Now you see it, now you don't: optimal parameters for interslice stimulation in concurrent TMS-fMRI

The powerful combination of transcranial magnetic stimulation (TMS) concurrent with functional magnetic resonance imaging (fMRI) provides rare insights into the causal relationships between brain activity and behaviour. Despite a recent resurgence in popularity, TMS-fMRI remains technically challenging. Here we examined the feasibility of applying TMS during short gaps between fMRI slices to avoid incurring artefacts in the fMRI data. We quantified signal dropout and changes in temporal signal-to-noise ratio (tSNR) for TMS pulses presented at timepoints from 100ms before to 100ms after slice onset. Up to 3 pulses were delivered per volume using MagVenture's MR-compatible TMS coil. We used a spherical phantom, two 7-channel TMS-dedicated surface coils, and a multiband (MB) sequence (factor=2) with interslice gaps of 100ms and 40ms, on a Siemens 3T Prisma-fit scanner. For comparison we repeated a subset of parameters with a more standard single-channel TxRx (birdcage) coil, and with a human participant and surface coil set up. We found that, even at 100% stimulator output, pulses applied at least -40ms/+50ms from the onset of slice readout avoid incurring artifacts. This was the case for all three setups. Thus, an interslice protocol can be achieved with a frequency of up to ~10 Hz, using a standard EPI sequence (slice acquisition time: 62.5ms, interslice gap: 40ms). Faster stimulation frequencies would require shorter slice acquisition times, for example using in-plane acceleration. Interslice TMS-fMRI protocols provide a promising avenue for retaining flexible timing of stimulus delivery without incurring TMS artifacts.

A Phantom Study of Crosstalk Artefact Reduction using Interleaved Method in MR Lumbar

Crosstalk artefacts appear as dark signal lines on MR images that occur due to imperfect multi-slice settings. There is still a good deal of emergence of crosstalk artefacts on T1 of SE and FSE sequence with interleaved slice acquisition methods. This study aims to determine the differences and the optimal value of reduction of crosstalk artefacts against changes in TR on T1 between SE and FSE sequences in interleaved slice acquisition methods. The experiment was carried out by scanning the water phantom with a variation of TR 500ms, 600ms, and 700ms in SE and FSE sequences. An assessment of the reduction of crosstalk artefacts was carried out using SNR values. There is a significant difference in the reduction of crosstalk artefacts between SE sequences and FSE on T1 (p-value = < 0.001). The reduction of crosstalk artefacts in SE sequences with TR values of 600ms and 700ms respectively, were 22.2% and 37.7%. Whereas the FSE sequences were 28.3% and 56%. Optimal crosstalk artefact reduction occurs in FSE sequences of T1 (56%). It is intended to use FSE sequences with TR 700ms on T1 if there are overlapping slices so that the crosstalk artefacts that occur will be reduced optimally.

Age- and Gender-Associated Liver Physiological T1rho Dynamics Demonstrated with a Clinically Applicable Single-Breathhold Acquisition

To understand women’s and men’s physiological ranges of liver T1rho relaxation time measured with a single breathhold black blood sequence, this healthy volunteer study was conducted in 62 women (mean age, 38.9 y; range, 18–75 y) and 34 men (mean age, 44.7 y; range, 24–80 y). Approval from the institutional ethics committee was obtained. Magnetic resonance imaging was performed with a 3.0T scanner with six spin-lock times of 0, 10, 20, 25, 35, and 50 ms and a single breathhold of 12 s per slice acquisition. Six slices were acquired for each examination. The results demonstrated that the female liver T1rho value ranged between 35.07 and 51.97 ms and showed an age-dependent decrease, with younger women having a higher measurement. The male liver T1rho value ranged between 34.94 and 43.39 ms, with no evidential age dependence. Postmenopausal women had similar liver T1rho values as men. For women, there was a trend that the liver T1rho value could be 4% to 5% lower during the menstrual phase than during the nonmenstrual phase. For both women and men, no evidential association was seen between body mass index and liver T1rho.