Mri Sequance

 1 Weighted Imaging or T1 Contrast  When we look at Fig.  3.2  carefully, we can easily realize that if we want to see a difference between different tissue types, we have to choose a relatively shorter TR time. For example, if we want a good contrast between GM and WM, we should choose TR somewhere around 800 ms not 3,000 ms. Because at 3,000 ms TR time, the signal from GM and WM practically becomes identical and we cannot see any difference. Please note that the 800 ms optimal TR time

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Magnetization (Mz)

Maksimum Mz Reached at TR of 3100msT1 measured as 750ms

T1 Relaxation Time Measurement

63% Level

  Figure 3.3    The T1 relaxation time is defined as the time required for z-magnetization to reach 63% of its original value. The times required for z-magnetization to reach its original value and 63% values are shown in the plot above.      

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THE RELAXATION CONCEPT IN MRI ● 31

is calculated from ( 3.2 ) by using the 1.5 T T1 relaxation time given in literature. If we ever want to see a great contrast between WM and CSF though for some unknown reasons, then TR time should be around 1,275 ms. Therefore, the TR time is a very critical parameter to create a good T1 weighted imaging in a simple spin echo sequence. In more complicated fast and ultrafast sequences, flip angle also becomes an additional parameter affecting the T1 weighting in the image. As we have given in Table  3.1 , we have quite many different tissue types and also a number of pathologies with different T1 relaxation times. Considering that in the brain T1 values range from 100 to 1,000 ms for most tissues, we recommend using a TR time somewhere between 400 and 800 ms in 1.5 T. In the later chapters of the book, the exact TR time choice is also given in each protocol, so that readers can have a reference TR value optimizing the T1 weighting with an excellent contrast and acceptable scan times.  A  typical T1 weighted image is shown in Fig.  3.4  acquired with a turbo spin echo sequence. In a T1 weighted imaging, there is a simple rule we should remember:  The tissue signal in a T1 weighted imaging is inversely proportional to its T1 relaxation time.  This simple statement comes from the fact that 1 −  e   − TR/T1  value of ( 3.2 ) will be small for a tissue with a long T1 value (CSF) and quite large for a tissue with short T1 value (fat). We can easily confirm this fact from the T1 weighted imaging by looking at hyperintense fat signal and hypointense CSF signal.  

T   2 Relaxation I  am hoping that you learned what the T1 relaxation is and how to measure it. Now, it is time to take a look at T2 relaxation and its great importance for MR imaging. Let’s again assume that we applied the 90° rf excitation pulse and tilted Mz-magnetization to XY-plane as shown in Fig.   2.6    . The excitation pulse will be turned off exactly when the spin tilted to XY-plane. As we remember from the T1 relaxation, the spins will start returning back to their original state in  Z -axis as soon as we turn off rf pulse. At this point, all spins rotate or move at the same speed (Larmor frequency) in XY-plane like a huge group of people (coherence). However, in time, spins start moving at slightly different speed and the coherence will be disappeared. To understand this spin behavior, we can consider marathon runners. All the runners will start running at the start signal all together and will look like the coherent group. After a short while, some of the runners will slow down because they will bump into the other runners (spins) nearby, get exhausted, or run slower than others. After 1 h or so, you will see