Radiographic Image Formation

1
Chapter 7

• Radiographic Image Formation
• and
• Exposure Factors

2
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.

3
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.

4
Radiographic Image Formation

• Since the body is made up of many materials of
  many various thicknesses, x-ray 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.

5
Exposure Factors

• There are several factors involved in
  radiography, all of which combined determine the
  quality of the radiograph produced.

6
Exposure Factors

• They are
• Kilovoltage
• Milliamperage
• Exposure Time
• Milliampere-Second
• Film-Anode Distance
• Part-Film Distance
• Size of Focal Spot

7
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.

8
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.

9
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.

10
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.

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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.

12
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.

13
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.

14
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.

15
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 distortion or magnification.

16
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.

17
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).

18
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.

19
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.

20
Range of Exposure Factors