Understanding PET scans



and welcome to high school physics explained and today I want to talk about positron emission tomography and positron emission tomography is a type of scanning technique that uses a radiopharmaceutical or a radioactive chemical that we utilize in the body that will allow us to perform scans and understand the inside of the body so ultimately positron is the term we use for the production of a small subatomic particle which is produced inside the body in a particular nuclear reaction emission the fact that the positron is actually admitted and interacts with their body which I'll explain in a moment and tomography that the information we get is fed into a computer and allows us to produce slices and that is what tomography means to produce images that are slices so let's see how this positron actually is utilized in terms of imaging before we go on we need to first of all understand the source of this positron and the source of the positron is a radiopharmaceutical in this case and the one I'm going to be concentrating on is FD G now I could use other examples of chemicals that involve radioactive isotopes of c11 nitrogen 13 and oxygen 15 but today I'm going to particularly concentrate on the most familiar form of radiopharmaceutical involve the pet and that is FD G which stands for fluro dioxide glucose and in essence it is a glucose analogue and if you look very carefully here at this model of the glucose molecule it is actually glucose with the only exception being is that an oxygen atom has been removed and replaced with a radioactive isotope of fluorine through an 18 so what's really important here is that FDG and I'll use FDG from now on to save me king tongue-tied is that FD g behaves chemically in every ways the same as glucose does so whatever glucose is utilized in your body and you that glucose is a very important chemical in the chemical reactions of respiration that is the important chemical reaction in every cell of your body to produce energy then wherever glucose works so we'll FDG the only difference is is that while fgg is actually being utilized in your cells it is also radioactive and so therefore it release some sort of some atomic particle as it decays and so we're going to explore how that actually is useful for imaging so let's concentrate first and foremost on specifically the fluid so here's flowing and flowing itself Lewin eighteen has a mass number of 18 and it has nine protons and if you can work that out it has 99 protons but nine neutrons and fluorine itself in this form is radioactive and so it is radioactive in such a way that it decays into oxygen now how does it decay into oxygen well if you can see by the numbers clearly my number of protons has decreased and yet my mass number has stayed the same so that means the only possible explanation for what's going on here is that a proton has converted into a neutron we won't go into the specifics of the nuclear physics involved here and how a proton can be converting into a neutron but it is then clear that we have certain how lost a positive one a charge of some sort and what we just use is that here a release of a positron and a positron is in every respect similar to an electron except that it's positively charged but it also is fundamentally different to all other forms of matter a positron is actually antimatter and despite what you might think in terms of alternate universes and so forth antimatter is a type of matter that is that cannot coexist with normal matter and as we'll see in a second that is really important but the most important thing is to understand is that we have a fundamental particle some some sort of subatomic particle that is in many respects very similar to an electron but is positively charged and is a form of antimatter but most importantly as you can see what it leaves behind after the positron decay the fluorine now becomes oxygen and oxygen at 18 in this case is another stable form of oxygen so if we were to go back to our FDG it's now actually converted into glucose proper there's no longer flowing atom there it now has an oxygen atom and therefore it's still glucose so what happens to this positron so here is my FDG and as i said to you this flowing will decay into oxygen and therefore converting this back to ordinary glucose so it releases this positive and that's how we write down the symbol positron we provide it as an e because it in many ways is fundamentally like an electron except it's positively charged but it won't travel very far as I said to you the positron cannot coexist with matter and so very shortly it will come across an electron now this electron could be a free electron but northern likely it will be an electron that is involved with another chemical close by so what happens when these two come together well the most important thing to understand here is they annihilate that is they completely destroy each other and that the mass that the both the positron and the electron actually have completely disappears and is ultimately converted into energy and that energy is of course can be calculated by e equals mc-squared so here we have a classic example of an application of Einstein's famous equation equals MC squared a positron and an electron and naya light and as a result produce energy but what's special about this energy is that it produces two photons of gamma radiation and what's really important too is that these two photons actually travel at a hundred and eighty degrees to each other and that's really significant as we'll explore shortly so what is happening so here is an example of a brain and brain is where a lot of this happens because the brain utilizes a lot of glucose that's why I've got a picture of brain here we've got the fgg SM is listed then clearly FDG accumulates