Spin Echo MRI Pulse Sequences, Multiecho, Multislice and Fast Spin Echo | MRI Physics Course #15

hello everybody and welcome back today's talk is the first in a three-part Series where we're going to be looking at different types of MRI pulse sequences we'll start off today by looking at spin Echo pulse sequences before then moving on to gradient Echo and inversion recovery sequences now the focus of today's talk is first going to be to understand what exactly a spin Echo is and why we would want to go about generating an echo within our sequence and we'll see that the generation of an echo is going to help us to recover some of that signal that's been lost during free induction Decay or T2 star decay once we understand what exactly a spin Echo is then we're going to move on and look at three different pulse sequences that utilize this spin Echo phenomenon now if you look at this pulse sequence here this is the kind of pulse sequence we've been looking at throughout this module now the first thing we do is apply a radio frequency pulse that will tip the net magnetization Vector out of the longitudinal plane and it will allow that Vector to gain some transverse magnetization as well as gaining transverse magnetization it's simultaneously losing longitudinal magnetization now why is this important the signal that we can measure in MRI is transverse magnetization we can't measure longitudinal magnetization because of that large magnetic field that we've applied along the patient that is always on there's no way that we can place a receiver coil and manage to somehow measure this longitudinal magnetization we can only measure what we flip into the transverse plane now we've applied a slide selection gradient at the same time that we apply that radio frequency pulse and that just allows us to select a specific slice and if you don't understand that concept go back to the slice selection talk we've then seen that we've applied a phase encoding gradient one gradient per complete cycle here to the entire slice and that phase encoding gradient will cause the spins to de-phase along the y-axis we've then previously touched on why we would apply a 180 degree RF pulse and how that goes about generating an echo within our sequence and we're going to revise that in a little bit more depth here today then when that Echo occurs when there's that rephasing of those spins we sample the signal at a time known as t e the time to Echo and it's enable te because that's where this Echo the spin Echo occurs and that's what we're going to be looking at today we sample over a period of time and we sample while we are applying the frequency encoding gradients as that frequency encoding gradient is being applied we're taking multiple discrete data points and then placing those data points within a single line on K space that corresponds to the specific phase encoding gradient that we applied for that specific pulse sequence we then wait a long period of time until we repeat the process again the time to repetition until we then flip those bins back into the 90 degree plane now te is a certain period of time after we have flipped those spins in this example to 90 degrees now what causes these spins to lose transverse magnetization it's not those spins returning into the longitudinal plane that contributes ever so slightly to signal loss the main contribution to loss of that transverse signal is the phasing of those spins if we were to take two spins here that we have flipped from the longitudinal plane into the transverse plane we then stop our 90 degree RF pulse those spins are going to D phase they're going to become out of phase with one another based on their local environments now that D phasing is causing loss of transverse signal because they are no longer in Phase with one another and that loss of transverse signal we can measure that rate and that rate is what's known as T2 and in an ideal world that loss of transverse signal from the D phasing would only be due to spin spin interactions with spins interacting with one another causing D phasing but as we'll see throughout this talk there are local magnetic field in homogeneities that are actually responsible for the majority of the dephasing of those spins and the loss of transverse magnetization and that loss of transverse magnetization because of the local magnetic field in homogeneities is what's known as T2 star or free induction Decay and that happens much more rapidly than T2 now these spins they've lost phase with one another so there's no longer any transverse magnetization the net magnetic moments of those spins though they're still in the 90 degree plane or they may have lost some of that angle there is still magnitude to those vectors if there's some way that we can cause those spins to re-phase they will start to regain some of that transverse magnetization that free induction Decay is a loss of transverse signal purely because of de-phasing we are not losing it because those spins have now gone into the longitudinal plane that's a really important concept to remember that's why T2 Decay happens so much faster than the longitudinal relaxation or T1 relaxation T1 relaxation takes a lot longer for the spins to ultimately lie back in parallel with that main magnetic field that's why TR is often so much longer than te so let's look at transverse Decay take three different tissues CSF fat and muscle now if this was