Various Aspects of Mri Physics

Practical Physics I

We have a come a long way already. We have discussed various aspects of MRI physics. In the next section I will discuss image contrast and a number of pulse sequences, which are commonly used in MRI. Without pulse sequences we can‟t do MRI. Our life depends on it in terms of which kind of image contrast we want to see or, even, which kind of pathology we want to detect. Understanding what a pulse sequence is and how it influences the image is vitally important.

Pulse Sequences

A pulse sequence is a sequence of events, which we need to acquire MRI images. These events are: RF pulses, gradient switches and signal collecting.

Figure 49 shows a "sequence diagram" in which the order of the events are shown schematically. These diagrams can be found in any book about MRI physics, so you better get used to them .

Let‟s go back to our first experiment. We started of with (1) switching on the Slice Select gradient (GSS). Simultaneously (2) a 90º RF-pulse was given to „flip‟ the net-magnetization into the X-Y plane. Then (3)

The Phase Encoding gradient (GPE) was switched on to do the first phase encoding. Then (4) the Frequency Encoding or Read Out gradient (GRO) was switched on during which (5) the signal, the Free Induction Decay (FID), was sampled.

This is a very simple and basic sequence. We also saw that the signal dies out very quickly. In the early days that was a problem. The hardware could not be switched quick enough to sample the entire signal. They could only sample the last part of the signal when most of the signal was gone. The resulting image showed it! It was signal starved. In order to improve the amount of signal the engineers came up with a brilliant solution.

Spin Echo (SE) Sequence

After the 90º-excitation pulse the net-magnetization is in the X-Y plane. It immediately starts to dephase due to T2 relaxation (spin-spin interactions). It is because of this dephasing that the signal drops like a stone. Ideally, we would like to keep the phase coherence because this gives the best signal. The brilliant solution the engineers came up with is this: a short time after the 90º RF-pulse a second RF-pulse is given. This time it is an 180º pulse. The 180º pulse causes the spins to rephase. When all the spins are rephased the signal is high again, and when we make sure we sample the signal at this instant we‟ll have a much better image. Figure 50 shows it better.

The signal we sample is called: an Echo, because it is "rebuilt" from the FID.

Notice that the 180º rephasing pulse is exactly in the middle of the 90º pulse and the echo.

There is a book (highly recommended, see reference) called: "MRI made easy... Well almost" by Schering, Germany, in which you can find a really nice analogy of rephasing:

Imagine a number of runners on a racetrack. When the whistle blows they all start to run (dephasing). Obviously they all run at the different speeds and after, let‟s say 30 seconds, the fastest one will be way ahead of the slower one. Then the whistle blows for the second time. The runners were instructed to turn around, without losing speed, at the second blow of the whistle (180º RF pulse). The fastest runner, now way behind the slower one, will catch up with the slower one (rephasing). After another 30 seconds they all arrive at the starting point at the same time (echo).

The effect of the 180º RF pulse is called: rephasing.

Figure 51 shows how this works. The spin system is mirrored around the Y-axis. Note that the rotation direction in the X-Y plane does not change.

A. It starts with a 90º excitation pulse. The magnetization is flipped into the X-Y-plane.

B. Immediately the spins dephase...

C. The spins dephase a bit more... then a 180 rephasing pulse is given.

D. The spins are mirrored around the Y axis.

E. The spins rephase until...

F. The spins are in phase again creating an "echo".

This is what is known as a Spin-Echo sequence.

As with everything in MRI, the spin-echo sequence is a compromise:

Advantages:

* The signal is strong

* Compensation for local field inhomogeneities: less artifacts.

Disadvantages:

* It takes time to do the rephasing step. This will increase the total scan time.

* It increases the amount of RF one has to put into the body (not that it‟s dangerous, but there are certain limits).

In spite of the increased scan time and the amount of RF the spin-echo sequence is widely used and has become the routine sequence in MRI.

Figure 53 shows the pulse sequence diagram. Notice that during the 180º rephasing pulse the slice select gradient (GSS) is switched on.

We start of with (1) switching on the Slice Select gradient (GSS). Simultaneously a 90º RF-pulse (2) is given to „flip‟ the net-magnetization into the X-Y plane. Then the Phase Encoding gradient (GPE) (3) is switched on to do the first phase encoding. GSS (4) is switched on again during the 180º rephasing pulse (5), so the same protons which were excited with the 90º pulse (2) are affected. Then the Frequency Encoding or Read Out gradient (GRO) (6) is switched on during which (7) the signal is sampled.

At this moment in time I can introduce a few sequence parameters.

TR (Repetition Time). As stated before, the whole process must be repeated as many times as the matrix in the phase encoding direction is deep. TR is the time between two 90º excitation pulses. In regular SE sequences the TR can be anything in the range of 100 to 3000 milliseconds.