The spin echo sequence is made up of a series of events : 90° pulse – 180° rephasing pulse at TE/2 – signal reading at TE. This series is repeated at each time interval TR (Repetition time). With each repetition, a k-space line is filled, thanks to a different phase encoding. The 180° rephasing pulse compensates for the constant field heterogeneities to obtain an echo that is weighted in T2 and not in T2*.
The rephasing lobe of the slice selection gradient, the phase encoding gradient and the dephasing lobe of the readout gradient are applied simultaneously, immediately after the excitation pulse.
The slice selection gradient applied for the 180° pulse requires no rephasing lobe. However, two identical gradient lobes are applied on either side of this gradient to eliminate the transverse magnetization created by the 180° rephasing pulse on the edge of the slice (where the protons will in fact be submitted to a flip angle of less than 180° due to the imperfect slice profile).
Duration = TR ∙ NPy ∙ Nex
A spin echo sequence has two essential parameters: TR and TE.
TR is the time interval between two successive 90° RF waves. It conditions the longitudinal relaxation of the explored tissues (depending on T1). The longer the TR, the more complete the longitudinal magnetization regrowth (Mz tends to M0). Reducing TR will weight the image in T1 as the differences between the longitudinal relaxation of the tissues’ magnetization will be highlighted .
In classic spin echo, after TR time, a single k-space line will be acquired. TR repetition is thus responsible for the duration of the sequence.
TE is the time interval between the 90° flip and receipt of the echo, the signal being produced by transverse magnetization. Transverse magnetization decreases according to the time constant T2 of each tissue (the field heterogeneities [which give T2*] being compensated by the 180° flip applied at TE/2).
In the T2–weighted spin echo sequence the TR and TE parameters are optimized to reflect T2 relaxation.
When the TR is long (over 2000 milliseconds), longitudinal magnetization recovery is complete and on the following flip, the influence of T1 on signal magnitude will be minimized. Associated with long TE (80 to 140 milliseconds), the different tissues are better highlighted according to their T2.
Long T2 tissues will appear as a hypersignal, as opposed to short T2 structures, which will appear as a hyposignal.
The proton density weighted spin echo sequence has optimized TR and TE parameters to minimize the influence of both T2 and T1. The contrast obtained will depend on the density of the hydrogen nuclei (i.e. protons).
A long TR (over 2000 milliseconds), associated with a short TE (10 to 20 milliseconds) will relatively suppress both the influence of T1 and the effect of T2 on signal magnitude.
Historically, spin echo was the first sequence to be used. It has been a benchmark for all subsequent developments, namely in terms of contrast. The 180° rephasing pulse gives a « true T2 » signal rather than a T2*signal.
Choosing the right sequence parameters (TR and TE) will produce images weighted in T1, T2 or proton density. The major disadvantage with T2 weighted spin echo sequences is linked to long TR resulting in prohibitive acquisition times.
While spin echo sequences can be used in clinical practice to obtain good quality anatomical T1-weighted images, faster types of sequence are preferred to obtain T2-weighted images.
This technique allows for simultaneous acquisition of several spin echo images, located in different positions without modifying the contrast. In fact, the spin echo sequence comprises a succession of periods of time (repetition time TR). Once the echo is obtained (at echo time TE, much less than TR), there is an interval of free time until the following repetition. This wasted time is used to acquire signals from other slices. This is done by applying selective 90°-180° pulses with adapted frequencies corresponding to other slice positions.
The multi-slice technique provides true spin echo imagery, without modifying T1 and T2 contrast. It is routinely used in clinical practice.
The number of slices that can be acquired simultaneously is proportionate to the free interval between each TR and inversely proportionate to TE.
The slices need to be spaced in the multi-slice technique to stop the imperfections in slice profile causing signal perturbations from one slice to the next.
Slice-interlacing is also used to space out the slices imaged during the same repetition.