CT Scanner Physics for Medical Physicists and Nuclear Medicine and Radiology. Shahid Younas, Sr. Medical Physicist &RPA, SKMCH&RC, Lahore, Pakistan

1. Introduction to CT Physics
Shahid Younas
Lecture 2

2. ANODE-ROTATING
PX # High energy electrons interact with matter and convert their
kinetic energy into
1 Characteristic X rays 2. Bremsstrahlung rays
3. White Rays 4. X Rays and Heat

3. ANODE-ROTATING
PX # 2: Which of the followings is not a part of x rays
production device?
1 Electron source 2. Gas filled tube
2 Target electrode 4. High voltage

4. ANODE-ROTATING
PX # 3: The selection of voltage, current, and exposure
time in X ray unit is controlled by,
1: Target Material 2. Filament Material
3:Power Generator
4.Distance between anode and cathode

5. ANODE-ROTATING
PX # 4: In a X rays production device which one is not
true,
1: The cathode is negatively charged and is the source of
electrons
2: The anode is positively charged and is the target of
electrons
3: Quenching Gas in tube balance Bremsstrahlung and
Characteristics X rays
4: Potential difference between anode and cathode
accelerates electrons

6. ANODE-ROTATING
PX# 7: An x-ray photon with energy equal to the kinetic energy
lost by the electron is called as,
1: Characteristic X Rays
2: Bremsstrahlung X Rays
3: Electrostatic Rays
4: Columbic Rays

7. ANODE-ROTATING
The probability of an electron's directly impacting a nucleus
during the production of x rays is extremely low, simply because,
at the atomic scale,
1: atom comprises mainly empty space
2: poses electrostatic forces
3: anode is made of inert material
4: cross-section of anode is very small

8. ANODE-ROTATING
In the figure below, what types of X rays are being produced?
1: Characteristic 2: Bremsstrahlung
3: Rayleigh Rays 4: Both 1 and 2

9. ANODE-ROTATING
PX # 13: Which is the major factor that affects x-ray
production efficiency?
1: Atomic number of the target material
2: Kinetic Energy of Incident Electron
3: impurity in Filament
4: All of above

10. Introduction to CT Physics
CATHODE- THE SOURCE OF
ELECTRON
 Helical Filament of tungsten wire
 Thermionic emission

11. ANODE-ROTATING
The probability of an electron's directly impacting a nucleus
during the production of x rays is extremely low, simply because,
at the atomic scale,
1: comprises mainly empty space
2: poses electrostatic forces
3: anode is made of inert material
4: cross-section of anode is very small

12. Introduction to CT Physics
CATHODE- THE SOURCE OF ELECTRON
Thermal energy given to the carrier
overcomes the binding potential- work
function of the metal.

13. Introduction to CT Physics
CATHODE- THE SOURCE OF ELECTRON
Electrons from the filament
flow through the vacuum
when a positive voltage is
placed on the anode w.r.t.
the cathode.

14. Introduction to CT Physics
CATHODE- THE SOURCE OF ELECTRON
Adjustments in the filament current
(filament temperature) control the tube
current.

15. Bremsstrahlung X Rays
Do you know which impurity is added to increase thermion in X
Ray tube filament?
Thorium
increases the efficiency of electron
emission and prolongs filament life.

16. FOCUSING CUP / CATHODE BLOCK

17. FOCUSING CUP / CATHODE BLOCK
 Helical filament of tungsten
wire is surrounded by
“Focusing cup or cathode
block”.
 shapes the electron beam
width and length.

18. FOCUSING CUP / CATHODE BLOCK
When no voltage is applied between the anode an electron cloud
- space charge cloud- builds around the filament.

19. Bremsstrahlung X Rays
Do you know side effect of space charge cloud?
The space charge cloud the limits the tube current. Increase in the
filament current does not increases the tube current.

20. FOCUSING CUP / CATHODE BLOCK
X-ray tube is space charge limited up to 40 kVp.

21. FOCUSING CUP / CATHODE BLOCK
 Higher kVp produces slightly higher tube current for the same
filament current.
 This relationship does not continue indefinitely. Beyond a
certain kVp, saturation occurs and a further increase in kVp
does not significantly increase the tube current.

22. Production of X Rays
“Take Risk in your life if you succeed you can lead . Albert Hitler”

23. Production of X Rays
“Me and my wife lived happily.... then we met. ”

24. ANODE

25. ANODE
 The anode is a metal target electrode that is maintained at a
positive potential difference relative to the cathode.
 Electrons striking the anode deposit the most of their energy as
heat, with a small fraction emitted as x-rays.

26. ANODE
The fixed anode x-ray tube.

27. ANODE
 Production of x-rays, in quantities necessary for acceptable
image quality, generates a large amount of heat in the anode.
 To avoid heat damage to the x-ray tube, the rate of x-ray
production must be limited.

28. ANODE
Tungsten (Z = 74) is the most widely used anode material because
of its high melting point and high atomic number.
Do you know what is the widely used anode metal?

29. ANODE
Better Brems
production
Handle
substantial
heat
deposition
Rough &
Tough
10%
Rhenium and
90% tungsten

30. ANODE
Molybdenum (Mo, Z = 42) and rhodium (Rh, Z = 45)
Provide useful characteristic X rays for breast imaging
Do you know which metal is used in Mammography as anode
and why?

31. ANODE
 Stationary and rotating anode configurations.

32. ANODE-STATIONARY
 The small stationary target area limits the heat dissipation rate
and consequently limits the maximum tube current and thus the
x-ray flux.
 Many dental x-ray units, portable x-ray machines, and portable
fluoroscopy systems use fixed anode x-ray tubes.

33. ANODE-ROTATING
 Rotating anodes have greater heat loading and higher x-ray
output capabilities.

34. ANODE-ROTATING
 Electrons impart their energy on a continuously rotating
target, spreading thermal energy over a large area and mass
of the anode disk.

35. ANODE-ROTATING
 X-ray machines are designed so that the x-ray tube will not
be energized if the anode is not up to full speed.
 This is the cause for the short delay (1 to 2 seconds) when
the x-ray tube exposure button is pushed.

36. ANODE-ROTATING
Do you know relation between Anode Angle and Focal Spot
Size?

37. ANODE-ROTATING
 The anode angle is
defined as the angle of
the target surface with
respect to the central ray
in the x-ray field.

38. ANODE-ROTATING
The anode angle is defined as the angle of the target surface with respect
to the central ray in the x-ray field.

39. ANODE-ROTATING
The actual focal spot size is the area on
the anode that is struck by electrons.
It is primarily determined by the length
of the cathode filament and the width of
the focusing cup slot.

40. ANODE-ROTATING
The actual focal spot size is the area on
the anode that is struck by electrons.
It is primarily determined by the length
of the cathode filament and the width of
the focusing cup slot.

41. ANODE-ROTATING
The effective focal spot size is the
length and width of the focal spot as
projected down the central ray in the x-
ray field.

42. ANODE-ROTATING

43. INTRODUCTION TO CT PHYSICS
In my country we go to prison first and then become President.

44. X ray Production Device
By by!