In this video lecture, we talked about the Contrast Mechanism in Spin Echo Imaging. So, one of major advantages of MRI is the great soft tissue contrast of the images. The definition of image contrast is the difference in color or in signal that makes an object distinguishable. This is general definition of a contrast. In case of MR images, they are mostly displayed in gray scale. So, the contrast is represented as intensity difference. The intrinsic properties that determine the image contrast of MRI is these three, as I mentioned in the previous video lecture, based on the equation for the spin echo. So, there's T1 relaxation time is different depending on the tissues. So, that can be a source of contrast between the tissues. T2 decay, T2 time constant is also different depending on the tissues. So, that can be another source of contrast. Proton density is also different. So, this T1 relaxation depends on the energy exchange between the hydrogen nuclei and the molecular tissue. It is about the spin lattice relaxation. T2 decay depends on interactions among those adjacent spins. Also this property is also different depending on the tissues. Of course, different tissues have different proton densities. Let's see an example of how T1 effect MR signal. This is flipping of 90-degree excitation and they exist on the transverse plane and no longitudinal component. This longitudinal component will recover back to original, but the tissue with the longer T1 like Kramer, so which has longer T1 than white matter. So, this figure is a little bit exaggerating the effect, but explains concept in easier way. So, gray matter takes longer time for the magnetization to recover back to original. So, as shown here, it takes longer time to recover. But, in case of white matter which is only shorter than T1 like white matter. Then, it takes shorter time to recover back to original. So, depending on the time to repeat. So, if we excite a flight for next RF pulse at this time point and then they may have no contrast because both of them recovered back to original. But, if we apply for next RF pulse here and then this one is partially recovered compared to the one with the shorter T1 has almost fully recovered. So, we may, by adjusting the time to repeat the TR, we may have certain contrast between different tissues. That is an example of contrast in MR imaging. We'll talk about this kind of concept in detail in this video lecture. This shows some examples of T1 recovery, T1 relaxation for the white matter which recovered fast and gray matter which recover slowly. There may be a maximum contrast exists this time to repeat TR, if it gets longer then the contrasts can reduce. T2 decay also similar, so gray matter decays slower, and white matter decays faster. So, in terms of signal decay, white matter decays faster, gray matter decays slowly. In terms of recovery of signal, white matter recovers fast while gray matter recovers slowly. So, this T1 effect and T2 effect may affect the signal in an opposite way. So, we want to empathize either one of them, but we don't want to emphasize both of them because they generally affect compensate each other. So, we may not have a contrast if we this effect are mixed together. Then, clinically, the information is less barrier. So, for example, the T1 and T2 relaxation times are different between gray matter and white matters. Then, how can we maximize the signal difference between gray and white matters in terms of T1 and T2 relaxation times with proton density? So, there are several parameters affecting image contrast that can be manipulated by the operator. So, there is time to repeat TR. So, it's the time interval between excitation RF pulses, and the echo time, so it's time interval between centers of excitation impulse and the echo. So, also, flip angle also affects the contrast in case of gradient equal. We can flip and then transverse and longitudinal and transverse magnetization decay and longitudinal magnetization recover. For the next excitation, it's partially recovered and flipped again. Then, we may have another signal. The time we acquire data from the RF excitation is called Echo Time. The time for the one RF pulse excitation to the next RF citation at the same location is called Repetition Time (TR). So, this is the signal recovery curve. This is time to repeat. During this time, white matter recover faster than gray matter. At this time point, we apply for RF pulse and then signal will decay. During this echo time and white matter decay faster than gray matter. The overall signal intensity of the spin echo imaging is determined by this S_0 x comma y, which is proton density and this portion determines signal decay during this echo time so which can be represented like that. This portion represent signal recovery from the previous excitation so which can be represented as shown here based on the T1 relaxation time. similar decay is about a T2 relaxation time. So, merge all these vectors together determines image contrast between for the tissues of our interest. How can we emphasize the signal difference for