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Ultra-strong gradient diffusion MRI at 7T with a head insert
Chantal Tax1,2, Edwin Versteeg1, Dennis J.W. Klomp1, Martijn F. Froeling1, Alberto de Luca1, and Jeroen C.W. Siero1,3
1University Medical Center Utrecht, Utrecht, Netherlands, 2CUBRIC, Cardiff University, Cardiff, United Kingdom, 3Spinoza Centre for Neuroimaging Amsterdam, Amsterdam, Netherlands

Synopsis

Diffusion weighting is achieved by the application of external field gradients typically for tens of milliseconds, during which the signal substantially decays due to inherent T2 relaxation. This work focuses on the benefits of strong gradients - here provided by a gradient head insert - for high SNR and short TE diffusion imaging at 7T. Proof-of-principle images show that a short TE (21 ms) at a b-value of 1000 s/mm2 is achievable at 7T using an EPI-readout.

Introduction

Diffusion MRI (dMRI) relies on the application of external field gradients to achieve diffusion weighting, commonly within the context of a spin echo experiment. Such diffusion encoding gradients are typically on for tens of milliseconds, during which the signal substantially decays due to inherent T2 relaxation. The duration of diffusion encoding gradients for a given degree of diffusion weighting (commonly quantified by the b-value) can be significantly shortened by the use of strong gradients, which are becoming more readily available in body systems1,2 and gradient head inserts3,4 at 3T. The reduced echo times (TE) result in significantly higher SNR and enable the study of diffusion properties in short-T2 compartments such as myelin water5,6. In addition to stronger gradients, stronger magnetic fields can further considerably boost the SNR. However, dMRI at 7T is often considered to be challenging or even disadvantageous because of the shorter apparent T2 - i.e. the additional loss of phase coherence when spins diffuse in local susceptibility fields - which negates the SNR gain. Nonetheless, the shorter TE enabled by strong gradients provides an exciting prospect to rekindle dMRI at 7T and truly benefit from its increased SNR7, as well as enhanced susceptibility-effects indicative of myelin properties8. This work presents a proof-of-principle of ultra-strong gradient dMRI at 7T using a gradient head insert.

Methods

Gradient head insert
The gradient head-insert consisted of a lightweight (45 kg) single-axis gradient coil operating in the z-direction9, which was powered by a dedicated amplifier (630A/990V Prodrive, Eindhoven). This combination yielded a maximum gradient amplitude and slew rate of 200 mT/m and 1300 T/m/s, respectively. The head insert was operated as an additional 4th gradient axis to the whole-body gradient setup and controlled via a separate gradient waveform generator.

Data
One healthy control was scanned with a dMRI acquisition protocol on 7T (Philips, NL) with and without the gradient head insert. As a proof-of-principle, the readout time was shortened by enabling partial Fourier 3/5 and parallel imaging with a SENSE factor of 2.3 for a 224x224 mm2 FOV and a 2 mm isotropic resolution. Diffusion-weighted images up to a b-value of 1000 s/mm2 were obtained with TE=21 ms using the head insert gradients. The data was corrected for susceptibility distortions with a reversed phase-encoding b=0 s/mm2 image10.

Results

Fig.1 shows the reduced TE as a function of b-value achievable with the gradient head insert compared to 40 mT/m body gradients, for different readout times ([0, 5, 10, 15] ms). Fig.2a shows that for sufficiently short TE, the signal gain from ultra high field outweighs the more rapid signal loss from the short apparent T2, where tTE is the TE threshold for which the 3T and 7T dMRI signals are equal. Here, a linear increase in signal was assumed as a function of field strength11 and the apparent T2 was set to 50 and 77 ms at 7T and 3T for white matter respectively12. Fig.2b shows how tTE varies for varying 7T signal gains and T2 settings. Fig.3 shows images acquired at TE=21 ms with the gradient head-insert operating at 1000 T/m/s and 200 mT/m.

Discussion and Conclusion

The simulation results show that using strong gradients - e.g. provided by head gradient inserts - is paramount for diffusion imaging at 7T. Here, the increased gradient performance allows for high b-values at echo times under 50 ms, which are unattainable with body gradients.Preliminary imaging results with a single-axis insert showed that TE at 21 ms for b=1000 s/mm2 was feasible at 7T using a conventional EPI-readout. Future work will focus on improving image quality, notably the signal loss in the temporal lobes originating from B1+ inhomogeneities, amplified noise due to spiking from a defect in the MR-scanner, and Gibbs ringing. B1+ inhomogeneities originate from the quadrature transmit coil currently used for the head insert. These inhomogeneities could be mitigated by replacing this coil by a multi-transmit array13. The image readout can furthermore be improved while maintaining short TE by adopting alternative strategies such as spiral readout5,14 and accelerated EPI readout enabled by the coil9.

Acknowledgements

CMWT was supported by a Veni grant (17331) from the Dutch Research Council (NWO) and a Sir Henry Wellcome Fellowship (215944/Z/19/Z).

References

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Figures

Maximum achievable b-value as a function of TE, for the gradient head insert (solid lines) and body gradients (dashed lines). RF pulse durations of 4 and 6 ms were assumed. The curves are shown for varying readout times [0, 5, 10, 15] ms e.g. depending on resolution and FOV, with a readout time of 0 ms representing a readout strategy starting at the centre of k-space (e.g. spiral imaging5,14).

a) Signal decay as a function of TE at 3T (blue) and 7T (red), with tTE the threshold where the signals are equal. A linear increase in signal was assumed as a function of field strength and the apparent T2 was set to 50 and 77 ms at 7T and 3T respectively. b) tTE as a function of other settings for the relative signal gain at 7T - which can depend on e.g. body noise11 - and T2.

dMRI images and ADC estimate with the gradient head insert at 7T and TE = 21 ms.

Proc. Intl. Soc. Mag. Reson. Med. 29 (2021)
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