An increase in tube current mA results in a higher production of electrons that are inside the x-ray tube which will, therefore, increase the quantity of x-radiation; more radiation will mean more photons reaching the detector and hence apparent structural density will decrease, yet the signal intensity will increase. The time factor s is a measure of the electrons production duration in the tube; meaning 's' prescribes how long mA will last. Increasing either the current or time will increase the quantity of radiation; therefore the amount of radiation in an examination is represented as mAs.
The reciprocity law states that the reaction of a photographic emulsion to light will be equal to the products of the intensity of that light and the time of the exposure 1. This law pertains to mAs in the sense that all combinations of mA x seconds that amount to an equal quantity will produce the same amount of density. It is due to this law that radiographers will have to take into consideration all other factors mA , focal spot , source image receptor distance SID , kVp to reduce time to avoid motion blur.
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X-rays, and How They Are Produced In simple terms, diagnostic x-rays are produced by accelerating electrons across a large voltage and bombarding them onto a heavy metal target usually tungsten. The energy of those crashing electrons is converted into a spectrum of x-rays. This occurs inside an x-ray tube. The peak value of this large voltage between the cathode and the anode is what we refer to as kVp kV is kilo Volts and p is for peak.
This determines the maximum energy the electrons have when they collide with the anode target, and in turn the maximum energy of x-rays that get produced. So for a 90 kVp tube, the maximum x-ray energy produced is 90 keV kiloelectron Volts. But, not all of the x-rays have this peak energy. The spectrum of the x-rays produced range from 0 to a maximum of whatever the kVp setting is.
In the end, the kVp setting determines the energy profile of the x-rays, which is really what interests us most. This energy profile is important because it determines the overall penetration of the x-rays.
For the scope of this article, suffice to say that higher energy x-rays are less likely to be absorbed and so penetrate deeper through tissue. In the first part of the experiment, the density of the film was kept constant by changing the kVp and mAs. In the second part of the experiment, different mAs's were chosen, and for each mAs, several kVp's were used.
Five observers read the radiographs. Time selected. Any of the above exposures will produce the exact same density on the resultant x-ray film. Faster exposure times are needed if the patient is having trouble holding still, as in the case of small children. When using a short exposure time and a high mA setting, be sure to refer to the tube rating chart to verify that the tube will be able to stand up to the extreme temperatures generated by such an exposure.
Regardless of which of the five possible combinations is selected in the example above, each will produce exactly the same number quantity of x-ray photons. An image produced by any of these combinations will have the same radiographic density amount of image blackening. Rules About mAs. If you just obtained an image of a hip, using 20 mAs, and the film is too light, increasing the mAs will make the film darker.
If the image was very light, you may need to double the mAs 40 mAs from the original exposure. Remember that any change in kVP affects image density, but primarily it changes image contrast. The primary role of kVp is to control strength and power of the x-ray beam and control contrast. Whenever possible, mAs should be used to increase or decrease the overall density darkness of an image- not kVp.
When adjusting technique settings, initially only one or the other kVp or mAs should be adjusted rather than making changes in both.
The background of an under penetrated image and some tissues will have acceptable densities, but the area of interest for example, a bone will look washed out with little or no detail. In such an image, the x-rays were so strong that most penetrated right through the patient, and no differential absorption took place.
Reduce kVp. Changing Density of an Image. When taking a repeat film for over or under exposure, the minimum amount of change in technique should be as follows: - At least double the mAs for an image that is being repeated because the film was too light - At least cut in half the mAs for a film being repeated because the film was too dark. Quantum Mottle. This increases the chance of a passing incident x-ray to have an interaction with the crystal.
A larger crystal will give off more light than a small crystal, thereby darkening a greater are of film. Using low mAs for an exposure may also lead to quantum mottle. If mAs is set too low, there are not enough x-ray photons in the beam to interact with a sufficient number of crystals.
It is similar to comparing a light rain to a big thunderstorm. Distance from x-ray tube to film source to image distance, or SID is one of the key components in producing a quality x-ray. Distance has a direct relationship with mAs.
Since x-rays obey all of the laws of light, we can better understand how distance affects x-rays by conducting a simple exercise. Take a flashlight into a darkened room.
Hold the light about 12 inches from a wall. You will see that the area covered by the light is a very small, well defined circle. You will also see that the intensity of the light is very strong and bright. Now slowly increase your distance away from the wall, keeping your eye on the area illuminated by the light. Also pay attention to the intensity of the light as you increase your distance.
As distance is increased, intensity of the light decreases. At the same time, the area illuminated by the light "fans out" and more of the wall is covered by light. X-rays behave in the very same way as light in our analogy above. When the distance from the source of x-rays x-ray tube to the image receptor film is increased, two things happen:. The distance between the source of the x-rays x-ray tube and the image receiver film is referred to as the source to image distance, or SID; also called focal film distance FFD.
The natural law that governs the relationship between intensity and distance is referred to as the inverse square law. It states that "the intensity of energy is inversely proportional to the square of the distance". The drawing below may help simplify this rather complex concept.
Notice that at the shorter distance 40" , the intensity of the x-ray beam is high. Even though the x-ray beam hasn't traveled very far, it has already started to fan out slightly and already covers 4 squares. By the time the x-ray beam has traveled roughly twice as far 72" , the intensity has dropped off considerably and the beam has fanned out wide enough to cover 16 squares.
That is essentially the inverse square law! Doubling the distance from the x-ray tube to the imaging cassette resulted in 4 time more squares being covered by the diverging beam. In our example, let's say that the intensity at 40" is 4 mR. Later we take another image, but this time, our distance is nearly doubled to 72". According to the inverse square law, we will need four times more mAs to maintain the same density as was seen in the first image.
Therefore, It would now take 16 mR to maintain the same density. In another example, let's say that we placed an instrument that measures radiation exposure in mR and we positioned it on the patient's skin where our central beam was centered. We took an exposure using a SID of 40". The measurement reads mR. Now we used the same technique factors we don't change kVp, Ma, or time , but we nearly double our distance from 40" up to 72".
Using the inverse square law, what would the mR reading be now? Keep these rules in mind when changing your subject to image SID :. Supine films are generally taken using a SID of 40". We set our technique at 2 mAs and get a well exposed film. Now the doctor tells you that he needs an upright view so that he can better see layering of fluid in the patient's lungs.
You nearly double your SID distance because upright films are taken at 72 inches. Using the inverse square law, you know that because you have doubled the distance, you need 4 times the intensity. You set your technique at 8 mAs 2 x 4 and get a perfect upright film! Example: We take a portable chest film on a patient who is sitting upright on a gurney. We use a SID of 72" and select 10 mAs. Our x-ray is well exposed and we are pleased with the results.
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