pure spin spin interaction this would what be known as T2 relaxation or transverse Decay now once the transverse signal has lost 63 percent of its magnetization that is what's known as the t2 time constant and we can see that the t2 values for these three different tissues will have different time based values that teach you value as a constant how quickly are those tissues losing their transverse magnetization now that loss of transverse magnetization is because of the phasing of those spins these spin spin interactions in CSF is much less in the spin spin interactions in fat and in muscle and it's this that provides us contrast within our image if we do a te that's based say at this period of time we can see that the signal from muscle will be much less in that effect and the signal from fat will be less than that of CSF now the problem I've mentioned is that local magnetic field in homogeneities caused the signal to be lost much quicker than the t2 constant and that's what's known as T2 star free induction Decay and you can see here on this graph just how much quicker that happens and this again occurs because of local magnetic field in homogeneities either from our machine it's hard to make a machine that has a perfectly uniform magnetic field and when you place anything into the magnetic field like a patient there are different molecules and atoms within that patient that are going to ever so slightly distort that magnetic field like we saw in our chemical shift talk now you might think how big do these magnetic field differences need to be in order to cause such a drastic loss of transverse magnetization why did the spins defaze so quickly well if we look at the gyromagnetic ratio of hydrogen if we were to place hydrogen within a one Tesla main magnetic field those hydrogens will process at a frequency of 42.58 million Hertz 42.58 megahertz and we can use this equation here to look at two separate spins first is experiencing the main magnetic field and the second is experiencing a magnetic field that is one millionth of a Tesla difference if this spin is processing at 42.58 million Hertz per second this spin instead of precessing at 42.58 million Hertz it's going to have an extra 42.58 precessions per second it's going to be spinning or it's going to be processing at a frequency that's 42.58 Hertz more now we're talking about million Hertz so that difference is ever so small we can use the rotational frame to compare how is this spin spinning in comparison to this spin if this bin stayed still as a rotational frame and we just looked at this spin relative to this spin this spin will process 42.58 Hertz more per second now all it has to do is process 180 degrees more to be completely out of phase with the spin we flip them into the transverse plane they're both processing in the megahertz millions of Hertz range but we know that every second this spin is processing 42.58 Hertz more than the spin it only takes a very small amount of time for the first time for these two spins to be completely out of phase from one another and it turns out if you use these equations we can see that the time taken for them to be out of Faith with one another is less than 12 milliseconds it's an extremely small amount of time and that is caused by the most minute difference in main magnetic field that's why we get this free induction Decay and it's very difficult to measure that signal so quickly in less than 12 milliseconds at a te that's going to give us accurate measurements and not only that you can see now that the contrast between these two tissues has been greatly reduced because of this free induction decay these spins have defaced we need a way of rephasing them and measuring it at t e and that is the basis of spin Echo pulse sequences so let's go to an example here where we're looking at some form of tissue we've applied a B1 we've flipped those spins into the 90 degree plane and we allow them now to lose phase over time now this loss of phase is the t2 relaxation spin spin interactions causing this dephasing and that actually happens over a very long period of time especially if we're looking at water in CSF that dephasing that loss of transverse magnetization is very slow now I've said to you that that loss of transverse magnetization happens much quicker in the real world free induction decay occurs as local magnetic field in homogeneities cause that signal to be lost very quickly because of that rapid defacing of those spins now if we were to look at two separate examples here the one on the left here is the laboratory frame how we've been looking at these spins spinning from the outside the one on the right is the rotational frame which is sometimes more easy to conceptualize when looking at spin Echo in the laboratory frame as we've seen now those spins are going to de-phase over time mainly due to those local magnetic field in homogeneities the rotational frame is going to show you how the spins D phase relative to the Llama frequency the Llama frequency at this specific location how are some of the spins going to defaze slower some of them D phase faster because of those slight differences in processional frequencies based on those local magnetic field in homogeneities now as we play this now with those local magnetic field in homogeneities we are losing signal rapidly compared to how we were losing signal when it was only spin to spin interaction see now