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Exposure Factors Chapter 4

Exposure Factors Chapter 4. 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. Exposure Factors Include. X-ray Machine mA

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Exposure Factors Chapter 4

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  1. Exposure FactorsChapter 4

  2. 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.

  3. 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

  4. 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?

  5. Quality and Quantity • Quality of an x-ray beam is determined by its penetrating power. • Higher frequency=greater penetrating power=shorter 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.

  6. What affects Quality and Quantity? • mA • Time • kV or kVp • Distance

  7. 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.

  8. 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.

  9. 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).

  10. 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

  11. 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.

  12. 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.

  13. 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

  14. 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.

  15. 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. • Sante’s Rule- a method of estimating needed kilovoltage in relation to area thickness. • 2 x thickness + 40= kVp

  16. Examples of Sante’s 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 • Sante’s rule is for table top x-ray but can be modified to fit other conditions.

  17. 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).

  18. 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

  19. 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.

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