1. Exposure Factors or Prime Factors

2. What is “technique” ? How does it affect the “image”
PRIME FACTORS
What is “technique” ?
How does it affect the “image”

3. Exposure Factors – 3 or 4 The four prime exposure factors are:
Voltage = kVp*
Current = mA*
Exposure time = seconds or fractions of a sec*
Source-to-image distance = SID

4. PRIME FACTORS
KVP
MAS
DISTANCE

5. kVp Kilovolts controls how fast the electrons are sent across the tube
Controls, quality, penetrability & contrast
Increasing kVp also increases scattered photons reducing image quality
Does kVp influence OD?

6. “SHORT” VS “LONG” SCALE

7. kVp
Low kVp (50 – 60)
Short scale
High contrast
“Bone work”

8. kVp
High kVp (90 – 120)
Long scale
Low contrast
“Chest images”

13. mA
Determines the number of photons, radiation quantity, OD & patient dose
Changing mA does not change the kinetic energy of e-
Available mA stations are usually 50, 100, 200, 300, 400 & 600

15. Exposure Time mA X s = mAs mAs controls OD
Should be kept as short as possible, for most examinations. To minimize the risk of patient motion
mA X s = mAs
mAs controls OD
mAs determines the number of photons in the primary beam

16. Distance Affects exposure of the IR because of the Inverse Square Law
SID largely determines the intensity of photons at the IR
Distance has no effect on radiation quality

17. INTENSITY IS SPREAD OUT…

18. Inverse Square Law Formula
Distance #2 - Squared
Intensity #1
Distance #1 - Squared
Intensity #2

19. SID Changes

20. Direct Square Law
New mAs = New distance 2
Old mAs Old distance 2

21. Focal-Spot Changes

22. Producing optimal radiographs … key for diagnosing disease

23. How much of the radiation received by the patient
Actually reaches the IR ?

24. About 1%

26. Creating the IMAGE
When x-rays pass through a patient's body, three things can happen:
(1) the x-ray photon is transmitted, passing through the body, interacting with the film, and producing a dark area on the film;
(2) the x-ray photon is absorbed in an area of greater tissue density, producing lighter areas on the film; and
(3) the x-ray photon is scattered and reaches the film causing an overall gray fog.

27. IMAGES DENSITY = THE AMOUNT OF BLACKENING “DARKNESS” ON THE RADIOGRAPH
CONTRAST – THE DIFFERENCES BETWEEN THE BLACKS TO THE WHITES

28. mAs Changes
at least % mas change needed to see a visible change in density

29. mAs DOUBLED = DENSITY DOUBLED

30. + 25% % mas

31. mAs Changes

32. Tube voltage (kVp) Determines the maximum energy in the beam
spectrum and affects the quality of the output spectrum
Efficiency of x-ray production is directly related to tube voltage

33. Influencing factors: kVp
15% rule:
15% kVp = doubling of exposure to the film
 15% kVp = halving of exposure to the film
15% rule will always change the contrast of the image because kV is the primary method of changing image contrast.
Remember :
15% change ( ) KVP has the same effect as doubling or ½ the MAS on density

34. kVp Changes
The kVp setting must be changed by at least 4% to produce visual changes an image

35. kVp Changes

36. + 15% kvp % kvp

37. kVp Changes

38. 4% kVp Changes

39. Determining Radiographic Technique The Patient Factor
The most difficult task for technologists… evaluating your patient
The patient size, shape, and physical condition greatly influences the technique selection

40. 4 general states of body habitus

41. Radiographic Technique
Technique charts are based on the “average patient”
The thicker the part the more x-radiation is required to penetrate. Calipers should be used
Keep in mind not only the measurement but the type of tissue you need to penetrate (fat vs muscle)

42. Technique In general, Soft tissue = low kVp and high mAs
Extremity (soft tissue & bone) = low kVp
Chest (high subject contrast) = high kVp
Abdomen (low subject contrast) = middle kVp

43. Low Subject Contrast

44. OD = amount of black on image

45. Film Screen Overexposed
Referring to a radiograph that is too dark because too much x-radiation reached the image receptor
Underexposed
Referring to a radiograph that is too light because too little x-radiation reached the image receptor

46. Technique - Pathology
Pathology can severely affect the technologist technique selection
Always question your patients about health status
If prior images are available…check them!

47. Pathology Can appear with increased radiolucency or radiopacity
Some pathology is destructive causing tissue to be radiolucent
Others can be additive causing tissue to be radiopaque

48. OD vs tissue attenuation

49. Technique selection – Fixed kVp
For each anatomic part there is an optimum kVp
mAs is varied based on part thickness or pathological condition