Eric Michael


Last Name

Michael

First Name

Eric

ORCID

0000-0002-2513-4254

Organisational unit

03628 - Prüssmann, Klaas P. / Prüssmann, Klaas P.

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Publications 1 - 2 of 2
  • Enhanced spectral response in frequency-dependent diffusion measurements using a linear encoding model
    Item type: Journal Article
    Michael, Eric; Hennel, Franciszek; Pruessmann, Klaas Paul (2025)
    Magnetic Resonance in Medicine
    PurposeTo devise a more comprehensive quantitative representation for spectral encodings in frequency-dependent diffusion measurements for improved estimation of D(omega).Theory and MethodsWhereas a spectral diffusion measurement is typically represented by a Dirac delta function at a single attributed frequency, spectral response is represented here by the encoding power in |Q(omega)|2 over a set of contiguous frequency intervals. Using this representation paradigm, a linear encoding model is formulated wherein diffusivity over each interval can be estimated by inverting the encoding process from a set of measurements. This strategy was validated in in vivo human brain imaging experiments evaluating D(omega) up to 50 Hz over 10-Hz intervals using high-performance gradients. The employed spectral encodings were selected using an accompanying framework devised to ensure robust encoding performance given the chosen frequency intervals. Additionally, simulated measurements were carried out to compare the performance in estimating D(omega) using the proposed encoding model versus using single-frequency attribution in relation to the form of D(omega) and the width of frequency intervals.ResultsIn vivo D(omega) determined using the proposed encoding strategy were found to increase with increasing frequency and could be mapped to spectral responses more spectrally selective than those characteristic of single-frequency attribution. In turn, simulated measurements demonstrated that the linear encoding model permitted D(omega) estimation with improved accuracy, especially for more nonlinear D(omega), at the expense of reduced precision, particularly for narrower frequency intervals.ConclusionBy devising a more holistic representation paradigm for frequency-dependent diffusion measurements, D(omega) can be recovered with higher fidelity.
  • Non-monotonic time- and frequency-dependent diffusion kurtosis in the human brain revealed by pulsed and oscillating gradient experiments
    Item type: Journal Article
    Hao, Runpu; Michael, Eric; Hennel, Franciszek; et al. (2025)
    Magnetic Resonance in Medicine
    Purpose: To investigate the time and frequency dependence of diffusion kurtosis in the in vivo human brain with pulsed (PGSE) and oscillating gradient spin-echo (OGSE) experiments over a range of diffusion times and frequencies using high-performance gradients. Methods: Four OGSE waveforms probing centroid frequencies of 11, 20, 34, and 51 Hz, corresponding to equivalent effective diffusion times of 22.7, 12.5, 7.4, and 4.9 ms; and three PGSE waveforms probing effective diffusion times of 50, 22.7, and 12.5 ms were designed for experimentation. Seven healthy volunteers were scanned using said waveforms at b-values of 500, 1000, and 2000 s/mm². Kurtosis metrics were computed and evaluated in various white and gray matter regions of interest. Complementary Monte-Carlo simulations were performed using a simple substrate with varying permeabilities and the same gradient waveforms as used for in vivo imaging. Results: Non-monotonic, biphasic kurtosis time- and frequency-dependences were observed in different white and gray matter regions. This behavior supports the notion that kurtosis is influenced by the combined effects of water-restriction interactions, inter-compartmental water exchange, and tissue heterogeneity. Kurtosis values measured using OGSE and PGSE waveforms at the same effective diffusion times showed significant discrepancies, with consistently higher values from PGSE than OGSE. These discrepancies are mainly attributed to differences in exchange sensitivities among gradient waveforms based on analogous findings from Monte-Carlo simulations. Conclusion: Kurtosis was found to exhibit biphasic time- and frequency-dependence in the human brain. When interpreting kurtosis measurements, the influence of exchange sensitivities of employed gradient waveforms should be considered.
Publications 1 - 2 of 2