Exposure Factor Selection And Technique Charts

Question 1

Why do we need a technique chart?

Answer 1

• to provide a level of consistency among images and radiographers
• standardizes selection of exposure factors to help the technologist produce quality radiographs and keep patient dose to a minimum
• does not account for unusual circumstances (still need to asses and make appropriate adjustments for body habitus, pathology etc)

Question 2

Digital systems

Answer 2

• wide dynamic range
• system compensates for poor exposure factor selection
  
  • could be over exposing patient without knowing
  • important to check EI numbers
  • images may still appear diagnostic regardless of over/under exposure

Question 3

Two main types of technique charts

Answer 3

variable kVp/fixed mAs

• based on the concept that kVp can be increased as anatomic part size increases
  
  • +2 kVp for every additional cm of part thickness
  • accurate measurement of part is critical for this type of technique chart (calipers)
  • changing kVp values for variations in part thickness is only effective for paediatrics or small extremities (small changes in kVp are more effective at low kVp levels than changing the mAs)
  • cons: more latitude for error, image contrast can vary greatly, adequate penetration of large parts is not always assured, may not have ideal contrast on image

Question 4

Fixed kVp/ variable mAs

Answer 4

-uses concept of selecting optimal kVp for exam, and adjusting mAs for variations in part thickness
-optimal kVp: high enough to penetrate the part, but not so high to decrease contrast too much
-for this chart, the optimal kVp is indicated, and mAs is adjusted for part thickness
X-rays are attenuated exponentially meaning: for every 4-5 cm change in part thickness, the mAs should be adjusted by a factor of 2
-ex: if a knee measures 10cm, and the technique is 70kVp@ 6 mAs, and another knee measures 14 cm, the new technique will be 70kVp @ 12 mAs

Question 5

Advantages of fixed kVp

Answer 5

• lower dose (higher kVp settings)
• adequate penetration of all anatomic parts
• consistent image contrast for exams
• greater latitude with exposures
• measurement of part is not critical