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THE PHYSICS OF RADIOLOGY

Production of X-ray

﴾ حیممنالرح مهللاالربس ﴿

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Photograph & radiograph ?

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What is radiation ?

Radiation is energy in

the form of waves or

particles.

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X- Rays

electron beam generator

tungsten target metal

resultant X-ray beam

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Dec. 22, 1895 Woman with fluoroscope, 1896

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Various x-ray unit designs Thomas Edison and his

skiascope, 1896

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Early medical x-ray techniques

1913 dental unit Chest fluoroscopy, 1901

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Characteristics of X-rays

- +

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Electrons Interaction with matter

عبور آزاد1.

(عبور الكترون از كنار الكترون، عبور الكترون از كنار هسته) انحراف 2.

(برخورد الكترون با الكترون، برخورد الكترون با هسته)برخورد 3.

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عبور آزاد

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عبور الكترون از كنار الكترون

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عبور الكترون از كنار هسته

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E1< E2 •برخورد الكترون با الكترون

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E1 > E2

X K

X L

برخورد الكترون با الكترون•

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Characteristic X Rays (II)

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Atom characteristics

A, Z and associated quantities

• Hydrogen A = 1 Z = 1 EK= 13.6 eV

• Carbon A = 12 Z = 6 EK= 283 eV

• Molybdenum A = 96 Z = 42 EK= 19.0 keV

• Tungsten A = 183 Z = 74 EK= 69.5 keV

• Uranium A = 238 Z = 92 EK= 115.6 keV

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برخورد الكترون با هسته

X- ray

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What do we need for production of

X-RAYS

1. Glass enclosure

2. Electron source

3. Potential difference

4. Target

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X-Ray tube

Filament

Rotor

Vacuum envelope Anode

Cathode

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2-Electron source (filament)

• Specialities of filament

1. Is made of tungsten(0.2 mm in diameter)

2. It is coiled(0.2 cm in diameter &1cm in length)

3. Potential difference 10V

4. Current 3-5A

-The X-RAY tube current measured in mA

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Example of a cathode

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Electron production

• Thermoionic emission (Edison effect)

• Space charge

• Space charge effect

• Residual space charge

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Function of focusing cup

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Kind of focus

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1: long tungsten filament

2 : short tungsten filament

3 : real size cathode 1: mark of focal spot

housing cathode

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3-Target

• Anode

• Is made of 90% tungsten and 10%

rhenium

• >99% of electron energy is converted into

heat

• Stationary anode

• Rotating anode(the purpose of the rotating anode is to

spread the heat produced during an exposure over a large area)

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Anode characteristic

1 : anode track

2 : anode track

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Line focus principle

• The anode angle is usually 15 to 20˚

• Heel effect(the intensity of film exposure on the anode side of

the X-ray tube is significantly less than that on the cathode side of the

tube)

• Actual focal spot size

• Apparent focal spot size

6: X Ray production 31

THE SMALLER THE ANGLE

THE BETTER THE RESOLUTION

Anode angle (II)

Angle

Incident electron

beam width

Apparent focal spot size

Actual focal

spot size

Film

Angle

Incident electron

beam width Increased

apparent

focal spot size

Actual focal

spot size

Film

„

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X-ray tubes

Coolidge tube Modern tube with

rotating anode

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4-Potential difference

• Cathode

• Focusing cap

• Grid-controlled X-ray tube

• Anode

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With alternating current, x-rays are only produced during the

positive half of the cycle (red areas above). There is a large

fluctuation in the voltage between the filament and the target,

contributing to a wide range of x-ray energies. X-ray machines

with constant potential (“direct current”) are preferred over the

standard alternating current. This provides more efficient x-ray

production and less exposure time per radiograph. Most of the

newer x-ray machines utilize constant potential.

Constant Potential (800 cycles.sec.)

60-cycle Alternating Current

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Basic elements of the X Ray

source assembly •Generator : power

circuit supplying the

required potential to

the X Ray tube

•X Ray tube and

collimator: device

producing the X Ray

beam

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X Ray tubes

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X Ray tube components

• Cathode: heated filament which is the source of the electron beam directed towards the anode – tungsten filament

• Anode (stationary or rotating): impacted by electrons, emits X Rays

• Metal tube housing surrounding glass (or metal) X Ray tube (electrons are traveling in vacuum)

• Shielding material (protection against scattered radiation)

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X-ray Tube molybdenum focusing cup

tungsten filament

electron flow

tungsten target

copper sleeve

unleaded glass window

leaded glass

vacuum inside tube

electrical connections

x-ray

When the exposure switch is depressed, the filament is heated,

producing a cloud of electrons around the filament. The high

voltage between the cathode (filament, focusing cup) and the

anode (target, copper stem) pulls the electrons across the x-ray

tube to interact with the target to produce x-rays.

