Our results show that S2
-SSFP is a viable technique for T2
-weighted whole brain fMRI at 7 T. The functional sensitivity of the S2
-signal at 7 T – as estimated by the average z-values in the activated area – is not significantly different from SE-EPI. The fact that no activation was found in three subjects for the TR15 protocol and that the average z-value was lower indicates a lower sensitivity of the TR15 protocol compared to the TR27 protocol. As the simulated contrast for the TR27 protocol is only reduced by 3% compared to the TR15 protocol, this might be due to higher sensitivity to motion in case of the short TR protocol as the measured signal is a composition of more components from past excitations (25
Due to the fact that this sequence is – in essence – spin-echo like and refocuses static magnetic field inhomogeneities, the small residual signal dropout is due to the residual T2
*-weighting caused by the necessary separation of the S2
-echo from the following RF pulse. In our study this was reduced as much as possible without reducing the readout window by using an asymmetric readout. The remaining T2
*-weighting in the 7 T experiment was thus reduced to that corresponding to a TE of about 3 to 5 ms for the different TRs used, which should be minimal assuming a T2
* of about 33 ms of grey matter at 7 T (26
). A further reduction of T2
*- weighting and signal dropout could be achieved by using a higher readout bandwidth at the cost of increased thermal noise. Also, in the case of SE-EPI some T2
*-weighting is present due to the EPI readout window.
A major advantage of T2-contrast images at high field strengths obtained by S2-SSFP is the reduced power deposition due to the low flip angles required. At 7 T, the SE-EPI protocol used had an about 3 times higher SAR value than the TR15 protocol and an about 5 times higher one than the TR27 protocol, which would lead to less volume coverage or longer measurement times in cases where less SAR efficient coils or pulses are used.
Another advantage is the reduced distortion compared to EPI methods when using no acceleration for EPI. When using an AF of 3 it becomes comparable to the distortion of S2-SSFP which occurs in the readout direction with the relatively low readout BW of 75 Hz/px of the TR27 protocol. A higher readout bandwidth (150 Hz/px) as used in the TR15 protocol due to the short TR further reduced geometric distortions.
While the use of a non-EPI readout has several above mentioned advantages it leads to long measurement times. The use of a 3D acquisition scheme enabled us to use 2D parallel acceleration and the combination of coil arrays with many elements and a high field resulted in a good image quality with a nine-fold acceleration. The advantage of parallel imaging at high fields has already been shown (27
). However, going to higher resolution is a limitation of the SSFP implementation in its current form as a higher resolution leads to a longer measurement time which is less of a problem for SE-EPI.
When performing the simulations using Eqs. 1
for a grey matter T2
of 60 ms (7 T) assuming a T2
change of 1.5 ms between rest and activation (estimated from the SE-EPI experiment at 7 T in (2
)), we obtain signal changes of 2.47% (SE), and 2.37% (S2
, TR15) and 3.05% (S2
, TR27) for our experimental parameters. This is in good agreement with the corresponding experimental results of 2.37% and 2.53% (SE), and 2.75% (S2
, TR15), respectively, however less so for the S2
-signal changes of the TR27 protocol (5.38%). As these calculations are based on a simple T2
change in grey matter this discrepancy at the longer TR protocol might be explained by partial volume effects of other tissues. The reason that the relative signal changes for S2
are larger than for SE, can be explained by the fact that the S2
-signal is intrinsically smaller and contains contributions from multiple coherence pathways where also higher order pathways contribute. These higher order contributions can be viewed as spin echoes with longer echo times which then have higher functional signal changes but a lower intensity which results in higher relative signal changes.
We conclude that whole brain fMRI using S2-SSFP is feasible at 7 T. Due to low power deposition and decreased susceptibility induced image artifacts, it may constitute an alternative to SE-EPI.