TR/TE weighting and tissue signal (T1/T2/PD)
A two by two matrix. Rows are repetition time (TR), short in the top row and long in the bottom row. Columns are echo time (TE), short on the left and long on the right. Short TR plus short TE gives a T1-weighted image (TR about 300 to 600 ms, TE about 10 to 20 ms; fat bright, water dark). Long TR plus short TE gives a proton density image (TR about 2000 ms, TE about 15 ms; CSF bright from high proton density). Long TR plus long TE gives a T2-weighted image (TR about 2000 to 4000 ms, TE about 80 to 120 ms; CSF and most pathology bright). Short TR plus long TE is not used because both T1 and T2 effects are strong at once.
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