contrast grid Flashcards
contrast
- 2nd photographic factors that allows detail to be visible (seen)
- Difference between adjacent densities (attenuating pattern)
- most difficult to evaluate
Visibility of detail
image is visible to the human eye only because sufficient contrast (and IR exposure/density) exists to permit the structural details to be seen
Contras in CR and DR
digital
- dynamic range
- adjusted by changing window width
image contrast
Difference between adjacent densities/image receptor exposure levels.
Total amount of contrast from the IR and the anatomic part
dynamic range
- range of brightness as display in the monitor
- describes the concept of contrast for digital images
terms to describe High contrast
Short scale
Short/narrow dynamic range
term used to describe Low contrast
digital
Long scale
Large/wide dynamic range
High Contrast differences
Few Shades of gray More Increased Short Scale Low kVp*(depends on exam) Short(narrow) dynamic range Narrow window width
Low Contrast (differences)
Many shades of gray Less Decreased Long Scale High kVp* ( depends on exam) Large (wide) dynamic range Wide window width
good contrast
- fulfil the purpose of the procedure
- should demonstrate all the structural differences that the body part has
scale of contrast
Number of useful, visible IR exposure values/density levels or shades of gray
Short scale
maximal differences between IR exposure values. Minimum number shades of gray
(whale)
Long scale
minimal differences between IR exposure values. Maximum number of shades of gray
(dolphin)
manipulating contrast (film)
Change in D log E curve of film
Adjustment to kVp (film*)
manipulating contrast (CR /DR)
Adjustment of window width
Controlling factors (DR)
- Window Width
- Histogram
- Look-up-Table (LUT)
Image receptor (film) factors
Range of densities film can record 4 factors: Use Intensifying screen Film density (overall blackening) Slope of D log E curve (density curve) Processing
using intensifying screen
contrast increases
Slope of D Log E curve (speed or sensitivity)
speed or sensitivity : measures the film’s
ability to respond to light or radiation
Slope of D Log E curve (latitude)
(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.
diagnostic range
- 25: anything below too light
2. 5: anything above too dark
toe
when curve starts going up
minimum density
shoulder
when curve start getting horizontal, maximum density
without Density
without proper density/IR exposure contrast cannot be evaluated
Processing
increase in time, temperature or replenishment rate will increase chemical fog. fog decreases contrast (decreases slope of the curve)
eliminate or minimize fog
increase contrast
Correctly exposed film range
all densities will fall within the visible range on the d log curve. (0.25 to 2.5)
what affects Digital Image Receptor Contrast
both Histogram and LUT affect final image
kvp still important
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
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
Kilovoltage and contrast
Inversely related
As kVp increases, contrast decreases (low contrast)
As kVp decreases, contrast increases (high contrast)
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
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
Increase in thickness and field size
- Increase scatter
- Longer scale of contrast
Tissue density
refers to how tightly the atoms of a substance are packed together
High atomic number and tissue density
Greater attenuation—high contrast
Large difference in atomic number and tissue density of adjacent tissues
high contrast
Increase in thickness and field size
Increase scatter
Longer scale of contrast
Any factor that increases the production of scatter/fog
decrease contrast
As kVp↑
fog/scatter↑ contrast↓
↓ Collimation/Beam Restriction
↓contrast
↓Grid Ratio
↓contrast
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
window width
describes digital processing the produces changes in brightness
- how many shades of gray we able to see in image
Grayscale bit depth
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
Digital image is recorded as
a matrix of small picture elements (pixels)
Each pixel is recorded as
a single numerical value, represented as a single brightness level on the monitor
Location of the pixel in an image
an area within the patient or a volume of tissue
Matrix
Made up of pixels and voxels
Field of view (FOV)
what we looking at
Image quality in digital is improved with
digital
larger matrix size which will result in greater number of pixels and smaller pixels
Numerical value assigned to each pixel is determined by
the way the body part attenuates the x-ray beam
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)
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)
Each pixel also has
a bit depth, or number of bits
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
Larger bit depth allows
more shades of gray to be displayed on the computer monitor
A 12 bit system would be capable of displaying
4096 shades of gray
Increase number of shades of gray available
improves contrast resolution
pixel bit depth
- Determines number of density values
- Affects density and contrast
- Controlled by ADC
bit systems
