Figure 13 illustrates that spin echo imaging can be employed to obtain two different types of tissue
contrast known as proton density contrast and T2 contrast. To obtain a proton density-weighted (PDw) image one uses a (relatively short) TE value of about 20 msec. Shorter TE values are desirable, but technical and engineering constraints usually prohibit their realization. In a PDw image the density of water protons is the predominant factor that defines what signal intensity is detected. Therefore, WM that has slightly fewer water protons per unit volume due to the presence of myelin tends to appear as lower (closer to black) signal compared to GM or CSF. CSF water density is slightly higher than in GM, causing the CSF to appear modestly more intense compared to GM. T2-weighted (T2w) images are typically obtained with TE values between 80 and 150 msec. This serves to maximize the contrast differences between the three tissue types as shown in the graph in Fig. 13. In the case of T2w images, the CSF shows the most intense signal, with GM and WM showing lower and almost equal signal intensities.
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