Introduction to CT Physics

Shahid YounasLecture 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

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

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

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 electrons3: Quenching Gas in tube balance Bremsstrahlung and

Characteristics X rays4: Potential difference between anode and cathode accelerates electrons

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 Rays2: Bremsstrahlung X Rays3: Electrostatic Rays4: Columbic Rays

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 space2: poses electrostatic forces3: anode is made of inert material4: cross-section of anode is very small

ANODE-ROTATING

In the figure below, what types of X rays are being produced?

1: Characteristic 2: Bremsstrahlung3: Rayleigh Rays 4: Both 1 and 2

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 Electron3: impurity in Filament4: All of above

Introduction to CT Physics

CATHODE- THE SOURCE OF ELECTRON

Helical Filament of tungsten wire

Thermionic emission

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 space2: poses electrostatic forces3: anode is made of inert material4: cross-section of anode is very small

Introduction to CT Physics

CATHODE- THE SOURCE OF ELECTRON

Thermal energy given to the carrier

overcomes the binding potential- work

function of the metal.

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.

Introduction to CT Physics

CATHODE- THE SOURCE OF ELECTRON

Adjustments in the filament current

(filament temperature) control the tube

current.

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.

FOCUSING CUP / CATHODE BLOCK

FOCUSING CUP / CATHODE BLOCK

Helical filament of tungsten wire is surrounded by “Focusing cup or cathode block”.

shapes the electron beam width and length.

FOCUSING CUP / CATHODE BLOCK

When no voltage is applied between the anode an electron cloud

- space charge cloud- builds around the filament.

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.

FOCUSING CUP / CATHODE BLOCK

X-ray tube is space charge limited up to 40 kVp.

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.

Production of X Rays

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Production of X Rays

“Me and my wife lived happily.... then we met. ”

ANODE

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.

ANODE

The fixed anode x-ray tube.

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.

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?

ANODE

Better Brems production

Handle substantial

heat deposition

Rough & Tough

10% Rhenium and 90% tungsten

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?

ANODE

Stationary and rotating anode configurations.

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.

ANODE-ROTATING

Rotating anodes have greater heat loading and higher x-ray

output capabilities.

ANODE-ROTATING

Electrons impart their energy on a continuously rotating

target, spreading thermal energy over a large area and mass

of the anode disk.

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.

ANODE-ROTATING

Do you know relation between Anode Angle and Focal Spot Size?

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.

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.

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.

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.

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.

ANODE-ROTATING

INTRODUCTION TO CT PHYSICS

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X ray Production Device

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