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Medical Physics: PET Scans (Positron Emission Tomography), Positron Annihilation, and Antimatter — Transcript

by Dr. Pierce's Physics & Math · 2,161 words · 366 segments · language en · Watch on YouTube

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0:00all right we're going to take a look
0:00today at anti-matter which sounds like
0:03science fiction but actually you'll come
0:06to see how it's used uh
0:07every day in hospitals and what's called
0:09a pet scan or positron emission
0:11tomography
0:13so first a little bit about matter and
0:14antimatter um you're probably familiar
0:17with the parts of an atom
0:18so here's a helium atom and inside the
0:21atom we have
0:22protons here so here's a proton
0:26and the proton has a charge of one and a
0:29mass number
0:30of one so we tend to write protons like
0:32this either
0:33one one with a lower case p or you might
0:35remember that it's actually just a
0:37hydrogen
0:38uh nucleus so you can write a proton
0:39like that
0:41uh these guys here are neutrons
0:44they have no charge so that's a neutron
0:48they don't have any charge so charge is
0:51zero they have a mass number of one
0:52you tend to write them like this and
0:54then buzzing around here on the outside
0:56we've made a neutral atom here
0:58so these are two um electrons that are
1:00cruising around in what's called the 1s
1:02orbital
1:02so here's an electron
1:07and you write electrons have charge of
1:09minus 1
1:10and a mass number of zero um so there's
1:12an electron
1:13or um these are actually called beta
1:15particles if you're doing nuclear
1:16physics so you might see them written
1:18like this
1:19right now it turns out you come to
1:23believe from kind of
1:24high school chem that the world's just
1:25kind of made up of out of uh
1:27this stuff but it turns out there are
1:29more particles than this uh and in fact
1:31uh every particle
1:33has a a pair particle
1:36that's the anti-matter version of
1:39matter that you see here and so the
1:42anti-particle for an electron
1:45anti-electron which is actually has a
1:47special name of a positron since it was
1:49the first one to be found
1:50so an anti-electron but we're actually
1:53just
1:54always going to call it a positron it
1:56has a special name because again it was
1:57the first one of these to be found
2:01the way that you would write a positron
2:03well it's just
2:04like an electron only it has a charge of
2:07plus
2:08one so you can either just write the one
2:09or write plus one
2:11zero and then e plus or you might see it
2:13as a beta plus
2:15uh you don't have to write the plus i
2:16just i just did you could write it as uh
2:18say one zero beta plus
2:23well where can you find these or where
2:25do they come from well what's going to
2:26be relevant for us today is we're going
2:28to look at
2:29these things can be made sometimes in a
2:31in a nuclear reaction
2:33and so one nuclear reaction that can
2:36produce these
2:38um is if you have a um
2:41an isotope of a fluorine
2:44so if you have uh the decay of fluorine
2:4818
2:59right so we'll suppose it will suppose
3:01that we've got fluorine 18
3:03and so here's here's fluorine it's
3:06element nine
3:07it's four and eighteen now it turns out
3:10that
3:11most of the time with this is an
3:13unstable nucleus
3:14um most of the time when it decays
3:18and it's about 97 of the time
3:22what it does is it decays by emitting
3:25one of these positrons
3:27and so what can happen is it will it
3:29will kick off a positron so you have a
3:31one
3:32zero e plus and then we can see
3:35what the other um species that must be
3:38left behind as well so the
3:40total charge here is is nine and so one
3:43plus mystery number has to be nine so
3:44why this thing has to have a charge of
3:46eight
3:47that's going to be an oxygen and then uh
3:50you have 18 as the mass number of
3:51protons plus neutrons um
3:54zero here so this needs to be 18. so
3:56you've made
3:57a a positron has come shooting out and
4:00you have
4:01oxygen 18.
