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EXPOSURE FACTORS

R Hussein Ahmed Hassan


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Exposure factors are factors that control

density (blackening) and contrast ofradiographic image.

They are some of the tools that

technologists use to create high-quality

radiographs


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Exposure Factors Controlled by the

Operator

kVp

mA times Exposure Time = mAs

etermines the quality and quantity of

the exposure

FF (SI ), Focal Spot and Filtration are

secondary factors


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1- EXPOSURE FACTORS:

VP. :

It controls the quality of the beam, i.e.

PENETRATION.

It influences :

a: penetration power, i.e. beam quality;

kVp. penetration power.

b: Radiographic contrast;

kVp. 1/radiographic contrast.

c: Radiation dose to patient.

kVp. 1/radiation dose.


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KVP

kVp controls radiographic contrast.

kVp determines the ability for the beam

to penetrate the tissue.

kVp has more effect than any other

factor on image receptor exposure

because it affects beam quality.


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KVP

To a lesser extent it also influences the

beam quantity.

As we increase kVp, more of the beam

penetrates the tissue with higher energy

so they interact more by the Compton

effect.

This produces more scatter radiation

which increases image noise and

reduces contrast.


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KVP

50 kV 79% is photoelectric, 21%

Compton, < 1% no interaction

80 kVp 46% is photoelectric, 52%

Compton 2% no interaction

110 kVp 23% photoelectric, 70%

Compton, 7% no interaction

As no interaction increases, less

exposure is needed to produce the

image so patient exposure is decreased.


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High kVp.

low radiographic contrast

Low kVp.

High radiographic contrast


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MA.:

1 Ampere = 1 C/s = 6.3 x 1018 electrons/

second.

The mA selected for the exposure

determines the number of x-rays

produced.

The number of x-rays are directly

proportional to the mA assuming a fixed

exposure time.

100 mA produced half the x-ray that 200

mA would produce.


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MA

Many x-ray machines are identified by

the maximum mA or mAs available.

A MP 500 has a maximum mAs of 500

mAs.

A Universal 325 has a maximum mA of

300 and maximum kVp of 125


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MA

More expensive three phase machines

will have a higher maximum mA.

A General Electric MST 1050 would have

1000 mA and 150 kVp.


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EXPOSURE TIME

The exposure time is generally always

kept as short as possible.

This is not to reduce patient exposure

but to minimize motion blur resulting

from patient movement.

This is a much greater problem with

weight bearing radiography.


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EXPOSURE TIME

Older machine express time as a

fraction.

Newer machines express exposure time

as milliseconds (ms)

It is easy to identify the type of high

voltage generation by looking at the

shortest exposure time.


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EXPOSURE TIME

Single phase half wave rectified fasted

exposure time is 1/60 second 17 ms.

Single phase full wave rectified fastest

exposure time is 1/120 second or 8 ms

Three phase and high frequency can

provide exposure time down to 1 ms.


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(4) MAS. :

It affect the total number of x-ray

produced by the tube during exposure, i.e.

QUANTITY.

It is the product of two quantities;

mA. the tube current;

s. the exposure time;


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MAS

mA and exposure time is usually

combined and used as one factor

expressed as mAs.

mAs controls radiation quantity, optical

density and patient dose.

mAs determine the number of x-rays in

the beam and therefore radiation

quantity.

mAs does not influence radiation quality.


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MAS

Any combination of mA and time that will

give the same mAs should provide the

same optical density on the film. This is

referred to as the reciprocity law.

As noted earlier for screen film

radiography, 1 ms exposure and

exposure longer than 1 seconds do not

follow this rule.


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MAS

On many modern machines, only mAs

can be selected. The machine

automatically gives the operator the

highest mA and shortest exposure time.

The operator may be able to select mA

by what is referred to as Power level.


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MAS

mAs is one way to measure electrostatic

charge. It determines the total number

of electrons.

Only the quantity of the photons are

affected by changes in the mAs.

Patient dose is therefore a function of

mAs.


