Inversion Recovery Pulse Sequences MRI | STIR and FLAIR | MRI Physics Course #19

hello everybody and welcome back we've made it to the final talk in this basic introduction to MRI pulse sequences we've looked now at spin Echo sequences as well as gradient echo pole sequences and we've seen the benefits and drawbacks of each of those we've also seen how selecting specific te's and TRS in combination with certain flip angles generates contrast within our image and that contrast can be predominantly T2 weighted or T1 weighted and the t e and TRS will generate that contrast based on the relaxation times of the tissue within the slice that we are Imaging now we're going to turn our attention to what's known as inversion recovery pulse sequences now the goal of inversion recovery is to null signal or negate signal coming from a specific tissue we are setting our sequence up with the goal of preventing signal from being generated from a specific tissue type and it's the T1 relaxation time the rate at which that tissue gains longitudinal magnetization that is going to allow us to set the specific parameters in that pulse sequence and prevent signal from coming from that specific tissue type now in order to build an inversion recovery sequence we start with a backbone of a spin echo pole sequence this is what we've seen before where we take a net magnetization Vector that's lying parallel to our main magnetic field flip it 90 degrees allow it to relax lose transverse magnetization at a rate of T2 flip that Vector 180 degrees with a second RF pulse and allow those then to re-phase and accumulate at a time that we sample the tissue which is known as the time to Echo when we're sampling the analog signal and we digitize it whilst we are applying the frequency encoding gradient putting those discrete data points into a single line of K space now what happens to the magnetization vectors during this pulse sequence well we have longitudinal magnetization within a specific tissue type we then flip it 90 degrees when that net magnetization Vector has been flipped 90 degrees it's got maximum transverse magnetization all of the spins are in Phase with one another and it has completely lost longitudinal magnetization it's then going to Decay at two separate rates the First Rate is T2 or T2 star where the spins D phase they're still in the transverse plane but they defaze and we lose transverse magnetization the second rate is T1 recovery where we are gaining longitudinal magnetization now if we look at this in a graphic form here at the time of our first 90 degree RF pulse we have fully lost longitudinal magnetization the magnetization in the Z plane here we've got zero percent of that original longitudinal magnetization now the tissues will regain or recover that longitudinal magnetization at different rates if we are looking at CSF and we're looking at fat is going to regain its longitudinal magnetization faster than CSF the spin lattice interactions are higher in fact than they are in CSF and we've got a shorter T1 time constant the T1 time constant is the period of time taken to regain 63 percent of the longitudinal magnetization now let's go back to that pulse sequence and see what happens if we were to precede that pulse sequence with a 180 degree RF pulse what would happen to the net magnetization vectors here we've got a longitudinal magnetization Vector along the main magnetic field the RF pulse causes those spins to process and fan out in Phase gaining transverse magnetization and it's allowed to go past the 90 degrees because we are applying energy into this system as it goes past 90 degrees we are having more magnetic vectors now facing anti-parallel to the main magnetic field remember initially there are vectors facing both parallel and anti-parallel and a slight predominance of those will be in the parallel Direction and that's why our net magnetization Vector is parallel a 180 degree RF pulse means that the spins are still lying parallel and anti-parallel to the main magnetic Vector but now a slight predominance are facing anti-parallel our net longitudinal magnetization is now negative relative to that initial net magnetization in the longitudinal plane and it turns out that this sequence here is what's known as an inversion recovery sequence you can see there's not that much difference between an inversion recovery sequence and a spin echo pole sequence now what effect will this have as we then progress through this pulse sequence well let's look at this initial graph here this zero percent of longitudinal magnetization happen because the spins were flipped 90 degrees we had lost longitudinal magnetization we've seen that with a 180 degree rfos we start to gain longitudinal magnetization in the negative Direction so we now need to redraw this graph for this specific pulse sequence and have a negative axis here on the longitude signal magnetization and if we apply a full 180 degree flip to that net magnetization Vector we are going to have a longitudinal magnetization that has a magnitude of negative 100 percent now when that 180 degree RF pulse is Switched Off what happens this net magnetization Vector doesn't flip around like this into the transverse plane gaining phase with all the spins giving off a transverse signal and then regaining longitudinal magnetization these spins lying in the longitudinal plane anti-parallel to the main magnetic field when the RF pulse is Switched Off are all out of phase within another we are not adding an RF pulse that is going to generate that transverse magnetization what's going to happen is they're going to lose their net transverse magnetization in the negative plane as they start to realign with the main magnetic field so what's going to happen to this net magnetization Vector it's going to get smaller and smaller over time at a rate of T1 the T1 time constant the spin lattice interactions is what's causing the individual spins to interact with the inert lattice within the tissue itself and slowly start to align with the main magnetic field so we lose longitudinal magnetization and to a specific point when we will have no net longitudinal magnetization within that sample the number of spins anti-parallel will equal the number of spins parallel then as time goes by we will get regrowth of longitudinal magnetization within the longitudinal plane parallel to the main magnetic field and we can draw this graphically for in this example fat muscle and CSF now what happens is we start at negative 100 of longitudinal magnetization we then get loss of that negative longitudinal magnetization to a specific point where the