4:04this reaction we're going to we're going
4:06to
4:07see how this gets used in medicine in a
4:10moment
4:11but just for the heck of it as it turns
4:13out about the other like three percent
4:14of the time
4:15um the way that this thing will decay is
4:18it will
4:19um do what's called an electron capture
4:22um so this would be a beta plus decay
4:27positron emission okay um
4:30and then um the other this is what's
4:32called an electron capture so the other
4:33way that this thing can decay because
4:35again it's an unstable nucleus
4:37um what it can do is it can snag an
4:40electron
4:42so this is called an electron capture
4:45you could write it as beta
4:47also and we can see what the species is
4:49that's made well the total charge here
4:51will be nine and we've pulled in
4:52electron minus one
4:54and so what that's going to do is make a
4:56total charge of eight
4:58and then likewise on the top our mass
4:59number is going to be 18.
5:01so it's another pathway to making an
5:04oxygen 18.
5:05but this one is a different
5:09type of reaction called electron capture
5:12or beta capture
5:17okay so the point being if you have this
5:20funny isotope of fluorine
5:22it can decay and then spit out and make
5:25in the world introduce into the world
5:27one of these particles a positron
5:29now where this gets really interesting
5:31is if one of these positrons
5:34happens to meet an electron right so
5:36we're going to take a quick uh
5:38look at what happens if we do that
5:40[Laughter]
5:47let's suppose we have a positron
5:49actually that we've made because
5:51by having this fluorine isotope decay
5:53and what we'll do is we'll have that
5:54positron meet an electron
5:56so let's look at what happens if we have
5:58a positron
6:02uh meet up with an electron
6:08all right we'll draw some pictures of it
6:09here's a little positron
6:11and it's going to join up with or meet
6:13an electron
6:16these guys are going to get together and
6:18we'll write a little uh
6:20reaction for that so the positron has a
6:22charge of plus one
6:24uh mass number zero e plus or beta plus
6:28uh plus the electron which has negative
6:30one charge zero
6:32e minus or beta minus and again
6:36what we've got here is a total charge of
6:38zero
6:39um so we've got to make something with a
6:41total charge of zero
6:43and we don't have a mass number we need
6:45something a mass number zero
6:46so total charge zero mass number zero um
6:49what fits the bill is actually a photon
6:52um but so uh or so those tend to be
6:56written with this symbol gamma for like
6:57a
6:58gamma ray now there's a problem with
7:00this this is saying you have a positron
7:02electron get together and make a photon
7:05uh make light unfortunately that would
7:08this would violate momentum because you
7:10can't have two things that just kind of
7:12come together and then you just fire one
7:14thing off in one direction um
7:16so what you actually end up getting here
7:17is a pair of photons
7:19two gamma rays one goes in one direction
7:23one goes the other direction
7:24right so you have a particle plus a
7:28particle
7:29makes no particles you just get light
7:30you get a pair of a pair of gamma rays
7:33that go off in in opposite directions
7:36okay
7:37so what's interesting with this is you
7:40have
7:40mass plus mass gives you something
7:42that's massless
7:44so this is as pure a conversion of mass
7:47into energy as there is
7:49when you get anti-matter and matter
7:51together
7:53they will annihilate each other and
7:55produce
7:56produce white in in in opposite
7:58directions
7:59um so these are again two gamma rays
8:01that head off in in opposite directions
8:04so a quick little story and then i'll
8:06prep you for a little movie clip that
8:08i've
8:08um that i've taken from uh from youtube
8:12is uh the way this ends up getting used
8:14in the hospital
8:15is you attach that radioactive fluorine
8:18that we talked about
8:20that we wrote the reaction for you
8:21attach that to a sugar
8:23so again just for records i'll put that
8:25fluorine reaction back up
8:26so we had fluorine 18 element 9
8:31decaying by emitting a positron 1 0
8:34e plus plus 8 18
8:38oxygen right so that's where these
8:40that's where this thing comes from in
8:41the first place
8:42so here's the deal you uh
8:45you have a chemist attach
8:49radioactive fluorine to a sugar and you
