Evaluating diffusion dispersion across an extended range of b‐values and frequencies: Exploiting gap‐filled OGSE shapes, strong gradients, and spiral readouts

Abstract

To address the long echo times and relatively weak diffusion sensitization that typically limit oscillating gradient spin-echo (OGSE) experiments, an OGSE implementation combining spiral readouts, gap-filled oscillating gradient shapes providing stronger diffusion encoding, and a high-performance gradient system is developed here and utilized to investigate the tradeoff between b-value and maximum OGSE frequency in measurements of diffusion dispersion (i.e., the frequency dependence of diffusivity) in the in vivo human brain. In addition, to assess the effects of the marginal flow sensitivity introduced by these OGSE waveforms, flow-compensated variants are devised for experimental comparison.Methods: Using DTI sequences, OGSE acquisitions were performed on three volunteers at b-values of 300, 500, and 1000 s/mm 2 and frequencies up to 125, 100, and 75 Hz, respectively; scans were performed for gap-filled oscillating gradient shapes with and without flow sensitivity. Pulsed gradient spin-echo DTI acquisitions were also performed at each b-value. Upon reconstruction, mean diffusivity (MD) maps and maps of the diffusion dispersion rate were computed. Results:The power law diffusion dispersion model was found to fit best to MD measurements acquired at b = 1000 s/mm 2 despite the associated reduction of the spectral range; this observation was consistent with Monte Carlo simulations.Furthermore, diffusion dispersion rates without flow sensitivity were slightly higher than flow-sensitive measurements. Conclusion:The presented OGSE implementation provided an improved depiction of diffusion dispersion and demonstrated the advantages of measuring dispersion at higher b-values rather than higher frequencies within the regimes employed in this study.

Tuned bipolar oscillating gradients for mapping frequency dispersion of diffusion kurtosis in the human brain

“…Experimentally, the ∆ D maps demonstrate comparable diffusion dispersion to previously reported studies 5,17,42 . The ∆ K maps exhibit distinct contrast from ∆ D , with larger magnitudes qualitatively observed in white matter relative to gray matter (Figure 4 and Supporting Information Figure S5).…”

“…Our technique will significantly benefit from the continued evolution of high‐performance gradients whereby higher gradient amplitudes will offset the reduced b ‐value while still yielding shorter TEs. Additionally, FTB oscillating gradients can also be used in combination with recent advances in OGSE optimization such as the gradient and spin‐echo technique presented by Wu et al, 40 or spiral readouts 41 as demonstrated by Michael et al, 42 to further improve image and map quality.…”

“…Experimentally, the ΔD maps demonstrate comparable diffusion dispersion to previously reported studies. 5,17,42 The ΔK maps exhibit distinct contrast from ΔD, with larger magnitudes qualitatively observed in white matter relative to gray matter (Figure 4 and Supporting Information Figure S5). This observation is consistent with the bimodal nature of the distributions in Figure 5C,D, and with the only other documented preliminary measurements of kurtosis with OGSE in the human brain presented by Yang et al 29 The generally larger negative ΔK in white matter suggests that, somewhat surprisingly, 23 Hz is a sufficiently high frequency to be in a regime in which diffusion becomes increasingly Gaussian and kurtosis begins to vanish, similar to findings at short diffusion times in the ex vivo spinal cord and mouse brain.…”

Revealing tumor microstructure with oscillating diffusion encoding MRI in pre‐surgical and post‐treatment glioma patients

“…Time-dependent diffusivity of human white matter and gray matter at short diffusion time regimes have been measured in recent years. [30][31][32] ADC or mean diffusivity (MD) has been described by power-law with the diffusion-weighted signal (normalized to b = 0 signal) expressed as:…”

“…The measured ADC (f ) or MD (f ) at OGSE frequencies of 0-120 Hz was reported to fit to a power 𝜃 of 0.5 or 1, a D 0 ranging from 0.6 to 1.2 μm 2 /ms, and a 𝛬 ranging from 2 to 15 μm 2 /s 0.5 (or μm 2 /s) in previous studies. [30][31][32] Figure 1E shows the simulation of ADC (f ) ADC (0 Hz) at f of 0-120 Hz, based on the reported time-dependent diffusivities in human white matter and gray matter. [30][31][32] Compared to solid tumor with high f IN (Figure 1C), ADC (f ) ADC (0 Hz) of white matter and gray matter is smaller.…”