Nikos G. Papadakis

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BACKGROUND High angular resolution diffusion tensor imaging (HARD) is an MRI technique that exploits the mobility of water molecules to yield maps of structural order and directionality of white matter tracts with greater precision than six-direction diffusion tensor imaging (DTI) schemes. OBJECTIVE To assess whether HARD is more sensitive than(More)
Functional magnetic resonance imaging (fMRI) signal variations are based on a combination of changes in cerebral blood flow (CBF) and volume (CBV), and blood oxygenation. We investigated the relationship between these hemodynamic parameters in the rodent barrel cortex by performing fMRI concurrently with laser Doppler flowmetry (LDF) or optical imaging(More)
Despite recent advances in alternative brain imaging technologies, functional magnetic resonance imaging (fMRI) remains the workhorse for both medical diagnosis and primary research. Indeed, the number of research articles that utilise fMRI have continued to rise unabated since its conception in 1991, despite the limitation that recorded signals originate(More)
Healthy human brain diffusion anisotropy maps derived from standard spin echo diffusion tensor imaging (DTI) were compared with those using fluid-attenuated inversion recovery (FLAIR) preparation prior to DTI to null the signal from cerebrospinal fluid (CSF). Consistent comparisons entailed development of DTI postprocessing methods, image masking based on(More)
A parameter, chi2p, based on the fitting error was introduced as a measure of reliability of DT-MRI data, and its properties were investigated in simulations and human brain data. Its comparison with the classic chi2 revealed its sensitivity to both the goodness of fit and the pixel signal-to-noise-ratio (SNR), unlike the classic chi2, which is sensitive(More)
We used functional magnetic resonance imaging of healthy subjects to investigate the neural basis for spontaneous "willed" action. We hypothesised that such action involves prefrontal cortex (PFC) and supplementary motor area (SMA), in addition to primary motor cortex. Furthermore, we predicted that PFC and SMA would demonstrate similar temporal response(More)
This study compares laser Doppler flowmetry (LDF) and arterial spin labeling (ASL) for the measurement of functional changes in cerebral blood flow (CBF). The two methods were applied concurrently in a paradigm of electrical whisker stimulation in the anaesthetised rat. Multi-channel LDF was used, with each channel corresponding to different fiber(More)
An analysis method for diffusion tensor (DT) magnetic resonance imaging data is described, which, contrary to the standard method (multivariate fitting), does not require a specific functional model for diffusion-weighted (DW) signals. The method uses principal component analysis (PCA) under the assumption of a single fibre per pixel. PCA and the standard(More)
This paper describes a method for correcting eddy-current (EC)-induced distortions in diffusion-weighted echo-planar imaging (DW-EPI). First, reference measurements of EC fields within the EPI acquisition window are performed for DW gradient pulses applied separately along each physical axis of the gradient set and for a range of gradient amplitudes. EC(More)
This work describes the use of a method, based on the projection onto convex sets (POCS) algorithm, for reduction of the N/2 ghost in echo-planar imaging (EPI). In this method, ghosts outside the parent image are set to zero and a model k-space is obtained from the Fourier transform (FT) of the resulting image. The zeroth- and first-order phase corrections(More)