how we've got Fanning out of that rotational frame those spins are de-phasing relative to one another the laboratory frame those spins have either gone faster or slower depending on the local magnetic field now we need a way to re-phase those spins to allow re-accumulation of phase and give us a better transverse magnetization signal now this is what's called a spin Echo where we regain signal and that signal is ever so slightly less in our true T2 signal what we've done here is accounted for those local magnetic field in homogeneities and the loss of signal at this time period is generally only due to spin spin interactions or molecules that have moved within the tissue during this period of time and have experienced a slightly different magnetic field strength then if we were to allow this to carry on for a further period of time we will again lose signal at the free induction Decay so let's look at exactly how this happens we're going to be looking at the rotational frame when we look at these examples because it's much more easy to conceptualize at least for me so let's now look we've flipped our spins into the 90 degree here we're looking from the side here we're looking end on as we allow those spins to then defaze we are losing phase we're losing transverse magnetization because of those local magnetic field in homogeneities we've said that this happens really quickly in a matter of milliseconds and it's very difficult to measure any good signal here at te we saw how rapidly that dropped off and even if we did measure it there wouldn't be much contrast between the tissues what we can do is as we look at this end on we can apply the 180 degree RF pulse now when we are flipping spins into the 90 degree we actually need to think of the sample as containing spins in the parallel and spins in the anti-parallel direction and as we apply that RF pulse those spins can gain transverse magnetization applying an RF pulse that's either stronger than this 90 degree RF pulse or applying the same strength but for double the period of time will allow that net magnetization Vector to surpass the 90 degree angle and actually form in a higher energy state in that anti-parallel side of our longitudinal magnetization that's how we can get past the 90 degree in our 90 degree RF pulse if we were to apply the 180 degree RF pulse here look what happens to these spins that have defased relative to our llama frequency some have de-phased slower and some have de-phased faster like this these spins down here are our leading spins they've de-phased faster these here are lagging spins let's apply a 180 degree RF balls and what that does is it spins that magnetization Vector 180 degrees along that X Y plane now these were our leading spins they're de-phasing faster these were our lagging spins also because we have now flipped these spins a full 180 degrees spins that were processing in say the clockwise direction as we've now flipped them are going to be spinning in the anti-clockwise direction so these leading spins are now going to be lagging behind the lagging spins but because they are dephasing faster they're experiencing a high a local magnetic field strength they are going to re-phase again as time passes by you can see that those leading spins now became the lagging spins and then re-phased we can now measure that signal at te our time to Echo and we can see how much higher that signal is and that signal now is much closer to the true T2 signal the signal loss because of spin spin interaction we've accounted for those local magnetic fields in homogeneities we're not reproducing signals here we're not adding more energy into the system technically what we are doing is allowing those magnetization vectors to re-phase and then that accumulation of phase is what's giving us that signal I hope this makes sense as to how we are accounting for those local magnetic field in homogeneities after all it's those in homogeneities that are causing this rapid defacing this spin here based on its location is experiencing a different magnetic field strength in this spin here and because that magnetic field strength is different it's rapidly defacing with the other spin once we apply that 180 degree RF pulse as long as these spins are in the exact same location this spin is still going to rapidly defaze but now the dephasing because of this change in orientation actually happens to be a re-phasing and that is the basis for spin Echo production now once we've generated that spin Echo and we've measured this signal here this analog signal that we've measured we've converted it to a digital signal we then place that Digital Signal within one line of k-space we then need to wait till TR so we can redo this entire process and fill another line of K space we've got many lines of K space to fill and not only that for each line of K space we repeat it multiple times and get multiple signal averages and then k space only represents one slice of our patient we still need to do that for all the slices within the patient you can see how if we're waiting a second to three seconds for TR this is going to start taking a very long time to take our MRI image so if we think about acquisition time we've seen that the time to take the total scan is the time from our first RF pulse all the way to tr what is our TR value then each phase encoding step is going to fill a different line of K space and the number of