Chapter 7
Radiographic Image
Formation
and
Exposure Factors
Radiographic Image
Formation
 In
this section we will see
how the image is formed on
the x-ray and what are the
effects of the various factors
that we control before
making the exposure.
Radiographic Image
Formation
X-rays can pass through solid objects the degree of penetration depending
upon the wavelength of the rays and the
composition of the object. Some rays
are completely absorbed; the others are
absorbed in varying degrees.
 The degree of absorption depends upon
the type of material, the thickness, and
the atomic number of the material that
the x-rays must pass through.

Radiographic Image
Formation
Since the body is made up of many
materials of many various thicknesses, xray is absorbed in varying degrees and
therefore reaches the film in varying
degrees. This is how the x-ray image (or
shadow) is formed. X-ray is a shadow
picture of differential absorption.
 This process is described in more detail
in the chapter on “darkroom procedures.”

Exposure Factors
 There
are several factors
involved in radiography, all of
which combined determine
the quality of the radiograph
produced.
Exposure Factors
They are:
 Kilovoltage
 Milliamperage
 Exposure Time
 Milliampere-Second
 Film-Anode Distance
 Part-Film Distance
 Size of Focal Spot
Kilovoltage (Quality)

Kilovoltage is the electromotive force or
electrical pressure that pushes the
electrons from the cathode to the anode
during exposure, therefore controlling
the speed that the electrons travel, the
force of the impact at the anode target
and the quality of penetrating power of
the x-rays produced.
Kilovoltage
 The
kilovoltage determines the
wavelength of the x-rays produced.
High kV. produces short
wavelength x-rays (hard rays), the
type with great penetrability; lower
kV results in longer wavelengths
(softer rays) and less penetrability.
Kilovoltage
We can then see that selecting the kV
predetermines the quality of the x-rays
produced. (the penetrating power of the
beam).
 The kV selected for the exposure
depends upon the thickness of the part
to be x-rayed. A general rule that has
sometimes been used to determine the
kV is twice the thickness of the part, in
cm, plus 25.

Milliamperage (Quantity)
Milliamperage determines the exact
number of electrons which will be forced
from the cathode to the anode in a
specific amount of time and under a
specific kilovoltage.
 This is explained by the fact that the MA
applied to the cathode filament
determines its temperature (its degree of
incandescence) and therefore the number
of electrons that will be boiled off.

Milliamperage
 It
is the MA together with the
time that determines the total
quantity of x-rays produced; for
each electron that strikes the
anode target, there will be an
equivalent amount of x-ray
energy produced.
Exposure Time (S) (Quantity)

Exposure time is the length of time (in
seconds) that the anode target will be
bombarded with electrons and therefore
during which x-rays are produced. The
exposure time and the amount of x-ray
produced per second (as determined by
the MA setting) determine the total
amount of radiation produced.
Milliampere-Second (MAS)

Milliampere-second refers to the
product of the milliampere and the
exposure time. It indicates the total
quantity of x-ray produced. The same
results will be had with high MA and
low time or low MA and high time. Low
MA at longer time will risk patient
movement and result in a poor quality
film, therefore, high MA and low
exposure time is preferred.
Milliampere-Second (MAS)

The MAS represent a combination of
the MA and the exposure time in
seconds. For example, the MAS used
for a lumbosacral (A-P) is 10 and any of
the following combinations or any other
combination making 10 MAS could be
used: 10 MA for one second, 30 MA for
1/3 second, 50 MA for 1/5 second, 100
MA for 1/10 second.
Film-Anode Distance
(FAD)
 AKA Tube-Film
Distance (TFD)
AKA Focal film Distance (FFD)
 refers to the distance from the
anode target to the film. It governs
radiographic distortion. The shorter
the FAD, the greater the distortion;
the longer the FAD, the more nearly
parallel the rays and the less
Film-Anode Distance
(FAD)
 The
previous statement would
seem to indicate that we should
always use an FAD as long as the
size of the room permits, but this is
not so. An increase in FAD reduces
the overall illumination so that the
MAS must be increased.
Film-Anode Distance
(FAD)
This increases the possibility of the patient
moving if you only increase the time.
 With modern day equipment you can
increase the MA and therefore less
chance of patient movement and better
diagnostic yield.
 It decreases the overall illumination due to
the inverse square law (so your technique
factors must be adjusted accordingly).

Part-Film Distance (PFD)
 AKA Object-Film
Distance (OFD)
 The part-film distance should be as
close to zero as possible. An
increase in the PFD/OFD will cause
distortion.
Size of the Focal Spot
The size of the focal spot is determined
by the size of the filament (the surface
area of the wire filament capable of
emitting electrons when heated to
incandescence). This is the size of the
electron stream.
 The larger the filament, the larger the
focal spot. The larger the focal spot, the
poorer the film detail.

Range of Exposure Factors
Factor
Radiography Radiotherapy
Kilovoltage
30 to 120 100 to 25000
M.A.
1 to 500
1 to 25
Time
1/120 to 30
1 s. to 30
s.
min
MAS
1 to 400
1 to 400
FAD
25 to 72 in. 25 to 50 cm
PFD
As close to 0 as possible
Focal Spot
Around 2
Around 2
mm
mm