Resources/MRI fundamentals
TR and TE
TR is the time between successive excitations of the same slice; TE is the time from excitation to echo readout. Together they decide which tissue property — T1 recovery or T2 decay — dominates your image contrast.
TR — who has recovered?
After each excitation, longitudinal magnetization regrows at each tissue's T1 rate. A short TR interrupts that recovery: tissues with short T1 (fat) have recovered and give strong signal; tissues with long T1 (CSF) have not, and stay dark. That difference is T1 contrast. A long TR lets everything recover — and erases T1 differences.
TE — who has decayed?
After excitation, transverse signal decays at each tissue's T2 rate. A long TE waits long enough for differences to show: short-T2 tissues have faded while long-T2 tissues (CSF, edema) still shine. That is T2 contrast. A short TE reads out before decay differentiates anyone.
The four combinations
| TR | TE | Result |
|---|---|---|
| Short (400–700 ms) | Short (10–25 ms) | T1-weighted |
| Long (2000+ ms) | Long (80–120 ms) | T2-weighted |
| Long | Short | Proton density |
| Short | Long | Avoid — mixed, low-signal contrast |
Try it in ScanFlo: Fix TE at 14 ms and scan at TR 400, 800 and 2000 ms. Watch gray–white contrast peak and then fade as recovery completes. Then fix TR at 2500 ms and step TE from 15 to 100 ms — CSF turns from dark to brilliant as T2 decay takes over the contrast.