arXiv subjects

Arthur Chakwizira

Publications and source records attributed to Arthur Chakwizira.

6 recordsLinked to original sources

arXiv2026

Characterising Water Exchange in Gliomas Using Diffusion MRI with Free Gradient Waveforms

Transmembrane water permeability, which regulates cellular water exchange and is influenced by water channels such as aquaporin-4 (AQP4), has been implicated in glioma progression and may affect tumour infiltration and treatment response. Non-invasive mapping of water exchange may therefore provide biomarkers of glioma pathology. This study investigates the feasibility of characterizing water exchange in gliomas using diffusion MRI with free gradient waveforms, known as the Restriction-Exchange (ResEx) approach, which enables exchange quantification independent of restricted diffusion effects. Thirteen patients with histologically confirmed gliomas (ten glioblastomas, three astrocytomas) underwent preoperative MRI at 3T using a custom ResEx protocol. Multiple diffusion-weighted acquisitions with selective exchange sensitivity were performed to estimate voxel-wise maps of the apparent diffusion coefficient (ADC), diffusion kurtosis, and water exchange rate. ResEx-derived maps revealed heterogeneous spatial patterns across and within tumours. Elevated exchange rates were commonly observed in enhancing tumour margins, potentially reflecting smaller cells, increased membrane permeability or AQP4 upregulation. In some cases, elevated exchange extended into non-enhancing peritumoural regions. Exchange values in oedema were slightly higher than in healthy tissue, suggesting potential infiltration or membrane disruption. Diffusion MRI with free gradient waveforms permits non-invasive mapping of water exchange in gliomas and reveals physiological information not captured by standard imaging. Exchange rate mapping may offer novel biomarkers of tumour aggressiveness, infiltration, and treatment response, and holds promise for surgical and radiotherapy planning.

physics.med-ph↗

arXiv2025

The role of dendritic spines in water exchange measurements with diffusion MRI: Time-Dependent Single Diffusion Encoding MRI

Time-dependent diffusion MRI (dMRI) with single diffusion encoding (SDE) probes water dynamics in biological tissues, but signal interpretation depends on microstructure. While prior work focused on restricted/hindered diffusion and membrane permeation, diffusion-mediated exchange between dendritic shafts and spines in gray matter (GM) remains understudied. We hypothesize that impermeable spiny dendrites produce time-dependent SDE signals mimicking permeative exchange and investigate how spine density biases exchange time estimates. Using Monte Carlo simulations and narrow escape theory, we quantify spine-shaft exchange times (3-26 ms), matching cortical permeative exchange estimates. A modified two-compartment Karger model characterizes time-dependent SDE signals but yields biased exchange estimates, reflecting spine volume fraction rather than morphology. Unaccounted diffusion-mediated exchange introduces up to 80% bias in NEXI/SMEX model estimates. We propose an extended three-compartment Karger model incorporating both diffusion-mediated (spine-shaft) and permeative (intra-extracellular) exchange. However, this model cannot uniquely separate membrane permeability from spine volume effects. Our findings emphasize that dendritic spines should be considered in SDE-based exchange studies and caution against attributing exchange solely to permeability. Advanced methods are needed to disentangle these mechanisms in GM.

physics.med-ph↗

arXiv2025

The role of dendritic spines in water exchange measurements with diffusion MRI: Double Diffusion Encoding and free-waveform MRI

Time-dependent diffusion MRI enables the estimation of water exchange rates in vivo, yet reported values in grey matter remain inconsistent. While most studies attribute these estimates to membrane permeability, non-permeative geometric exchange has also been proposed. The present study investigates the contribution of geometric exchange between dendritic spines and shafts to diffusion MRI-derived exchange estimates. Monte Carlo simulations were performed in synthetic dendrites with varying spine morphology, density, and membrane permeability. Diffusion-weighted signals were generated using multiple protocols - including single diffusion encoding, double diffusion encoding, and free waveforms - and were analysed using four frameworks: the Kärger model (via kurtosis time-dependence), correlation tensor imaging, Restriction-Exchange, and Multi-Gaussian Exchange with transient kurtosis (tMGE). Dendritic spines were found to impart similar time-dependence signatures on the diffusion-weighted signal as permeative exchange (signal decrease with diffusion time). The effect was modulated by both spine morphology and density. Both the exchange rate and microscopic kurtosis increased with spine density. The tMGE method demonstrated the ability to disentangle geometric from permeative exchange. Non-permeative exchange in dendritic spines has a non-negligible impact on exchange estimates obtained with diffusion MRI and should be considered in future studies. Diffusion MRI exchange estimates may provide a non-invasive proxy for dendritic spine density, with potential applications in studies of neurological disorders.

