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E XPOSURE FACTORS : Contrast, Density, Image Quality, technique math 1.

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Presentation on theme: "E XPOSURE FACTORS : Contrast, Density, Image Quality, technique math 1."— Presentation transcript:

1 E XPOSURE FACTORS : Contrast, Density, Image Quality, technique math 1

2 R EVIEW S LIDE -W HAT DO WE KNOW ABOUT X - RAYS SO FAR ? 2

3 U NITS OF M EASURE -R EVIEW Measure of current, amount of electron charges passing through a point Measure of potential difference, value of potential electrical difference as one unit of current passes 3

4 U NITS OF M EASURE - REVIEW 4

5 E XPOSURE FACTORS 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 mA, timekVp, kV 5

6 E XPOSURE F ACTORS 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 mAskVp 6

7 D ENSITY Density The overall darkening of the image Viewing the overall, general image darkness (or lightness), a general term, referred to as optical density Density Controlled by the amount of electrons sent to the target anode Density Formula for mAs mA x seconds = mAs 200mA x.25 seconds= 50mAs 7

8 C ONTRAST Contrast The differences between the light and dark areas on the image Since x-rays are only black, white and gray, we tend to say the difference between the black and the white areas Contrast The Scale of Contrast can range from completely black to stark white and various shades of gray in between. The scale of contrast is referred to as a long scale or a short scale. Contrast When the difference between the grays is large, it is considered High Contrast. When the difference between the grays is small, it is considered Low Contrast. t 8

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

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

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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 13

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15 M ILLIAMPERAGE 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

16 T IME In seconds How long x-rays will be produced to 6 seconds 16

17 MASMAS mA X s = mAs 17

18 M A S R ECIPROCITY 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

19 W HAT CHANGES TO M A S ARE NEEDED FOR HUMAN EYE TO DETECT ? 19

20 T O 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

21 DENSITY DIRECTLY PROPORTIONAL TO M A S + 25%mAS = 25% increase in density 100 mAs +50% mAs = 50% increase in density 21

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23 D ENSITY 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? 23

24 W HAT 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? 24

25 V ARIABLES THAT A FFECT D ENSITY 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 25

26 D ENSITY M ATH W ORK This is posted on the website. Please Download and turn in the next class Oct 2 nd 26

27 S AMPLE 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? 27

28 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 28

29 S HORT SCALE OF CONTRAST  Not very many differences  Between grays  Also known as high contrast 29

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32 L ONG SCALE OF CONTRAST Many different shades of gray Also known as low contrast 32

33 K ILOVOLTAGE PEAK ( K V P ) Controls both Contrast and Density Controls contrast Although mAs is considered the primary controlling factor of density, kVp also can affect density of the image. Determines the quality of the x-ray beam Penetrating Ability The higher the kVp, the more penetrating the beam is, the denser tissue it can travel through. Referred to as the strength of the beam. The x-ray beam has multiple photons with many different energy levels. Poly- energetic Beam Created by both Brems (multiple energy photons) and characteristic types of radiation, the x-ray beam quality is absolutely controlled by the kVp. The energy ranges from 0 to the peak kV. It can also be termed heterogeneous beam energy. 33

34 B EAM A TTENUATION AKA ABSORPTION High kVp Penetrates more easily Causes more grays Low contrast Low kVp Decreases penetration Causes more black-white High contrast Different parts of body attenuate differently The difference in attenuation is the basis for contrast 34

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36 O PTIMAL K V P I S 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 #) 36

37 + 15% K V P - 15% K V P 37

38 15% R ULE 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 38

39 C ONTRAST M ATH W ORK This is posted on the website. Please Download and turn in the next class (Oct 2 nd ) 39

40 C ONTRAST M ATH W ORK 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 40

41 C ONTRAST M ATH W ORK Image was shot at 75 kV. What is the new kV if you want to narrow the contrast? The new kV should be In order to narrow the contrast, you must reduce kV. 41

42 C ONTRAST M ATH W ORK 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

43 V ARIABLES THAT A FFECT D ENSITY 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 43

44 S OURCE TO I MAGE R ECEPTOR D ISTANCE SID -controlled and manipulated in the x-ray room 44

45 I NTENSITY OF THE BEAM 1. As distance _______: intensity ________ 2. As distance _______: intensity ________ 3. Inverse relation 45

46 INTENSITY IS SPREAD OUT… 46

47 I NVERSE 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 47

48 H OW DOES DISTANCE AFFECT IR EXPOSURE ? 1. Increased distance: decreased exposure ________________ 2. Decreased distance: increased exposure ________________ 3. Inversely proportional to the square of the distance ________________ Intensity is ¼ of original ________________ Intensity increases to 4 x’s the original exposure 48

49 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 49

50 I NVERSE S QUARE L AW

51 A PPLICATION OF INVERSE SQUARE LAW PRINCIPLES CAN YIELD SIGNIFICANT REDUCTIONS IN PATIENT AND OPERATOR RADIATION EXPOSURE. 51

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