chapter 3: x-ray production ionization and excitation · pdf fileslide 1 of 4 chapter 3: x-ray...

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Chapter 3: X-RAY PRODUCTION

Ionization and Excitation

Ionization is “knocking” an electron out of an atom

– Creates a free electron + ion (an atom with +1 charge)

– Occurs when radiated with energy above the electron binding

energy

Excitation is “knocking” an electron to a higher orbit

– When the radiation energy is lower than the binding energy

After either ionization or excitation, an atom has higher energy

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X-Ray Beam Quality and Intensity

The x-ray beam has two main properties you need to understand.

Beam QUALITY is the ability of the beam to penetrate an object,

its all about the penetrating power of the x-ray photons.

Beam INTENSITY this is the number of x-ray photons in the beam.

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Factors Affecting X-ray Quality

1. Filtration.

The primary purpose of adding filtration to the x-ray beam is to

remove low energy x-rays that have no chance of getting to the film.

As filtration is increased, so is the beam quality, but quantity is

decreased.

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There are three types of filtration:

i. Inherent Filtration: The glass envelope window equals about 0.5

mm Al

ii. Added Filtration: Added in collimator

iii. Compensating Filtration: Used to improve image quality or

radiation reduction

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i. Inherent Filtration

The glass envelope of the tube filters the emerging beam. In

diagnostic x-ray tubes the glass is equal to about 0.5 mm Al.

As tube ages and more tungsten is vaporized, tungsten will build up on

the inside of the tube that will add more filtration.

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ii. Added Filtration

One or two mm of aluminum is added filtration placed in the

collimator. This filtration is generally placed on the mirror of the

collimator.

This filtration attenuates x-rays of all energies emitted from the tube.

This shifts the spectrum to the high side.

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iii. Compensating Filters

Compensating filters are added to the beam by the operator to

compensate for differences in subject tissue density or type.

In areas of the body where there are great differences in tissue

density, compensating filters are used to reduce exposure in the area

of less density.

This reduced patient exposure and improves image quality. The

thoracic spine and full spine x-rays need filtration.

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Compensating Filters

This is the 40” Cervicothoracic Compensating Filter.

It may be called the thyroid filter as it reduces the exposure to the upper thorax.

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Compensating Filters

This heart shaped filter is used to reduce exposure to the ovaries of females of child bearing age.

It reduces exposure by about 85%.

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2. Kilovoltage

As the kVp is increased, so is beam quality and therefore HVL.

An increase in kVp results in a shift of the x-ray emission spectrum

towards the higher energy side.

This increases the effective energy of the beam, making it more

penetrating.

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Factors Affecting X-ray Intensity

A number of factors affect X-ray Intensity @ Quantity. These same

factors also control radiographic film density:-

a) Milliamperage- Seconds (mAs)

b) kVp

c) Distance

d) Filtration

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1. mA x time (s) = miliamperage seconds = mAs

The X-ray quantity is directly proportional to the mAs. If we double

the mAs, the number of electrons striking the target is doubled.

300 mA @ 1/30 second = 10 mAs

200 mA @ 1/20 second = 10 mAs

100 mA @ 1/10 second = 10 mAs

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2. Kilovoltage

X-ray intensity varies rapidly with changes in kVp.

The change in quantity is proportional to the square of the ratio of the

change.

If the kVp is doubled, the intensity would increase by a factor of four.

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What really happens when the kVp is increased?

When kVp is increased, the penetrability of the x-rays is increased and

relatively fewer x-rays are absorbed in the patient.

More rays pass through the patients to interact with the film.

To maintain a constant exposure of the film, an increase of 15% in

kVp should be accompanied by a reduction of one half the mAs.

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3. Distance

Radiation intensity from an x-ray tube varies inversely with the square

of the distance from the target. This is referred to as the inverse

square law.

It is the same for any type of electromagnetic energy.

We will explores distances in the Lab.

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4. Filtration

X-ray machines have metal filters inserted into the useful beam.

The primary purpose is the remove the low energy beam that reach

the patient and are absorbed superficially.

These low energy photons contribute nothing to the formation of the

radiographic image.

