Spin echo sequences - Detailed
We have already covered the spin echo sequence in some detail earlier. The difference between this sequence and the one outline on the previous page is the addition of the 180° RF pulse at TE/2.
In the previous sequence and in gradient echo sequences the signal decays quickly due to magnetic field inhomogeneities. The spin echo sequence preserves the MRI signal for longer with the addition of the 180° pulse as described previously.
The second difference is the frequency encoding gradient. A positive gradient is applied prior to the 180° RF pulse. This has the same effect as a negative FE gradient applied after the 180° pulse.
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The TR is necessary to allow sufficient time for the T1 relaxation to complete in order to have enough Mz to give a signal when it is flipped by the 90° pulse.
A typical brain scan contains 18 slices
- The TR is 540ms
- A matrix size of 256 x 512 (256 phase encoding steps are required per slice)
- The scan time is:
TR x PE steps x Number of slices / 60,000
540 x 256 x 18 / 60,000 = 41.4 minutes
Considering the TE is only 30ms, this is a very long scan with a lot of dead time in which no signal is being created. We can use this dead time by selecting another slice and starting a cycle, then selecting a third slice and starting a cycle etc. After 540 ms it is time to start the second cycle for the first slice. In 540 ms we can scan 18 lines of 18 different k-spaces. Now we just need to repeat this enough times to get every line of every k-space (i.e. multiple by the number of phase encoding steps). Recalculating the scan time gives us:
540 x 256 / 60,000 = 2.3 minutes
This technique is used in nearly every scan to make the scan times shorter.
Multi echo sequence
So far, only one echo per cycle is being created. We can acquire more echoes in one cycle.
From the previous chapter "T1, T2 and PD Weighted Imaging" we saw that selecting different TEs we can create different weighted images:
- PD weighted uses a short TE of 15 ms
- T2 weighted uses a long TE of 1000-3000 ms
We can transmit two 180° pulses two create two echoes with different TEs of the same row of the same k-space. In this way we create a PD and a T2 image in the same amount of time as it takes to create one image.
We can take the multi-echo sequence further. We can repeat the 180° RF pulses many times, creating many echoes, within one cycle. If we apply a different GPE each time we can fill up different lines of k-space. The number of echoes we create is called the "echo train length (ETL)". In the above example the ETL is 5 but we can use an ETL of 212. The TE measured is taken to be the echo created when the GPE is zero and is called the "effective TE (Teff)".
N.B. The phase encoding gradient is reversed prior to the next 180° RF pulse to rephase the spins.
This will shorten the sequence time:
Normal spin echo = TR x no. GPE x number of slices
Turbo spin echo = TR x no. GPE x number of slices / ETL
- Very fast - useful for MR angiography in which very fast scan times are needed.
- Can create two images of different contrasts by filling two different k-spaces. E.g. if we have an ETL of 14, we can use the first 7 echoes for a PD image (first k-space) and the last 7 echoes for a T2 image (second k-space). This is called a Double-Echo TSE sequence
- Only really able to achieve heavily T2 weighted images
- Mix of contrasts: Each echo that fills a different line of k-space is at a different time and, therefore, a different contrast.
Fast advanced spin echo or HASTE sequence
We can take the TSE one step further and fill an entire k-space in one cycle. If we use an ETL of 212 that reduces the scan times significantly. Furthermore, we only really need to fill up just over half of k-space (i.e. 212 rows). We can then use a Half Fourier Imaging to extrapolate the rest of k-space and complete the image.
The very late echoes are put in the centre of k-space (heavily T2 weighted) which results in an image that only shows free water. This is the sequence used in an MRCP study.
We've now gone through one type of sequence - the Spin Echo sequence. Next we'll cover Gradient Echo sequences.