|Type of sequence||Philips||Siemens||GE||Hitachi||Toshiba|
|Multi-echo SE||Multi SE||
|Fast SE||Turbo SE||Turbo SE||Fast SE||Fast SE||Fast SE|
In fast spin echo sequences, the interval of time after the first echo, is used to receive the echo train, to fill the other k-space lines in the same slice . Because of the reduced number of repetitions (TR) required, the k-space is filled faster and slice acquisition time is reduced.
This is done by applying new 180° pulses to obtain a spin echo train. After each echo, the phase-encoding is cancelled and a different phase-encoding is applied to the following echo.
The number of echoes received in the same repetition (during TR time) is called the Turbo Factor or Echo Train Length (ETL).
These sequences allow several images of the same slice position without increasing overall acquisition time. The advantage is that the images are obtained with a different contrast, which is useful in characterizing certain lesions (for example, highlighting contrast at long TE for hepatic angioma, which appears as a relative hypersignal).
After the first echo is obtained, there is a free interval until the next TR. By applying a new 180° pulse, a new echo is received, with the same phase encoding, to build the second image . The echo time of the 2 images differs and the second image will be more T2 weighted than the first.
Typically, these sequences are used to obtain simultaneously PD- and T2-weighted images.
The contrast in fast spin echo is modified in relation to a standard spin echo sequence. As the echoes are received at different echo times, the echoes corresponding to the central k-space lines are the ones that will determine image contrast. The moment at which theses echoes are acquired is called effective TE.
In T1 weighted sequences, the need to choose a short TR limits echo train length. This type of sequence is very commonly used in T2 weighting, namely in pelvic imagery.
Within lipid molecules a spin-spin coupling (J coupling) occurs between the atomic nuclei. This coupling shortens relaxation time T2. Fast repetition of 180° pulses in fast spin echo sequences will perturb J coupling, causing fat T2 to lengthen.
Thus, fat has a higher T2 signal in fast spin echo than in standard spin echo, the latter respecting J coupling.
A DIET (Delayed Interval Echo Train) sequence is a fast SE sequence where the delays between 180° pulses are designed to respect J coupling: as a result, the fat maintains an appearance closer to that observed in a standard SE sequence.
In fast spin echo, slice acquisition time is reduced by a factor corresponding to the number of echoes received in the same TR time.
There may be poorer spatial resolution, with a blurred image, if the k-space lines corresponding to high spatial frequencies are filled with late echoes (weak intensity due to T2 relaxation).
Conversely, the use of strong intensity early echoes to fill the high spatial frequencies in k-space is likely to produce truncation or Gibbs artifacts.
Duration = TR · number of phase encodings · number of acquisitions / turbo factor
However it should be noted that the reduction in acquisition time resulting from measuring several echoes in the same TR also means a reduction in the time interval available to simultaneously acquire other slices (using the multi-slice technique).
The interest of fast SE sequences resides in their speed (around ten seconds) added to their low sensitivity to magnetic susceptibility artifacts and magnetic field heterogeneities.
Modifications in contrast and fat signal must be taken into account in interpreting the images.
The risk of artifacts and the large quantity of radiofrequency energy deposited by 180° pulses restricts the parameters (TR, effective TE, echo train length) of this type of sequence.
Fast spin echo can be combined with the technique developed for multi-echo sequences to obtain images faster with different contrasts in the same zone of interest.