Signal weighting (T1, T2, PD) and sequences parameters : TR, TE

Spin Echo sequence is based on repetition of 90° and 180° RF pulses. Spin Echo sequence have two parameters:

• Echo Time (TE) is the time between the 90° RF pulse and MR signal sampling, corresponding to maximum of echo. The 180° RF pulse is applied at time TE/2.
• Repetition Time is the time between 2 excitations pulses (time between two 90° RF pulses).

Each tissue has a specific proton density, T₁ and T₂ time. The NMR signal depends on these 3 factors.
After time T₁, longitudinal magnetization has returned to 63 % of its final value. T₁ defines the recovery rate of longitudinal magnetization.
For example, here are longitudinal magnetization recovery curves for 2 tissues A and B with different T₁s.

After time T₂, transverse magnetization has returned to 37 % of its initial value. T₂ defines the decay rate of transverse magnetization.
For example, here are transverse magnetization decay curves for 2 tissues A and B with different T₂s.

How do TR and TE modify tissue signals?

Let's consider 2 tissues A and B with different T₁s. If TR is very long, even if tissue A has a longuer T₁ than tissue B, the longitudinal magnetization of both tissues will recover completely before the next excitation.
Thus, the transverse magnetization amplitude will be the same for both tissues after each excitation.

If TR is short and if tissue A has a longer T₁ than tissue B, the longitudinal magnetization of tissue A will recover less than the longitudinal magnetization of tissue B.
Thus, the transverse magnetization amplitude of tissue B will be higher after the next excitation.

In the graphs below :

• The first part of the following curves corresponds to the longitudinal magnetization recovery after the first excitation.
• TR is the delay between excitations.
• The second part corresponds to the transverse magnetization decay after the second excitation.
• The MR signal is acquired at time TE after excitation.

By setting the TR to short values, tissue contrast will depend on differences in longitudinal magnetization recovery (T₁).

By setting the TR to long values, the T₁ effect on tissue contrast will be reduced. If TE is long enough, differences in transverse relaxation will alter tissue contrast (the T₂ effect).
(But if TE is too long, the signal will have disappeared !)

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.

For example:

• 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.

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!