where there is glucose metabolized we have positron emission the positron annihilates with an electron to produce energy and producers to gamma photons which are released at 180 degrees some of you may be asking 180 degrees and that is again explained by another important law that is that is the law of conservation of energy so now that we've got two gamma photons being released what actually happens next so here I have a very simple diagram to represent what happens when a person gets a scan and over here I have a ring of detectors now these are gamma detectors then over here I've got a body and in this section here I have representative of something that releases positrons in other words this is where there is a high rate of glucose uptake and so this is the area that is going to be releasing in gamma radiation and so how does this ring of detectors know that this is the place of the gamma bursts well the first thing you need to understand of course is I have two bursts of gamma radiation going in opposite directions and what's really important to understand is is that this detection of the gamma burst and this gamma burst is for all intents and purposes arriving at the same time in other words this and this are simultaneous events and the computer can detect that these two are opposite to each other and therefore also that these two bursts arrive at the same time however what is important also to understand is that these particular gamma bursts is traveled through more tissue and this Gabor's has traveled through less tissue and so therefore the amount of attenuation or the amount of energy loss of this gamma bursts over here is going to be a little bit more on this side than this side and so with some complex mathematics and connected of course to a computer is that these sensors can determine the actual place of the gamma burst by working out the difference between the intensities of these two gamma bursts now clearly this is not enough and of course there is going to be multiple gamma bursts in all different directions and in fact in order to produce an image this calculation this determination has to be done up to a million times for the actual computer to determine the actual place of this FDG uptake and hence the positron emission but that in essence is what happens so now all of this information of course is fed into a computer analyzed to produce an image so here is an image of PET scan let's have a close look though at the features of it the first thing you should notice is at least in terms of structure we have low resolution so doctor is not going to provide you at a PET scan simply because he wants to get a good structural feature of your body clearly PET scan is not useful for that so why does he take it well let's have a look at the image and you see that there's areas that are really dark up here and diffuse over here and what's really important to understand is that PET scans are really high in function what I mean by that well clearly there is more metabolic uptake of FDG in the brain than anywhere else in the body now clearly there's obviously FDG or glucose metabolism occurring in lots of part of the body but far more is occurring in the brain which is what you would expect so it appears darker so in other words PT skin gives you no information about structure but about functional activity now in this case we have a elderly woman who's had a PET scan and the doctors noted the fact that there are areas that are darker such as here and here and here which is higher in terms of what normally would occur in a patient so in this case we have a case where a woman has signs of colon cancer and colon cancer is a ultimately your own cells rapidly dividing and rapidly mutating and obviously they need glucose so there is a lot more glucose uptake then there would be normally and so the patient is showing signs of cancer simply by the darkness here now of course the bladder as well has a dark area that isn't necessarily meaning that they've got cancer but if the glucose is broken down and your kidneys will ultimately take those waste products into the urine and so therefore there's a good chance that some of the FDG actually ends up in the urine or at least the byproducts thereof and so therefore you're going to get some sort of image in terms of the bladder but if you look down over here this is the same patient but here we've got another slice and you can see here this slice is pretty much straight down the center and this slice is little bit back you can't see the head over here but clearly the doctor also notices there is a little bit more uptake of FDG in the lung that's suggestive of some sort of tumor so let's summarize very quickly well FDG ultimately is a example of a radio farmer school used in positron emission tomography positrons emitted as the ftg decays into normal glucose that positron is then annihilated as it encounters an electron releasing two gamma photons in opposite directions due to the conservation of energy and they are in turn detected by a ring of gamma kilometres or a gamma camera which then can determine the actual position of those bursts to produce an image hope that has helped you understand p e– t thanks listening bye for now I hope you found that video useful and remember like share and subscribe oh and if you have a comment or a question or your like a concept for me to explain to you please drop a comment down below i'm paul from high school physics explained bye for now

27 thoughts on “Understanding PET scans

  1. such a great video!! thank you sooo much!! this is the best explanation I found after looking at many resources and even better than what I learnt in my class! Super thanks for your work! Also looking to understand SPECT, I hope I can another video of yours explaining SPECT.