anti-parallel and parallel spins are equal in number now it turns out we can figure out what this point is here and for tissues such as fat and muscle it's 69 of the T1 time constants for those tissues now when we try and do this for CSF it becomes a little bit more complicated because our TR can't be long enough to include the T1 values here and it becomes a combination of both the TR and the te that we select in this pulse sequence so here we can figure out where fat is going to cross that threshold where fat is going to go from a negative longitudinal magnetization into a positive longitudinal magnetization and then regain that longitudinal magnetization remember at all points there's no transverse magnetization accumulating here it's just purely a function of T1 within the tissues the same will happen for muscle here at a specific rate that's different to fat and the same will happen for CSF in this example CSF has a longer te1 value it's going to take longer to lose that negative longitudinal magnetization because there's less spin lattice interaction now at some point we are going to apply a 90 degree RF pulse we know that the 90 degree RF pulse will take this longitudinal magnetization wherever it is and flip it 90 degrees into the transverse plane causing those spins to become in Phase with one another they're going to process in phase and gain transverse magnetization we know that the degree of transverse magnetization is dependent on the magnitude of the longitudinal magnetization at the time that we're applying the 90 degree RF pulse when we apply that 90 degree RF pulse we know that we're going to lose all transverse magnetization and we know that it's going to gain transverse magnetization at the specific rate that we've seen before now we know the rates for the regaining of longitudinal magnetization when a specific tissue has zero longitudinal magnetization and gains then positive longitudinal magnetization at these points in our graph each of these tissues has zero longitudinal magnetization so we can predict then what the regrowth of that longitudinal magnetization is going to be at these specific time points here now look what's happened here initially when we didn't invert the spins we started at a 90 degree iron pulse there was T1 differences within tissues if we first apply a 180 degree RF pulse we can see that the differences in T1 is going to be Amplified between these tissues because we've now got dabbling of that T1 time you can see that depending on the time that we choose to flip the spins 90 degrees we are going to get an increase in T1 differences in those tissues now this is getting a bit big for the slide let's narrow it down it's the same graph but I've just shortened the x-axis here it still represents the same amount of time now you can see here that I've labeled the time between this 180 degree rifles and the 90 degree oracles and I've introduced a separate parameter known as the ti the time of inversion we've seen te the time of Echo and TR the time of repetition now we've added a different parameter the ti time of inversion this is specific to inversion recovery pulse sequences the timing here is going to show us where along this graph are we going to flip these spins into the 90 degree plane remember what this graph is showing here this graph is showing the degree of longitudinal recovery it says nothing at the moment about the transverse signal here don't get confused by this representing transverse signal that we're measuring now what happens if we place this 90 degree rifles a specific time period away from our initial 180 degree rfos this here represents our TI time and we make the t i time equal to the period of time where fat is going to have zero longitudinal magnetization what's going to happen to these tissues when we apply a 90 degree rfos let's take fat and CSF as an example initially prior to this 180 degree articles they're both lying with full magnetization in the longitudinal plane we apply a 180 degree RF pulse that is going to cause the net magnetization Vector to be negative 100 in the longitudinal plane now we know that CSF loses its longitudinal magnetization at a rate of T1 that is much longer than fat fat is going to lose longitudinal magnetization much quicker because it's got a shorter T1 time if we apply this 90 degree RF pulse at the period of time where fat has got no positive or negative longitudinal magnetization and we've still got this negative longitudinal magnetization in water because of that 180 degree RF pulse and water has a longer T1 so it hasn't lost that longitudinal magnetization yet if we apply the 90 degree iron pulse at this time period represented by this dotted line and we flip those magnetization vectors into the transverse plane only CSF is going to be giving a signal the longitudinal magnetization Vector for fats at this point in time is zero we have now created a sequence where we've got no transverse magnetization coming from fat and that is the basis for inversion recovery sequences now when we use a short time to inversion here we've got what's known as stir a short Tau inversion recovery or short time to inversion inversion recovery now this allows us to negate the signal coming from fat and we can still get bright signal coming from muscle or from CSF this graph here is what's known as a phased recovery graph showing the changes in longitudinal magnetization accounting for both the direction and the magnitude of that longitudinal magnetization we can also you might see this graph represented like this which is known as a magnitude sequence where just the magnitude of the longitudinal magnetization is recorded on the graph not the direction because once we apply the 90 degree RF poles those spins are going to be in phase and in the transverse plane doesn't matter if it comes from the negative side or if it comes from the positive side now if we were to flip these spins into the 90 degree plane we can see that CSF muscle are going to have some signal here whereas fat is going to give off no signal it's going to look dark in our image hypointense we flip them into the 90 degree plane they then relax at T2 and if we sample quickly with a te we are going to get signal that is corresponding to the degree of longitudinal magnetization that it had now you can see that we followed this up with a spin Echo pulse sequence we are accounting for local magnetic field in homogeneities that means these sequences are good for Imaging near places where we're going to get artifact like metal artifact or we're going to get chemical shift artifact because we account for those local magnetic