Exposure Factors or Prime Factors 1 PRIME FACTORS

Exposure Factors or Prime Factors 1

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

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

PRIME FACTORS • KVP • MAS • DISTANCE 4

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

“SHORT” VS “LONG” SCALE 6

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

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

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

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

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

INTENSITY IS SPREAD OUT… 17

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

SID Changes 19

Direct Square Law • New m. As = New distance 2 Old m. As Old distance 2 20

Focal-Spot Changes 21

Producing optimal radiographs … key for diagnosing disease 22

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

About 1% 24

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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. 26

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

m. As Changes • at least 20 - 30 % mas change needed to see a visible change in density 28

m. As DOUBLED = DENSITY DOUBLED 29

+ 25% + 50 % mas 30

m. As Changes 31

Tube voltage (k. Vp) • 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 32

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

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

k. Vp Changes 35

+ 15% kvp - 15% kvp 36

k. Vp Changes 37

4% k. Vp Changes 38

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

4 general states of body habitus 40

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

Technique n In general, Soft tissue = low k. Vp and high m. As n Extremity (soft tissue & bone) = low k. Vp n Chest (high subject contrast) = high k. Vp Abdomen (low subject contrast) = middle k. Vp n 42

Low Subject Contrast 43

OD = amount of black on image 44

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 45

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

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

OD vs tissue attenuation 48

Technique selection – Fixed k. Vp n For each anatomic part there is an optimum k. Vp n m. As is varied based on part thickness or pathological condition 49