[MUSIC] We will talk about Spin Echo imaging here. We already talked about Spin Echo in the previous week. But we will talk about Spin Echo in a slightly different viewpoint again in this video lecture. Let's try to review the concept again. So when we apply for 90 degree excitation, the longitudinal magnetization will move to the transverse magnetization. A transverse plane, and then these spins will lose phase coherence because of spin-spin interaction random dephasing, and also progressive dephasing,called magnetic field inhomogeneity. So these two vectors continue through this dephasing together. So these are the, looks like it's stationary again. They actually rotate at the level frequency. And this rotation speed is going to be different, depending on the magnetic field strengths. So because of magnetic field inhomogeneity, magnetic field is not uniform, their intrinsic rotation speed is going to be slightly different. Okay? And then also they interact each other as both interact each other. And then the rotation speed is going to be slightly changing. So, in this procedure, spin-spin interaction is random code, so it cannot be recovered. But this progressive dephasing due to magnetic field inhomogeneity can be recovered by applying for 180 degree RF pulse, as shown here. You applied for 180 degree RF pulse to the X prime direction, then this whole spin will flip to this plane, okay? And then rotation direction is the same. So they will gather the focus to here because some of them rotate fast and now catch up, and then some of them rotate slower, is now going to merge together here. And then they will form another high called echo, or spin echo, okay? That they. But this intensity is slightly lower, smaller than this initial intensity because of T2 decay. Okay, that is caused by random spin-spin interaction, okay? But we only recover signal due to magnetic field inhomogeneity. Okay, please try to keep in mind that. So echo is formed here. Okay, we can apply for 180 degree RF pulse and fly to different direction, too. So up to this point, this procedure is going to be the same as the previous flight. But at this point, we can apply for 180 degree. So in the previous slide, we applied for that along x prime direction. Now we can apply for 180 degree along y prime direction. And then this spins and each are distinguished based on their colors. They will be flipped along y prime direction now, okay? And then what will happen? The same thing happens. With those spins rotating fast, will catch up, and those spins will [INAUDIBLE] catch [INAUDIBLE] by and then they will merge together as shown here. And then new echo is going to form, in the same similar way as the previous slide case. But in this case, though echo is formed at the same polarity. So, at the same plus y prime axis. When in case of previous of slide, the echo was formed along the minus y prime direction. But now they are going to be formed at the same direction, so plus y prime direction, okay? So this application of applying 180 degree RF pulse, the phase can be modulated, by the pulse sequence problem. This is explanation for the spin echo case. And what will happen? Let's see, this is in time domain though consequence for the spin echo. 90 degree and 180 degree and then there will be free induction decay after 90 degree. Then T2 decay and then we can apply for 180 degree and then the signal will recover to the T2 decay point. And then we can form it's spin echo. And then we can read the data during this period. And then this procedure is going to be repeated. So if we see the point, number one, two, three, four here. So K-space Trajectory is going to be like that. So now, after magnetization [INAUDIBLE] applying for 180, 90 degree, and then it will in the center of K-space, and then it will move all the way up, if we apply for [INAUDIBLE] ingredient. And the Readout [INAUDIBLE] ingredient and it will move all the way up, and then 180 degree pulse will change the polarity of all the phases. So it moved to the opposite direct side of the k-space. And then this readout gradient will move, fill the data along this k-space line. So this is going to be performed during this ADC sampling, okay? I'm trying to review the concept that we talked in previous weeks. Okay let's continue the discussion that we mentioned in the previous slides. For the spin echo case, so spin echo is formed along the same direction as vibrative resonance, and then what would happen if we just keep waiting? And then these spins will dephase again, which is obvious, okay? In the same way, as two different regions, magnetic field inhomogeneity and spin-spin interaction. And they will dephase again. And then, if you can apply for 180 degree RF pulse along y prime axis in the same way as the previous slide. And then they will refocus again and then we can see average of second Spin Echo. Okay? And then this procedure can be repeated, although this echo intensity is going to be slightly smaller than the first echo. But, this procedure can be repeated. And then we can, fill multiple K-space lines. After 190 RF pulse, we can apply for multiple 180 degree RF pulses and then we can acquire multiple K-space lines. And then they will accelerate their location and that is the concept of fast spin echo. So spin echo creation is repeated by applying for multiple 180 degree RF pulses. So that is the basic concept of fast spin echo. But this this is a sequence diagram for the fast spin echo. So 90 degree RF pulse is applied and the first 180 degree and then day location and then second 180 degree and then another day location. And this process is going to be repeated and then the decay following T2, okay? And then application of each 180 degree RF pulse recover the up to T2 and which generate multiple spin echoes. And then we can fill case base lines for the multiple case base lines after this 90 degree and then combining multiple 180 degree RF pulses. And then we can manipulate which K-space lines should be filled by modulating the phasing encoding gradient strength before their location. So location or case based lines can be manipulated by the pulse sequence program. So first spin echo is favourable for T2 weighted imaging. And because T2 weighted imaging case, what TR and TE should we use for T2 weighted imaging? Then lone TR. Okay. Lone TR means twice or three times longer than T1 or T2. And long T should be used, and the long T means T is similar to the T2 over interest. So in that case the fast spin echo is very favorable because