To sum up:
- A long TR and short TE sequence is usually called Proton density -weighted
- A short TR and short TE sequence is usually called T1-weighted
- A long TR and long TE sequence is usually called T2-weighted
Nearly all MR image display tissue contrasts that depend on proton density, T1 and T2 simultaneously. PD, T1 and T2 weighting will vary with sequence parameters, and may differ between different tissues in the same image.
The following table shows T1 and T2 relaxation times for various tissues at 1.5 T.
- A tissue with a long T1 and T2 (like water) is dark in the T1-weighted image and bright in the T2-weighted image.
- A tissue with a short T1 and a long T2 (like fat) is bright in the T1-weighted image and gray in the T2-weighted image.
- Gadolinium contrast agents reduce T1 and T2 times, resulting in an enhanced signal in the T1-weighted image and a reduced signal in the T2-weighted image.
|| T1 (ms)
|| T2 (ms)
| Gray matter
| White matter
|| Reduces T1
|| Reduces T2
In clinical practice:
- TE is always shorter than TR
- A short TR = value approximately equal to the average T1 value, usually lower than 500 ms
- A long TR = 3 times the short TR, usually greater than 1500 ms
- A short TE is usually lower than 30 ms
- A long TE = 3 times the short TE, usually greater than 90 ms
It is your turn now! Change the TR and TE sequence parameters and the T1 / T2 times of the tissues and observe the contrast and acquisition time variations.
Don't forget: a good MRI sequence gives high tissue contrast but lasts the shortest time possible!