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Description of a Gradient  | Physics of MRI 8: Advanced Concepts - Part 1b
Description of a Gradient | Physics of MRI 8: Advanced Concepts - Part 1b
2012additionalchapteressentiallyfull videogradientsmagneticMRIUHN
Dangers of a Gradient - Minimal and Nerve Stimulation | Physics of MRI 8: Advanced Concepts - Part 1b
Dangers of a Gradient - Minimal and Nerve Stimulation | Physics of MRI 8: Advanced Concepts - Part 1b
2012chapterdangerferromagneticfieldfull videogradientsmagneticpatientperipheralradiofrequencysafetystrengthsuddenUHN
Radiofrequency Pulses Danger - Heat | Physics of MRI 8: Advanced Concepts - Part 1b
Radiofrequency Pulses Danger - Heat | Physics of MRI 8: Advanced Concepts - Part 1b
2012calculationschapterdepositdepositedelectromagneticexceedferromagneticfrequencyfull videoheatheatingradiofrequencyscannerUHNwaves
SSFP Pulse Sequence - Gradient Echo and Signal Decays | Physics of MRI 8: Advanced Concepts - Part 1b
SSFP Pulse Sequence - Gradient Echo and Signal Decays | Physics of MRI 8: Advanced Concepts - Part 1b
2012acquirechapterdirectionechofull videogradientgradientsmagneticmoleculesorthogonalpresencepulsesignalUHNvoxelwater
SSFP Tissue Contrast | Physics of MRI 8: Advanced Concepts - Part 1b
SSFP Tissue Contrast | Physics of MRI 8: Advanced Concepts - Part 1b
2012anglechaptercomparisoncontrastflipfull videogradientimageincreasenoisepulseratiosequencesignaltissuetissuesUHN
SSFP Off Resonance | Physics of MRI 8: Advanced Concepts - Part 1b
SSFP Off Resonance | Physics of MRI 8: Advanced Concepts - Part 1b
2012artifactbehaviorchapterfieldfrequencyfull videomagneticmetallicrangesignalUHN
Transcript

of lectures on basic MRI physics, and the topic of today's lecture is going to be the Advanced Concepts in MRI. So next safety/g issue we're gonna talk about is that related to the gradients. So just briefly recall, from our previous lectures, so again, we have this large external magnetic field, so that's essentially uniform everywhere

inside the bore of the magnet. And we then apply additional magnetic field gradients. And these gradients vary both spatially and temporally. And we saw, from the earlier lectures, that the purpose of these gradients is to encode spatial information, so essentially these are how we encode our image. Now the gradients are responsible essentially for the knocking sound that people often associate with MRI. So

as you turn the gradients on and off, this creates a knocking sound. Now in terms of the dangers associated with gradients, the projectile danger is minimal. So again, this is an additional magnetic field.

So you may at first think well, if we turn on these gradients, we're gonna make that magnetic field strength even stronger, and we're gonna even have more of a danger associated with any projectiles,

say ferromagnetic projectiles. In practice, this really isn't the case, because the strength of gradients is much, much less than the main magnetic field, so, orders the magnitude less. So, while theoretically, yes, the magnetic field strength increases, it's only by such a small fraction. There's no additional danger associated with turning the gradients on and off. So of course, there's always the ever-

present danger of the main magnetic field being on, but it's not the case that if you're standing ten feet away from the magnet, and you turn the gradients on, all of sudden your ferromagnetic materials are all gonna be sucked into the magnet when you turn the gradients on, that's not the case. The only real issue associated with the gradients is that, because they vary over time, because

you have a magnetic field varying over time, it can actually cause a peripheral nerve stimulation. So they can actually stimulate the nerves, so it often feels sort of like a poking sensation. So it's like someone's poking you, and this isn't really so much of a safety issue, there's really no adverse consequences as a result to peripheral neural stimulation, but it's more an issue of patient

comfort, especially if someone is already fairly anxious when they go into the scanner, then all of a sudden they feel themselves being poked in the back. So this is really something you wanna avoid, just more from a patient safety perspective, rather than a real serious danger. The last piece of hardware that I'm gonna talk about, in terms of safety issues, is the radiofrequency pulses,

or the RF. Now the main issue associated with RF pulses is that they deposit energy into the body. And if you deposit energy, this can lead to heating. So just by comparison, really this is the same principle that microwave ovens work on. So you deposit electromagnetic waves, in this case, microwaves, into food and that heats up the food. So the same principles that work in MRI. The difference is

