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

Abstract

Oscillating gradient spin-echo (OGSE) sequences have demonstrated an ability to probe time-dependent microstructural features, although they often suffer from low SNR due to increased TEs. In this work we introduce frequency-tuned bipolar (FTB) gradients as a variation of oscillating gradients with reduced TE and demonstrate their utility by mapping the frequency dispersion of kurtosis in human subjects.Methods: An FTB oscillating gradient waveform is presented that provides encoding of 1.5 net oscillation periods, thereby reducing the TE of the acquisition. Simulations were performed to determine an optimal protocol based on the SNR of kurtosis frequency dispersion-defined as the difference in kurtosis between pulsed and oscillating gradient acquisitions. Healthy human subjects were scanned at 7T using pulsed gradient and an optimized 23 Hz FTB protocol, which featured a maximum b-value of 2500 s/mm 2 . In addition, to directly compare existing methods, measurements using traditional cosine OGSE were also acquired. Results: FTB oscillating gradients demonstrated equivalent frequencydependent diffusion measurements compared with cosine-modulated OGSE while enabling a significant reduction in TE. Optimization and in vivo results suggest that FTB gradients provide increased SNR of kurtosis dispersion maps compared with traditional cosine OGSE. The optimized FTB gradient protocol demonstrated consistent reductions in apparent kurtosis values and increased diffusivity in generated frequency dispersion maps. Conclusions: This work presents an alternative to traditional cosine OGSE sequences, enabling more time-efficient acquisitions of frequency-dependent diffusion quantities as demonstrated through in vivo kurtosis frequency dispersion maps.

Frequency-dependent diffusion kurtosis imaging in the human brain using an oscillating gradient spin echo sequence and a high-performance head-only gradient

“…With clinical MRI gradient systems, recent time-dependent DKI studies with PGSE and STEAM have captured the long-time limit of kurtosis (Δ eff from 21.2 to 100 ms for PGSE; Δ eff from 100 to 800 ms for STEAM) ( Lee et al, 2020 ; Solomon et al, 2023 ). The short-time limit of kurtosis for in vivo human brains has been preliminarily evidenced in a recent study with single-frequency OGSE and single-time PGSE acquisitions ( Borsos et al, 2023 ). In our study, by leveraging the high-performance MAGNUS gradient MRI system, we characterize the trend of decreasing kurtosis over frequency in the short-time limit (or equivalently, the positive correlation between kurtosis and effective diffusion times Δ eff from 4.0 to 14.2 ms) more comprehensively using multi-frequency OGSE, which agrees with and expands previous findings ( Borsos et al, 2023 ).…”

“…The short-time limit of kurtosis for in vivo human brains has been preliminarily evidenced in a recent study with single-frequency OGSE and single-time PGSE acquisitions ( Borsos et al, 2023 ). In our study, by leveraging the high-performance MAGNUS gradient MRI system, we characterize the trend of decreasing kurtosis over frequency in the short-time limit (or equivalently, the positive correlation between kurtosis and effective diffusion times Δ eff from 4.0 to 14.2 ms) more comprehensively using multi-frequency OGSE, which agrees with and expands previous findings ( Borsos et al, 2023 ). The trend of decreasing kurtosis over frequency may mainly be attributed to more frequent interactions of water molecules with membranes or barriers at lower frequencies (or equivalently, longer diffusion times) ( Aggarwal et al, 2020 ).…”

“…Denser sampling and/or a broader frequency range will be desired with further OGSE waveform optimization ( Borsos et al, 2023 ; Hennel et al, 2021 ) and/or hardware advancement, which would likely further facilitate the power-law fitting process and may allow more rigorous statistical analysis. Moreover, higher-frequency sampling may allow us to study the short-time limit of diffusivity within the Mitra regime ( Aggarwal et al, 2020 ; Lemberskiy et al, 2017 ; Mitra et al, 1993 ; Novikov and Kiselev, 2011 ; Reynaud et al, 2016 ; Sukstanskii, 2013 ).…”

“…For image acquisition such as echo planar imaging (EPI), high-performance gradient systems can enable faster traversal of k-space, thus allowing higher resolution, higher signal-to-noise ratio (SNR), and less distortion and image blurring ( Setsompop et al, 2013 ; Tan et al, 2016 ; Ugurbil et al, 2013 ). For diffusion encoding, the high gradient strength and slew rate enable more efficient diffusion weighting at short diffusion times with OGSE (e.g., f max × b max =0.075 μm −2 ) ( Arbabi et al, 2020 ; Borsos et al, 2023 ; Hennel et al, 2021 ; Michael et al, 2022 ; Tan et al, 2020 ). Still, most in vivo human time/frequency-dependent diffusion studies with OGSE mainly focus on either only DTI measurements ( Arbabi et al, 2020 ; Hennel et al, 2021 ; Michael et al, 2022 ; Tan et al, 2020 ) or a comparison between OGSE measurements at a single oscillating frequency and PGSE measurements at a single diffusion time ( Borsos et al, 2023 ).…”