Hi students. We will talk about the image contrast, filled overview, and resolution in this week. These are the contents of this week. We will talk about the gradient echo and spin echo imaging sequences, and contrast mechanism in spin echo imaging, and also contrast mechanism in gradient echo Imaging. After that, we'll talk about the concept of a Field of View and Resolution. Let's talk about the gradient echo and spin echo first. So, this is gradient echo imaging sequence, pulse sequence diagram. As we talked in the previous week, we talked about RF pulse and slice selection and slice refocusing gradient, and also we talked about readout prephase gradient and readout gradient and which generates an echo. Also we talked about phase encoding gradient. ADC analog-to-digital converter to data sampling is performed during the readout gradient is turned on and we get signal called echo. This procedure is repeated by changing phase encoding gradient and for the next excitation, we get another phase encoding line and the phase encoding strength will get changed from different from the previous excitation. This is the overall gradient echo imaging sequence, so this prephase and frequency encoding gradient generate gradient echo. The time from the RF pulse center to the echo center from here to here. So this is called echo-time, typically is denoted as TE. The time from one RF pulse center to the next RF pulse center, so in this figure from here to here. This is this time is called time to repeat, typically denoted as TR, time to repeat or repetition time, either way, it's typically denoted as TR. This is defined for RF pulses exciting the same slice location. This is important. If we acquire data in multi-slice mode, in that case we acquire one phase encoding line for one slice and then for the next slice folder next RF pulse excitation, we excite a different location. Location of RF excitation can be changed by changing the modulation frequency, or transmission frequency, or Larmor frequency. Just changing the frequency, resonance frequency. And then we can excite a different location. This TR definition is not about just RF pulse centers, but it's about the RF pulses exciting the same location. Meaning the transmission frequency, the resonance frequency should be the same to define a time-to-repeat (TR). Please keep in mind that. The multi-slice imaging, one excitation to the next excitation will have different location. It will take longer time for the one slice to be excited again. The time to repeat for the multi-slice imaging is longer, which improved a little bit beyond but I'm trying to tell you the concept over time to repeat here. Typically, the slice refocusing, pre-phasing, and phase encoding gradients are applied simultaneously as shown here. This slightly focusing, and pre-phasing, and phase encoding they're all independent to each other. So, we can apply for them at the same time event as shown here. This will minimize this minimum possible echo time, which is sometimes very desirable to minimize the signal decay. That is sometimes desirable. These events are typically applied at the same time. Let's talk about the spin echo. This is spin echo RF pulses and also signals generated by the RF pulses. There is 90 degree RF pulse and 180 degree RF pulse. We have free induction decay signal right after the 90 degree RF pulse, and we have a spin echo signal after 180 degree RF pulse. So, again I'm trying to explain you the concept of gain. There's a longitudinal magnetization, and 90 degree RF pulse will flip this to the transverse plane, we are talking about on the rotating reference frame, so it looks like a stationary, but it actually rotate at the frequency of called Larmor frequency. Magnetization is flipped towards the transverse plane, and then some of the spins may rotate fast or some of them rotate slow, they get dephased. There are two mechanism of de-phasing, one is spin-spin interaction, which are random. The spins interact each other to some of them may rotate fast or slow. Like balls flowing on the floor they interact each other, then move fast or slow, very similar. This is very random, that's called T2 relaxation. And some of the spins have different frequency than the other spins which is called magnetic field inhomogeneity. So that also cause spins de-phase. Those two mechanisms contribute to signal decay, which is called the free induction decay, which called T2 star. It follows T2 star signal decay, but among the two mechanisms that I just mentioned, the magnetic field inhomogeneity can be and refocused. That procedure is reversible by applying for 180 degree RF pulse. Those spins dephased by the magnetic field inhomogeneity can be recovered because their intrinsic rotation speed is the same but those spins get dephased but applying for 180 degree pulse will refocus, changes the polarity of phases and then there speed is maintained, those spins will catch up and then generate higher intensity, injected at the