that the radiofrequency waves are much less efficient at heating than microwaves, based on the difference in their frequencies. So microwaves have a frequency of about 3 GHz whereas, the RF waves we use in MRI have a frequency of about 63 MHz. So the principle is the same, but the effect is much less. Nonetheless, if you deposit

enough RF energy into the body, you can that can lead to heating. And if you're not careful, this actually can cause burns in the patient. Now there's a concept called the Specific Absorption Rate or what's often abbreviated as SAR, which is calculated to determine the amount of heat per unit time that's deposited into the body. So the amount of heat per unit time deposited into the body.

So SAR calculations are performed on the MR scanner by the MR scanner itself. And under normal circumstances, scanner software will not let you exceed predetermined SAR thresholds. So these thresholds are set to, obviously, prevent burning. So for whatever you're doing, the scanner will calculate how much SAR that will be deposited,

and if it's not sufficient to cause burning, it will let you proceed, and if not, it will prevent you from scanning. Now the SAR calculations themselves depend on a number of factors. So first of all, is the type of pulse sequence, and secondly, is the person's weight. So one important thing is, when you're scanning a person, don't overestimate their weight, because if you do that, then the scanner

will think that you can deposit more heat than you really should be allowed to do. So this is when you always go in to get an MR scan, one of the questions they always ask you is to get your weight. And it's not because they're prying for personal information, it's just simply because they wanna make sure that you don't overheat during scanning. Now an important issue that arises is that, under

normal circumstances as I said, the scanner software will not allow you to exceed SAR threshold. However, those SAR calculations are based on the assumption that, what's in the scanner is essentially a human being or just simply tissue. And if you have things that are not tissue-like, in particularly metal, then the SAR calculations will not actually take this into account. So anything metal, especially

things that form circular loops, can give rise to local heating hot spots. So some examples of things that can cause that are wires, for example jewelry, so necklaces, bracelets, rings, or bras, especially those with underwire. So all of these sort of things can create local heating hot spots that will not be accounted for by the scanner software. So as a result, you have to remove any sort of metal on

the body prior to scanning. So again, with the metal there's really two issues. So one is a projectile dangers, so if you have ferromagnetic materials, that's an absolute danger because things can get sucked into the magnet. But even if something's non-ferromagnetic and it's metal, so let's say something like copper, that could still be a danger because as a result of the RF it could give rise to

a local hot spot. Next I'm gonna move on to talk about SSFP. So this stands for steady-

state free precession, and has various different names on the different vendor scans. So on Philips it's called Balance FFE. On Siemens, TrueFISP, and on GE, FIESTA. So to describe what goes on with an SSFP pulse sequence. I'm gonna start out with gradient echo

cause it's actually very closely related. So again, recall from our previous lectures, we have an RF pulse, tips and magnetization down, we have our y gradient turns on, which moves us in the vertical direction of k-space. We then turn on our x gradient, that moves us in the orthogonal direction of k-space, and we acquire data as we sweep out a line of k-space. So the SSFP pulse

sequence, in fact, starts out in exactly the same manner. So we actually acquire the data in exactly the same way. We go through k-space in exactly the same manner. But the key is that in SSFP, we actually balance out all of the gradients. So any gradient that's positive, has be balanced out by an equal amount of negative gradient area. So the total area of all

the gradients and SSFP is zero, and that's different than your conventional gradient echo pulse sequence. Now this seems like an almost trivial sort of change, but as I'll show you, it can have a profound impact on the resulting properties of the scan. So the key reason is that, in the presence of a magnetic field gradient, this can lead to signal decays. So just to illustrate that point, here we have let's

say, three different protons that eat in, sorry, three different water molecules inside a particular voxel of tissue. And as we saw before, they add up produced a net signal. Now, in the presence of a gradient, each of these water molecules will see a slightly different magnetic field, because again, there's a linear gradient across the voxel. So, over time, as we know, when we have different magnetic fields, we have different frequencies

of rotation, and therefore over time, those three different water molecules will begin to dephase relative to each other, and there will be a signal loss. So we encountered this many times in the previous lectures. Now, with SSFP, we invert the polarity of the gradient. So recall that if we had a positive polarity gradient and we have to invert it, and