  2. If the, let's say tumour, is not in the middle of the ring of gamma cameras, how do both y-photons reach the camera at the same time?? I mean do they not have the same velocity? Therefore one should arrive earlier than the other, when the origin is not in the centre of the gamma cameras, right?

  3. Hey! Student from Cognitive Neuroscience! Very useful! Thanks a lot 🙂 way more fast and interesting way to learn than good old books. You made the subject passionating! 😀

  4. Updated to correct unfinished thought:
    Your explanation of the intensity of the gamma bursts detected (being 180° from one another) in one direction having to travel through more tissue than the bursts in the opposite direction as the way to locate where the bursts are originating, does not take into account the different distances traveled for each photon (of a particular photon pair) from the point of annihilation to the detector ring, nor the angles of incidence of each photon (of that photon pair) to the tangents of the detector ring at the points of detection: obviously if the annihilation occurrence is not dead center of the detector ring (circle), then either the resulting distances traveled by each photon (of that photon pair) to the detector ring will be different, or the angles of incidence of each photon (of that photon pair) to the tangents of the detector ring at the points of detection will not be 90°, or both. Re: your explanation from about 6:48 to about 10:36. Therefore, I submit that determining the locations from where the gamma bursts are originating is more a function of the values of their different distances from the detector ring, their angles of incidence to the tangents at the points of detection on the detector ring, or both. For any photon pair, knowing the difference in distance each photon has to travel to the detector ring, the diameter of the detector ring, and the angle of incidence to the tangents of the detector ring at the points of detection (angle will be the same [mirror-imaged] for both photons as their paths of travel are 180° in opposite directions to the detector ring: combined paths of both photons can be represented as a chord on a circle) is what is needed to locate where the gamma bursts are originating. The other information is known: speed of light, wavelength of photon and subsequent properties, mapping instrumentation, etc.

  5. Your explanation of the intensity of the gamma bursts detected (being 180° from one another) in one direction having to travel through more tissue than the bursts in the opposite direction as the way to locate where the bursts are originating, does not take into account the different distances traveled for each photon (of a particular photon pair) from the point of annihilation to the detector ring, nor the angles of incidence of each photon (of that photon pair) to the tangents of the detector ring at the points of detection: obviously if the annihilation occurrence is not dead center of the detector ring (circle), then either the resulting distances traveled by each photon (of that photon pair) to the detector ring will be different, or the angles of incidence of each photon (of that photon pair) to the tangents of the detector ring at the points of detection will not be 90°, or both. Re: your explanation from about 6:48 to about 10:36. Therefore, I submit that determining the locations from where the gamma bursts are originating is more a function of the values of their different distances from the detector ring, their angles of incidence to the tangents at the points of detection on the detector ring, or both. For any photon pair, knowing the difference in distance each photon has to travel to the detector ring, the diameter of the detector ring, and the angle of incidence to the tangents of the detector ring at the points of detection (angle will be the same [mirror-imaged] for both photons as their paths of travel are 180° in opposite directions to the detector ring; combined paths of both photons can be represented as a chord on a circle). The other information is known: speed of light, wavelength of photon and subsequent properties, mapping instrumentation, etc.

  6. I don't get how the detector can know the location of the Annihilation when both 180° y-rays are detected at the same time.
    Does the detector compare the amplitude of the Photon energy? It must compare something to know the difference in distance of the travel from the 2 y-rays right?

  7. thank you for your clarification, but could you please what are the different between PET, and SPECT and as you explain on this video the main aim of the PET is for functions, so the SPECT for what, if you have another video which explains the SPECT please guide me.

    regards,,,
    Ali

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