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Outline | Physics of MRI 8: Advanced Concepts - Part 1a
Outline | Physics of MRI 8: Advanced Concepts - Part 1a
2012advancedchapterconceptsdescribingfull videolecturelecturesMRIspectroscopyUHN
Spectroscopy | Physics of MRI 8: Advanced Concepts - Part 1a
Spectroscopy | Physics of MRI 8: Advanced Concepts - Part 1a
2012chapterchemicalconcentrationcoupleelectricalfrequencyfull videomagneticmetabolitesmoleculeMRIrotationrotationalsignalUHN
Fourier Transform Theory - Metabolites | Physics of MRI 8: Advanced Concepts - Part 1a
Fourier Transform Theory - Metabolites | Physics of MRI 8: Advanced Concepts - Part 1a
2012applychaptercomponentscompositedomainfrequencyfull videoindividuallowestmetaboliteoccurrenceplotsignalsignalssimplyspatialspectroscopyspectrumtransformUHNvivovoxel
Types of Spectroscopy | Physics of MRI 8: Advanced Concepts - Part 1a
Types of Spectroscopy | Physics of MRI 8: Advanced Concepts - Part 1a
2012chapterconcentrationessentiallyfull videometabolitespectraspectroscopyspectrumUHNwater
Spectroscopy of Non Proton Molecules | Physics of MRI 8: Advanced Concepts - Part 1a
Spectroscopy of Non Proton Molecules | Physics of MRI 8: Advanced Concepts - Part 1a
2012chapterfull videometabolitesrarelyrisespectroscopyUHN
MR Safety and MR Magnet Dangers | Physics of MRI 8: Advanced Concepts - Part 1a
MR Safety and MR Magnet Dangers | Physics of MRI 8: Advanced Concepts - Part 1a
2012aneurysmchapterclipsdeviceferromagneticfieldfull videohardwareimplantablelyingmagnetmagneticmagnetsmainMRIpacemakerpulledsafetyscanUHN
Transcript

Hi. My name is Marshall Sussman. I'm an MRI physicist at the University Health Network and the University of Toronto. I'm giving a series of lectures on basic MRI physics. And the topic of today's lecture is going to be Advanced Concepts in MRI. And this lecture is going to be divided up into two parts, and the first part's gonna begin right now. So just giving/g an outline of what I'm gonna be speaking about today. I'm gonna start out by describing some of the more advanced concepts

that we have in MRI. And this goes beyond the more basic topics that have been covered in previous lectures. I'm gonna start off by describing a little bit of both spectroscopy in MRI. I'm gonna touch on MR safety. I'm gonna talk about SSFP or steady-state free precession imaging, parallel imaging, diffusion imaging, and finally, PROPELLER. [BLANK_AUDIO] So spectroscopy. We know from previous lectures that

the signal in MRI arises from magnetic dipole moments associated with water molecules. And we have an example of that in the illustration below. And by far, the most abundant source of hydrogen in the body is water, which is pervasive/g throughout the body, obviously. However, there's also other molecules that contain hydrogen as well within the body. And these too, under some circumstances, can also emit electrical signals. So a couple of examples are fat as well as various lower concentration metabolites in the body. So a couple of examples, lactate, choline, creatine, then there's many others. Now, just recall from our previous lecture, the reason we have signal in MRI is because the magnetic dipole moment essentially spins around the magnetic field. And the frequency of rotation is equal to the gyromagnetic ratio, which is a constant, times the magnetic field. So that's often illustrated aswell within the body. And these too, under some circumstances, can

also emit electrical signals. So a couple of examples are fat as well as various lower concentration metabolites in the body. So a couple of examples, lactate, choline, creatine, then there's many others. Now, just recall from our previous lecture, the reason we have signal in MRI is because the magnetic dipole moment

essentially spins around the magnetic field. And the frequency of rotation is equal to the gyromagnetic ratio, which is a constant, times the magnetic field. So that's often illustrated as gamma times B. So just as an example, in the case of water, let's just say for the purpose of this talk that water rotates at 63.0000 MHz. However, it so happens that the local chemical environment

