Factors affecting the

X-Ray output

Dr. Nahla Nagy

What is intensity?

1.Quantity

2.Quality

Intensity:

Total energy contained in the beam (product of quality and quantity of x-ray photons) per unit area per unit time

Quantity is the number

Quality is the energy

Together = the intensity of the X-ray

X-Ray Quantity

X-ray quantity: measure of the number of x-ray photons in the useful beam.

AKA x-ray output, intensity, or exposure Unit Roentgen (R) Directly affected by: Miliamperage-second (mAs) Kilovoltage (kVp) Distance (d)

X-Ray Quality

X-ray quality: : measurement of the penetrating ability of the x-ray beam.

Describes the distance an x-ray beam travels in matter. High energy x-ray photons travel farther in matter- more penetrating Numerically represented by the Half-value layer (HVL).

Factors Affecting IntensityFactors affecting intensity of x-ray beam actually

affect no. of photons or penetration power of

photon in x-ray beam.

1.Target Material

2.X-ray Tube Current (Miliamperage- mA)

3.Tube voltage (kVp)

4.Exposure time (s)

5.Filtration

6.Collimation

7.Source to film distance

Target Material

●Determines efficiency of X-ray production

●Higher the atomic number- greater will be

the efficiency of the production of x rays

X-ray Tube Current (mA)

1.Produced by flow of electrons from their point of origin (filament) to the anode (target) of x-ray tube

2.A change in filament voltage of about 5% result in 20-30% change in x-ray tube current (mA)

3.Hotter filament emits more electrons and adds electrons to the current electrons (mA) across the x-ray tube

4.Increased mA means increase in quantity of electrons available to hit the target -that results an increase in the quantity of x-ray photons

mA is directly proportional to quantity of x-ray beam

y axis = number of X-ray photons (quantity)x axis = energy of X-ray photons (quality)Area under the curve = intensity (product of quantity and quality)

Exposure Time (s)

1) Increasing time (s) heats filament for a

longer time.

2) Longer the filament is heated more

electrons are produced.

3) Increase in quantity of electrons

mAs (miliamperage seconds)

●mA and exposure time have a direct effect on the quantity of X-ray photons produced

●mAs is the product of mA (current) and exposure time (seconds); determine total number of x-ray photons produced

●The miliamperage required for a given exposure is inversely proportional to the exposure time

mAs Rule:

Original mA x Original time = New mA x New time

mA multiplied by s

ExampleIn an examination controls are set for 10 mA

and time is 30 sec, what is the mAs?

10mA x 30s = 300 mAs

If mA is increased to 15, how much time is

required to produce same mAs?

(original mA x original time = new mA x new time)

10mA x 30s = 15mA x Xs

           X = 20 sec.

Tube Voltage (kVp)

1.Increasing kVp increases potential difference between the cathode and anode.

2.kVp controls the speed of the electrons, increasing kVp will

1.increase the number of x ray photons produced per unit time

2.Shorter wavelength and more penetrating photons of x-ray radiation are produced (i.e., increases speed)

3.Increase in quantity and quality of x-rays.Quantity = number (speed)Quality = energy

Increasing kVp, increases both number and

energy of x-rays Drastic increase in intensity (area under the graph)

Tube Voltage (kVp)

●Intensity is proportional to (kVp)2

●In dentistry to maintain same density :rule

of thumboIncrease of 15 kVp requires halving the exposure

time

oDecrease of 15 kVp requires doubling the

exposure time

Filtration

Filtration is the removal of low energy,

less penetrating photons from x-ray

beam by placing filter in the path of x-

ray beam. Filters are aluminum sheets placed at the end of the tube where x-rays exit

FiltrationTypes of Filtration

1.Inherent-X-ray tube and its housing

2.Added-Sheets of metal placed in

the path of the beam

3.The patient

Filtration1.No. of photons decreas

2.Mean (average) energy of photons increase

3.Maximum energy is unchanged

Effects quality and quantity of x-ray beam

Half value Layer

●Definition: The thickness of an absorber

required to reduce the number of x-ray

photons passing through it by one half of

its original value

●In dentistry :

69 kVp/ below         HVL of 1.5mm Al

70-90 kVp               HVL of 2.5 mm Al

Collimation

A collimator is a metallic barrier with an

aperture in the middle used to reduce the

size of the x-ray beam, thus, the volume of

irradiated tissue within patient is also

reduced

Main Functions

●Protect the patient

●Decrease the scatter radiation

Collimation

1.Reduces patient exposure

2.Reduces scatter radiation

3.Increases film quality

Round beam limiting device, 20

cm (8”) focus-film distance, 6 cm

(2,36”) output field diameter. 

Rectangular beam limiting device as an

option to reduce patient’s exposure to

radiation.

www.jzimaging.com/SigmaBiomedics_Intraos_70.htm

Collimation

1.Number of photons decreases

2.Mean energy of photons is unchanged

3.Maximum energy of the photons is

unchanged

Inverse Square LawThe intensity of the X ray beam is inversely proportional to square of the source-film distance

I1/I2 = (D2)2/(D1)2

Source –film distance

●The x-rays of primary beam emerge from the

protective tube housing not as parallel rays but as divergent rays

●Intensity of the x-ray beam decreases as the distance from

the

source increases

Source –film distance

●Doubling the SFD distance ,exposure time

will have to be increased by the factor of 4

to maintain the same density on the film

Inverse Square Law

1.No of photons decreases

2.Mean energy of photons is unchanged

3.Maximum energy of the photons is

unchanged

Review

Factors affecting intensity

Tube voltage (kVp)

Current (mA)

exposure time (s)

Filtration

Collimation

Source to film

distance

Factors affecting Quantity

kVp

mA

Exposure time Filtration Collimation

Distance

Factors affecting

Quality

kVp

Filtration

Thank You