-10 bit (210 = 1024)
-12 bit (212 = 4096)* standard for diagnostic
radiography
-16 bit (216 = 65,536)
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
Pixel values on
x axis
number of pixels with that brightness value on
y-axis
Far left of graph represents
minimum useful signal (metal, bone)
Far right represents
maximum useful signal (skin line, air/gas)
Soft tissues are recorded (histogram)
near center
Look up table data
-“Ideal” histogram or reference histogram
stored in computer
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
why kvp still affects histogram
-controls subject contrast
-Proper kVp assures the desired differential
attenuation through the patient (subject
contrast)
Contrast resolution
digital
to distinguish between small objects that attenuate the beam in a similar way
Changing Window width
digital
can adjust the contrast of the image
Wide window (digital)
more grays or lower contrast
Narrow window (digital)
fewer grays or higher contrast
Avoid using above 80 kVp for non-grid exams
digital
higher kVp produces excessive scatter and digital image receptors are more sensitive to scatter
Advantage of digital imaging
wide dynamic range response (more latitude)
beam restriction/collimation increase
decrease scatter
increase contrast
Increase grid ratio
Increase contrast
SID
*
increase OID
increase contrast
decrease OID
decrease contrast
increase mAs
decrease mAs
*
increase filtration
decrease contrast
decrease filtration
increase contrast
Chemical fog
Decreases slope of the D log E curve
evaluating contrast
-Appropriate range of densities/image receptor
exposures present
-Anatomical structures of interest present
focal spot size
*
anode heel effect
*
Steeper the slope
greater the contrast
increasing amount of irradiated tissue
decrease contrast
decreasing amount of irradiated tissue
increase contrast
increasing the differences between the atomic number of the tissues
increase contrast
decreasing the differences between the atomic number of the tissues
decrease contrast
increasing density of tissues
decrease contrast
decreasing density of tissues
increase contrast
using contrast media
increase contrast
additive pathological conditions
decrease contrast
destructive pathological conditions
increase contrast
increasing grid ratio
increase contrast
decreasing grid ratio
decrease contrast
use of intensifying screen
increase contrast
Enters body, 1 of 3 things can happen to the photons:
- Pass through unaffected
- Be absorbed by the patient
- Interact and change directions (scatter)
In the diagnostic range of 30-140 kVp
30 - 140 kvp
the scatter generated is produced by
Compton interaction and characteristic radiation
Characteristic photons
low energy and are absorbed
Compton
energy nearly equal to primary beam
As kVp increases
the percentage of Compton interaction also increases
Scatter↑ when kVp↑
contrast↓
Compton photons are able to
pass through the body and approach the IR from many directions
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
Purpose of the grid
-Improves the radiographic contrast of the image
-Absorbs scattered radiation before it reaches the
image receptor
grids invented by
-Dr.gustav bucky 1913
- crosshatched, wide lead strips 2cm apart and running
in 2 directions
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
grid use when
- body part is more than 10 cm
- kvp>60
- in digital kvp>80
Basic grid construction
-Radiopaque lead strips
-Separated by radiolucent interspace material
-Typically aluminum
Thin and flat
disadvantages of grid use
Grid lines on film
Increased patient dose
grid Design
linear (most common) or crosshatch(rare)
grid Type
parallel ( they never meet) or focused (match xray beam divergence)
grid ratio
-height/distance
-height of lead strips divided by the distance between -
lead strips
-higher the ratio the more efficient
grid Frequency
of lead strips per inch
- Stationary (portable)
- or moving
grid cut-off
grid improperly absorbs the primary beam
convergence point
defines grid radius
where x-ray beam and lead strips come together
grid radius
sid that has to be used with certain focused grid
Higher grid ratio
More efficient in removing scatter
Less latitude in positioning the grid
Frequency range
60-200 lines/in
25-80 lines/cm
Most commonly 85 to 103 lines/inch
grid frequency
- # of lead strips per cm
- higher frequency grids have thinner lead strips
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
Very high-frequency grids #
103-200 lines/in
41-80 lines/cm
Recommended for use with digital systems
178-200
Lead content
Most important factor in grid’s efficiency
Measured in mass per unit area
g/cm2
High ratio, low frequency grids
Tend to have highest lead content
Reciprocating
Motor drives grid back and forth during exposure
Oscillating
Electromagnet pulls grid to one side
Releases it during exposure
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
GCF
mAs with grid/mAs without grid
Used when going from non-grid technique to
a grid ex. Knee TT or Bucky
moire effect
When grid lines are
parallel to scan lines in
the plate reader
-Occurs with stationary grids (port)
off-level error
when the tube in angle across the long axis of the grid strips.
off-focus
wrong SID
