contrast grid Flashcards
(139 cards)
1
Q
contrast
A
- 2nd photographic factors that allows detail to be visible (seen)
- Difference between adjacent densities (attenuating pattern)
- most difficult to evaluate
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
3
Q
Contras in CR and DR
digital
A
- dynamic range
- adjusted by changing window width
4
Q
image contrast
A
Difference between adjacent densities/image receptor exposure levels.
Total amount of contrast from the IR and the anatomic part
5
Q
dynamic range
A
- range of brightness as display in the monitor
- describes the concept of contrast for digital images
6
Q
terms to describe High contrast
A
Short scale
Short/narrow dynamic range
7
Q
term used to describe Low contrast
digital
A
Long scale
Large/wide dynamic range
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
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
10
Q
good contrast
A
- fulfil the purpose of the procedure
- should demonstrate all the structural differences that the body part has
11
Q
scale of contrast
A
Number of useful, visible IR exposure values/density levels or shades of gray
12
Q
Short scale
A
maximal differences between IR exposure values. Minimum number shades of gray
(whale)
13
Q
Long scale
A
minimal differences between IR exposure values. Maximum number of shades of gray
(dolphin)
14
Q
manipulating contrast (film)
A
Change in D log E curve of film
Adjustment to kVp (film*)
15
Q
manipulating contrast (CR /DR)
A
Adjustment of window width
16
Q
Controlling factors (DR)
A
- Window Width
- Histogram
- Look-up-Table (LUT)
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
18
Q
using intensifying screen
A
contrast increases
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
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.
21
Q
diagnostic range
A
- 25: anything below too light
2. 5: anything above too dark
22
Q
toe
A
when curve starts going up
minimum density
23
Q
shoulder
A
when curve start getting horizontal, maximum density
24
Q
without Density
A
without proper density/IR exposure contrast cannot be evaluated
25
Processing
increase in time, temperature or replenishment rate will increase chemical fog. fog decreases contrast (decreases slope of the curve)
26
eliminate or minimize fog
increase contrast
27
Correctly exposed film range
all densities will fall within the visible range on the d log curve. (0.25 to 2.5)
28
what affects Digital Image Receptor Contrast
both Histogram and LUT affect final image
| kvp still important
29
use of kVp in digital IR contrast
```
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
```
30
Subject contrast
- 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
31
Kilovoltage and contrast
Inversely related
As kVp increases, contrast decreases (low contrast)
As kVp decreases, contrast increases (high contrast)
32
kvp and subject contrast
Primary controller of subject contrast
kVp controls energy of photons in the beam
Energy of the photons determines attenuation and the type of interaction
33
As kVp increases what happens?
```
we have a wider range of photon energy
Greater penetrability
Greater range of exposures
Greater amount of scatter
Longer scale of contrast
```
34
Increase in thickness and field size
- Increase scatter
| - Longer scale of contrast
35
Tissue density
refers to how tightly the atoms of a substance are packed together
36
High atomic number and tissue density
Greater attenuation—high contrast
37
Large difference in atomic number and tissue density of adjacent tissues
high contrast
38
Increase in thickness and field size
Increase scatter
| Longer scale of contrast
39
Any factor that increases the production of scatter/fog
decrease contrast
40
As kVp↑
fog/scatter↑ contrast↓
41
↓ Collimation/Beam Restriction
↓contrast
42
↓Grid Ratio
↓contrast
43
Digital Image Receptors
- 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
44
window width
describes digital processing the produces changes in brightness
- how many shades of gray we able to see in image
45
Grayscale bit depth
is the number of shades of gray the system is capable of displaying
-
46
Digital Imaging steps
- Image data is acquired from the exposure
- Data is electrical signal (analog)
- Electrical signal sent to ADC and converted into digital or numerical data
47
Digital image is recorded as
a matrix of small picture elements (pixels)
48
Each pixel is recorded as
a single numerical value, represented as a single brightness level on the monitor
49
Location of the pixel in an image
an area within the patient or a volume of tissue
50
Matrix
Made up of pixels and voxels
51
Field of view (FOV)
what we looking at
52
Image quality in digital is improved with
| digital
larger matrix size which will result in greater number of pixels and smaller pixels
53
Numerical value assigned to each pixel is determined by
the way the body part attenuates the x-ray beam
54
If the photons are highly attenuated or absorbed would result
(digital)
in pixels being assigned a low numerical value = higher brightness on monitor (lighter or less density)
55
Photons passing through tissue with low attenuation
would be assigned a higher numerical value, resulting in less brightness on the monitor (darker or more density)
56
Each pixel also has
a bit depth, or number of bits
57
Number of bits determine
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
Larger bit depth allows
more shades of gray to be displayed on the computer monitor
59
A 12 bit system would be capable of displaying
4096 shades of gray
60
Increase number of shades of gray available
improves contrast resolution
61
pixel bit depth
- Determines number of density values
- Affects density and contrast
- Controlled by ADC
62
bit systems
-10 bit (210 = 1024)
-12 bit (212 = 4096)* standard for diagnostic
radiography
-16 bit (216 = 65,536)
63
Histogram
