2: radiation physics and x ray Machine

Matter

Anything that has mass and takes up space

The fundamental unit of matter

atom

Components of an atom

protons, neutrons, electrons

Mass Number

the sum of the number of neutrons and protons

atomic number

number of protons

How are atoms arranged on the periodic table?

by increasing atomic number

Electrons

revolve around the nucleus in defined paths - orbits or shells

Maximum shells contained by an atom

7 shells

K shell

the orbital shell closest to the nucleus

Lettering of shells

K, L, M, N, O, P, Q

binding energy

electrons are held in orbit by electrostatic attraction between the positive nucleus

Is binding energy different for each shell?

Yes

What is binding energy dependent on?

the distance from the nucleus

Binding Energy (BE) Units

electron volts or kilo electron volts

Ionization in a neutral/stable atom

# of protons = # of electrons

Ionization

removal of an electron from an atom

the energy required to remove an electron must exceed...

the binding energy of the election

What is an ion pair?

positive ion and ejected electron

What is radiation?

the emission and propagation of energy through space

What are the two types of radiation?

particulate and electromagnetic

examples of particulate radiation

-electrons (beta particles and cathode rays)
-protons
-neutrons
-alpha particles

What is electromagnetic radiation?

movement of wave-like energy as a combination of electric and magnetic fields

Examples of electromagnetic radiation (7)

- x rays
- y rays
- UV
- visible light
- IR Radiation (AKA heat)
- microwaves
- radio waves

ionizing radiation

radiation w/enough energy to free electrons

non-ionizing radiation

Radiation that has enough energy to move atoms, but not enough to remove electrons

ionizing radiation examples

x-rays and gamma rays
(some UV)

non-ionizing radiation examples

Microwaves, Radio Waves, Infrared light and the visible light spectrum
(and some UV Rays)

3 properties of electromagnetic radiation

1) does not have mass or weight
2) no electrical charge
3) travels at the speed of light

Wilhelm Roentgen

discovered x-rays

Components of X ray Tube

- cathode
- anode
- leaded glass housing (vacuum)
- power supply

What is the cathode made of?

Tungsten wire filament

Molybdenum focusing cup

where cathode is placed -
focuses electrons on target (in a narrow beam)

Is the cathode negatively or positively charged?

negatively charged

what happens when the cathode is heated?

electrons are produced and accelerate towards a positive anode

What are the two types of anodes?

stationary and rotating

Are rotating anodes used in dental units?

NO

Is the anode positively or negatively charged?

positively charged

What is the anode made of?

a small tungsten target 2-3 mm thick that is embedded in a copper stem

Why is the anode embedded in a copper stem?

because copper is a better heat conductor than tungsten

Focal spot

area on the anode that electrons hit during x-ray production

Metal housing of x-ray tube head characteristics

Has transformers
Filled with insulating oil

Metal housing of x-ray tube head function

Protection of x ray tube
grounds the high voltage components

insulating oil (tubehead)

Surrounds x-ray tube and transformers, prevents overheating

Transformers (tube head)

alter voltage

Tube head seal

ALUMINUM or LEADED glass covering of the tube head that permits the exit of x-rays

Aluminum disks (tube head)

1. Filter out the non penetrating, longer wave length X-rays
2. sheets of 0.5 mm thick (aluminum)

X rays consist of a....

continuous spectrum of photon energies

only photons with sufficient energies can...

penetrate through anatomic structures and reach image receptors or sensors

Low energy photons cannot...

reach image receptors and cause unnecessary exposure to patients

how are low energy photons removed?

small metallic disc or filter

Types of filters

Inherent
Added

Examples of inherent filters

X ray tube housing
glass envelope
oil

Example of added filter

sheets of aluminum

Required total filtration for 1.5mm Aluminum

50-70 kVp

Required total filtration for 2.5mm Aluminum

Above 70 kVp

Collimator

lead plate with a hole that fits directly over the opening of metal housing
restricts the size of x ray beam

Position indicating device (PID)

lead lined cylinder that extends from the metal housing of the tubehead
aims and shapes the X-ray beam.

what do collimators regulate?

the shape and size of beam

What is most dental equipment collimated at? (measurement)

2.75 inches or 7 cm

step down transformer

a transformer that decreases voltage
110-220V to 3-5V

Step up transformer

a transformer that increases voltage
65,000 to 100,000 voltage

Autotransformer

voltage compensator that corrects minor fluctuations

Tube current

flow of electrons through the tube from cathode to anode

What is tube current controlled by?

mA

Tube voltage is controlled by

kVp

Timer (x ray unit)

controls duration of x ray exposure

thermionic emission

the release of electrons from the outer shell of tungsten filament

What are the two types of interactions when high energy electrons interact with the tungsten target?

Bremsstrahlung or Breaking Radiation
Characteristic Radiation

Bremsstrahlung radiation

1) electron passing close to a tungsten atom is attracted towards the nucleus
2) the electron changes direction
3)The kinetic energy lost by this electron is emitted as radiation

Most x rays are produced by what reaction?

Bremsstrahlung (70%)

Characteristic radiation

- results when the electrons bombarding the target eject inner orbit electrons
- removal of electrons from tungsten atom makes it positively charged
- the atom returns to neutral stage by emitting excess energy as radiation
(10-28% of x rays)

No interaction (patient)

area top 1/4 of head
9%

Coherent scattering (patient)

eyebrow/forehead region
7%

Compton Scatter (patient)

chin region
57%

Photoelectric absorption/effect (patient)

nose/center of face region
27%

what is the primary contributor of image formation?

photoelectric absorption

what happens during photoelectric absorption?

incident photon interacts with the inner shell electron of an atom in the patient's tissues
the photon gives all of its energy and disappears

incident photon has energy greater than...

the binding energy of the electron with which it interacts

Compton scattering

1) higher energy photon strikes a loosely bound outer electron
2) electron is removed it from its shell
3) the remaining energy is released as a scattered photon

does coherent scatter contribute to diagnostic imaging?

NO

Coherent scattering

(occurs with low energy x-rays, typically below the diagnostic range.)
1) The incoming photon interacts with the atom
2) atom becomes excited
3) The x-ray does not lose energy but changes direction.

X-ray beam quality

mean energy or Penetrating ability of the x-ray beam

Shorter wavelength x rays

more energy
more penetrating

Longer wavelength x rays

low energy
less penetrating capability

What does tube voltage control?

quality
wavelength
energy

Tube voltage

The potential difference between the cathode and the anode
(makes electrons move from cathode to anode)

what does voltage determine?

speed of electrons

What happens if speed of electrons increases?

electrons strike target with greater force and energy
the beam is penetrated with shorter wavelength/higher energy

Dental radiography voltage

65 to 100kV

What happens if:
85 to 100kV
Shorter wavelength
more energy

over penetration

What happens if:
65 to 75kV
Longer wavelength
less energy

under penetration

when is a higher kVp used?

when the area that needs to be examined is dense or thick

Amperage

determines the amount of electrons passing through the cathode filament

Ampere

the unit used to measure number of electrons or current flowing through cathode filament

1mA is how many amperes?

1000 amperes

1 milli ampere is how many amperes?

1/1000 of an ampere

Does a change in mA change the kinetic energy of electrons?

NO, only quantity is changed

how many mA required in dental radiography

7 to 15 mA

exposure time

time during which x rays are produced

how is exposure time measured

in impulses as x rays are created in a series of bursts rather than a continuous stream