Unlike imaging using radiation, in which the contrast depends on the different attenuation of the structures being imaged, the contrast in MR images depends on the magnetic properties and number of hydrogen nuclei in the area being imaged. Different contrasts in the area being imaged can be selected for by running different sequences with different weightings. The main three sequences are:
- T1-weighted (maximum T1 contrast shown)
- T2-weighted (maximum T2 contrast shown)
- Proton density (PD) weighting (density of hydrogen protons shown)
There are other more complicated sequences as well (e.g. fluid attenuated inversion recovery (FLAIR) and short tau inversion recovery (STIR)) which we will cover later.
To recap, T1 relaxation is the recovery of the longitudinal magnetisation (Mz). The higher the Mz at the time of applying the 90° RF pulse the greater the transverse signal (Mxy). The TR (time to repetition) is what determines the length of time between 90° RF pulses:
The longer the TR
The longer the time to the next 90° RF pulse
The more time Mz will have had to recover
The higher the transverse signal when the 90° RF pulse is applied
*** i.e. it is the TR that determines the T1 signal ***
The time constant, T1, is a measure of the time it takes for the nuclei to reach 63% of their original Mz. Hydrogen nuclei in different molecules have different T1s. Those with a short T1 will recover their Mz quicker than those with a long T1.
To maximise the contrast between the T1 properties of tissues in the sample being imaged, we need to set the TR so that it occurs at the point in the curve at which there is the greatest difference. As seen on the curve above, this is at a short TR.
Note about T2 weighted imaging
In order to maximise T2 weighted imaging we want to minimise the contribution of T1 contrast. Looking at the above chart, the smallest T1 contrast is at long or short TR’s. At short TR’s the signal is too small to be of use and so a long TR is used.
To recap, T2 decay is the decay of the transverse magnetisation (Mxy) after application of the 90° RF pulse.
The longer the time after the 90° RF pulse, the more the Mxy decays and the smaller the transverse signal. As we saw in the spin echo sequence, TE is the “time to echo”. If we have a long TE there is more time for the Mxy to decay and we get a smaller signal.
The longer the TE
The longer the time allowed for Mxy to decay
The smaller the transverse (T2) signal
*** i.e. it is the TE that determines the T2 signal ***
The time constant, T2, is the time it takes for the signal to decay to 37% of its excited Mxy. Hydrogen nuclei in different molecules have different T2’s. Those with a short T2 will take a shorter time to decay than those with a long T2.
To maximise the T2 contrast a long TE is used, although not so long that the signal is negligible.
Note About T1 Weighted Imaging
To maximise the T1 signal in T1-weighted imaging we want to minimise the contribution of the T2 signal. From the figure above we see that the smallest contrast occurs at a small TE or a very long TE. However, at very long TE’s the signal is too small and so a short TE is used in T1 weighted imaging.
Proton Density Imaging
Unlike T1 and T2 weighted images, proton density (PD) does not display the magnetic characteristics of the hydrogen nuclei but the number of nuclei in the area being imaged. To get a PD weighted image we want to minimise the contribution of both T1 and T2 contrast.
- T1 minimised with a long TR: large signal and small T1 contrast
- T2 minimised with a short TE: large signal and small T2 contrast
|Water signal||Water has a long T1. T1-WI uses a short TR so the signal from water is still low, therefore, water appears dark|
|Fat signal||Fat has a short T1, so even though the TR is short the signal is still high and fat appears bright|
|TR||Short. 300-600 ms|
|TE||Short. 10-30 ms|
|Water signal||T2-WI uses a long TE so the signal from water is high, therefore, water appears bright|
|Fat signal||Fat has a short T2, so at a long TE the signal is less bright and it will be darker than water|
|TR||Long. 2000 ms|
|TE||Long. 90-140 ms|
|Water signal||A long TR results in a high water signal, but a short TE means that this is less than the signal of a T2 scan. The signal of water is in the middle|
|Fat signal||A long TR results in a high fat signal, and short TE means this signal is higher than on a T2-WI. Fat appears bright|
|TR||Long. 1000-3000 ms|
|TE||Short. 15 ms|