-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
Pixel values on
x axis
65
number of pixels with that brightness value on
y-axis
66
Far left of graph represents
minimum useful signal (metal, bone)
67
Far right represents
maximum useful signal (skin line, air/gas)
68
Soft tissues are recorded (histogram)
near center
69
Look up table data
-“Ideal” histogram or reference histogram
| stored in computer
70
LUT
-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
why kvp still affects histogram
-controls subject contrast
-Proper kVp assures the desired differential
attenuation through the patient (subject
contrast)
72
Contrast resolution
| digital
to distinguish between small objects that attenuate the beam in a similar way
73
Changing Window width
| digital
can adjust the contrast of the image
74
```
Wide window
(digital)
```
more grays or lower contrast
75
```
Narrow window
(digital)
```
fewer grays or higher contrast
76
Avoid using above 80 kVp for non-grid exams
| digital
higher kVp produces excessive scatter and digital image receptors are more sensitive to scatter
77
Advantage of digital imaging
wide dynamic range response (more latitude)
78
beam restriction/collimation increase
decrease scatter
| increase contrast
79
Increase grid ratio
Increase contrast
80
SID
*
81
increase OID
increase contrast
82
decrease OID
decrease contrast
83
increase mAs
| decrease mAs
*
84
increase filtration
decrease contrast
85
decrease filtration
increase contrast
86
Chemical fog
Decreases slope of the D log E curve
87
evaluating contrast
-Appropriate range of densities/image receptor
exposures present
-Anatomical structures of interest present
88
focal spot size
*
89
anode heel effect
*
90
Steeper the slope
greater the contrast
91
increasing amount of irradiated tissue
decrease contrast
92
decreasing amount of irradiated tissue
increase contrast
93
increasing the differences between the atomic number of the tissues
increase contrast
94
decreasing the differences between the atomic number of the tissues
decrease contrast
95
increasing density of tissues
decrease contrast
96
decreasing density of tissues
increase contrast
97
using contrast media
increase contrast
98
additive pathological conditions
decrease contrast
99
destructive pathological conditions
increase contrast
100
increasing grid ratio
increase contrast
101
decreasing grid ratio
decrease contrast
102
use of intensifying screen
increase contrast
103
Enters body, 1 of 3 things can happen to the photons:
- Pass through unaffected
- Be absorbed by the patient
- Interact and change directions (scatter)
104
In the diagnostic range of 30-140 kVp
30 - 140 kvp
105
the scatter generated is produced by
Compton interaction and characteristic radiation
106
Characteristic photons
low energy and are absorbed
107
Compton
energy nearly equal to primary beam
108
As kVp increases
the percentage of Compton interaction also increases
109
Scatter↑ when kVp↑
contrast↓
110
Compton photons are able to
pass through the body and approach the IR from many directions
111
Scatter increases with the following factors:
Increase in size of the field and thickness of the body part
Increased kVp
As tissue density increases
Atomic number of tissues decreases
112
Purpose of the grid
-Improves the radiographic contrast of the image
-Absorbs scattered radiation before it reaches the
image receptor
113
grids invented by
-Dr.gustav bucky 1913
- crosshatched, wide lead strips 2cm apart and running
in 2 directions
114
Dr. Hollis Potter made
-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
grid use when
- body part is more than 10 cm
- kvp>60
- in digital kvp>80
116
Basic grid construction
-Radiopaque lead strips
-Separated by radiolucent interspace material
-Typically aluminum
Thin and flat
117
disadvantages of grid use
Grid lines on film
| Increased patient dose
118
grid Design
```
linear (most common)
or crosshatch(rare)
```
119
grid Type
```
parallel ( they never meet)
or focused (match xray beam divergence)
```
120
grid ratio
-height/distance
-height of lead strips divided by the distance between -
lead strips
-higher the ratio the more efficient
121
grid Frequency
of lead strips per inch
- Stationary (portable)
- or moving
122
grid cut-off
grid improperly absorbs the primary beam
123
convergence point
defines grid radius
| where x-ray beam and lead strips come together
124
grid radius
sid that has to be used with certain focused grid
125
Higher grid ratio
More efficient in removing scatter
| Less latitude in positioning the grid
126
Frequency range
60-200 lines/in
25-80 lines/cm
Most commonly 85 to 103 lines/inch
127
grid frequency
- # of lead strips per cm
| - higher frequency grids have thinner lead strips
128
higher frequency grids have thinner lead strips
-have thinner lead strips
-Increases the chances of scattered photons passing
through the strips and reaching the film especially at
high kVp
129
Very high-frequency grids #
103-200 lines/in
| 41-80 lines/cm
130
Recommended for use with digital systems
178-200
131
Lead content
Most important factor in grid’s efficiency
Measured in mass per unit area
g/cm2
132
High ratio, low frequency grids
Tend to have highest lead content
133
Reciprocating
Motor drives grid back and forth during exposure
134
Oscillating
Electromagnet pulls grid to one side
| Releases it during exposure
135
Whenever a grid is placed in the beam to remove scatter
Density of radiograph will go down
Exposure factors must be increased to compensate for lack of density
Patient dose increases
136
GCF
mAs with grid/mAs without grid
Used when going from non-grid technique to
a grid ex. Knee TT or Bucky
137
moire effect
When grid lines are
parallel to scan lines in
the plate reader
-Occurs with stationary grids (port)
138
off-level error
when the tube in angle across the long axis of the grid strips.
139
off-focus
wrong SID