Filters therefore reduce patient exposure.

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Primary and Secondary Radiation

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Primary Radiation

The radiation that coming directly from a source, such as a radioactive

substance or an x-ray tube, without interactions with matter.

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Secondary Radiation

This type of radiation is produced by a scattering of the primary x-ray

beam.

The x-ray photons and photo-electrons in the beam undergo a change

of direction after interaction with atoms and molecules as they pass

through a substance.

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X-ray Tube

The major components of the modern X-ray tube are: -

a) cathode (electron source)

b) anode (acceleration potential)

c) rotor/stator (target device)

d) glass/metal envelope (vacuum tube)

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a) Cathode

The cathode consists of:

consists of the filament,

focusing cup, and associated

wiring.

a spiral of heated low

resistance tungsten wire

(filament) is for electron

emission. Wire is heated by

filament current.

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b) Anode

Consists of the anode, stator,

and rotor.

Positively charged so that the

electrons from the filament

(cathode) are attracted to it

to produce x-rays.

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c) Rotor and Stator

The rotor is the non-

stationary part of a rotary

electric motor or alternator,

which rotates because the

wires and magnetic field of

the motor are arranged so

that a torque is developed

about the rotor's axis.

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The stator is the stationary

part of an electric motor or

alternator. Depending on the

configuration of the motor the

stator may act as the field

magnet, interacting with the

armature to create motion. The

stator may be either a per

magnet or an electromagnet.

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Rotating and Stationary Anode

Stationary Anode: An anode assembly that is immobile.

Rotating Anode: An anode assembly that turns during exposure.

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d) Glass and Metal Envelope

The envelope is the glass

housing that protects the

tube. It is also used to help

protect from excessive

exposure to x-rays. The

envelope is the first part of

the filtration system.

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Basic X-ray Circuit

There are two main parts of the circuit, one is the main circuit and

the second is the filament circuit.

A. Main part of X-Ray Circuit: supplies power to the x-ray tube so that

x-rays are produced.

B. Filament Circuit: supplies power to the filament of the x-ray tube so

that the filament supplies enough electrons by thermionic emission.

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Basic X-ray Circuit

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a) Main X-ray Circuit

Main Switch: The switch that generates the power to the x-ray tube.

Exposure Switch: A remote control device that permits current to

flow through the circuit.

Timer: Device used to end the exposure at an accurately measured

preset time.

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In the diagram below are the important parts of the circuit. The blue

part is the main x-ray circuit and the tan part is the filament circuit.

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Components

1. main breaker - this is where the alternating current comes from to power the circuit.

2. exposure switch - when you push the button to start an exposure this switch closes to

start the exposure.

3. autotransformer - this is where you adjust the kVp for the exposure.

4. timer circuit - this part of the circuit stops the exposure.

5. high-voltage step-up transformer - this transformer bumps the voltage up so that the x-

ray tube has very high voltage to make the electrons have enough energy to form x-rays.

6. four-diode rectification circuit - this makes the current only go in one direction through

the x-ray tube.

7. filament circuit variable resistor - this variable resistor adjusts the current going to the

filament.

8. filament step-down transformer - this transformer steps the voltage down and therefore

the current up.

9. x-ray tube - this is where the x-rays are created.

10. rotor stator - this rotates the anode.

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b) Filament Circuit

Control Factor Electrical Device and Location in Circuit

kVp Selection kVp LevelAutotransformer (between incoming line and

exposure switch)

mA Selection Filament CurrentVariable resistor (in filament circuit between

incoming line and step-down transformer)

Time Selection Length of exposureTimer circuit (between exposure switch and

step-up transformer)

Rotor Switch Speed of rotating anodeStator (separate circuit from stator of anode

motor)

Exposure Switch Moment of exposureSwitch (between autotransformer and timer

circuit)

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References

John Ball, Adrian D. Moore, Steve Turner (2008) Essential Physics for

Radiographers, Blackwell

Richard R. Carlton, Arlene McKenna Adler (2005) Principles of

Radiographic Imaging, Delmar

chapter 3: x-ray production ionization and excitation · pdf fileslide 1 of 4 chapter 3: x-ray...

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