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)