have a negative polarity gradient of equal size. Now, the significance of that is that water molecules that initially saw a lower magnetic field will now see a higher magnetic field. So they all start to rotate in the opposite direction of what they did initially. And if the water molecules are rotating in an opposite direction, they'll begin to rephase. And when they rephase, the size of the signal

will regrow. So with SSFP we tend to have much greater signal and noise ratio because we're rephasing all of the magnetization by directly balancing all of our gradients. So if we look back to pulse sequences with SSFP, you can see that in all cases, for both the x and y gradients, and the z as well, all the gradients are completely balanced. So

they have exactly the same area positive as they do negative, and that re-phases all of the magnetization so that we achieve a greater signal to noise ratio. So what about the tissue contrast with SSFP? Well, if you go through the math, which I'm not gonna do in this lecture,

you can show that the signal we get from tissue is proportional to the square root of the T2/T1 of the tissue. In

general, water tends to have the largest signal because T1 is approximately equal to T2 in water. All other tissues have T1 greater than T2, so they'll have a smaller signal than water. Now, a rather unique property of SSFP, is the tissue contrast is largely independent of the pulse sequence parameter. So that means, within reasonable limits, no matter what the TE is, no matter what the TR is,

no matter what the flip angle is, the overall contrast or the overall signal between the different tissues will be approximately the same. This is very distinct from the other types of pulse sequences where, as we saw in earlier lectures, varying TE and TR has a very strong influence on the resulting contrast between the tissues. Now, as a consequence of this, what this means is that the signal

to noise ratio increases directly with flip angle. So the contrast does not change, but the signal to noise ratio will continue to increase as we increase our flip angle. Now as a result, with SSFP, we tend to use relatively large flip angles to get a very large signal to noise ratio. So, for example, 60 degree flip angle is quite common, in comparison with gradient echo, we typically use a flip angle

more in the range of say, 15, or 20 degrees, or sometimes even smaller. In fact, the signal-to-noise ratio is often not limited by the pulse sequence itself, but rather by the Specific Absorption Ratio, or SAR, because what happens is that the amount of SAR you deposit depends on the square of the flip angle. So if we increase our flip angle by the factor of two, we increase our SAR by the factor of

four. So in fact, we can often actually get away with higher signal-to-noise ratio by using larger flip angles, but we often have to limit ourselves because we're starting to see some problems with SAR. So here we have two examples of the image of the heart. So the image on the left is a gradient echo pulse sequence and the image on the right is an SSFP-based pulse sequence. So the one thing is, when you compare these two, you can

see that the signal-to-noise ratio of the SSFP-based pulse sequence is much better, and this is fairly typical of SSFP. We have really high quality, higher SNR pulse sequences. Also notice in the SSFP pulse sequence that the brightest signal tends to be the blood because, again, the T1 and T2 are closest to each other in comparison with, say the myocardium, which has a much larger difference between

T1 and T2. So one of the unusual features of SSFP is the way that off-resonance

artifacts appear. So recall that, off-resonance refers to the range or the type of magnetic field that exists within the tissue. So in general, a range of magnetic fields exist because we don't have a perfectly homogeneous magnetic field. So in general, anything

that's sort of at the central magnetic field strength is called on-resonance, and anything that away from the central peak is called off-resonance. Now, in SSFP, the behavior of the signal varies as a function of the resonant frequency or the magnetic field strength. You can see the black line, in the plot that I've shown you here, illustrates the behavior of the SSFP signal. And you can see that over a

relatively of the broad range, the signal is fairly flat. So in this region, regardless of the magnetic field strength, the SSFP signal will appear approximately the same. However, you can see that at some point the signal drops down to very close to zero. So essentially, there's a null in the SSFP signal. And that occurs at a particular resonant frequency.

So in the image, this appears as essentially a banding-type artifact or a null. You can see the arrows that I've illustrated in the slide here, in the image on the right, it indicates region of these bands or these nulls in SSFP. And again, these are related to the resonant frequency or the range of magnetic fields that are present in the image. Now if we have, say something like metal in our image, so

this is the person with some kind of metallic device, this can cause a severe distortion in the magnetic field around the device. And as a result, when we acquire SSFP images, you have these sort of bizarre-looking banding pattern appearances. And again, that's because we have a very strange looking magnetic field around the region of the metallic box. So that ends the first part of this

lecture. We've covered spectroscopy, MR safety, and SSFP. I hope

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