Turbo spin echo (TSE) sequences with a long train of non-selective, variable flip angle, refocussing pulses are very time-efficient¹. However, they are also affected by the transmit field (B₁⁺) inhomogeneity that arises at high field strength MRI (≥3T). Recently, signal homogenisation of such sequences using Direct Signal Control (DSC) has been presented². Here, RF shim settings are dynamically updated between pulses to generate an optimal signal over the sequence.

Alternatively, several methods are available that improve the homogeneity of excitation pulses, mainly targeted at gradient echo sequences. In general, the calculation and implementation of excitation pulses is more straightforward than that of a set of RF shim settings, for which the entire sequence needs to be simulated. Custom excitation pulses, such as Spiral Nonselective (SPINS) pulses³, are usually designed for small flip angles and are ignorant to the phase of the excitation. TSE sequences, however, require a 90˚excitation flip angle at a 90˚phase with respect to the refocusing pulses, to realise the highest possible signal.

In this work, SPINS excitation pulses have been adapted for use in TSE sequences and were compared to dynamic RF shimming using DSC.

This study has been conducted on a 3T Achieva system (Philips) with an 8-channel transmit body coil and 8-channel receive head coil. Following local safety guidelines, the system-calculated head SAR was kept below 12% of the 3.2W/kg limit to prevent local SAR hotspots. B1+ and B0 were mapped as described in (2).

SPINS pulse design³, based on magnitude least squares optimisation, was combined with reVERSE⁴ to stay within peak RF power limits(Fig.1). Bloch-simulations⁵ showed a non-zero phase across the excited volume, with only a small spread around the mean value (Fig.2). The mean value was added as a global phase offset to the RF pulses to steer the effective phase towards zero: the system-defined phase setting for excitation pulses in CPMG sequences. The fidelity of 90˚SPINS pulses was measured in a 3D-AFI sequence⁶ with TR1/TR2=50/250ms, to stay within SAR limits.

DSC optimisation was targeted at the T1 and T2 of CSF. CSF is present in the centre and periphery of the brain, so an improvement upon the periphery-shaded quadrature mode is expected.

The T2w-3D-TSE sequence has 106 echoes per shot (including 6 dummies), TR=2500ms, ESP=4.0ms, asymptotic refocusing flip angle 35˚, resolution (1mm)^3, 2D-SENSE=1.7x1.7 and NSA=2. Scan time is 7m35s for whole head coverage.

The B1+-maps in Figure 3 show a flattened excitation using SPINS, approximately 7% below target. The phase relative to quadrature is close to zero (±20˚).

Figure 4 shows that SPINS-excited TSE, compared to quadrature TSE, results in slightly higher signal in grey and white matter, and a decrease in CSF signal. DSC has a more pronounced effect, showing a more homogeneous intensity distribution of the CSF over the brain volume.

The phase distribution of SPINS pulses is narrow, even though phase wasn’t constrained in pulse optimisation. This is probably due to only low spatial frequencies being visited in transmit k-space. Numerical optimisation specifically targeting a flat phase might be beneficial in the future. Some degradation of the excitation magnitude is seen in the frontal cortex in the simulation, but a matching signal gap hasn’t been found any of the experiments.

The excitation magnitude is quite homogeneous, but does not reach the desired FA. This is partially inherent to the measuring method: 2-3˚ underestimation is expected at this FA⁶. The FA discrepancy could also be due to the small tip angle approximation which was used to design the pulse. Still, a slightly lower-than-ideal, but homogeneous signal, would be of more use than the inhomogeneous signal generated by quadrature pulses.

Using SPINS excitation, a decreased TSE signal ratio is observed in the CSF near the top of the brain. This coincides with a measured phase offset, suggesting signal loss due to violation of CPMG conditions. This wasn’t predicted in the Bloch simulation.

Even though the SPINS excitation is flatter than quadrature excitation, when used in a TSE sequence it improves the homogeneity only slightly. The >100 refocusing pulses in quadrature mode following every SPINS excitation are probably the cause of this. A more homogenous signal is found using DSC.

This work is conducted at 3T, but has obvious extension to 7T where the illustrated effects are more severe.