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oil filter

timer

exposure

switch

collimator

PID

Step-up Trans.

Step-down Trans.

kVp

Auto

mA

The low-voltage circuit (green in diagram above) controls the

heating of the filament in the x-ray tube. The mA control regulates

the amount of voltage that passes through the step-down

transformer, which in turn reduces the voltage to about 5 volts;

this is enough to heat the filament and produce electrons.

X-ray Machine Components

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The high-voltage circuit (red in diagram) controls the voltage across

the x-ray tube. It is regulated by the kVp selector (a rheostat) and the

step-up transformer, resulting in a very high voltage which pulls the

electrons from the filament to the target. The higher the kVp, the

greater the energy of the electrons

oil filter

timer

exposure

switch

collimator

PID

Step-up Trans.

Step-down Trans.

kVp

Auto

mA

X-ray Machine Components

7: X Ray beam 42

Tube filtration

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kVp determines the voltage across the x-ray tube. This

ultimately determines the energy (penetrating ability) of the x-

ray beam. Higher kVp = higher average energy (dotted lines

above) and higher maximum energy. There is also an increase

in the number of x-rays produced when kVp is increased.

Increasing the kVp allows you to reduce exposure time (An

increase of 15 kVp allows you to cut the exposure time in half).

It is recommended that at least one x-ray machine in the office

have the capability of varying the kVp (to image children,

patients with tremors, etc., which requires minimum exposure

time). In general, a higher kVp is preferred, especially for

periapical and periodontal diagnosis.

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An increase in the mA setting or the exposure time

results in an increase in the number of x-rays

produced. There is no change in the average energy

of the x-ray beam. A machine with variable mA

settings would normally be set at the highest mA,

allowing for a reduced exposure time.

Exposure time: 60 impulses = 1 sec.

IAEA 45

X Ray spectrum: tube current

400 mA

200 mA

X Ray Energy (keV)

Number of X

Rays per unit

Energy

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X Ray spectrum: Target Z

Higher Z

Lower Z

X Ray Energy (keV)

Number of X

Rays per unit

Energy

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X Ray spectrum: Target Z

Three Phase

Single

Phase

X Ray Energy (keV)

Number of X

Rays per unit

Energy

IAEA 7: X Ray beam 48

Factors affecting

• X Ray Quantity

• TUBE CURRENT (mA)

• EXPOSURE TIME (s)

• TUBE POTENTIAL

(kVp)

• WAVEFORM

• DISTANCE (FSD)

• FILTRATION

• X Ray Quality

• TUBE POTENTIAL

(kVp)

• FILTRATION

• WAVE FORM

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Density = degree of darkening. Affected by:

Exposure factors (Increase = increase in density)

Size of head (soft tissue, bone): Increase will result

in decrease in film density

Object density (bone, teeth, restorations): Increase

will result in decrease in film density

Film fog (scatter, storage): Results in increase in

overall film density

Contrast = density differences. Increased by:

Lowering kVp

An increase in subject contrast

Technically, higher contrast (lower kVp) is preferred for

caries detection. Lower contrast (higher kVp) is

recommended for imaging periapical and periodontal

changes. For general use, a medium kVp (70-75) is usually

selected.

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Measures how well the details (boundaries) of an object are reproduced on a radiograph

Increased by:

Source-object distance

Object-film distance

Film crystal size

Motion will decrease sharpness

Sharpness

Decreased by:

Source-object distance

Object-film distance

Magnification

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Increasing the distance from the target of the x-ray tube (focal

spot, focus) to the object (teeth/film) (FFD = focus-film

distance) will result in an increase in sharpness and a

decrease in magnification. This results when a longer PID

(cone) is used.

Target

16” from film

Film

Target

8” from film 8” FFD image

16” FFD image

Moving the film closer to the teeth will also increase sharpness

and decrease magnification.

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