EXPOSURE FACTORSDR Hussein Ahmed Hassan

Exposure factors are factors that control

density (blackening) and contrast of

radiographic image.

They are some of the tools that

technologists use to create high-quality

radiographs

Exposure Factors Controlled by the Operator

kVp mA times Exposure Time = mAsDetermines the quality and

quantity of the exposureFFD (SID), Focal Spot and

Filtration are secondary factors

1- EXPOSURE FACTORS:

KVP. : 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.

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.

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.

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.

High kVp.low radiographic

contrast

Low kVp.High radiographic

contrast

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.

MA

Patient dose is also directly proportional to the mA with a fixed exposure time.

A change in mA does not affect kinetic energy of the electrons therefore only the quantity is changed.

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

MA

More expensive three phase machines will have a higher maximum mA.

A General Electric MST 1050 would have 1000 mA and 150 kVp.

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.

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.

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.

(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;

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.

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.

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.

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.

20 mA. X 1.0 s = 20 mAs40 mA. X 0.5 s = 20 mAs80 mA. X 0.25 s = 20 mAs200 mA. X 0.1 s = 20 mAs400 mA. X 0.05s = 20 mAs

Ampere is 1 coulomb (C) of electrostatic

charge flowing each second.

1A = 1C/s = 6.3 X 1018 electron/s20 mAs = 0.2 Amperes.

This charge releases this No. of

electrons:

6.3 X 1018 X 0.2 = 1.26 X 1018 electron/s

(5) Focal spot:Most x-ray tubes offer two focal

spot sizes:

a. Fine focus:

b. Broad focus:

a/ Fine focus: (0.3 – 0.6 mm2)

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 FFD ... etc).

a/ Broad focus: (0.6 – 1.2 mm2)

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.

Two focal spot

FOCAL SPOT SIZE The focal spot size limits the tube’s

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 patient movement.

FOCAL SPOT SIZE

If the mA is properly calibrated, the focal spot will have no impact on the quantity or quality of the beam.

(6) F.F.D. :

The intensity of x-ray beam reduces

with increased FFD.

It follows the Inverse Square Law

( I.S.L.) .

I 1/d2.

DISTANCE

Distance affects the intensity of

the x-ray beam at the film but has

no effect on radiation quality.

Distance affects the exposure of

the image receptor according to

the inverse square law.

INVERSE SQUARE LAW

mAs (second exposure) SID2 2nd

exposure

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

------------------------

mAs (first exposure) SID2 1st

exposure

DISTANCE

The most common source to image distances are 40” (100 cm) and 72”(182 cm)

Since SID 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.

DISTANCE

Increasing the distance will impact the geometric properties of the beam.

Increased SID reduces magnification distortion and focal spot blur.

With the need to increase the mAs 3.5 times for the 72” SID, tube loading becomes a concern.

DISTANCE

72” SID is used for Chest radiography and the lateral cervical spine to reduce magnification.

72” SID used for the full spine to get a 36” beam.

(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;

FILTRATION

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.

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.

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.

FILTRATION

CHIROPRACTIC RADIOGRAPHY IS A LEADER IN THE USE OF COMPENSATING FILTERS. WE HAVE TOTAL CONTROL OVER COMPENSATING FILTRATION.

IN AREAS OF THE BODY WITH HIGH SUBJECT CONTRAST OR WIDE DIFFERENCES IN DENSITY, COMPENSATING FILMS IMPROVE IMAGE QUALITY AND REDUCE PATIENT EXPOSURE.

THE END