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20 mA. X 1.0 s = 20 mAs

40 mA. X 0.5 s = 20 mAs

80 mA. X 0.25 s = 20 mAs

200 mA. X 0.1 s = 20 mAs

400 mA. X 0.05s = 20 mAs

Ampere is 1 coulomb (C) of electrostatic

charge flowing each second.

1A = 1C/s = 6.3 X 10

18

electron/s

20 mAs = 0.2 Amperes.

This charge releases this No. of electrons:

6.3 X 10

18

X 0.2 = 1.26 X 10

18

electron/s


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(5) Focal spot:

Most x-ray tubes offer two focal spot

sizes:

a. Fine focus:

b. Broad focus:


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a/ Fine focus: (0.3 0.6 mm

2

)

It records fine details.

It can not withstand too much heat.

Its usage may require long exposure

time.

Used whenever geometric factors are

more (long subject-film distance, short

FF ... etc).


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a/ Broad focus: (0.6 1.2 mm

2

)

It can withstand too much heat.

Always used in combination with short

(s) and fast film/screen system.

Used whenever voluntary or

involuntary motion is highly expected.

Used when radiosensitive organ is

within exposed area or 10 cm from

collimation border.


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Two focal spot


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F

OCAL

S

POT

S

IZE

The focal spot size limits the tubes

capacity to produce x-rays. The

electrons and resulting heat are

placed on a smaller portion of the

x-ray tube.

The mA is therefore limited for the

small focal spot. This results in

longer exposure times with greater

chance of atient movement.


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FOCAL SPOT SIZE

If the mA is properly calibrated, the

focal spot will have no impact on the

quantity or quality of the beam.


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(6) F.F.D. :

The intensity of x-ray beam reduces with

increased FF .

It follows the Inverse Square Law ( I.S.L.) .

I 1/d

2

.


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D

ISTANCE

istance affects the intensity of the x-

ray beam at the film but has no effect on

radiation quality.

istance affects the exposure of the

image receptor according to the inverse

square law.


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INVERSE SQUARE LAW

mAs (second exposure) SI 2 2nd exposure

---------------------------- = ------------------------

mAs (first exposure) SI 2 1st exposure


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ISTANCE

The most common source to image

distances are 40 (100 cm) and 72(182

cm)

Since SI does not impact the quality of

the beam, adjustments to the technical

factors are made with the mAs.

To go from 40 to 72 increase the mAs

3.5 time.


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ISTANCE

Increasing the distance will impact the

geometric properties of the beam.

Increased SI reduces magnification

distortion and focal spot blur.

With the need to increase the mAs 3.5

times for the 72 SI , tube loading

becomes a concern.


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ISTANCE

72 SI is used for Chest radiography

and the lateral cervical spine to reduce

magnification.

72 SI used for the full spine to get a

36 beam.


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(7) FILTERATION:

Thin sheet of Al (aluminum) 1mm or 2mm

thick added to the pathway of radiation to

filter the low energy radiation.

Increasing filtration will increase the

quality and reduce the quantity of the

beam.

It removes low energy radiation:

Reduce skin dose;

Harden the beam;


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F

ILTRATION

All x-ray beams are affected by the

filtration of the tube. The tube housing

provides about 0.5 mm of filtration.

Additional filtration is added in the

collimator to meet the 2.5 mm of

aluminum minimum filtration required by

law.

2.5 mm is required for 70 kVp.


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FILTRATION

3.0 mm is required for at 100 kVp.

3.2 mm is required for operations at 120

kVp.

Most machines now are capable of over

100 kVp operation.

We have no control on these filters.


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FILTRATION

3.0 mm is required for at 100 kVp.

3.2 mm is required for operations at

120 kVp.

Most machines now are capable of

over 100 kVp operation.

We have no control on these filters.


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FILTRATION

C

HIROPRACTIC RA IOGRAPHY IS A LEA ER IN

THE USE OF COMPENSATING FILTERS

. W

E

HAVE TOTAL CONTROL OVER COMPENSATING

FILTRATION

.

IN AREAS OF THE BO Y WITH HIGH SUBJECT

CONTRAST OR WI E IFFERENCES IN ENSITY

,

COMPENSATING FILMS IMPROVE IMAGE

QUALITY AN RE UCE PATIENT EXPOSURE

.


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THE END

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