Elizabeth A. Moore

Orientationally invariant indices of axon diameter and density from diffusion MRI

This paper proposes and tests a technique for imaging orientationally invariant indices of axon diameter and density in white matter using diffusion magnetic resonance imaging. Such indices potentially provide more specific markers of white matter microstructure than standard indices from diffusion tensor imaging. Orientational invariance allows for combination with tractography and presents new opportunities for mapping brain connectivity and quantifying disease processes. The technique uses a four-compartment tissue model combined with an optimized multishell high-angular-resolution pulsed-gradient-spin-echo acquisition. We test the method in simulation, on fixed monkey brains using a preclinical scanner and on live human brains using a clinical 3T scanner. The human data take about one hour to acquire. The simulation experiments show that both monkey and human protocols distinguish distributions of axon diameters that occur naturally in white matter. We compare the axon diameter index with the mean axon diameter weighted by axon volume. The index differs from this mean and is protocol dependent, but correlation is good for the monkey protocol and weaker, but discernible, for the human protocol where greater diffusivity and lower gradient strength limit sensitivity to only the largest axons. Maps of axon diameter and density indices from the monkey and human data in the corpus callosum and corticospinal tract reflect known trends from histology. The results show orientationally invariant sensitivity to natural axon diameter distributions for the first time with both specialist and clinical hardware. This demonstration motivates further refinement, validation, and evaluation of the precise nature of the indices and the influence of potential confounds.

British Association of MR Radiographers (BAMRR) Safety Survey 2005: Potential impact of European Union (EU) Physical Agents Directive (PAD) on Electromagnetic Fields (EMF)

To measure the potential impact on clinical MRI practice in the UK of the European Union (EU) Physical Agents Directive (PAD) on electromagnetic fields (EMF). There is evidence that the exposure limit values contained in the PAD will make it impossible for members of staff to stand close to the magnet during scanning; currently this is common practice in order to provide care and support for vulnerable patients. Interventional MR procedures will also be impossible.