CT numbers, window width and window level

1. CT NUMBERS,
WINDOW WIDTH AND
WINDOW LEVEL
G.YOGANANTHEM
PROF.DR.I.GURUBHARATH

2. The Linear Attenuation Coefficient (µ)
The linear attenuation coefficient (µ) describes
fraction of a beam of x-rays or gamma rays that
absorbed or scattered per unit thickness of the
medium.
Where:
I=the intensity of photons transmitted across
some distance x
I0=the initial intensity of photons
µ=the linear attenuation coefficient
x=distance traveled ( thickness )

3. The Linear Attenuation Coefficient (µ)

5. CT PRINCIPLE
• The internal structure of an object can be reconstructed from
multiple projections of the object.
• Linear attenuation coefficient (μ) is the base

6. CT: SCANNING PRINCIPLE…

8. CT :TERMINOLOGY…
• Voxel: Volume element 3D)
• Pixel: Picture element (2D)
• Field of view (FOV): diameter of the area
being scanned
FOV (250mm)
• Pixel size= --------------------
Matrix size(512) – no of pixels
=0.5mm

10. Why we are giving importance to CT numbers and
why not
LINEAR Attenuation coefficient (μ)

11. μ 80kV 100kV 150kV
μ muscle 0.1892 0.1760 0.1550
μ water 0.1835 0.1707 0.1504
Because, when we are changing kvp – Linear attenuation
coefficients are μ varying.

12. but, CT value of muscle at different kvp are constant
(0.1892-0.1835)
80 kV: 1000 x ---------------------- = 31
0.1835
100 kV :1000 x (0.1760-0.1707) / 0.1707= 31
150 kV :1000 x (0.1550-0.1504) / 0.1504= 31
μ 80kV 100kV 150kV
μ muscle 0.1892 0.1760 0.1550
μ water 0.1835 0.1707 0.1504

13. It is easy to remember CT value compare to Linear attenuation
coefficient value.

16. CT IMAGE DISPLAY
• The range of CT numbers above and below the window level
is called the window width,
which determines the contrast
• A narrow window width provides higher contrast than wide
window width
• Window level & window width can be set to any desired
value of CT number

17. Our normal eye will not recognize 2000 divisions of CT illuminations

18. GRAY SCALE
By using gray scale we made it 20 illuminations only

19. -1000 to -901
-900 to -801
-800 to -701
-700 to -601
-600 to -501
-500 to -401
-400 to -301
-300 to -201
-200 to -101
-100 to -1
0-100
101-200
201 -301
301-400
401-500
501-600
601-700
701-800
801-900
901-1000

20. Brain window

21. -20 to -16
-15 to -11
-10 to -6
1-5
6-10
11-15
16-20
20-24
21-25
26-30
31-35
36-40
41-45
46-50
51-55
56-60
61-65
66-70
71-75
76-80

22. Abdomen window

23. -50 to -41
-40 to -31
-30 to -21
-20 to -11
-10 to 0
1-10
11-20
21-30
31-40
41-50
51-60
61-70
71-80
81-90
91-100
101-110
111-120
121-130
131-140
141-150

24. Lung window

25. -1250 to -1201
-1200 to -1151
-1150 to -1101
-1100 to -1051
-1050 to1001
-1000-to -950
-951to -901
-900 to -851
-850 to-801
-800 to-751
-750 to -700
-699 to -650
-649 to -600
-599 to -550
-549 to -500
-499 t0 -450
-449 to -400
-399 to -350
-349 to -300
-299 to -250

26. SOME CLINICAL WINDOW
LEVEL & WIDTH
Examination Window width Window Level
Head 80 40
Chest (mediasti) 450 40
Chest (lung) 1500 -500
Abdomen (liver) 150 60

27. Window width
The window width (WW) as the name suggests is
the measure of the range of CT numbers that an
image contains. That is range of CT numbers
displayed in the CT image.
A wider window width (2000 HU), therefore, will
display a wider range of CT numbers.

28. Window
level/centre
The window level (WL), often also referred
to as window centre, is the midpoint of the
range of the CT numbers displayed.

29. Upper and lower grey level
calculation
When presented with a WW and WL one can calculate
the upper and lower grey levels i.e. values over x will be
white and values below -y will be black.
 the upper grey level is calculated via
WL + (WW ÷ 2)
 the lower grey level is calculated via
WL - (WW ÷ 2)

30. Simply saying window width
controls contrast
and
Window level controls
brightness.

31. WINDOW LEVEL & IMAGE CONTRAST

33. WINDOW LEVEL & WINDOW WIDTH