SPINS excitation versus DSC dynamic RF shimming for homogenising high field strength TSE imaging
Ronald Mooiweer1, Shaihan J Malik2, Joseph V Hajnal2, Nico van den Berg1, Peter R Luijten1, and Hans Hoogduin1

1UMC Utrecht, Utrecht, Netherlands, 2Division of Imaging Sciences and Biomedical Engineering, King’s College London, London, United Kingdom


In this work the design of SPINS excitation pulses has been expanded for use in TSE sequences and was compared to dynamic RF shimming using DSC in a standard T2w TSE sequence. We have demonstrated homogeneous 90 degree excitation, but in itself this was not sufficient to make TSE images uniform. Manipulating the refocusing pulses (using DSC) remains a necessity.


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.


We have demonstrated homogeneous 90˚ excitation, which by itself is not sufficient to make TSE images uniform. Manipulating the refocussing pulses remains a necessity. Further improvements might be made when dynamic RF shimming and excitation pulse design are combined.


No acknowledgement found.


1. Busse RF, Hariharan H, Vu A, Brittain JH. Fast spin echo sequences with very long echo trains: design of variable refocusing flip angle schedules and generation of clinical T2 contrast. Magn. Reson. Med. 2006;55:1030–7. doi: 10.1002/mrm.20863.

2. Malik SJ, Beqiri A, Padormo F, Hajnal J V. Direct signal control of the steady-state response of 3D-FSE sequences. Magn. Reson. Med. [Internet] 2014;00. doi: 10.1002/mrm.25192.

3. Malik SJ, Keihaninejad S, Hammers A, Hajnal J V. Tailored excitation in 3D with spiral nonselective (SPINS) RF pulses. Magn. Reson. Med. 2012;67:1303–15. doi: 10.1002/mrm.23118.

4. Lee D, Grissom W a., Lustig M, Kerr AB, Stang PP, Pauly JM. VERSE-guided numerical RF pulse design: A fast method for peak RF power control. Magn. Reson. Med. 2012;67:353–362. doi: 10.1002/mrm.23010.

5. Bloch simulator: http://www-mrsrl.stanford.edu/~brian/blochsim/

6. Yarnykh VL. Actual flip-angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field. Magn. Reson. Med. 2007;57:192–200. doi: 10.1002/mrm.21120.


90˚SPINS pulse after reVERSE. Top: RF waveforms for 8 transmit channels, the dashed red line marks the maximum allowed RF amplitude limit for this system (13.5 muT). Bottom: waveforms describing a 3D spiral in k-space, taking into account the impulse response function of the gradient system.

Simulations of the SPINS pulse at 90˚FA. Over the brain volume, the %FA achieved has a mean of 98.7 and a standard deviation of 3.7. The phase has a mean of 46.2 and standard deviation of 7.6 degrees.

AFI B1+ mapping using 90˚ excitation in quadrature and with SPINS. Left: Percentage of Flip Angle achieved, viewed in three orientations. The phase of the SPINS pulse is found by subtracting the measured phase of quadrature mode from the SPINS measurement. Right: Profile across the line in the sagittal plane.

TSE images acquired using: (top to bottom) quadrature excitation, SPINS excitation with quadrature refocusing, DSC optimised pulse train. Ratios with respect to quadrature are also calculated, after image registration to the quadrature image.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)