Conventional contrast

PD-weighting (proton density, water content):
Long repetition time: TR >> T1
Full T1 relaxation.
Short echo time: TE << T2
No T2 signal decay.

T2-weighting:
Long repetition time: TR >> T1
Full T1 relaxation.
Long echo time: TE ~ T2
Significant T2 signal decay.

T1-weighting:
Short repetition time, TR ~ T1
No time for relaxation (saturated measurement).
Short echo time, TE << T2
No T2 signal decay.