Exposure factors are factors that control
density (blackening) and contrast of
They are some of the tools that
technologists use to create high-quality
Exposure Factors Controlled by
times Exposure Time = mAs
Determines the quality and
quantity of the exposure
FFD (SID), Focal Spot and
Filtration are secondary factors
controls the quality of the beam, i.e.
a: penetration power, i.e. beam
kVp. α penetration power.
kVp. α 1/radiographic
Radiation dose to patient.
kVp determines the ability for the
beam to penetrate the tissue.
kVp has more ef fect than any other
factor on image receptor exposure
because it af fects beam quality.
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 ef fect.
This produces more scatter
radiation which increases image
noise and reduces contrast.
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
Ampere = 1 C/s = 6.3 x 1018
The mA selected for the exposure
determines the number of x-rays
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.
dose is also directly
proportional to the mA with a fixed
A change in mA does not af fect
kinetic energy of the electrons
therefore only the quantity is
x-ray machines are identified
by the maximum mA or mAs
A MP 500 has a maximum mAs of
A Universal 325 has a maximum mA
of 300 and maximum kVp of 125
expensive three phase
machines will have a higher
A General Electric MST 1050 would
have 1000 mA and 150 kVp.
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
This is a much greater problem
with weight bearing radiography.
machine express time as a
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.
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
(4) MAS. :
af fect the total number of x-ray
produced by the tube during exposure,
is the product of two quantities;
the tube current;
the exposure time;
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 xrays in the beam and therefore
mAs does not influence radiation
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.
many modern machines, only
mAs can be selected. The machine
automatically gives the operator
the highest mA and shortest
The operator may be able to select
mA by what is referred to as Power
is one way to measure
electrostatic charge. It determines
the total number of electrons.
Only the quantity of the photons
are af fected by changes in the
Patient dose is therefore a
function of 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
6.3 X 10 18 X 0.2 = 1.26 X 10 18 electron/s
(5) Focal spot:
x-ray tubes of fer two focal
a. Fine focus:
b. Broad focus:
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
Used whenever geometric factors
are more (long subject-film
distance, short FFD ... etc).
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
Used whenever voluntary or
involuntary motion is highly
Used when radiosensitive organ is
within exposed area or 10 cm from
FOCAL SPOT SIZE
The focal spot size limits the
tube’s capacity to produce xrays. The electrons and
resulting heat are placed on a
smaller portion of the x-ray
The mA is therefore limited for
the small focal spot. This
FOCAL SPOT SIZE
the mA is properly calibrated,
the focal spot will have no impact
on the quantity or quality of the
(6) F.F.D. :
intensity of x-ray beam reduces
with increased FFD.
follows the Inverse Square Law
( I.S.L.) .
α 1/d 2 .
af fects the intensity of
the x-ray beam at the film but has
no ef fect on radiation quality.
af fects the exposure of
the image receptor according to
the inverse square law.
INVERSE SQUARE LAW
most common source to image
distances are 40” (100 cm) and
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.
the distance will impact
the geometric properties of the
Increased SID reduces
magnification distortion and focal
With the need to increase the mAs
3.5 times for the 72” SID, tube
loading becomes a concern.
SID is used for Chest
radiography and the lateral
cervical spine to reduce
72” SID used for the full spine to
get a 36” beam.
sheet of Al (aluminum) 1mm or 2mm
thick added to the pathway of radiation
to filter the low energy radiation.
filtration will increase the
quality and reduce the quantity of the
removes low energy radiation:
Reduce skin dose;
Harden the beam;
x-ray beams are af fected by the
filtration of the tube. The tube
housing provides about 0.5 mm of
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.
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
3.0 mm is required for at 100
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
A LEADER IN THE USE OF
COMPENSATING FILTERS. WE
HAVE TOTAL CONTROL OVER
IN AREAS OF THE BODY WITH
HIGH SUBJECT CONTRAST OR
WIDE DIFFERENCES IN DENSITY,
COMPENSATING FILMS IMPROVE
IMAGE QUALITY AND REDUCE