during this long echo time, for the just spin echo, we're just waiting time, waiting to get the one phase encoding line. But, in case of fast spin echo, up to the point of Readout of echo, time it can acquire multiple phase encoding lines. Okay, with the phase encoding lines to fill the k-space, so their location can be significantly accelerated for this T2 weighted imaging. This fast spin echo imaging also can be used for T1 weighted imaging too, okay? So we'll going to talk about that, okay. The most important imaging sequence. This is most important imaging sequences in clinical diagnosis. So, one thing you may have to remember is the echos filling the k-space center portion determines imaging contrast. So, which echo fills k-space center? Then that determines image contrast. So if you want T2 and if, for instance, this fifth sequel is, these are at T2 contrast. This is 80 millisecond at the 90th degree excitation. Then this is going to be the child echo to fill the k-space center portion. And then last of them can fill the edge portion of the k-space. Then we can get T2 weighted imaging, okay? Or if we want a T1 weighted imaging, or proton-density weighted imaging, then show the echo, show T is the child. Then the first echo has the shortest T possible among these multiple echoes. So the first echo will fill the k-space center portion, okay? So we can adjust, again, we can adjust this location by changing the phase encoding gradient strength, okay? And first echo fills, then the rest of them can fill the k-space edge portion. Then we can get T1 weighted imaging or proton density weighted image. So this number 180 RF pulses are called turbo factor or echo train length. And then this is the vector accelerating data application. So short echo train length is effective to get the short effective TE. And so effective TE means the echoes filling the k-space center. So that determines the image contrast. So that is called effective echo time for that image. Short effective TE and that increases T1 weighting for the short echo train length. And this requires longer scan time because the echo train length is small, so the acceleration factor is small. So, this skin type is slightly longer than the case with the longer echo train. And both slices can be covered per TR. Because echo train is short, then each slice excitation will have a shorter scan time to accommodate multiple slices in one TR. Then there will be some limited number of slices, but that limitation is a little bit smaller because we use shorter echo train length, and more severe for the longer echo train length. And it causes less blurring for the image in the artifact, because if we use longer echo train length, then data location can be accelerated too much more. But there will be also some [INAUDIBLE] learning due to some T2 decay. Okay, advantages of fast spin echo imaging is that reduces scan times significantly. That is the most important advantage of fast spin echo and it can provide High-resolution matrices and multiple number of excitations can be used because scan time is accelerated, data location is accelerated. So during that period we can increase number of to increase. What you can improve image quality and also this can be more advantageous for T2 weighted images. So the disadvantages of the Fast Spin Echo imaging is that increase some flow and motion related effects, and also the fat signal is bright on T2 weighted images. If we acquire T2 weighted imaging with just a Spin Echo Imaging, then scan time will get longer compared to the fast spin echo. But the problem of this fast spin echo imaging is that until acquiring the desired echo, we keep acquiring multiple echoes. By applying for multiple 180 degree RF pulses, as shown in the previous slide. So this induces some signal for the fast signal, that is through the j coupling, okay? So initially people didn't know why fast signal is bright, T2 fast spin echo, T2 weighted images, but now people know that is about the j coupling. So that is one disadvantage, but people now or clinicians know T2 weighted, fast spin echo based, the T2 weighted provide bright fat signals. So they now take that into account for the clinical variation. So that does not cause a big problem. There is some image blurring with very long echo trains NEX because of some significant T2 decay. So, again, proton density and, T1 and T2 weighted imaging can be performed by the fast spin echo imaging tool. Similar to the spin echo imaging. And it's very similar, almost the same as spin echo weighted imaging, but slightly can be accelerated more by using fast spin echo. So the concept is the same as spinecho case. For the T1-weighted imaging, short TR, like a tissue T1 values and then short T, as short as possible. And, this portion will minimize T2 contribution. And, this portion maximizes T1 contrast. But this short TR usually accommodates all the slices necessary for clinical diagnosis of a target region. If we use spin echo or gradient echo, well, first spin echo case is slightly different. There will be limited number of slices for T1 weighted imaging by using fast spin echo because multiple 180 degree RF pulses are applied and multiple case baselines are acquired. So that event per slide gets longer. So, T1 weighted imaging typically used short TR, so that will limit for the T1 weighted imaging, for the fastest spin echo. But, if we have some best spatial coverage, it is small, like a digital imaging photo, spine imaging. Then, or, in that case fast spin echo imaging can be used for T1 weighted imaging too. And that will decrease scan time significantly. Or you can apply for more higher number of averaging for better. And T2 weighted imaging or imaging is very for the fast spin echo imaging as I mentioned before. Because to get the vertical echos and then to get the contrast you can acquire multiple echos until the point we get echo. Okay, again the T2 weighted imaging is long T like a tissue T2 values, like a 60 to 100 millisecond. And short TE is imaging, it's as shown as a possible. And they're determined by k-space center echo. So that is called effective echo. And multiple slices can be excited and encoded in one TR because of this long TR. So this long TR accommodates all the multiple slices necessary for covering the target region, okay? And spin echo T2 weighted imaging is used when bright fast signal, while blurring is not desired. But in most cases, T2 weighted imaging or proton density weighted imaging is performed by fast spin echo sequence.
