 Processing
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18
Q

using intensifying screen

A

contrast increases

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19
Q

Slope of D Log E curve (speed or sensitivity)

A

speed or sensitivity : measures the film’s

ability to respond to light or radiation

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20
Q

Slope of D Log E curve (latitude)

A

(how much can we mess up)
is the range of log relative exposure
values that will produce densities in the diagnostic range. Determined by the
composition of the film’s emulsion.

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21
Q

diagnostic range

A
  1. 25: anything below too light

2. 5: anything above too dark

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22
Q

toe

A

when curve starts going up

minimum density

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23
Q

shoulder

A

when curve start getting horizontal, maximum density

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24
Q

without Density

A

without proper density/IR exposure contrast cannot be evaluated

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25
Q

Processing

A

increase in time, temperature or replenishment rate will increase chemical fog. fog decreases contrast (decreases slope of the curve)

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26
Q

eliminate or minimize fog

A

increase contrast

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27
Q

Correctly exposed film range

A

all densities will fall within the visible range on the d log curve. (0.25 to 2.5)

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28
Q

what affects Digital Image Receptor Contrast

A

both Histogram and LUT affect final image

kvp still important

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29
Q

use of kVp in digital IR contrast

A 
Use of proper kVp still important
_ kVp controls subject contrast because it 
  controls how the beam will be
  attenuated by the anatomy 
- Need differential attenuation through 
  the patient for any given exam
- Produces signal differences to the digital 
  detector
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30
Q

Subject contrast

A
  • subject means the patient
  • how much radiation transmitted by a body part and how that body part absorbs the radiation that passes through it , giving us characteristics of the tissues and structures making up that body part.
  • how the beam interacts with the body
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31
Q

Kilovoltage and contrast

A

Inversely related
As kVp increases, contrast decreases (low contrast)
As kVp decreases, contrast increases (high contrast)

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32
Q

kvp and subject contrast

A

Primary controller of subject contrast
kVp controls energy of photons in the beam
Energy of the photons determines attenuation and the type of interaction

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33
Q

As kVp increases what happens?

A 
we have a wider range of photon energy 
Greater penetrability
Greater range of exposures
Greater amount of scatter
Longer scale of contrast
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34
Q

Increase in thickness and field size

A
  • Increase scatter

- Longer scale of contrast

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35
Q

Tissue density

A

refers to how tightly the atoms of a substance are packed together

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36
Q

High atomic number and tissue density

A

Greater attenuation—high contrast

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37
Q

Large difference in atomic number and tissue density of adjacent tissues

A

high contrast

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38
Q

Increase in thickness and field size

A

Increase scatter

Longer scale of contrast

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39
Q

Any factor that increases the production of scatter/fog

A

decrease contrast

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40
Q

As kVp↑

A

fog/scatter↑ contrast↓

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41
Q

↓ Collimation/Beam Restriction

A

↓contrast

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42
Q

↓Grid Ratio

A

↓contrast

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43
Q

Digital Image Receptors

A
  • dynamic range describes contrast as it is displayed on a monitor for digital images
  • range of brightness on the display monitor
  • number of density values displayed on image on monitor
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44
Q

window width

A

describes digital processing the produces changes in brightness
- how many shades of gray we able to see in image

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45
Q

Grayscale bit depth

A
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46
Q

Digital Imaging steps

A
  • Image data is acquired from the exposure
  • Data is electrical signal (analog)
  • Electrical signal sent to ADC and converted into digital or numerical data
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47
Q

Digital image is recorded as

A

a matrix of small picture elements (pixels)

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48
Q

Each pixel is recorded as

A

a single numerical value, represented as a single brightness level on the monitor

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49
Q

Location of the pixel in an image

A

an area within the patient or a volume of tissue

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50
Q

Matrix

A

Made up of pixels and voxels

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51
Q

Field of view (FOV)

A

what we looking at

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52
Q

Image quality in digital is improved with

digital

A

larger matrix size which will result in greater number of pixels and smaller pixels