can slightly alter the rotational speed of the molecule. So for example, let's say we have this other molecule that contains hydrogen. Because of its different structure and different chemical environment, the rotation frequency of this molecule will be slightly different than that of water. So just say, in this case, the example I've given here, it rotates at 63.0005 MHz. So in other words, because of the

local difference in chemical environment, there's a slight change in the rotational frequency of the molecule. Now, let's say we have an example. Here we have, say a pixel or a region in the body, and in general, that will contain many different types of chemicals that all contain hydrogen. So the overall signal that we detect in our coil is going to be the sum of the signals from all of those

individual components. So the question is, how do we discriminate the signal coming from these different molecules? Because again, we've just received the overall sum in our coil. And the way we do that goes back to the concept that I introduced in the k-space and gradients talk. And that is, we use

Fourier transforms.

So I'm just gonna display this slide which I actually had in my earlier lectures, that just outlines the basic concepts underlying Fourier transform theory. So in Fourier transform theory, let's say we have a series of signals added together that are oscillating in different frequencies. So in this case, I have one signal oscillating

at the lowest frequency and two signals oscillating at a higher frequency. Now, if I add those signals together, I'm gonna get a composite signal that's obviously gonna be the sum of those individual components. Now, instead of plotting the signal in the time domain, I can also plot that signal as a histogram representation. So the histogram indicates the relative contribution of different frequencies

to that signal. So in this case I have one occurrence at a lower frequency and two occurrences at a higher frequency. Now if you recall from the earlier lectures, the idea behind a Fourier transform is it just simply allows you to transform between those two representations of a signal. So if I had the time domain representation, I could apply

a Fourier transform, and it would tell me what the various different frequency components that made up that signal are. And then I could also do the converse. If I had the frequency components I could apply a Fourier transform and determine what the time domain representation is. Now, in my previous lectures, I used this to illustrate how we can localize spatial information and encode spatial information

using gradients. But in the context of spectroscopy, if you think about it, this is exactly what goes on in spectroscopy. Because remember these metabolites, which each have a different chemical environment, are gonna be rotating at slightly different frequencies. So in this case, let's say I have one occurrence of one metabolite that's oscillating at the lowest frequency and I have two occurrences

or twice as much of a different metabolite that oscillating with a higher frequency. So again, what I'm measuring is the plot on the right. That's the overall composite signal from all of the individual components within the body. Now, if I wanna distinguish those individual components I can just simply apply a Fourier transform to that signal, and I can get the relative contribution

of those different metabolites. So in this case, again, based on the Fourier transform, I can see that there's essentially one unit of metabolite one and two units of metabolite two. And that's simply all there is to streposcopy. Now, in this simple example here I've just shown you two isolated metabolites, but obviously in a real in vivo spectrum there could be many more. And the plot I've shown

you here illustrates an example of a real spectrum that we get from the body. And you can see that in this case there's three different peaks in the spectrum. And again, this is obtained just simply by applying a Fourier transform to the composite signal that we receive from the body. [BLANK_AUDIO] Now, there's a couple of different ways of doing spectroscopy. The first

type of spectroscopy is called single voxel spectroscopy. So this

is essentially where we do a spectroscopic analysis of only a single voxel, or a single region within a tissue. Next type is called chemical shift imaging. So that's essentially where we acquire data from many different voxels, and we obtain a spectrum at each individual voxel. So you can see an example of that on the image on the right

where we have spectra at many different voxels within the brain. Now, one other point I should note is, if you look at all these spectra, one thing you can notice that's not there is the water. There's actually no water peak, like for example on the spectrum on the left, you can see there's choline, creatine, and N-acetylaspartate

or NAA, but there's no water peak. Why is that? Well, the problem is that water has a much, much greater concentration than many of these other metabolites. So it could be a thousand times greater or perhaps even higher. So the problem is if we have water present then it would absolutely swamp out and obscure all these other much lower concentration

metabolites. So any time we do spectroscopy, we always have to work very hard to eliminate the signal from water so that we can then uncover these residual metabolite peaks that have much lower concentrations. I should also mention that it's also possible to do spectroscopy of molecules other than protons. So we saw in

earlier lectures that other molecules will also give rise, rather

atoms, will also give rise to signals, or can give rise to signals. So the image below gives you an example of phosphorous spectroscopy, which is very important biologically because it's to, for example, to illustrate energy usage in, or metabolism, in the body. Unfortunately, any spectroscopy other than proton spectroscopy is relatively rarely done in a clinical environment.