field in homogeneities we can use Stir close to say an implant in the hip which has metal within it is generally going to give us a metal artifact this spin echoed as well for accounting for those local magnetic field in homogeneities you'll see later on in this course that there are other ways to suppress signal coming from fat which are based on the frequency or processional frequency differences between fat and CSF those sequences don't work as well when we're getting chemical shift artifact or magnetic field in homogeneities that's where stir comes into its own now there are downsides here we've added an extra portion here our TR has gotten longer the time to take this image is going to get along our acquisition time is going to get longer now importantly when we are selecting this time to inversion we're not actually specifically selecting for fat we're selecting for any tissue that has a T1 time constant of fat we are selecting for tissues or reducing signal from tissues that have short T1 time constants now later on we're going to look at contrast agents within MRI imaging we're going to look at something specifically known as catalinium now what gadolinium does is it shortens the T1 time constant of the tissues so we can't use Stir when we are providing gadolinium as contrast because we might actually null the signal from tissues that have gadolinium in them opposite of what we want to do when we're adding a contrast agent it also turns out that in sequences such as stir we actually get a slight reduction in signal to noise ratio so we need to weigh out these positive and negatives when it comes to using stir now another major benefit of using stir is that often we are trying to image a lesion that we don't know anything about we're trying to figure out does it have fat in it is it does it have fluid in it what are the inherent properties of this specific lesion that we're seeing in our scan now many pathologies have both long t1s and long t2s much like CSF CSF as a long T1 and it has a long T2 now when we are using our traditional pulse sequences either we're making a T1 weighted or T2 weighted and the contrast in those tissues are at odds with one another when both the T1 and T2 are along within that tissue what stir does is despite if we look at CSF despite tsf having a very long T1 we are getting a high signal from the CSF despite it having a long T1 because of this inversion here it allows for additive effects of the both inherent T1 and T2 properties of a lesion so instead of lesions looking slightly darker because the T1 and T2 is competing in stir images lesions often look even brighter and are more easy to spot within the image now what happens if we were to increase this time to inversion here if we were to make it match up with the point at which CSF crossed this barrier here now these points you might also be heard of as bounce points here you can see how it looks like the graph bounces off of the x-axis here now these bounce points represent the specific TI where that tissue is going to be knulled where signal from that tissue is going to be lost as we move our TI longer here you can see that this sequence is going to take longer we've increased this period of time but the contrast in our tissue is going to be very different now we have attenuated fluid within our image and this is what's known as flare fluid attenuated inversion recovery here we've got no signal coming from our CSF we've got a higher signal coming from fat and intermediate signal coming from muscle if we were to look at a traditional T2 weighting of the brain this is not a flare image this is T2 here we can see the CSF is bright in T2 our gray matter is lighter than our white matter we've got bright fat in the subcutaneous tissues here if we were to apply a flare inversion recovery here we were to apply this 180 degree pulse add a TI that matched up with the bounce point of CSF our image would look like this within the gray and white matter interface here we've still got T2 waiting that's dependent on the te that we've selected so we're still doing a T2 weighted image but we've knulled or suppressed the signal coming from fluid notice how there is a region here that is stayed bright in that initial image it was quite hard to tell that there was a lesion here because it was difficult to tell whether the brightness was coming from the lesion the inherent properties of the lesion all the brightness was coming from the bright fluid within our image Flair has allowed us to know the signal coming from CSF and keep the signal within the lesion here because this signal is not coming purely from fluid within the lesion it's a T2 inherently T2 bright lesion this allows us to get contrast between a lesion and fluid and flare can be very useful especially when we are dealing with lesions that are but the CSF or lesions that are at the interface between the gray and white matter here so you can see how we can utilize these sequences in order to suppress signal coming from a specific tissue and it allows us to do a couple of things if we suppress the signal and use a stir image and the lesion suddenly becomes dark we can infer that perhaps that lesion is made of fat or if we suppress the signal coming from fluid and we see that there's still a bright region within our image we know that brightness is not coming from fluid and it's coming from a specific lesion or a specific tissue that has other inherent T2 brightness to it so that is inversion recovery in a nutshell it's not too difficult to understand as long as you understand that principle of flipping their magnetization Vector into the negative longitudinal plane and that longitudinal magnetization gains longitudinal magnetization in the parallel direction of the main magnetic field and the rate at which it gains it is T1 and because we know those rates we can select our time to inversion to null specific signal coming from tissues that have specific T1 values this is something that comes up over and over again in exams because many people make the mistake of confusing these recovery graphs with the transverse magnetization or the t2 star Decay graphs and hopefully when you see these in exams you will now avoid that confusion I've got questions that you can practice on Linked In the description below if that interests you go and check it out we are now going to move on to more Advanced Techniques in MRI imaging I can't wait to get into it with you so until then goodbye everybody

Channel: Radiology Tutorials

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Length: 20min 44sec (1244 seconds)

Published: Wed Sep 27 2023