phase encoding steps is going to give us our resolution in the y-axis but each one requires a repetition of this entire cycle the next represents the number of excitations or the number of signal averages that we do for each line in case space every time we add another excitation we add a full TR onto our scan time so we need to account for the number of excitations then we need to do all of these steps for every single slice so we need to multiply that by the number of slices that's going to give us our total acquisition time now the three different pulse sequences that we're going to look at here that utilize spin Echo pulse sequences we're going to see how this downtime between t e and TR is going to be utilized to reduce this total scan time now before we move on to these pulse sequences you may have noticed some subtle changes here our D phasing frequency encoding gradient Now lies before the 180 degree pulse and this is a common site for this frequency encoding gradient to be placed but you see now it's no longer below this line it's above the line here now why is it here previously we place the frequency encoding gradient below this line a d phase ingredient prior to our data acquisition time when we applied the frequency encoding gradients along the x-axis now we apply that D phasing frequency encoding gradient to allow those spins despite having different frequencies along the x-axis to re-phase at t e and D phase again allow us to increase signal and decrease signal something we covered in the frequency encoding talk if we were to do that before the 180 degree rfo so we would require that frequency encoding gradient to be in the same direction along the x-axis as when we reapply it here at our time to Echo and that's because this 180 degree RF pulse causes a flipping of those net magnetization vectors we can apply our phase encoding gradient prior or after this 180 degree RF pause and I've Illustrated it here to show you that we can use those two separate timings so let's get into our first pulse sequences what is known as the multi-eco spin Echo Imaging we've done exactly what we've looked at in this talk so far flip the spins to 90 allow them to de-phase flip the 180 degrees allow them to reface giving us an echo at te and we read out that Echo during the frequency encode ingredients we then got all this time to wait until our TR what we can do here is apply another 180 degree rfos those spins when we first flip them to 90 degrees started to defaze we flip them 180 degrees and they started to re-phase again boom we read out our signal at this point now they will defaze at that free induction Decay again they've still got magnitude we haven't lost the transverse signal it hasn't flipped into the longitudinal plane we've seen that takes a very long time for T1 relaxation to occur what's happened after that first 180 degree rfls once they've refaced given us our Echo they are defacing again at T2 star we can apply another 180 degree pulse and that again is going to cause that rephasing we can generate another Echo here and measure out a separate te now this te is going to have slightly less signal than our first ee because the spin Echo is only accounting for the local magnetic field in homogeneities we can't do anything to recover more than T2 Decay T2 Decay true T2 Decay is going to happen no matter what that spin spin interaction is going to cause loss of transverse magnetization that is unrecoverable not even by spin Echo Imaging now what have we done here we've created two separate data acquisition points that have different Tes one a very short te and one a slightly longer te now which sequences use a very short te well a very short t e is used in proton density Imaging because we haven't allowed those differences in T2 relaxation to occur and in T1 weighted Imaging because we don't want differences in T2 to provide contrast to our image the second te here is a longer te like we're using T2 Imaging we've allowed a longer period of time for that true T2 relaxation to occur and we've separated the contrast in our image based on those T2 values we can now place this data acquisition into one k space and this data acquisition into a separate k space during this one pulse sequence now we've created one k space for this specific slice at this phase encoding gradient we filled one part of K space here our TR is going to be long we are waiting a long TR and that's what we want in proton density Imaging we want those spins to regain their full longitudinal relaxation before then flipping them again into the transverse plane so a short t e and a long TR is going to give us a proton density weighted image in this case space and when we fill this k space with our second data acquisition we filled now this line here it's the same slice that we've selected with the same phase encoding gradient the only thing that's changed is our te that slightly longer t e is going to bring out those T2 differences in tissue and we are going to generate a slice that has T2 weighting or more T2 waiting in that image you can see how we've utilized this downtime to now create two separate images now this is a little bit of an old technique with the development of flare which we're going to look at when we look at inversion recovery sequences we generally don't use proton density weighted Imaging especially when we're looking at brain Imaging but this was a good way to see