kVp vs ma for x-ray production affect x-ray and CT radiologic technologists every day as these are the two major factors that can be changed when generating x-rays with a diagnostic x-ray tube. This video discussed the x-ray production factors of tube potential (kVp), tube current (mA) and time (s) and describes the effect of these factors on the x-ray spectrum. We use the waterfall analogy to x-ray tube in order to gain insight on the physics of the tube.
For a free self check study guide on x-ray generation see our website:
howradiologyworks.com/xraygen/
Chapters:
00:00 Intro
03:54 X-ray Tube Overview
04:34. kVp X-ray spectrum
05:18. mA X-ray spectrum
05:50. time (s)
06:20. takeaways
07:13. outro
As discussed above the main technical parameters that can be changed when generating an x-ray exposure are the kVp and the mA. These each change the x-ray spectrum or x-ray distribution differently so it is important to keep the effect of each one in mind.
The kVp changes both the overall shape of the x-ray spectrum as well as how many photons are produced, as can be seen in the kVp Figure. In this example Figure we can see the effect on the x-ray spectrum of changing from 80 kVp to 140 kVp.
Important points to remember for the kVp dependence:
The maximum x-ray energy is determined by the kVp
The total dose of each exposure is strongly dependent on the kVp, Exposure~kVp2 (*a power law slightly higher than 2, but just remember this approximation)
The kVp changes the overall shape of the x-ray spectrum, it does not just scale it.
The effect of the mA is more straightforward and hence typically the kVp is changed first and then the mA is then used to fine-tune the delivered x-ray exposure.
The relative number of photons at each energy bin stays the same when the mA is changed. This is to say that the shape of the x-ray spectrum stays the same when the mA is changed and it is just scaled up or down. This is called being directly or linearly dependent on the mA so when the mA is changed by a given amount the patient exposure is changed by the same amount, ie. If the mA is doubled the x-ray exposure to the patient is also doubled.
Also, just for completeness, we’ll mention the exposure time. When you do an exposure of the x-ray tube, you turn that x-ray tube on and you have the exposure on for a given amount of time. And if you leave the kV and the mA constant then the exposure is also linearly proportional to time, just like the patient exposure is linearly dependent on the mA. So, the longer that the x-ray tube is on, the more exposure and the more dose the patient will receive.
Take-home Point: The kVp changes the average energy of the x-ray spectrum and the dose dependence goes like (kVp^2 ), whereas the shape of the x-ray spectrum is not changed with mA but just scaled with mA
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