Andrew M. Kiruluta
Department of Physics
tel: (617) 384-8138 fax: (617) 724-3338
Neuronal Currents: The ability to detect neuronal magnetic fields directly using MRI would help investigators achieve the “holy grail” of neuroimaging, namely both high spatial and temporal resolution. This would be a significant improvement over the current leading neuroimaging techniques such as functional MRI (fMRI), which is indirect, has low temporal resolution, and magnetoencphalography (MEG) which offers no unique localization of activity. We are working on new approaches for magnifying these currents and their associated magnetic fields in-vivo.
Diffusion: High-resolution diffusion spin tagging to study transport phenomena in porous media with complex flow, and to ‘inverse-image’ the underlying confining geometry to determine compartment sizes and their related apparent diffusion coefficients in the presence of homonuclear scalar couplings. A related current area of research interest is diffusion tensor imaging as applied to mapping neural interconnectivity in the brain.
Traveling Wave MRI: At ultra high field strength ($>$4T), the excitation wavelength relative to the dimension of the human body leads to significant B_1 inhomogeneity. The dielectric constant of tissue with a high water content can be as high as 70, leading to a wavelength inside tissue of less than 15 cm at 7T, which is commensurate or less than most body parts. The excitation field is thus spatio-temporal and in conjunction with the increase in tissue conductivity. We find that in the propagation of radiation at ultra high fields, new phenomena commonly observed in quantum optics but traditionally negligible in NMR, such as spatio-temporal phase modulation of the excitation field such that the identity between pulse area and flip angle is no longer valid. It is shown that in addition to the well-studied dielectric resonance phenomena at high magnetic fields, field propagation effects transform the excitation pulse into an adiabatic excitation. The high field strength also means that nonlinear effects such as self-induced transparency, propagation phenomena such as transient four wave mixing, are now possible in NMR experiments. Additional constraints due to phase matching considerations are imposed on the formalism of echo formation in high field NMR. The concept of a directional echo introduces new phenomena in MR but common in quantum optics such as holography, wave mixing and self-induced transparency.
NMR Holography: Since its inception, both MRI and NMR have operated in the long wavelength regime where far field electrodynamic effects are negligible. With the use of high field strength imagers (>4T), on large samples such as a human head, and coupled with the high dielectric constant of tissue (>50), the excitation wavelength becomes on the order of or smaller than the imaging sample. Under these conditions, new phenomena previously unknown in MR such as holography, arising from the spatial and spectral interference of the traveling excitation fields become possible. The spectral interference (grating) component has always been part of conventional MR. It arises naturally from either the intrinsic chemical shift anisotropy of the spin system or the field inhomogeneity due to the applied spatial encoding gradients. The spatial grating component is new and arises from the emergence of the propagating wave vector with traveling wave excitation fields. Spatial-spectral holographic properties of storage, programmable time-delay, phase conjugation or time-reversal and Bragg selectivity are experimentally demonstrated for the first time in an MR sample. These ideas are shown to be extendable to complex holographic signal processing functions such as recognition, correlations and triple products. This approach has potential for new spatial localization techniques using quasi-optical techniques for focusing excitation fields, slice selection through volume Bragg selectivity, phase conjugate distortion free imaging as well as higher resolution encoding limited only by the spacing of spatial interference fringes and T2.
Grant, P.E., Pienaar, R., Bolar, D.S., Kiruluta, A.M. and Alsop, D.C., “Endogenous Whole Brain Blood Flow (Arterial Spin Labeling) MRI in the Neonatal Brain Injury”, RSNA Nov (2006).
Kiruluta, A.M., “Field Propagation Phenomena in Ultra High Field NMR: A Maxwell-Bloch Formulation”, Proc. Intl. Soc. Mag Reson. 16 (2007).
Kiruluta, A.M., “A Spectral Filtering View of Diffusion Gradient Encoding”, Proc. Intl. Soc. Mag Reson. 16 (2008).
Kiruluta, A.M. and Qasmieh, I.A., “Diffusion at Short Time Scales: q-space Imaging with Chirped Gradient Waveforms”, Proc. Intl. Soc. Mag Reson. 16 (2008).
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