Oscillating diffusion-encoding with a high gradient-amplitude and high slew-rate head-only gradient for human brain imaging

Abstract

Purpose: We investigate the importance of high gradient-amplitude and high slew-rate on oscillating gradient spin echo (OGSE) diffusion imaging for human brain imaging and evaluate human brain imaging with OGSE on the MAGNUS head-gradient insert (200 mT/m amplitude and 500 T/m/s slew rate).

Methods: Simulations with cosine-modulated and trapezoidal-cosine OGSE at various gradient amplitudes and slew rates were performed. Six healthy subjects were imaged with the MAGNUS gradient at 3T with OGSE at frequencies up to 100 Hz and b = 450 s/mm² . Comparisons were made against standard pulsed gradient spin echo (PGSE) diffusion in vivo and in an isotropic diffusion phantom.

Results: Simulations show that to achieve high frequency and b-value simultaneously for OGSE, high gradient amplitude, high slew rates, and high peripheral nerve stimulation limits are required. A strong linear trend for increased diffusivity (mean: 8-19%, radial: 9-27%, parallel: 8-15%) was observed in normal white matter with OGSE (20 Hz to 100 Hz) as compared to PGSE. Linear fitting to frequency provided excellent correlation, and using a short-range disorder model provided radial long-term diffusivities of D_∞,MD = 911 ± 72 µm² /s, D_∞,PD = 1519 ± 164 µm² /s, and D_∞,RD = 640 ± 111 µm² /s and correlation lengths of l_c,MD = 0.802 ± 0.156 µm, l_c,PD = 0.837 ± 0.172 µm, and l_c,RD = 0.780 ± 0.174 µm. Diffusivity changes with OGSE frequency were negligible in the phantom, as expected.

Conclusion: The high gradient amplitude, high slew rate, and high peripheral nerve stimulation thresholds of the MAGNUS head-gradient enables OGSE acquisition for in vivo human brain imaging.

Keywords: diffusion imaging; head-gradient; microstructure.

Figure 1.

Waveforms and spectra for 80-Hz (a-c) sine-OGSE and (d-f) cosine-OGSE, with (a,d) sinusoidal or (b,c,e,f) trapezoidal modulation (with ramps constrained by either (b,e) whole-body or (c,f) head-only gradient PNS), and the resulting maximum b-values, assuming a nominal maximum gradient amplitude of 200 mT/m and maximum waveform duration of 50 ms (half of the symmetric pair of diffusion-encoding waveforms is shown). In (d), the ascending and descending ramps of sinusoidal cosine-OGSE assume head-only PNS.

Figure 2.

Achievable sine-OGSE (a-c) and cosine-OGSE (e-f) frequencies and b-values based on gradient performance. (a,d) The minimum required gradient amplitude (in mT/m) for sinusoid sine-OGSE for a waveform duration of 50 ms; (b,e) maximum b-value of trapezoid-modulated OGSE vs. b-value of sinusoidal OGSE at the same gradient amplitude for whole-body gradient performance and PNS; (c,f) maximum b-value of trapezoid-modulated sine-OGSE vs. b-value of sinusoidal OGSE at the same gradient ampltidue for head-only gradient performance and PNS.

Figure 3.

Plots of DTI metrics for three white matter parcels (genu, body, and splenium of the corpus callosum) and three gray matter parcels (temporal lobes, posterior fossa, insula/cingulate gyri) for PGSE and various OGSE frequencies from a subject (male, 59 years), showing the general trend in the majority of the parcels of increased (a,e) mean diffusivity, (b,f) parallel diffusivity, (c,g) radial diffusivity, but decreased (d,h) fractional anisotropy going from PGSE to OGSE, and with higher OGSE frequency.

Figure 4.

Representative DTI maps from a subject (male, 59 years) of mean diffusivity (MD), parallel diffusivity (PD), radial diffusivity (RD) and fractional anisotropy (FA) from PGSE and the various OGSE frequencies (20 Hz to 100 Hz), demonstrating visible increase in diffusivity measures and decrease in FA from PGSE to OGSE and with higher OGSE frequency.

Figure 5.

Representative maps of long-term diffusivity (D_∞) and correlation length (l_c) obtained from mean and radial diffusivity (MD, RD respectively) using the disordered model from two subjects (male, 41 years-old) and (female, 32 years-old).

Figure 6.

OGSE maps of radial diffusivity (long-term diffusivity and correlation length) obtained with different pulse sequence optimization parameters for another subject (male, 42 years-old).