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53
Q

Numerical value assigned to each pixel is determined by

A

the way the body part attenuates the x-ray beam

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54
Q

If the photons are highly attenuated or absorbed would result
(digital)

A

in pixels being assigned a low numerical value = higher brightness on monitor (lighter or less density)

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55
Q

Photons passing through tissue with low attenuation

A

would be assigned a higher numerical value, resulting in less brightness on the monitor (darker or more density)

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56
Q

Each pixel also has

A

a bit depth, or number of bits

57
Q

Number of bits determine

A

by the ADC (analog to digital conversion)

-the accuracy of the digitized analog signal so controls the exact pixel brightness level or gray level

58
Q

Larger bit depth allows

A

more shades of gray to be displayed on the computer monitor

59
Q

A 12 bit system would be capable of displaying

A

4096 shades of gray

60
Q

Increase number of shades of gray available

A

improves contrast resolution

61
Q

pixel bit depth

A
  • Determines number of density values
  • Affects density and contrast
  • Controlled by ADC
62
Q

bit systems

A

-10 bit (210 = 1024)
-12 bit (212 = 4096)* standard for diagnostic
radiography
-16 bit (216 = 65,536)

63
Q

Histogram

A

-A data set in a graphical form
-Graph of the pixel digital values versus those
values in the image
-used to eliminate unnecessary information
outside the collimated field
-compared to LUT

64
Q

Pixel values on

A

x axis

65
Q

number of pixels with that brightness value on

A

y-axis

66
Q

Far left of graph represents

A

minimum useful signal (metal, bone)

67
Q

Far right represents

A

maximum useful signal (skin line, air/gas)

68
Q

Soft tissues are recorded (histogram)

A

near center

69
Q

Look up table data

A

-“Ideal” histogram or reference histogram

stored in computer

70
Q

LUT

A

-final step in imaging processing
-Controls the visible contrast of the image on
the display monitor
-is the primary factor affecting contrast in
digital imaging

71
Q

why kvp still affects histogram

A

-controls subject contrast
-Proper kVp assures the desired differential
attenuation through the patient (subject
contrast)

72
Q

Contrast resolution

digital

A

to distinguish between small objects that attenuate the beam in a similar way

73
Q

Changing Window width

digital

A

can adjust the contrast of the image

74
Q 
Wide window 
(digital)
A

more grays or lower contrast

75
Q 
Narrow window
(digital)
A

fewer grays or higher contrast

76
Q

Avoid using above 80 kVp for non-grid exams

digital

A

higher kVp produces excessive scatter and digital image receptors are more sensitive to scatter

77
Q

Advantage of digital imaging

A

wide dynamic range response (more latitude)

78
Q

beam restriction/collimation increase

A

decrease scatter

increase contrast

79
Q

Increase grid ratio

A

Increase contrast

80
Q

SID

A

*

81
Q

increase OID

A

increase contrast

82
Q

decrease OID

A

decrease contrast

83
Q

increase mAs

decrease mAs

A

*

84
Q

increase filtration

A

decrease contrast

85
Q

decrease filtration

A

increase contrast

86
Q

Chemical fog

A

Decreases slope of the D log E curve

87
Q

evaluating contrast

A

-Appropriate range of densities/image receptor
exposures present
-Anatomical structures of interest present

88
Q

focal spot size

A

*

89
Q

anode heel effect

A

*

90
Q

Steeper the slope

A

greater the contrast

91
Q

increasing amount of irradiated tissue

A

decrease contrast

92
Q

decreasing amount of irradiated tissue

A

increase contrast

93
Q

increasing the differences between the atomic number of the tissues

A

increase contrast

94
Q

decreasing the differences between the atomic number of the tissues

A

decrease contrast

95
Q

increasing density of tissues

A

decrease contrast

96
Q

decreasing density of tissues

A

increase contrast

97
Q

using contrast media

A

increase contrast

98
Q

additive pathological conditions

A

decrease contrast

99
Q

destructive pathological conditions

A

increase contrast

100
Q

increasing grid ratio

A

increase contrast

101
Q

decreasing grid ratio

A

decrease contrast

102
Q

use of intensifying screen

A

increase contrast

103
Q

Enters body, 1 of 3 things can happen to the photons:

A
  • Pass through unaffected
  • Be absorbed by the patient
  • Interact and change directions (scatter)
104
Q

In the diagnostic range of 30-140 kVp

A

30 - 140 kvp

105
Q

the scatter generated is produced by

A

Compton interaction and characteristic radiation

106
Q

Characteristic photons

A

low energy and are absorbed

107
Q

Compton

A

energy nearly equal to primary beam

108
Q

As kVp increases

A

the percentage of Compton interaction also increases

109
Q

Scatter↑ when kVp↑

A

contrast↓

110
Q

Compton photons are able to

A

pass through the body and approach the IR from many directions

111
Q

Scatter increases with the following factors:

A

Increase in size of the field and thickness of the body part

Increased kVp

As tissue density increases

Atomic number of tissues decreases

112
Q

Purpose of the grid

A

-Improves the radiographic contrast of the image

-Absorbs scattered radiation before it reaches the
image receptor

113
Q

grids invented by

A

-Dr.gustav bucky 1913
- crosshatched, wide lead strips 2cm apart and running
in 2 directions

114
Q

Dr. Hollis Potter made

A

-improvements to the use of grids
-Realigned lead strips to run in one direction
-Moved grid during exposure to make lines invisible on
image

115
Q

grid use when

A
  • body part is more than 10 cm
  • kvp>60
  • in digital kvp>80
116
Q

Basic grid construction

A

-Radiopaque lead strips
-Separated by radiolucent interspace material
-Typically aluminum
Thin and flat

117
Q

disadvantages of grid use

A

Grid lines on film

Increased patient dose

118
Q

grid Design

A 
linear (most common)
or crosshatch(rare)
119
Q

grid Type

A 
parallel ( they never meet)
or focused (match xray beam divergence)
120
Q

grid ratio

A

-height/distance
-height of lead strips divided by the distance between -
lead strips
-higher the ratio the more efficient

121
Q

grid Frequency

A

of lead strips per inch

  • Stationary (portable)
  • or moving
122
Q

grid cut-off

A

grid improperly absorbs the primary beam

123
Q

convergence point

A

defines grid radius

where x-ray beam and lead strips come together

124
Q

grid radius

A

sid that has to be used with certain focused grid

125
Q

Higher grid ratio

A

More efficient in removing scatter

Less latitude in positioning the grid

126
Q

Frequency range

A

60-200 lines/in
25-80 lines/cm
Most commonly 85 to 103 lines/inch

127
Q

grid frequency

A
  • # of lead strips per cm

- higher frequency grids have thinner lead strips

128
Q

higher frequency grids have thinner lead strips

A

-have thinner lead strips
-Increases the chances of scattered photons passing
through the strips and reaching the film especially at
high kVp

129
Q

Very high-frequency grids #

A

103-200 lines/in

41-80 lines/cm

130
Q

Recommended for use with digital systems

A

178-200

131
Q

Lead content

A

Most important factor in grid’s efficiency
Measured in mass per unit area
g/cm2

132
Q

High ratio, low frequency grids

A

Tend to have highest lead content

133
Q

Reciprocating

A

Motor drives grid back and forth during exposure

134
Q

Oscillating

A

Electromagnet pulls grid to one side

Releases it during exposure

135
Q

Whenever a grid is placed in the beam to remove scatter

A

Density of radiograph will go down
Exposure factors must be increased to compensate for lack of density
Patient dose increases

136
Q

GCF

A

mAs with grid/mAs without grid

Used when going from non-grid technique to
a grid ex. Knee TT or Bucky

137
Q

moire effect

A

When grid lines are
parallel to scan lines in
the plate reader
-Occurs with stationary grids (port)

138
Q

off-level error

A

when the tube in angle across the long axis of the grid strips.

139
Q

off-focus

A

wrong SID

hobson contrast (3 decks)

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