X-ray Interactions Photoelectric and Compton Scattering for Radiologic Technologists [Rad Physics]
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- čas přidán 17. 06. 2024
- X-ray Interactions photoelectric and compton scattering are the primary means that x-rays interact with matter. For this reason it is important for Radiologic Technologists and Radiographers to be familar with these effects.
chapters:
00:00 Intro
0:41 Photo-electric effects
03:03 Compton Scattering
04:05 Relative Contribution
05:30 Summary
For more details and figures from this video on x-ray interactions please see this post on our site:
howradiologyworks.com/x-ray-i...
When x-rays interact with the human body during an x-ray exposure, they form an image that is highly dependent on the type of interactions of matter and x-rays. Diagnostic x-ray interactions are dominated by two different physical interactions - the photoelectric effect and Compton scatter.
Understanding the impact of the photoelectric effect and Compton scatter and their behavior as a function of energy can greatly improve your ability to select the best technical parameters for a given clinical situation.
We start with a high level summary graphic that demonstrates the differences between the x-ray interactions of: photoelectric, compton and coherent scattering and then go into detail on each of the interactions.
X Ray Interactions
The photoelectric effect is the dominant contributor to the generation of signal in an x-ray image as the x-ray is coming in and will be stopped and deposit its energy locally.
The photoelectric effect occurs when an x-ray interacts with an electron in the matter. The photo is completely absorbed and its energy is transferred to an electron that is removed from the electron cloud.
Since the electrons that are in the inner shells are at a more stable configuration the electrons in the outer shells will transition to an inner shell and a characteristic x-ray will be emitted. These secondary events are very low energy and are absorbed relatively locally and do not contribute to the measured image signal.
The likelihood of such interactions with inner shells depends strongly on atomic number Z (i.e. Z^3), or how many protons are in nucleus.
Therefore, image contrast in x-ray and CT is much better for materials with high Z elements.
During this interaction, electrons which move to the inner shell, preserve energy and emit secondary x-ray photon.
Another important point is that the likelihood of interaction is much higher for lower diagnostic x-ray energies, i.e. (1/E^3), where E is the energy of the x-ray photons.
Therefore, when possible it is typically beneficial to use lower energy photons for a given imaging task, provided that they can penetrate the patient.
Rad Take-home Point: In the photoelectric effect an x-ray comes in and deposits its energy locally mostly in an energetic electron (which then deposits its energy locally).
Compton Scattering
Compton Scattering is the second dominant effect in x-ray imaging. In this case, the x-ray photon interacts with an electron in the outer shell, and hence the likelihood of Compton Scattering doesn’t depend on Z.
X-Ray Interactions with Matter and Image Signal Generation in X-Ray Imaging
As shown in the Figure the X-Ray photon knocks the electron out. Then the photon goes out in an opposing direction from the knocked out electron in order to conserve momentum.
It is important to remember here is that unlike in the photoelectric effect, the energy is not all deposited locally.
The scattered photon may still have a significant fraction of the energy of the incoming photon. It can still travel through the patient and potentially could have a secondary scatter effect or could get measured on the detector.
For more information on the impact of x-ray scatter on image quality and the effect of technical parameters on x-ray scatter please see our post on x-ray scatter.
Rad Take-home Point: In the Compton effect an x-ray interacts with a weakly bound electron and the electron and photon both continue on in opposing directions. - Zábava
![Coherent Scattering [a.k.a. elastic scattering, Rayleigh]](http://i.ytimg.com/vi/Z4uSygAixQY/mqdefault.jpg)
![Coherent Scattering [a.k.a. elastic scattering, Rayleigh]](/img/tr.png)
Lol. Great cold open. This video is an entertaining and accurate educational resource. Keep them coming Dr. Nett!
Thanks for dropping a comment Prof Tim. We will keep em coming.
thank you! Great graphics as well. These videos really help me in my diagnostic imaging class at Veterinary School!
Great to hear. Yeah X-rays in other animals have the same physics. Good luck in vet school;)
It helped me a lot while studying for my up-coming committee exam. I am really appreciate it!!!