physics.med-ph↗

arXiv2024

Diffusion MRI with double diffusion encoding and variable mixing times disentangles water exchange from intrinsic kurtosis

Double diffusion encoding (DDE) makes diffusion MRI sensitive to a wide range of microstructural features, and the acquired data can be analysed using different approaches. Correlation tensor imaging (CTI) uses DDE to resolve three components of the diffusional kurtosis: isotropic, anisotropic, and microscopic. The microscopic kurtosis is estimated from the contrast between single diffusion encoding (SDE) and parallel DDE signals at the same b-value. Another approach is multi-Gaussian exchange (MGE), which employs DDE to measure exchange. Sensitivity to exchange is obtained by contrasting SDE and DDE signals at the same b-value. CTI and MGE exploit the same signal contrast to quantify microscopic kurtosis and exchange, and this study investigates the interplay between these two quantities. We perform Monte-Carlo simulations in different geometries with varying levels of exchange and study the behaviour of the parameters from CTI and MGE. We conclude that microscopic kurtosis from CTI is sensitive to the exchange rate and that intercompartmental exchange and the intrinsic kurtosis of individual compartments are distinct sources of microscopic kurtosis. In an attempt to disentangle these two sources, we propose a heuristic signal representation referred to as $μ$MGE (MGE incorporating intrinsic kurtosis) that accounts for both effects, by exploiting the distinct signatures of exchange and intrinsic kurtosis with varying mixing time: exchange causes a slow dependence of the signal on mixing time while intrinsic kurtosis arguably has a much faster dependence. We find that applying $μ$MGE to data acquired with multiple mixing times for both parallel and orthogonal DDE may allow estimation of the exchange rate as well as the isotropic, anisotropic, and intrinsic kurtosis.

physics.med-ph↗

arXiv2023

Diffusion MRI with free gradient waveforms on a high-performance gradient system: Probing restriction and exchange in the human brain

The dependence of the diffusion MRI signal on the diffusion time carries signatures of restricted diffusion and exchange. Here we seek to highlight these signatures in the human brain by performing experiments using free gradient waveforms that are selectively sensitive to the two effects. We examine six healthy volunteers using both strong and ultra-strong gradients (80, 200 and 300 mT/m). In an experiment featuring a large set of gradient waveforms with different sensitivities to restricted diffusion and exchange (150 samples), our results reveal unique time-dependence signatures in grey and white matter, where the former is characterised by both restricted diffusion and exchange and the latter predominantly exhibits restricted diffusion. Furthermore, we show that gradient waveforms with independently varying sensitivities to restricted diffusion and exchange can be used to map exchange in the human brain. We consistently find that exchange in grey matter is at least twice as fast as in white matter, across all subjects and all gradient strengths. The shortest exchange times observed in this study were in the cerebellar cortex (115 ms). We also assess the feasibility of future clinical applications of the method used in this work, where we find that the grey-white matter exchange contrast obtained with a 25-minute 300 mT/m protocol is preserved by a 4-minute 300 mT/m and a 10-minute 80 mT/m protocol. Our work underlines the utility of free waveforms for detecting time-dependence signatures due to restricted diffusion and exchange in vivo, which may potentially serve as a tool for studying diseased tissue.

physics.med-ph↗

arXiv2021

Probing restricted diffusion and exchange using free gradient waveforms: validation by numerical simulations

Monitoring time-dependence with diffusion MRI yields observables sensitive to compartment sizes (restricted diffusion) and membrane permeability (water exchange). However, restricted diffusion and exchange have opposite effects on the diffusion-weighted signal, which can confound parameter estimates. In this work, we present a signal representation that captures the effects of both restricted diffusion and exchange up to second order in b-value and is compatible with gradient waveforms of arbitrary shape. The representation features mappings from a gradient waveform to two scalars that separately control the sensitivity to restriction and exchange. We demonstrate that these scalars span a two-dimensional space that can be used to choose waveforms that selectively probe restricted diffusion or exchange, in order to eliminate the correlation between the two phenomena. We found that waveforms with specific but unconventional shapes provide an advantage over conventional pulsed and oscillating gradient acquisitions. We also show that parametrisation of waveforms into a two-dimensional space can be used to understand protocols from other approaches that probe restricted diffusion and exchange. For example, we find that the variation of mixing time in filter-exchange imaging corresponds to variation of our exchange-weighting scalar at a fixed value of the restriction-weighting scalar. Numerical evaluation of the proposed signal representation using Monte Carlo simulations on a synthetic substrate showed that the theory is applicable to sizes in the range 2 - 7 micrometres and barrier-limited exchange in the range 0 - 20 s$^{-1}$. The presented theory constitutes a simple and intuitive description of how restricted diffusion and exchange influence the signal as well as how to design a protocol to separate the two effects.

physics.med-ph↗