8:51purposefully put it into a patient
8:54all right and then that sugar goes
8:56throughout their body
8:57but so you have this radioactive sugar
8:58that you've purposefully given to a
9:00patient
9:00goes all over the body that sugar uh
9:04cancer cells will tend to gobble the
9:06sugar
9:07at a greater rate than other cells so
9:09you'll get a greater concentration of
9:11the
9:11the sugar say where there's like cancer
9:13cells
9:14what will happen is that radioactive
9:17fluorine in the sugar
9:18will will make a positron in your body
9:21it will decay and make a positron
9:23well that positron doesn't have to go
9:24very far to find an electron in your
9:26body there's plenty of them
9:27all right so what will happen is the
9:30elect
9:30the positron will meet an electron in
9:32your body and then
9:34annihilate and zing you get a pair of
9:35gammas that go in in different
9:37directions
9:38well so you have the patients laid out
9:40on the slab
9:41and the positrons and electrons are
9:44meeting and annihilating each other and
9:45shooting out a pair of gammas
9:47and what you can do is detect those
9:49gammas and figure out where the
9:52where the gamma rays came from and that
9:54actually tells you where the sugar was
9:56being used
9:57all right so um i wanted to tell you
9:59that story first but i'll show you here
10:00in a second a more
10:02effective animation um that shows that
10:04process
10:05but again just so you know what to look
10:06for in the animation patient's going to
10:08be given some
10:09a patient with possible cancer well that
10:11we want to image is going to be given a
10:13radioactive sugar
10:14sugar is going to go throughout the body
10:16collect preferentially in certain places
10:18that use sugar a lot
10:20the brain the heart and then in
10:22particular cancer cells
10:24and then what you do is you look for
10:26where these gammas are coming from when
10:27they're produced
10:29and wherever the sugar collects well you
10:31can get a little bit of contrast in the
10:32image around
10:34say where a cancer cell is so i'll show
10:36you a little video clip about that
10:38so here is this patient who may or may
10:40not have
10:41some kind of a cancer and what's
10:43happening here is the patient
10:45is being given the radioactive sugar the
10:47sugar with the fluorine isotope that's
10:49attached to the
10:51type of glucose with fluorine attached
10:54and the sugar goes throughout the body
10:55right it's carried around everywhere
10:57in the bloodstream and so we have to
10:59wait a bit for the
11:01sugar to collect in tissues
11:06as you can see as it says down there the
11:08tissues that tend to gobble the sugar
11:11you know it collects at a greater
11:13concentration
11:14in those tissues
11:19so the little green ball is supposed to
11:20be the radioactive fluorine
11:23and the hexagons there
11:26are the sugar there it decays it made a
11:30positron
11:31that positron will have an easy time
11:33finding an electron in your body
11:35and there's the two gammas say going in
11:37the opposite directions
11:38and so what's done is your body is put
11:41in
11:42this ring of detectors and then um
11:45from the detectors uh an image is
11:49able to be constructed that shows where
11:52the
11:52counts where the gamma rays are coming
11:54from so there's this australian
11:57agency that's made this video one thing
12:00that's really fascinating with this if
12:02you stop and think about the chain of
12:03events
12:03that's happened here is you needed the
12:06you needed a
12:07chemist to um make the radioactive sugar
12:11in the
12:11in the first place um so you needed that
12:14to happen you need that to get to the
12:16um the hospital where doctors who know
12:19how much
12:20radioactive sugar you need to be given
12:22can properly dose
12:24it you need people who know how long
12:26that you should wait
12:28how long it takes for the sugar to get
12:29around in your body um
12:32before the um the scan can be made and
12:35then you need people who know how to
12:36operate the mri and then of course you
12:38need radiologists who know how to read
12:40the mri
12:41so this is just a spectacular amount of
12:43collaboration between
12:45all kinds of scientists and so that is
12:47how antimatter
12:49is used in pet scans in the hospital

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