1. Seeram Chapter 9: Image Manipulation in CT

2. 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 emphasized
Overall information content reduced

3. Windowing
Manipulation of image gray scale using image’s CT numbers

4. Windowing Manipulation allows customization of visibility soft tissues
brain
dense structures
bone

5. Window Width & Level WL WW Window width Window level
range of CT #’s imaged
determines maximum # of gray shades which could be displayed on monitor
Window level
center or midpoint of CT # range WW WL

6. Window Width & Level
Pixels outside of window displayed as Black or White WW WL

7. Window Width & Level 3000 >200 151-200 2000 101-150 51-100 1-50
1000
(-49)-0
(-99)-(-50)
Window: 400
Level: 0
(-149)-(-100)
(-199)-(-150)
<(-199)
-1000

8. Small Window Width Short gray scale
1000
3000
Short gray scale
Small block of CT #’s assigned gray levels
Small transition zone of white to black
Window: 400
Level: 0
-1000
-1000

9. Small Window Width
1000
3000
Used to display soft tissues within structures containing different tissues of similar densities
Level centered near average CT # of organ of interest
Window: 400
Level: 0
-1000
-1000

10. Large Window Width Long gray scale
1000
Long gray scale
Large block of CT #’s assigned gray levels
Large transition zone of white to black
Window: 2000
Level: 0
-1000

11. Large Window Width Used where large latitude required
1000
Used where large latitude required
Used to simultaneously display tissues of greatly differing attenuation
Window: 2000
Level: 0
-1000

12. Window Example
100
WL =0
WW = 200
All pixels with CT #’s > 0 +(200/2) = 100: White
200
All pixels with CT #’s < 0 -(200/2) = -100: Black
-100

13. Another Window Example
140
WL = 40
WW = 200
All pixels with CT #’s > 40 + (200/2) = 140: White
200 40
All pixels with CT #’s < 40 - (200/2) = -60: Black
-60

14. Still Another Window Example
200
WL = 0
WW = 400
All pixels with CT #’s > 0 + (400/2) = 200: White
400
All pixels with CT #’s < 0 - (400/2) = -200: Black
-200

15. Larger Window Means Obscuring Small Differences in Tissue Attenuation
WW=200
WW=400
200
100
Range 20 40
One gray shade encompasses larger range of CT #’s
-100
-200

16. Window Width & Contrast
As WW increases
contrast decreases
latitude (range of CT #’s imaged) increases
As WW decreases
contrast increases
latitude decreases
Clinical goal:
Largest available contrast at the latitude required by study

17. Window Width & Image Contrast
Large window width
different structures more likely to have same gray shade
Narrow window width
Gray shade differences more likely visible between structures
Very narrow window width
Small differences in attenuation seen as black & white

18. Preset Window & Level Available for all commercial CT
initial WW and WL pre-sets for specific study types
Can be overridden by operator

19. Specialized CT Image Manipulation Programs
Region of interest (ROI) analysis
average CT #
standard deviation

20. Specialized CT Image Manipulation Programs
Image annotation
grids
arrows
notes
Bad Thingee

21. Specialized CT Image Manipulation Programs
Histogram analysis
graph of CT # frequency
# of pixels with given range of CT#’s
can apply to
ROI
entire image

22. Specialized CT Image Manipulation Programs
Multiplanar reconstruction
3D reconstruction
Quantitative CT
Osteo CT / bone densitometry
Xenon CT
Radiation therapy treatment planning using CT data

23. Multiplanar Reconstruction
Creates non-axial images from stack of contiguous transverse axial scans
stack contains 3 dimensional CT data
pixels new cut identified & selected from each axial image without scanning

24. Cuts
coronal
sagittal
paraxial
oblique

25. Coronal Reconstruction Example
Stack of
Axial Slices
Reconstructed
Slice

26. Coronal Reconstruction Example
Stack of
Axial Slices
Reconstructed
Slice

27. Sagittal Reconstruction Example
Stack of
Axial Slices
Reconstructed
Slice

28. Oblique Reconstruction Example
Stack of
Axial Slices
Reconstructed
Slice

30. Reformatting Advantages
enables visualization of specific structures relative to surrounding structures
aids in determining / localizing true extent of
lesions
fractures
bone fragments
foreign bodies

31. Reformatting Disadvantages
Image quality can be poorer than axial images if plane thickness > pixel size
affects blurring
less problem in spiral scanning

32. Reformatting Disadvantages
More prone to motion / breathing artifacts
reformatted image taken from many slices
reformatted image represents longer time interval than single slice

33. Quantitative CT: Measurement of Bone Mineral Density (BMD)
Competes with DEXA

34. Quantitative CT: Measurement of Bone Mineral Density (BMD)
Scout image used to define midvertebral plane
Axial images contains both patient & reference phantom
Reference phantom used to account for slight CT # changes over time
phantom has water & bone equivalent parts

35. Quantitative CT: Measurement of Bone Mineral Density (BMD)
Software automatically defines region of interest
Mean CT #’s calculated
Bone Mineral Density calculated
BMD compared to
“standard” values
previous studies for this patient

36. Xenon CT for Regional Cerebral Blood Flow Imaging
Patient inhales xenon gas
serves as a contrast agent
Xenon
gas
inert element
high atomic #
provides good CT contrast
high fat solubility
crosses blood - brain barrier

37. Xenon CT Applications Brain metabolism Potential applications
regional perfusion in cerebrovascular disease
Potential applications
dementia
sleep disorders
migraines
epilepsy

38. CT in Radiation Therapy Treatment Planning
Conventional CT study performed
patient positioned exactly as for therapy
CT data communicated to treatment planning computer

39. CT in Radiation Therapy Treatment Planning
CT data used to develop depth dose data
Isodose data superimposed on CT image

40. Other CT Software Features
Multiple window settings within a slice
Image magnification
Measurement of distances / angles
Highlighting of selected pixel values in image
Cine viewing of image stack

41. CT Applications Split image Dental Angiography
one thicker slice reconstructed into two thinner slices
Dental
Angiography
Plastic Surgery Reconstruction
Multimodality Image Fusion
Overlaying of images of various modalities
One anatomical (CT), one functional (Nuc med)