Week 6b Chapter 18,19 Radiographic Exposure

Week 6b Chapter 18,19 Radiographic Exposure - Chapter 18...

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Chapter 18 Radiographic Exposure Exposure Factors influence and determine the quantity and quality of the x-radiation to which the patient is exposed. Radiation quantity refers to the radiation intensity referred to as mR or mR/ mAs. Radiation Quality refers to the beam penetrability and measured in HVL.
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Radiographic Exposure The radiographic exposure factors are under the control of the operator except for those fixed by the design of the x-ray machine. There are two choices for focal spot. With the exception of compensating filters, added filtration is fixed. The type of high voltage power is also fixed.
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Exposure Factors Controlled by the Operator kVp mA times Exposure Time = mAs Determines the quality and quantity of the exposure SID, Focal Spot and Filtration are secondary factors
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kVp As we have discussed in the laboratory, kVp controls radiographic contrast. kVp determines the ability for the beam to penetrate the tissue. kVp has more effect than any other factor on image receptor exposure because it affects beam quality.
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kVp To a lesser extent it also influences the beam quantity. As we increase kVp, more of the beam penetrates the tissue with higher energy so they interact more by the Compton effect. This produces more scatter radiation which increases image noise and reduces contrast.
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kVp 50 kV 79% is photoelectric, 21% Compton, < 1% no interaction 80 kVp 46% is photoelectric, 52% Compton 2% no interaction 110 kVp 23% photoelectric, 70% Compton, 7% no interaction As no interaction increases, less exposure is needed to produce the image so patient exposure is decreased.
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mA 1 Ampere = 1 C/s = 6.3 x 10 18 electrons/ second. The mA selected for the exposure determines the number of x-rays produced. The number of x-rays are directly proportional to the mA assuming a fixed exposure time. 100 mA produced half the x-ray that 200 mA would produce.
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mA Patient dose is also directly proportional to the mA with a fixed exposure time. A change in mA does not affect kinetic energy of the electrons therefore only the quantity is changed.
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mA Many x-ray machines are identified by the maximum mA or mAs available. A MP 500 has a maximum mAs of 500 mAs. A Universal 325 has a maximum mA of 300 and maximum kVp of 125
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mA More expensive three phase machines will have a higher maximum mA. A General Electric MST 1050 would have 1000 mA and 150 kVp.
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Exposure Time The exposure time is generally always kept as short as possible. This is not to reduce patient exposure but to minimize motion blur resulting from patient movement. This is a much greater problem with weight bearing radiography.
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Exposure Time Older machine express time as a fraction. Newer machines express exposure time
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Week 6b Chapter 18,19 Radiographic Exposure - Chapter 18...

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