How TR and TE make tissue look different
Tonight · ~20 min · read · energy: low · setup: none
Relaxation happens; weighting is a choice. This lesson is the two timing parameters — TR and TE — that decide whether the image’s contrast is dominated by T1, T2, or proton density, and what “T1-weighted” actually means (hint: not “measuring T1”).
The anchor: weighting is about read-out timing
From lesson 2, T1 and T2 relaxations happen simultaneously after each excitation. The scanner does not measure them directly; it reads out the signal at chosen moments, and two timing parameters decide which relaxation the image is dominated by:
- TR (repetition time) — time between excitations. Drives T1 weighting (how much longitudinal recovery happens before the next pulse).
- TE (echo time) — time from excitation to readout. Drives T2 weighting (how much transverse decay has happened when you read).
Choose short TR + short TE and the image is T1-weighted (contrast dominated by T1 differences). Choose long TR + long TE and it is T2-weighted (contrast dominated by T2 differences). “Weighted” is the key word: you are not measuring T1 or T2 as numbers — you are making a picture whose contrast is dominated by that parameter.
What each weighting looks like
The clinical shorthand (orienting, not exact):
| Weighting | Typical TR/TE | What is bright | What is dark | Good for |
|---|---|---|---|---|
| T1-weighted | short / short | fat, post-contrast enhancement, subacute blood | fluid (CSF, oedema) | anatomy; contrast enhancement |
| T2-weighted | long / long | fluid (CSF, oedema, cysts) | muscle, cortical bone, air | pathology (oedema, fluid) |
| PD-weighted | long / short | proton-dense tissue | — | proton density |
So CSF is dark on T1, bright on T2; fat is bright on T1. That single pair of facts is the fastest way to orient yourself in an MRI you have never seen. Note how lesson 1’s point returns: “bright” and “dark” are relative within one image under one weighting — there is no absolute intensity scale.
Gadolinium contrast, briefly
Gadolinium-based contrast shortens T1, so enhancing tissue (vascular, inflamed, many tumours) becomes bright on T1-weighted post-contrast images. The standard comparison is pre-contrast vs post-contrast T1. Enhancement is a property of the tissue’s perfusion/permeability, not of the pre-contrast scan — which is why synthesising post-contrast from pre-contrast is asking the model to invent something absent (lesson 8).
The sequence families (reference, after the model)
Now that “weighting” is concrete, the family table is compression, not introduction:
| Family | Idea | Typical use |
|---|---|---|
| Spin echo (SE) | 90° excitation + 180° refocus | clean T1/T2 contrast; the reference |
| Turbo / fast spin echo (TSE/FSE) | many 180° refoci per excitation → faster | routine T1/T2/PD |
| Gradient echo (GRE) | no 180° refocus; flip angle < 90°; faster, T2*-sensitive | dynamic, 3D, susceptibility |
| Inversion recovery (IR) | 180° inversion pulse before excitation; uses inversion time (TI) to null a tissue | fat suppression, FLAIR |
| STIR (Short TI Inversion Recovery) | IR that nulls fat signal | fat-suppressed, edema-sensitive |
| Fat suppression (several techniques) | reduces bright fat signal so pathology is visible | T2 fat-sat, STIR |
Lesson 4 takes STIR specifically, because inversion recovery is the cleanest example of “choose timing to null a tissue”.
Stop and think — then reveal
You are handed an unknown brain MRI. CSF (the ventricles) is bright and the brain looks generally fluid-sensitive. Is this more likely T1- or T2-weighted, and why does that not tell you the number T2 of any tissue?
T2-weighted — CSF is bright on T2 and dark on T1, so bright CSF is the T2 signature. But it does not tell you any tissue’s T2 value, because the image is weighted, not a measurement: it shows contrast dominated by T2 differences, encoded as relative brightness under that sequence’s TR/TE on that scanner. To get an actual T2 number you would need a quantitative T2-mapping sequence (multiple TEs, a fit), not a routine clinical T2-weighted image (lesson 1).
What to retain
- TR drives T1 weighting; TE drives T2 weighting. “Weighted” means contrast dominated by that parameter, not a measurement of it.
- T1: fat bright, fluid dark, good for anatomy + contrast enhancement. T2: fluid bright, good for pathology/oedema. CSF dark-on-T1/bright-on-T2 orients you fast.
- Gadolinium shortens T1 → enhancement bright on T1-weighted; it reflects perfusion, not the pre-contrast scan.
- Sequence families (SE/TSE/GRE/IR/STIR) are compression of the weighting idea, with speed/suppression trade-offs.
Next: the cleanest “choose timing to null a tissue” example — why STIR suppresses fat.