CT Image Quality and Display Flashcards
Primarily controlled by resolution and noise consideration
CT IMAGE QUALITY
Varied with window level and
window width controls
DENSITY AND CONTRAST
CT image: requires __ contrast
difference
4%
Diagnostic image: requires __
contrast difference
10%
Depends on pixel, voxel and matrix size
RESOLUTION
resolves high-contrast
objects 1.5 times its size
Pixel size:
+Voxel size: __mm
8-13mm
seen better in smaller pixel size
+High contrast objects:
seen better in a larger pixel size
Low contrast objects:
+CT: __mm resolving power
+Conventional: __mm resolving power
CT: 0.35mm
Conventional: 0.25mm
+Undershoot and overshoot
+Occurs when two objects have >60% difference in attenuation
coefficients
INTERFACE ARTIFACT
Occurs in objects of greatly different contrast
INTERFACE ARTIFACT
+ Star pattern artifact is an example of?
INTERFACE ARTIFACT
Compensation: large matrix size & reconstruction algorithm
INTERFACE ARTIFACT
Achieved by choosing a smaller
display FOV
CT MAGNIFICATION
Produces an image with less tissue
per voxel
CT MAGNIFICATION
+Provides a larger image of a smaller
area of interest
CT MAGNIFICATION
+Provides higher resolution
CT MAGNIFICATION
Magnified CT matrix appears?
Pixelated & Less sharp
Can be found in all digital system
+Directly related to the amount of
data collected by the detector
+Comprises 3-5% of the image
Noise
Affects the low contrast resolving
ability of the CT unit
+Noise evaluation: water phantom
+Average Reading: at least 100 pixels
(10x10 matrix)
Noise
+Can be found in all digital system
+Directly related to the amount of
data collected by the detector
+Comprises 3-5% of the image
Noise
+A function of exposure time
+Controlled by the quantity of data
detected
Motion
The last phase in the creation of
the CT image
CT IMAGE DISPLAY
Includes all of the system
components necessary to convert
digital data created from the
reconstruction process to electrical
signals needed by the CT display
monitor (DAC)
CT IMAGE DISPLAY
Controls the digital image
production process
+Series of mathematical formulas
(algorithms)
PROCESSING AND DISPLAY PROGRAMS
Display parameters?
+Display parameters
+ Window width
+ Window level
+ Reconstructions
+ Enhancements
Used to display CT images
+– >2000 different HU (hounsfield units) values
GRAY SCALE
+High HU: assigns __ shades of gray
+Lower HU: assigns __ shades of gray
(High - Lighter and Low - Darker)
+____: displays only __ shades of gray
Monitor & 256
Human Eye:
+Limited to <__ shades of gray
+Cannot appreciate contrast differences <__
40% & 10%
Can differentiate contrast <1%
CT
Mechanism that determines the quantity of HU represented as shades
of gray on a specific image
+Window Width
+ 500-2000 HU
+ Decrease image contrast
+ Best for imaging tissues (CT numbers vary greatly)
+ e.g. Lung imaging
WIDE WINDOW WIDTH (WIDE WW)
+50-500 HU
+Increase image contrast
+Best in imaging the brain (CT numbers not vary greatly)
+Narrow WW
+ Mechanism that selects the center CT value of the window width
+Window Center/Level:
+___ WL: makes the image darker
+___ WL: makes the image brighter
WINDOW LEVEL
INCREASE
DECREASE
+Localizer scan, Scout, Surview,
Topogram, Preview or Pilot
+A localization image (2D)
+The first image of an examination
+Slightly poorer image quality
SCANOGRAM
Used by RT to confirm exact
centering of the patient
+Used by radiologist to
describe the structural
locations
GRID APPLICATION
+Used to outline the area of interest
+Used to determine the HU
+Used to display the area’s
measurement within the patient
+Used for volume estimates
CURSOR
Reverse the image from left to
right & vise versa
+Reverse the density display
from black to white & vice versa
REVERSE DISPLAY
+Uses only the image data
+Does not improve resolution
+Not the same as decreasing the
FOV
+Allows the image detail to be clearly
seen
IMAGE MAGNIFICATION
+Added text on the images for
descriptions or anatomical labeling
+Arrows
+Markers
+Used to display optional information
+Allows operator to annotate images
with
+Words
+Phrases
IMAGE ANNOTATION
