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Exposure factors:

Exposure factors:. Contrast, Density, Image Quality, technique math. Review Slide-What do we know about x-rays so far?. Units of Measure-Review. Ampere. Measure of current, amount of electron charges passing through a point. Volt.

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Exposure factors:

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  1. Exposure factors: Contrast, Density, Image Quality, technique math

  2. Review Slide-What do we know about x-rays so far?

  3. Units of Measure-Review Ampere Measure of current, amount of electron charges passing through a point Volt Measure of potential difference, value of potential electrical difference as one unit of current passes

  4. Units of Measure-review

  5. Exposure factors mA, time kVp, kV • mAs • Milliamps-Amount of electrons burned off filament • Time-measured in seconds, fractions of seconds, determines how long the electrons will flow across to anode(another amount) • Kilovoltage • Determines the strength of the x-ray • Determines the wavelength of the x-ray • Determines the power of the x-ray • Determines the penetrating ability of x-ray

  6. Exposure Factors mAs kVp • Controls the density of the image by controlling the amount of electrons sent to anode target • Controls the contrast of the image by controlling the penetrating power of the x-ray photon • Also controls the density of the image because more photons are able to penetrate the part being imaged

  7. Density

  8. Contrast

  9. What changed?....contrast or density?

  10. Now what changed?....very subtle, often subjective

  11. 2 4 3 1

  12. 4 5 3 2 1

  13. DENSITY • Anatomic Density • Body part/object being x-rayed • Atomic # • Thickness of part • Optical Density • Amount of x-ray photons reaching the image receptor • The mA applied • The time applied • Also referred to as x-ray output

  14. Milliamperage • mA • One milliampere is equal to one thousandth of an ampere. • The amount of current supplied to the x-ray tube • Range 10 to 1200 mA

  15. Time • In seconds • How long x-rays will be produced • 0.001 to 6 seconds

  16. mAs mA X s = mAs

  17. mAs Reciprocity • 100 mA x 1/4 = 25 mAs • 200 mA x 1/8 = 25 mAs • 400 mA x 1/16 = 25 mAs This works ONLY when you are trying to keep the mAs the SAME………

  18. What changes to mAs are needed for human eye to detect?

  19. To change density • The human eye needs a 20-30% change in density on an image in order to visibly see it. • Most frequently radiographers will change the density by doubling or by halving the density. • What do you do in order to double density on an image?

  20. DENSITY directly proportional to mAs 100 mAs + 25%mAS = 25% increase in density +50% mAs = 50% increase in density

  21. Density • Density is like toast…too much and the toast is burned, too little and it is underdone. • The images differ in density only. Which one looks optimal to you?

  22. What would you do? This image was taken at 60 mAs. What would you do to fix this image? This image was taken at 300 mA. What was the time of the exposure? If we wanted to change the mA but keep the mAs the same, what would we do?

  23. Variables that Affect Density • Patient size • Thickness of body part • Tissue composition • Z# • Bone, muscle, soft tissue, water, air • mAs • kVp • Source image receptor distance (SID) • The distance from the tube to the image receptor • The closer the tube, the more photons hit target • Beam modification • The use of filter (we will cover later) • Image receptor • The use of grid vs. non-grid, film , CR, DR (we will cover later) • Processing • Chemistry, time in chemistry (we will cover later

  24. Density Math Work This is posted on the website. Please Download and turn in the next class Oct 2nd

  25. Sample problems When mA is unknown… The image was shot at 45mAs using a .75second exposure. What is the mA? When s is unknown…. The image was shot at 80mAs using the 400mA station. What was the time of exposure?

  26. CONTRAST • THE DIFFERENCES BETWEEN: • Blacks • Whites • Dark gray • Light gray THERE IS A SCALE OF CONTRAST • many colors of black, white, gray= long scale • Few colors of black, white, gray=short scale

  27. Short scale of contrast • Not very many differences • Between grays • Also known as high contrast

  28. 2 4 3 1

  29. 4 5 3 2 1

  30. Long scale of contrast • Many different shades of gray • Also known as low contrast

  31. Kilovoltage peak (kVp)

  32. High kVp Penetrates more easily Causes more grays Low contrast Low kVp Decreases penetration Causes more black-white High contrast Beam Attenuation AKA absorption • Different parts of body attenuate differently • The difference in attenuation is the basis for contrast

  33. Optimal kVpIs there such a concept? • YES and NO • Depends on the body part • The anatomic area of interest • More energy is needed to penetrate through bony tissue (high z #) than soft tissue (low z #)

  34. + 15% kVp - 15% kVp

  35. 15% Rule 15% kVp = doubling of exposure to the image receptor  15% kVp = halving of exposure to the image receptor 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

  36. Contrast Math Work This is posted on the website. Please Download and turn in the next class (Oct 2nd)

  37. Contrast Math Work Request: Widen the contrast Asking for a long scale contrast Looking for more grays, more grays that look alike Solution: Increase kV Request: Narrow the contrast Asking for a short scale contrast Looking for less grays, more black and whites Solution: Decrease kV Use the 15% rule

  38. Contrast Math Work Image was shot at 75 kV. What is the new kV if you want to narrow the contrast? In order to narrow the contrast, you must reduce kV. The new kV should be 63.75

  39. Contrast Math Work Image was shot at 65kV. What would the new kV be if you wanted to widen the contrast? To widen the contrast, you must increase kV. Add 15% of 65 to 65. The new kV would be 74.75

  40. Variables that Affect Density • Patient size • Thickness of body part • Tissue composition • Z# • Bone, muscle, soft tissue, water, air • mAs • kVp • Source image receptor distance (SID) • The distance from the tube to the image receptor • The closer the tube, the more photons hit target • Beam modification • The use of filter (we will cover later) • Image receptor • The use of grid vs. non-grid, film , CR, DR (we will cover later) • Processing • Chemistry, time in chemistry (we will cover later

  41. Source to Image Receptor Distance SID -controlled and manipulated in the x-ray room

  42. Intensity of the beam • As distance _______: intensity ________ • As distance _______: intensity ________ • Inverse relation

  43. INTENSITY IS SPREAD OUT…

  44. Inverse square law • Farther the distance of the x-ray tube to the IR • Photons have less chance of getting to IR • Due to divergent beam

  45. How does distance affect IR exposure? • Increased distance: decreased exposure • ________________ • Decreased distance: increased exposure • ________________ • Inversely proportional to the square of the distance • ________________ • Intensity is ¼ of original • ________________ • Intensity increases to 4 x’s the original exposure

  46. INVERSE SQUARE LAW • Used for RADIATION PROTECTION • When you change your distance from the “radiation source” • The intensity of radiation will be reduced by a square of the distance • MOVING AWAY FROM THE SOURCE • INCREASED – CLOSER TO SOURCE

  47. Inverse Square Law 2 2

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