And the reason for that is because the signal-to-noise ratio of these other metabolites tends to be very, very low. And as a result, its just simply difficult to do. So spectroscopy itself is relatively rarely used, and non-proton spectroscopy is even more rare. Okay, now let's move on to talk about MR safety. So safety is essentially

an issue associated with the various different hardware components

of our MR system. So associated with each hardware component there's some associated safety issues. And in particular, the three types of hardware that we have in MRI is the main magnet, the gradients and the radio frequency or RF generator. So in the next couple of slides I'm gonna discuss the issues associated with each of these components, and some possible strategies you can take to minimize

any safety risks. So the first is the main magnet. So the main danger here is that anything that's ferromagnetic will essentially get sucked into the bore of the magnet, and this is an issue because the magnetic field used in MRI is very powerful. So to just give you an illustration of that, the earth's magnetic field is about half a Gauss, a "safe" magnetic field, one where there's

no safety concerns is generally one considered to be less than about 5 Gauss. So by comparison, the magnetic field we're using in MRI has strength to about 15,000 Gauss or 1.5 Tesla. So you can see, it's thousands of time higher than the earth magnetic field, and thousands of times higher than what is to be essentially a safe magnetic field, or one that has no effect. So as you can imagine,

there is a very large force associated with these very high magnetic fields. Another important issue to be aware of is that, in MRI, the type of magnets we use are superconductive magnets. And these magnets are always on. They don't make noise, they don't make make any sounds. You wouldn't know they're on just by looking at them, but they're always on. And this is often a danger because people often assume, unless a scan is going

on or unless there's noises that everything's okay, it's not on on. But this isn't true. In fact, the magnet is always on, 24 hours a day, 365 days a year. In fact, turning it off can be quite dangerous if it's not done properly, and in fact, can cost a lot of money. So for those reasons, the

magnetic field is always kept on, and this in itself creates a safety issue, because just because the magnet isn't in use does not mean there isn't a danger. And just to sort of drive the point home, deaths have occurred but from objects being pulled into the magnet when someone was lying inside the magnet. So just to give you an example of the strength of the main magnet, here we have a couple

of pictures of things that have been pulled into the magnet. So you're gonna see a table being pulled into the magnet, a couple of examples of chairs, a dolly/g. So you can see that the magnetic field has a lot of strength, and obviously, if someone's lying inside the magnet or even lying between the magnet and that object, some serious injury can occur. So really the solution

is, you have to make sure not to be bring an ferromagnetic materials inside the magnet room. Now, there are some additional dangers associated with the main magnetic field, and these primarily revolve around implantable devices that patients may have. So for example, a pacemaker, main magnetic field can interfere with the function of the pacemaker, and if a person is reliant on that pacemaker

to pace their heart, then if the function of that pacemaker is interfered with, then obviously, that could have some adverse consequences on the patient. Another issue, implantable device issue, is aneurysm clips. So the older aneurysm clips tended to have, or in some cases, contained ferromagnetic materials in their construction. And as a result, when placed in the magnetic field, the aneurysm clips

could experience a twerk, which could actually dislodge them from the aneurysm clip, and cause the blood vessel to burst. Now, obviously, these can have very serious health consequences. But in more recent times, the device makers have recognised this problem, and more modern aneurysm clips are constructed from non ferromagnetic materials. Nonetheless, obviously, as we encounter patients, many of them

may have these older clips. So it's certainly something that must be investigated. Another issue, which isn't so much danger, but more of an inconvenience, is that any device that has a magnetic storage media such as, computers, credit cards, bank cards, things of that nature, will tend to be deleted if you place them in the presence of a magnetic field. So this is, again, more of an inconvenience, but

something you should be aware of. So walking to the scan room make sure you remove all your credit cards, and obviously don't bring computers into the magnet room. [BLANK_AUDIO]

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