if there were lesions close to water within the brain we could acquire a T2 image and if the lesion was T2 bright it's quite difficult to see the difference between the CSF and the lesion and this proton density image would allow us to tease out some of those differences no longer really used because of flair Imaging which we're going to look at later but this is the first way that we can utilize some of that spare time we can then repeat the sequence over and over again the only thing we're changing is the phase encoding gradient here and as we change the phasing coding gradient we're going to fool the rest of k-space now the next pulse sequence we're going to look at is a much more common pulse sequence we start again with exactly the same sequence here what are we going to do with this extra period of time now the slice that we have selected here we've only caused a very specific slice along our patient to then be flipped into the transverse plane all the other protons within our patient are still just experiencing the main magnetic field they weren't processing up the frequency of this RF pulse that we flipped into the 90 degrees we've then generated our spin Echo we've measured that spin Echo from that specific slice the rest of the tissue is just waiting around processing at the Llama frequency again here the only magnetic field that's on is the main magnetic field what we can do in multi-slice spin Echo Imaging is apply a different RF pulse with the same slice selection gradient we are going to create a gradient along the z-axis that causes the spins to process at different frequencies along that z-axis we can select a different RF pulse with a different RF bandwidth to select a different slice while this is happening that First Slice is still relaxing waiting for it to get all its longitudinal relaxation before we flip it again what we're doing here is firstly we have selected one specific slice we have excited that slice we flip those spins into the 90 degree we've generated our spin Echo and we've measured that signal in the second signal here we are using a different rfos a different radio frequency pulse that different frequency is going to then account for a different slice within our patient these spins in that First Slice are still regaining their longitudinal relaxation and we can now at the same te this te here is the same as this te here so we've got the same te but on different slices we can now fill two separate K spaces with these two Echoes and we can repeat this process for multiple different slices while we're waiting for that next TR to occur the TR here is going to correspond to this First Slice once we repeat this process again we are going to get a separate TR that's going to correspond to this slice the t e and the TR for each slice will be exactly the same and they are filling different K spaces remember k space encodes for a specific slice in our patients so let's look at what that looks like we again filling two separate K spaces in multi-eco imaging those two k spaces represent a different weighting in multi-slice spin Echo we are representing the same weighting here we've got the same te per slice and the same TR per slice but these two k spaces are encoding four different slices now in multi-eco imaging with different waiting for the same slice in multi-slice spin Echo Imaging we've got different slices with the same weighting there's a subtle difference between the two now we don't want to excite slices that are exactly next to each other because when we looked at that RF bandwidth remember there was a range of frequencies in that RF bandwidth and there's some overlap between slices here so what we can do is as we're looking at our patient we can select slices that are further apart from one another and as we select slices that are further apart from one another when we repeat the cycle again we can choose slices that lie between those slices and afterwards we can combine those signals this is what's known as interleaving of our signals we take all those separate case based data points and put them in their correct positions corresponding to the correct slices in our patient you can see how this is going to drastically reduce the amount of time it takes to acquire our MRI signal and the number of slices that we can put into a single TR is going to correspond to the total reduction within our scan time so multi-slice Imaging allows us to use that downtime to then image other slices within our patient but before combining all of that to generate our whole MRI image that we can scroll through from slice to slice now the last spin Echo sequence that we're going to look at is what's known as fast spin echoimaging now again we've started all these sequences the same apply the 90 degree rfas allow them to defaze flip them 180 degrees allowing them to reface creating an echo and that Echo is then sampled and placed in k space we've now created sample that's going to correspond to this region in k space that corresponds to this specific phase encoding gradients we can then apply another 180 degree RF pulse like we did in our multi-eco Imaging but we can see that something different is happening here in our phase encode ingredients we've applied an equal and opposite phase encoding gradient to the initial phase encoding gradient that we used prior to the 180 degree RF pulse remember when we apply a phase encoding gradient that phase