Exposure Factors Chapter 4

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Exposure FactorsChapter 4
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Exposure Factors

• Are adjustable variables that can influence the
  diagnostic qualities of a radiograph.
• Proper selection of the exposure factors will
  lead to a decreased number of retakes safer and
  more cost effective.

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Exposure Factors Include

• X-ray Machine
• mA
• Kvp
• mAs
• FFD
• Collimation
• Technique Chart
• Patient
• Thickness of patient
• Pathological changes
• Movement
• Appliances
• Cassette
• Dark Room / Film Processing

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Producing x-rays

• Electrical currents must be applied to both
  cathode filament and field between the cathode
  and the anode.
• What are these two applied currents called?

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Quality and Quantity

• Quality of an x-ray beam is determined by its
  penetrating power.
• Higher frequencygreater penetrating
  powershorter wavelength.
• Quantity (intensity) of x-ray beam is defined as
  the amount of energy flowing per second through a
  unit area perpendicular to the direction of the
  beam.
• Basically number of x-rays traveling from x-ray
  tube to image receptor in a period of time.

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What affects Quality and Quantity?

• mA
• Time
• kV or kVp
• Distance

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Milliamperes (mA)

• 1/1000 of an ampere
• Source of heat that is applied to filament
  causing electrons to start forming electron
  cloud.
• This process is called thermionic emission
• Once electrons are pulled away from atom, they
  start to travel to target spot on anode.
• Amount of electrons available is directly
  dependent on the heat applied to the filament.

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Quantity continued

• Number of x-rays produced at the anode depends on
  the size of electron cloud.
• mA affects intensity of x-ray beam and is measure
  of quantity of x-radiation produced.

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Time

• Time- total quantity of x-rays produced during a
  given exposure depends on the length of the
  exposure.
• There exists a direct relationship between the mA
  and the length of exposure (time).
• Exposure time- period during which the x-rays are
  permitted to leave the x-ray tube.
• Measured in fractions of seconds (1/60, 1/40,
  etc).

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How many x-rays are available?

• Determined by the number of electrons and the
  period of time set for their release.
• Available quantity of x-rays is expressed as the
  product of the mA and the time.
• mA x time mAs
• s seconds

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Advantages of high mA

• Allows for shorter time setting with the same
  number of x-rays produced.
• Shorter time, possibility of motion is decreased.
• Decreases the exposure of restraining personnel.
• Greater amount of x-rays produced.
• Allows examination of thicker anatomic areas.

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mA continued

• Suitable mA settings depends on the thickness and
  type of tissue being radiographed.
• Machines with higher mA potential are more
  powerful and have increased diversity of use in
  practice.
• Smaller x-ray machines have a constant mA
  capability with no provision for alteration.

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mAs

• mA time are inversely related but often
  combined settings on the x-ray machine
• As mAs increase the x-ray becomes blacker in
  color and as mAs decrease they become whiter

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Kilovoltage

• Describes the electrical potential (or
  difference) between the cathode and anode.
• Responsible for accelerating the electrons from
  the cathode to the anode.
• Relates to the penetrating power of x-rays.
• Measured in kilovolts.
• kVp maximum energy available at that kV.

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Affects of changing kilovoltage.

• Changes penetrating power
• Increase kVp Increase in penetrating power
• Increase kVp allows for lower mAs settings which
  call for shorter exposure time.
• Santes Rule- a method of estimating needed
  kilovoltage in relation to area thickness.
• 2 x thickness 40 kVp

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Examples of Santes Rule

• If measured thickness is 10 cm.
• (2 x 10) 40 60 kVp
• If measured thickness is 12 cm.
• (2 x 12) 40 64 kVp
• Santes rule is for table top x-ray but can be
  modified to fit other conditions.

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Why do we use 40?

• Represents the distance from the x-ray tube focal
  spot to image receptor (film) in inches.
• This distance can be referred to as the Focal
  Film Distance (FFD) or Source-image Distance
  (SID).

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Distance

• Distance between x-ray source and image receptor
  affects the intensity of the image produced.
• Decrease in distance (SID or FFD) Increase in
  intensity of x-rays.
• Increase in distance decrease in intensity of
  x-rays.
• Decrease in distance decrease in image sharpness

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SID/FFD continued

• Should be kept constant.
• In vet practices, most likely will be 36-40
  inches.
• Where can I find this measurement?
• The use of this knowledge aids with the
  preparation of a technique chart.