Steven Delputte

The design of anisotropic diffusion phantoms for the validation of diffusion weighted magnetic resonance imaging

Diffusion weighted magnetic resonance imaging offers a non-invasive tool to explore the three-dimensional structure of brain white matter in clinical practice. Anisotropic diffusion hardware phantoms are useful for the quantitative validation of this technique. This study provides guidelines on how to manufacture anisotropic fibre phantoms in a reproducible way and which fibre material to choose to obtain a good quality of the diffusion weighted images. Several fibre materials are compared regarding their effect on the diffusion MR measurements of the water molecules inside the phantoms. The diffusion anisotropy influencing material properties are the fibre density and diameter, while the fibre surface relaxivity and magnetic susceptibility determine the signal-to-noise ratio. The effect on the T(2)-relaxation time of water in the phantoms has been modelled and the diffusion behaviour inside the fibre phantoms has been quantitatively evaluated using Monte Carlo random walk simulations.

Absolute quantification of carnosine in human calf muscle by proton magnetic resonance spectroscopy

Carnosine has been shown to be present in the skeletal muscle and in the brain of a variety of animals and humans. Despite the various physiological functions assigned to this metabolite, its exact role remains unclear. It has been suggested that carnosine plays a role in buffering in the intracellular physiological pHi range in skeletal muscle as a result of accepting hydrogen ions released in the development of fatigue during intensive exercise. It is thus postulated that the concentration of carnosine is an indicator for the extent of the buffering capacity. However, the determination of the concentration of this metabolite has only been performed by means of muscle biopsy, which is an invasive procedure. In this paper, we utilized proton magnetic resonance spectroscopy (1H MRS) in order to perform absolute quantification of carnosine in vivo non-invasively. The method was verified by phantom experiments and in vivo measurements in the calf muscles of athletes and untrained volunteers. The measured mean concentrations in the soleus and the gastrocnemius muscles were found to be 2.81 +/- 0.57/4.8 +/- 1.59 mM (mean +/- SD) for athletes and 2.58 +/- 0.65/3.3 +/- 0.32 mM for untrained volunteers, respectively. These values are in agreement with previously reported biopsy-based results. Our results suggest that 1H MRS can provide an alternative method for non-invasively determining carnosine concentration in human calf muscle in vivo.

POSTPROCESSING OF BRAIN WHITE MATTER FIBER ORIENTATION DISTRIBUTION FUNCTIONS

By acquiring high angular resolution diffusion weighted magnetic resonance images (HARDI), Q-ball analysis can disclose the 3D organization of fibrous tissue such as the brain white matter. The resulting orientation distribution functions (ODFs) typically contain a lot of noise. Therefore we here present an ODF noise filtering technique as well as a regularization algorithm. In order to facilitate efficient fiber tractography (the 3D reconstruction of axonal fasciculi) a data reduction scheme was developed that extracts the most important fiber directions together with an estimate of their uncertainty as well as 2 new scale invariant white matter characteristics. We also present a new framework, with proof of concept, for fully ODF-based local clustering of major fiber tracts, based on matching neighbouring voxels using symmetric criteria.

Dipole estimation errors in EEG source localization due to not incorporating anisotropic conductivities of white matter in realistic head models

The electroencephalogram (EEG) is a useful tool in the diagnosis of epilepsy. EEG source localization can provide neurologists with an estimation of the epileptogenic zone. Many EEG source localization approaches assume head models with isotropic conductivity, while in reality the conductivity of white matter is anisotropic. The conductivity along the nerve bundle is higher than the conductivity perpendicular to the nerve bundle. Using diffusion weighted magnetic resonance images (DW-MRI), we can determine the directions the anisotropic diffusion. Using the latter we can derive the anisotropic conductivity tensor. These anisotropic conductivities can be fused with the realistic head model, derived from MR images. Using a grid of dipoles, placed in white and grey matter regions, we can compare the head model with white matter anisotropy with a head model with isotropic conductivity for the white matter compartment. As quantification measures we used the dipole location and orientation error. Results show that the location error was very small in both white and grey matter regions (<5 mm). The dipole orientation error had a mean of 3.8 degrees and 6.1 degrees in grey and white matter regions. This would indicate that the systematical error due to not incorporating anisotropic conductivities of white matter is very small.

SIMULATION OF THE DIFFUSION IN THE INTERSTITIAL SPACE OF A FIBER PHANTOM

The diffusion in the extra cellular space of brain white matter in vivo can be considered as the analogue of diffusion in porous media and can be studied ex vivo by fiber phantoms with diffusion weighted magnetic resonance imaging. Based on phantom studies and Monte Carlo simulations, the diffusion properties in the interstitial space were studied in the short and long time limit, whereby the effect of the fiber geometry and density was evaluated.