encoding has memory throughout the sequence once we've de-phased them we turn off the phase encoding gradient and those spins have the same frequency because it's exposed to the main magnetic field they have the Llama frequency in fact defacing has had memory in our slice if we apply an equal and opposite phase encoding gradient that is going to re-phase that phase encoding gradient cancel out the effect of this initial facing coding gradients we can then use a different phase encoding gradients and the echo that we generate here is going to be a different line of K space we haven't selected another slice we've just created another Echo they defased flipped the 180 rephased then after that 180 they've again started to defaze at T2 star flip them 180 re-phase getting this Echo here this Echo and when we measure out the signal here is going to correspond to a different phase encoding gradient a different part of K space we can then repeat this again in equivalent opposite phase encoding gradients a new phase including gradient to give us another Echo at te that's going to correspond to a different part of K space you can see now that these Echoes are filling different lines of K space obviously the signal is going to decrease over time because of that T2 relaxation and the contrast throughout k space is not going to be identical now remember contrast predominantly comes from the middle of K space so we can start filling k space initially with small phase encoding gradients allowing us to get good contrast within our image before then filling out k space with higher and higher phase encoding gradients to give us that detail in our image now why would we do this we're obviously going to lose some of that classical contrast that we want and we're going to lose signal the more and more tees we cram in between our first 90 degree rfos and the TR well it drastically reduces the amount of time it takes to fill k-space here and the number of Echoes that we have within our pulse sequence between the first RF pulse and TR is what's known as the echo train length the ETL the number of Echoes here and here I've just included three Echoes if we had an echo train length of three that would reduce our total scan Time by a factor of three this can lead to a drastic reduction in total scan time when we use hundreds of Echoes between the first RF files and the TR and that's what we can do in fast spin Echo that's why it's fast I've got multiple Echoes within one single TR that's filling one slice one k space here now of course this is going to come with some consequences we are going to get a reduction in signal to noise ratio and we are going to get a change of contrast as we are getting this T2 relaxation with Time Each Echo is only going to represent the degree of T2 relaxation at that period of time for that specific tissue so The Echoes at the end of our sequence nearing the TR here are going to have very little signal very little transverse magnetization left now there are certain tissues that keep their transverse magnetization for quite a long time like fluid and when we are creating images like an MRCP where we're only interested in water these can be really useful because that water around pains its signal for such a long period of time because of its long T2 relaxation times now we can go one step further and make this even quicker when we looked at k space before we saw that k-space had what's known as conjugate symmetry we can separate k space flip one half of K space and we see that they're exactly symmetrical here this tells us that we could probably get away with only filling half of K space so we can create an image where we only fill half of K space and the order in which we fill that case space will also have consequences for the contrast that we're going to get in our image we've reduced the time it takes to create that single slice by another factor of two and we can get really quick images even though we're using spin Echo sequences now spin Echo sequences can't be used for every single application when we try and create T1 weighted images we want really short te times to negate that T2 differences in tissue and really short te times are often quite difficult within a spin Echo sequence because we need time to apply the 90 degree RF pulse and apply the 180 degree rfos there are other sequences that allow us to flip spins at smaller flip angles not using that full 90 degree flip angle and allow us to retain that longitudinal magnetization that then gets flipped into the transverse plane and give us better T1 weighted images and that's what we're going to cover when we look at gradient Echo signals we're then going to round this off by looking at inversion recovery seeing how we can null signal coming from a specific tissue because we know that T1 and T2 relaxation rates now this is a complicated and long talk I would encourage you to go over these Concepts to make sure that you have them in your head I hope I haven't lost you here let me know if you found this useful in the comments below let me know if you've learned anything today and as always I will have a question bank with curated questions Linked In the description you can go and test your knowledge on these Concepts so until next time goodbye everybody

Channel: Radiology Tutorials

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Length: 33min 33sec (2013 seconds)

Published: Thu Aug 24 2023