Unit 3 Radiation Characteristics

Recap the production of x- rays

Machine turned “on”

 Electrical current (Amperage mA) goes to cathode

 Electrons “boil off”

 Thermionic Emission

 Exposure button pressed so Voltage (kV) sends electrons to anode

 Collide with target & become x-rays

 Exit through:

—> Aluminum Filter

—> Collimator

—> PID cathodeanode

How does X-ray work?

X-Rays pass through easily —→ X-rays aren’t permitted to pass

through

Higher density structures and materials absorb more radiation permitting less x-rays to travel through the structure or material &

reach the dental x-ray film or senso

Radiolucent (RL)

• structures appear dark or black

• Radiolucent structures lack density; they permit x-rays to pass with little or no resistance

• Examples: air spaces and pulp tissue

Radiopaque (RO)

• structures appear white or light

• Radiopaque structures are dense and resist passage of the x-ray beam

• Examples: enamel, bone, amalgam

Radiographic Density

refers to the overall darkness or blackness of a processed

radiograph/image.

Radiographic Contrast

refers to the degrees of blackness between adjacent areas on an image

What are the exposure values?

mA

kV

T

Distance

Why understand the concepts of density and contrast?

 Changes in settings will affect:

• a patient’s exposure to radiation

• the quality of the image for diagnosis

Voltage

• A measurement of force

• the penetrating ability of the x-ray beam.

• X-ray Beam QUALITY

• The power behind the x-ray

When voltage is increased, the speed of the electrons is?

• Increased

• The electrons strike the target with greater force and energy, resulting in a more penetrating x-ray beam

Lower voltage (kVp)

Altering the voltage alters the wavelengt

Higher voltage (kVp)

kilovoltage peak (kVp) = maximum peak voltage

^kVp = ^ penetrating power

^kVp= ^ darkness

Electromagnetic spectrum

Wavelength determines energy and penetrating power

Lots of shades of grey

low contrast, long scare contrast

Few shades of grey

high contrast, short scale contrast

Contrast equals what?

shades of grey

kVp affects what?

Contrast

Contrast is controlled by?

• kV only

• low kV= high contrast

• high kV= low contrast

Amperage

• the amount of x-rays.

• X-ray Beam Quantity

mA regulates the temperature of the filament, therefore:

• More mA

• More heat

• More electrons

• More x-rays

An increase in____ affects the ___ of

the resulting radiograph

mA, density

• mA has no effect on contrast

Exposure time (T)

• Exposure time affects the length of time that electrons move across the tube gap:

high T = increase in number of x-rays

high T = increase in overall density of the radiograph

high T = increase in intensity of beam

• INVERSELY RELATED TO mA: if one increases the other must decrease to maintain density.

Distance

As x-rays travel away from the anode they lose?

energy

Intensity

• the total energy in an x-ray beam.

 The sum of all the factors:

 kVp

 mA

 T

 distance

The INTENSITY of the x-ray beam is affected by :

high kVp = high energy and high intensity

high mA = high number of x-rays and high intensity

high T = high number of x-rays and high intensity

high distance = low intensity

As x-tray travels they?

• lose energy / intensity

• We have to compensate this loss of intensity by increasing

other exposure factors (kVp, mA, exposure time).

• INVERSE SQUARE LAW: double the distance = 1/4 th

The x-ray beam that exits from a short PID is?

more intense than the beam exiting from a longer one

Kilovoltage (kV)

Kilovoltage affects the speed of the electrons moving across the

gap:

high kVp = faster electrons

high kVp = shorter wavelength x-rays

high kVp = a higher energy beam

Decrease kV

• Fewer shades of grey

• High contrast

• Short Scale

Increase kV

• Lots of shades of grey

• Low contrast

• Long Scale

KVP RULE

if increase kV by 15 then decrease T by ½ OR if decrease kV by 15

then double T

high kVp = increased penetrating ability

high kVp = increase intensity

high kVp = an increase in density

high kVp = a decrease in contrast

Milliamperage (mA)

Milliamperage affects the number of the electrons

available to move across the gap:

high mA = increase in number of x-rays

high mA = increase in overall density

high mA = increase in intensity of beam

Time (T)

Exposure time affects the length of time that electrons move across the tube gap:

high T = increase in number of x-rays

high T = increase in overall density

high T = increase in intensity of beam

INVERSELY RELATED TO mA: if one increases the other must decrease to maintain density

Target to Receptor Distance (TRD)

• Any time TRD is changed there must be a change to other exposure factors

• higher TRD = reduce intensity of beam because they have further to travel

What does kV control?

•Quality (Power)

•Density

•Contrast

What does mA control?

•Quantity (amount)

•Density

What are some of the exposure factors that will affect the density (darkness) of the radiograph?

mA, T, kV, and distance

• mA affects number of x-rays

• T affects number of x-rays

• kV affects intensity

• distance affects intensity

What exposure factors affect the contrast of a radiograph?

kVp only!

• Kilovoltage affects the quality of the beam - its

penetrating ability!

• The more penetrating the beam, the lower the contrast.

What exposure factors affect the intensity of the x- ray beam?

kVp, mA, T & distance

• kVp affects the energy of the beam

– more energy = more intensity.

• mA affects the amount of x-rays

– more x-rays = more intensity.

• T affects the amount of x-rays

– more x-rays = more intense.

• distance - the further x-rays must travel, the less intense

they will be.

If