6 research outputs found
Biophysically motivated efficient estimation of the spatially isotropic R*2 component from a single gradientârecalled echo measurement
Purpose
To propose and validate an efficient method, based on a biophysically motivated signal model, for removing the orientationâdependent part of R*2 using a single gradientârecalled echo (GRE) measurement.
Methods
The proposed method utilized a temporal secondâorder approximation of the hollowâcylinderâfiber model, in which the parameter describing the linear signal decay corresponded to the orientationâindependent part of R*2. The estimated parameters were compared to the classical, monoâexponential decay model for R*2 in a sample of an ex vivo human optic chiasm (OC). The OC was measured at 16 distinct orientations relative to the external magnetic field using GRE at 7T. To show that the proposed signal model can remove the orientation dependence of R*2, it was compared to the established phenomenological method for separating R*2 into orientationâdependent and âindependent parts.
Results
Using the phenomenological method on the classical signal model, the wellâknown separation of R*2 into orientationâdependent and âindependent parts was verified. For the proposed model, no significant orientation dependence in the linear signal decay parameter was observed.
Conclusions
Since the proposed secondâorder model features orientationâdependent and âindependent components at distinct temporal orders, it can be used to remove the orientation dependence of R*2 using only a single GRE measurement
The Influence of Radio-Frequency Transmit Field Inhomogeneities on the Accuracy of G-ratio Weighted Imaging
G-ratio weighted imaging is a non-invasive, in-vivo MRI-based technique that aims at estimating an aggregated measure of relative myelination of axons across the entire brain white matter. The MR g-ratio and its constituents (axonal and myelin volume fraction) are more specific to the tissue microstructure than conventional MRI metrics targeting either the myelin or axonal compartment. To calculate the MR g-ratio, an MRI-based myelin-mapping technique is combined with an axon-sensitive MR technique (such as diffusion MRI). Correction for radio-frequency transmit (B1+) field inhomogeneities is crucial for myelin mapping techniques such as magnetization transfer saturation. Here we assessed the effect of B1+ correction on g-ratio weighted imaging. To this end, the B1+ field was measured and the B1+ corrected MR g-ratio was used as the reference in a Bland-Altman analysis. We found a substantial bias (â-89%) and error (â37%) relative to the dynamic range of g-ratio values in the white matter if the B1+ correction was not applied. Moreover, we tested the efficiency of a data-driven B1+ correction approach that was applied retrospectively without additional reference measurements. We found that it reduced the bias and error in the MR g-ratio by a factor of three. The data-driven correction is readily available in the open-source hMRI toolbox (www.hmri.info) which is embedded in the statistical parameter mapping (SPM) framework
Biophysically motivated efficient estimation of the spatially isotropic Râ2 component from a single gradient-recalled echo measurement
Purpose To propose and validate an efficient method, based on a biophysically motivated signal model, for removing the orientationâdependent part of Râ2 using a single gradientârecalled echo (GRE) measurement. Methods The proposed method utilized a temporal secondâorder approximation of the hollowâcylinderâfiber model, in which the parameter describing the linear signal decay corresponded to the orientationâindependent part of Râ2. The estimated parameters were compared to the classical, monoâexponential decay model for Râ2 in a sample of an ex vivo human optic chiasm (OC). The OC was measured at 16 distinct orientations relative to the external magnetic field using GRE at 7T. To show that the proposed signal model can remove the orientation dependence of Râ2, it was compared to the established phenomenological method for separating Râ2 into orientationâdependent and âindependent parts. Results Using the phenomenological method on the classical signal model, the wellâknown separation of Râ2 into orientationâdependent and âindependent parts was verified. For the proposed model, no significant orientation dependence in the linear signal decay parameter was observed. Conclusions Since the proposed secondâorder model features orientationâdependent and âindependent components at distinct temporal orders, it can be used to remove the orientation dependence of Râ2 using only a single GRE measurement
Fiber-orientation independent component of R(2)* obtained from single-orientation MRI measurements in simulations and a post-mortem human optic chiasm
The effective transverse relaxation rate (R(2)*) is sensitive to the microstructure of the human brain like the g-ratio which characterises the relative myelination of axons. However, the fibre-orientation dependence of R(2)* degrades its reproducibility and any microstructural derivative measure. To estimate its orientation-independent part (R(2,iso)*) from single multi-echo gradient-recalled-echo (meGRE) measurements at arbitrary orientations, a second-order polynomial in time model (hereafter M2) can be used. Its linear time-dependent parameter, β(1), can be biophysically related to R(2,iso)* when neglecting the myelin water (MW) signal in the hollow cylinder fibre model (HCFM). Here, we examined the performance of M2 using experimental and simulated data with variable g-ratio and fibre dispersion. We found that the fitted β(1) can estimate R(2,iso)* using meGRE with long maximum-echo time (TE(max)âââ54âms), but not accurately captures its microscopic dependence on the g-ratio (error 84%). We proposed a new heuristic expression for β(1) that reduced the error to 12% for ex vivo compartmental R(2) values. Using the new expression, we could estimate an MW fraction of 0.14 for fibres with negligible dispersion in a fixed human optic chiasm for the ex vivo compartmental R(2) values but not for the in vivo values. M2 and the HCFM-based simulations failed to explain the measured R(2)*-orientation-dependence around the magic angle for a typical in vivo meGRE protocol (with TE(max)âââ18âms). In conclusion, further validation and the development of movement-robust in vivo meGRE protocols with TE(max)âââ54âms are required before M2 can be used to estimate R(2,iso)* in subjects
Biophysically motivated efficient estimation of the spatially isotropic R*â component from a single gradientârecalled echo measurement
PURPOSE: To propose and validate an efficient method, based on a biophysically motivated signal model, for removing the orientationâdependent part of R*â using a single gradientârecalled echo (GRE) measurement. METHODS: The proposed method utilized a temporal secondâorder approximation of the hollowâcylinderâfiber model, in which the parameter describing the linear signal decay corresponded to the orientationâindependent part of R*â. The estimated parameters were compared to the classical, monoâexponential decay model for R*â in a sample of an ex vivo human optic chiasm (OC). The OC was measured at 16 distinct orientations relative to the external magnetic field using GRE at 7T. To show that the proposed signal model can remove the orientation dependence of R*â, it was compared to the established phenomenological method for separating R*â into orientationâdependent and âindependent parts