This is referred to as the reciprocity law. MAs Any combination of mA and time that will give the same mAs should provide the same optical density on the film. mAs does not influence radiation quality. mAs determine the number of x-rays in the beam and therefore radiation quantity. mAs controls radiation quantity, optical density and patient dose. MAs mA and exposure time is usually combined and used as one factor expressed as mAs.
#Under exposure x ray full
Single phase full wave rectified fastest exposure time is 1/120 second or 8 ms Three phase and high frequency can provide exposure time down to 1 ms. Įxposure Time Single phase half wave rectified fasted exposure time is 1/60 second 17 ms. Newer machines express exposure time as milliseconds (ms) It is easy to identify the type of high voltage generation by looking at the shortest exposure time. Įxposure Time Older machine express time as a fraction. This is a much greater problem with weight bearing radiography. This is not to reduce patient exposure but to minimize motion blur resulting from patient movement. Įxposure Time The exposure time is generally always kept as short as possible. A General Electric MST 1050 would have 1000 mA and 150 kVp. MA More expensive three phase machines will have a higher maximum mA. A Universal 325 has a maximum mA of 300 and maximum kVp of 125 MA Many x-ray machines are identified by the maximum mA or mAs available. A change in mA does not affect kinetic energy of the electrons therefore only the quantity is changed. MA Patient dose is also directly proportional to the mA with a fixed exposure time. 100 mA produced half the x-ray that 200 mA would produce. The number of x-rays are directly proportional to the mA assuming a fixed exposure time. The mA selected for the exposure determines the number of x-rays produced. MA 1 Ampere = 1 C/s = 6.3 x 10 18 electrons/ second. KVp 50 kV 79% is photoelectric, 21% Compton, 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. This produces more scatter radiation which increases image noise and reduces contrast. As we increase kVp, more of the beam penetrates the tissue with higher energy so they interact more by the Compton effect. KVp To a lesser extent it also influences the beam quantity. kVp has more effect than any other factor on image receptor exposure because it affects beam quality. kVp determines the ability for the beam to penetrate the tissue. KVp As we have discussed in the laboratory, kVp controls radiographic contrast. Įxposure 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 The type of high voltage power is also fixed. With the exception of compensating filters, added filtration is fixed. 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. Radiation Quality refers to the beam penetrability and measured in HVL. Radiation quantity refers to the radiation intensity referred to as mR or mR/ mAs. Radiographic Exposure Exposure Factors influence and determine the quantity and quality of the x-radiation to which the patient is exposed.