same time point later at 180 degree RF from the 90 degree RF pulse. If the signal is called spin echo and the diagram is shown here. Two successive RF pulses induce a spin echo, that signal, this spin echo signal is maximized when the first RF pulse at 90 degrees flipping and the second RF pulse at 180 degree. In that case, the spin echo signal gets maximized. So after the first RF pulse, demagnetization is dephased, by spin-spin interaction (irreversible) and magnetic field inhomogeneity which is, spin-spin interaction is irreversible, because they are random, magnetic field inhomogeneity dephasing by magnetic field inhomogeneity is reversible, by applying for 180 degree RF pulse, and transverse magnetization are flipped around the transverse plane. This makes fast spins lagging and slow spins leading the phase. Then dephasing magnetization regroups and generate a spin echo. The time between the 90 degree and 180 degree RF pulse, and time between the 180 degree RF pulse and the center of spin echo is identical. This is spin echo pulse sequence diagram. We have 90 degree RF pulse, 180 degree RF pulse and the spin echo. We have a slice selection gradient, and slice refocusing gradient, in the same way as gradient echo and lead out pre-phasing gradient, and lead out gradient which is similar to the gradient echo but the difference is this pre-phasing gradient polarity is now changing to plus rather than minus and I'm going to talk about it again. We have phase encoding gradient which is the same as gradient echo. We have data accretion readout ADC analog-to-digital converter we are sampling here, during the readout gradient is turned on. The only difference from the gradient echo imaging from the spin echo imaging are 180 degree RF pulse is applied in the middle, and also tell the change in polarity in the prephase gradient as shown here. The region that this polarity or prepaid gradient changed is the 180 degree RF pulse changes the phase of all the spins. So it changes the phase of prephase gradient, too. If any spins that has phase is induced by this gradient, will have the opposite polarity after this 180 degree RF pulse. Because of this 180 degree RF pulse, this prephase gradient polarity should change. Then what happen? You may have some questions. What happens if we move this location of this pre-phase gradient which doesn't have to be applied at the same at the same time as the other slight refocusing or phase encoding gradient? It can be applied at a different time, because these procedures are independent. We can move these lead out prephase gradient after the 180 degree RF pulse. What should changed? That the change is that the prephasing gradient polarity should change it to minus in the same way as the gradient echo. Again, because this 180 degree RF pulse changes polarity of all the spins. These are prephasing gradient which have negative polarity, after 180 degree RF pulse, should now change to positive polarity plus, if it is placed before the 180 degree RF pulse. Please keep in mind that. This is overall spin echo pulse sequence diagram. And the division of echo time is from 90 degree to the center of spin echo, which is almost the same as gradient echo and time to repeat definition is also the same. There are hundreds of MR imaging techniques, and most of them can be classified into gradient echo imaging, and spin echo imaging. In routine clinical scans, spin echo imaging is typically preferred to gradient echo imaging, because of higher signal to noise ratio and also robustness to magnetic field inhomogeneity. So the signal is T2 rather than T2 star. The signal intensity of spin echo depends on the recovery of longitudinal magnetization during TR and decay of transverse magnetization during TE. So, overall signal intensity or spin echo imaging can be described as shown here. This portion determines how much proton spins exist within one pixel, and this portion determines how much signal recovered after previous excitation. This represent magnitude over longitudinal magnetization before applying for RF excitation. This component describes how much signal left when we acquire data. After flipping to the transverse plane, so we acquire data after the time called echo time (TE) we acquire data. There will be slight signal decay. Again, we need this echo signal because we need time for the encoding, frequency encoding and phase encoding to have special information to form an image, we use echo. So in this case, spin echo. The resultant image had information of proton density and T1 dependence and also T2 dependence. For the anatomical imaging and also some clinical purposes, we need to empathize, not just improving the SNR. There is more important things than just improving SNR. That is improving the contrast of the tissues of our interest. Improving the contrast based on the proton density where T1 time differences, or T2 time differences are very important to have a meaningful clinical information. We'll talk about them.