T2, or T1, or proton density? Okay let's talk about that. Okay, this is the case when a long TR and long TE is used. So, when TR is pretty long, so two times or say three times T1 of tissue of interest. So, in case of gray matter and white matter, so T1 is typically around this range. So, if TR is two twice or three times longer than these T1 over tissue of interest. then this longitudinal magnetization for the gray matter and white matter, both of them will recover back to original almost completely. Though there will be almost no contrast in terms of T1. But if there's time to repeat echo time it'll be a little bit long. So, as shown here if that echo time is long enough to emphasize the difference between gray matter and white matter, and then that will maximize the signal difference between gray matter and white matter in terms T2. So, this T2 decay maximize signal difference in case of echo time is long enough. So, that echo time is around the level of T2 of tissue interests. In case of gray matter and white matter, it's around at 60 to 100 milliseconds within this range. Then, the T2 signal difference is going to be maximized between these two. If T2 long longer than this T2 tissue of interest and they merged around here so that difference is going to be a little bit reduced in terms of contrast to noise ratio. But that signal difference is going to be maximized when TE is around the T2 over tissue of interest. So, that it's called long TE. So, this long TR, which means twice or three times longer than T1 of tissue and long TE which means TE in the range of T2 of tissue interest. Then, this is called a T2 weighted imaging. Because that signal difference mostly comes from the T2 difference between the tissues. Long TR and short TE case. So, as shown here, this long TR will minimize signal difference in terms of T1 as mentioned in the previous slide. If time to repeat is also quite short in this figure and then the signal difference between gray matter and white matter in terms of T2 is also reduced. Okay so short TE means, minimum possible TE to minimize the signal difference between tissues of interest, gray matter and white matter. So, in this case, there will be almost no T2 contrast too. No T1 contrast and no T2 contrast because of a long TR and short TE. In this case, this signal intensity is going to be almost proton density-weighted image. So, it's about proton density-weighted imaging, because there will be slight contribution of T1 and slight contribution of T2. So, it cannot be completely zero. So, it's called proton density-weighted image. Also in the previous slide we called T2 weighted imaging, but they're still contribution of proton density difference, so that's why we call T2 weighted imaging or proton density-weighted image. Also, this short TE case, where we sometimes need short TE to minimize the signal difference and also maximize signal intensity minimizing a decay. Because of that we applied for slightly focusing gradient and lead our prevailing gradient and phase encoding gradient at the same time for this purpose. Also, the short cases, short TR and short TE case. So, this TE is minimized shorts, which minimize T2 contrast as mentioned in the previous slide. But, if we use TR a little bit shorter than the one we talked in the previous slide, longer TR. So, if we use a shorter TR and T1 recovery between white matter and gray matter is going to be now maximized. So, it can be maximized when this time to repeat TR is in the range of T1 over tissues of interest, which is around the range of 0.8 or 1.5 seconds. So, if TR is short, and the signal difference between tissues of interest can be maximized. Because short or longer T1 will have partial signal incomplete T1 relaxation. So, there will be signal difference between tissues with a short TR, a short T1, and long T1, if we used short TR. So, in this imaging, so overall it's about a T1-weighted imaging. Again, there is light contribution of a proton density here too. So, that's why it's called a T1-weighted imaging. So, if we acquire [inaudible] just explained in the previous slides, and then this is T1-weighted imaging, and this is T2-weighted imaging, and this is proton-density-weighted imaging. So, here in the T1-weighted imaging, white matter shows brighter. It's brighter than gray matter, and that means signal over the white matter recovers faster than gray matter. So, that's why this signal is brighter. In case of T2-weighted imaging, it's opposite. Okay in that case. White matter signal, decayed faster than gray matter. Thats why signal intensity is lower in the T2-weighted imaging in case of white matter compared to gray matter. Proton-density-weighted imaging, white matter signal is lower than gray matter. That means white matter has lower proton density than gray matter. We can understand like that. So, TR, TE distribution is as shown here. So, proton-density-weighted imaging shows overall high proton density and T2-weighted imaging are typically useful for the pathologic evaluation and T1 weighted images are typically helpful to show anatomical details.
