CT
Seeram Chapter 9:Image
Manipulation in CT
Image Manipulation Defined
“Those techniques (operations) or processes which modify an image or group of images to enhance the visibility of useful information while suppressing ‘noise’ or non-useful information”
No additional or new information produced
Content of interest emphasizedOverall information content reduced
WindowingManipulation of image gray scale using
image’s CT numbers
Windowing
Manipulation allows customization of visibilitysoft tissues
braindense structures
bone
Window Width & Level
Window widthrange of CT #’s imageddetermines maximum # of gray
shades which could be displayed on monitor
Window levelcenter or midpoint of CT #
range
Window Width & LevelPixels outside of window displayed as Black or White
>200
151-200
101-150
51-100
1-50
(-49)-0
(-99)-(-50)
(-149)-(-100)
(-199)-(-150)
<(-199)
3000
-1000
0
1000
2000
Window: 400Level: 0
3000
-1000
0
1000
-1000
Window: 400Level: 0
Short gray scale Small block of CT
#’s assigned gray levels
Small transition zone of white to black
3000
-1000
0
1000
-1000
Window: 400Level: 0
Used to display soft tissues within structures containing different tissues of similar densities
Level centered near average CT # of organ of interest
0
1000
Window: 2000
Level: 0
-1000
Long gray scale Large block of CT #’s
assigned gray levels Large transition zone
of white to black
0
1000
Window: 2000
Level: 0
-1000
Used where large latitude required
Used to simultaneously display tissues of greatly differing attenuation
WL =0WW = 200
All pixels with CT #’s > 0 +(200/2) = 100: White
All pixels with CT #’s < 0 -(200/2) = -100: Black
100
-100
200 0
WL = 40WW = 200
All pixels with CT #’s > 40 + (200/2) = 140: White
All pixels with CT #’s < 40 - (200/2) = -60: Black
140
-60
200 40
WL = 0WW = 400
All pixels with CT #’s > 0 + (400/2) = 200: White
All pixels with CT #’s < 0 - (400/2) = -200: Black
200
-200
400 0
Larger Window Means Obscuring Small Differences in Tissue Attenuation
One gray shade encompasses larger range of CT #’s
200
-200-100
100
20 - 40 40 - 8020 40
WW=200 WW=400
Range
Window Width & ContrastAs WW increases
contrast decreaseslatitude (range of CT #’s
imaged) increasesAs WW decreases
contrast increaseslatitude decreases
Clinical goal:Largest available contrast at
the latitude required by study
Window Width & Image ContrastLarge window width
different structures more likely to have same gray shade
Narrow window widthGray shade differences more likely visible
between structuresVery narrow window width
Small differences in attenuation seen as black & white
Preset Window & Level
Available for all commercial CTinitial WW and WL pre-sets for specific study
typesCan be overridden by operator
Specialized CT Image Manipulation Programs
Region of interest (ROI) analysisaverage CT #standard deviation
Specialized CT Image Manipulation Programs
Image annotationgridsarrowsnotes
Bad Thingee
Specialized CT Image Manipulation Programs
Histogram analysisgraph of CT # frequency# of pixels with given range of CT#’s
can apply toROIentire image
Specialized CT Image Manipulation Programs
Multiplanar reconstruction3D reconstructionQuantitative CT
Osteo CT / bone densitometryXenon CTRadiation therapy treatment planning using
CT data
Multiplanar ReconstructionCreates non-axial images from stack of contiguous transverse axial scansstack contains 3 dimensional CT data
pixels new cut identified & selected from each axial image without scanning
Cutscoronalsagittalparaxialoblique
Stack ofAxial Slices
ReconstructedSlice
Stack ofAxial Slices
ReconstructedSlice
Stack ofAxial Slices
ReconstructedSlice
Stack ofAxial Slices
ReconstructedSlice
Reformatting Advantages
enables visualization of specific structures relative to surrounding structures
aids in determining / localizing true extent oflesionsfracturesbone fragments
foreign bodies
Reformatting DisadvantagesImage quality can be
poorer than axial images if plane thickness > pixel sizeaffects blurring
less problem in spiral scanning
Reformatting Disadvantages
More prone to motion / breathing artifactsreformatted image taken from many slicesreformatted image represents longer time
interval than single slice
Quantitative CT: Measurement of Bone Mineral Density (BMD)Competes with DEXA
Quantitative CT: Measurement of Bone Mineral Density (BMD)Scout image used to define midvertebral planeAxial images contains both patient & reference
phantomReference phantom used to account for slight CT #
changes over timephantom has water & bone equivalent parts
Quantitative CT: Measurement of Bone Mineral Density (BMD)
Software automatically defines region of interest
Mean CT #’s calculatedBone Mineral Density calculatedBMD compared to
“standard” values previous studies for this patient
Xenon CT for Regional Cerebral Blood Flow Imaging
Patient inhales xenon gasserves as a contrast agent
Xenongasinert elementhigh atomic #
provides good CT contrasthigh fat solubilitycrosses blood - brain barrier
Xenon CT ApplicationsBrain metabolism
regional perfusion in cerebrovascular disease
Potential applicationsdementiasleep disordersmigrainesepilepsy
CT in Radiation Therapy Treatment PlanningConventional CT study performed
patient positioned exactly as for therapyCT data communicated to treatment planning
computer
CT in Radiation Therapy Treatment Planning
CT data used to develop depth dose data
Isodose data superimposed on CT image
Other CT Software Features
Multiple window settings within a sliceImage magnificationMeasurement of distances / anglesHighlighting of selected pixel values in
imageCine viewing of image stack
CT ApplicationsSplit image
one thicker slice reconstructed into two thinner slices
DentalAngiographyPlastic Surgery ReconstructionMultimodality Image Fusion
Overlaying of images of various modalitiesOne anatomical (CT), one functional (Nuc
med)