Your welcome. All we ask is to Just share our channel with your colleagues
This video changed my life and I was able to finally understand these concepts. Thank you so much!
Mahmoud that is high praise, thanks for the great comment 😉
Thank you for this video and the time put in for it!
You’re welcome Dalia. Please share the channel with others.
Thank you so much sir 😊. Great effort👏
Most welcome
Exactly what I was looking to understand!
Glad it helped, pass it along
Thanks so much. It is so helpful.
Your welcome, thanks for taking the time to leave message.
@@HowRadiologyWorks Great job, Brian. You motivated people to learn the wonderful knowledge in order to help other people. Always bless you to succeed in your career. Cheers.
This message means a lot. I’m trying to help technologists master the physics behind the equipment so that they can get the best images for the patients. Thanks for your appreciation.
Understand well thanks
Great. This is important when studying technical parameters for x-ray and CT
Well explained
Thanks Miss Joel I appreciate your time also to leave us the comments. Check out our other videos too.
Thank you for the helpful videos on your channel from a first year student. I have a question about the depiction of photoelectric absorption please: I see that you have an x-ray emitted as the characteristic radiation from the atom along with the photoelectron. I've learned in one of my modules that effectively all of the elements that make up the body have such relatively low Z numbers (and therefore lower binding energies and electron energy states) that the characteristic radiation they emit during this process would be much lower frequency - in the infrared. How common would it be for a naturally occuring element in the human body to emit x-rays as characteristic radiation?
You are right, it should be labeled photon rather than x-ray. I will update in a future version
Thanks for the vid! So the incoming photon for photoelectric is of low energy? Happens when kVp of less than 70 kVp?
As the energy goes down the photoelectric effect increases. There is not one specific energy that is low. For Mamo very low energy is used
@@HowRadiologyWorks thank you 🙏
@@sgtpeppers3021 , no problem and thanks for teaching the band to play. I think it was 20 years ago today .😏
Thank you sir. I have a question here. If Compton effect makes the outer shell electron to eject, then why do we need higher energy of X-rays for it to happen?? why Compton effect is not dominant at lower energies??
In Photoelectric effect the atom as a whole absorbs the photon and therefore there it a higher cross section, or it is more likely than Compton where the interaction is just between the outer electron and and the photon. Photoelectric goes like 1/(hv)^3 which is why it is dominant at low energies.
Can u explain it again in simpler terms ?
In compton sacttering where does the recoil electron goes,
Is it ramain in its orbit or get knocked down from its orbit
The election is free and will deposit its energy relatively locally
Compton is not necessarily dependent on Z but more dependent on electron density.
Thanks atomic cowboy. The target audience for these videos is radiologic technologists. For this case we describe it as dependent on Z as for the most case Z is correlated with electron density.
Why it happens inner side of shell ?
When the electron binding energy matches the photon energy the photoelectric interaction can occur. This can happen in other shells but is more likely in the k shell given the energy of the X-rays.
What happens after with an electric photon ?? Is it possible to produce bremsstrahlung inside the human body through this interaction ??
I’m not sure what you by an electric photon. We use electrons to make x-ray photons that are used for imaging
@@HowRadiologyWorks I would like to know what happens to an electron ejected from the human body? Is it possible that this electron produces bremsstrahlung radiation?
pubmed.ncbi.nlm.nih.gov/25261767/ Yes it is possible when electrons are used in external beam radiation therapy that x-rays are generated
@@HowRadiologyWorks thank you so much.
@@lucasfeitosa_ your welcome!
I wanna ask a Q ... where the Where does the emitted electron disappear?
It will most likely slow down and stop in the tissue close to the first interaction since is has limited energy.
@@HowRadiologyWorks Well then we can say that it decays and does not go to another atom or becomes a free electron outside the body or crystal structure
Please tell me why KVp for bone is 45 and abdomen is 70 whyyy and why MAs is opposite
First set kVp and use lower kVp for bones due to higher contrast for high Z materials at low Z. Then to get a similar noise level higher mA is needed at the lower kVp since a lower kVp has a lower beam quantity so we can compensate with higher mA
The opening joke.....Christ
And that was before the King Richard movie, which I actually would recommend
Glad you kept watching