Magnetic resonance microscopy (MRM)
MRM is a non-invasive, non-destructive technology that can be used to probe optically opaque systems. By judicious design of magnetic resonance (MR) methodologies, this mode of visualization can be made sensitive to a number of phenomena including chemical composition and both coherent (velocity, acceleration) and incoherent molecular motion (diffusion, dispersion). While it is better known in the context of medical diagnosis, MR imaging has found application in fields ranging from Physics to Engineering and Geology – with applications as varied as microfluidics, catalysis, materials characterization, multiphase flows, fluidization, oil recovery, and drug delivery.
Multinuclear MR and multiphase systems
The extent to which chemically-specific information can be resolved in a magnetic resonance experiment depends on both the physical attributes of the system being studied and the nature of the particular NMR technique employed. Frequently, in a magnetic resonance imaging experiment, the 1H nucleus is observed, because it has the highest detection sensitivity of all the NMR-active nuclei. However, in the study of heterogeneous porous media, observation of the 1H nucleus has some distinct disadvantages. Although NMR signal is inherently both chemical and nucleus-specific, the frequency characteristics that allow individual resonances to be identified can easily become obscured by a number of effects. Here, a multinuclear approach to chemical mapping has been adopted as a means of improving the versatility of the MRM toolkit for the study of heterogeneous systems. Advantages of the multinuclear approach will be highlighted; these include: (i) improved frequency resolution via the inherent superior chemical shift dispersion of X-nuclei, (ii) reduced vulnerability to magnetic susceptibility-induced field gradients at interfaces, (iii) longer signal lifetimes, permitting observation of transport phenomena on longer time-scales (and hence, length-scales), and (iv) inherent or highly efficient solvent suppression in aqueous systems. The studies reported here focus primarily on the detection of 13C at natural abundance. However, the techniques presented are applicable to other spin-˝ nuclei (e.g. 31P, 15N).
Expanding the MRM toolkit to access new information about complex systems
Tailored MRM methods have been implemented to achieve the following.
Heteronuclear imaging methods for the study of 13C at natural abundance, more specifically: In situ, quantitative mapping of conversion and selectivity in heterogeneous catalytic reactors1-3.
Hybrid polarization transfer and pulsed-field gradient (PFG) methods for the study of species-specific diffusion and dispersion in multicomponent systems, including: Flow in packed beds4, mass transport in biofilm systems5, and characterization of emulsion structure6 with superior phase resolution and extended observation time-scales.
Characterization of anomalous diffusion as probed by MR, focusing on: The relevance of observation time-scale5, reconciliation of proposed physical models with the observed displacement dynamics, and a new approach to characterizing structure and improving image contrast by application of fractional calculus7.
Quantitative visualization of mixing processes in a microfluidic device, including: Concentration mapping in optically opaque microchannels of arbitrary cross-section and mapping of evolving flow fields8, 9.
References:
1. B. S. Akpa, M. D. Mantle, et al. In situ 13C DEPT-MRI as a tool to spatially resolve chemical conversion and selectivity of a heterogeneous catalytic reaction occurring in a fixed bed. Chemical Communications. 21, 2741 (2005)
2. L. F. Gladden, B. S. Akpa, et al. (2005). In situ reaction imaging in fixed-bed reactors using MRI. NMR Imaging in Chemical Engineering. S. Stapf and H. Song-I. Weinheim, Wiley-VCH: 590-606.
3. A. J. Sederman, M. D. Mantle, et al. In situ MRI study of 1-octene isomerisation and hydrogenation within a trickle-bed reactor. Catalysis Letters. 103, 1-8 (2005)
4. B. S. Akpa, D. J. Holland, et al. Enhanced C-13 PFG NMR for the study of hydrodynamic dispersion in porous media. Journal of Magnetic Resonance. 186, 160-165 (2007)
5. D. A. Graf von der Schulenburg, B. S. Akpa, et al. Non-invasive mass transfer measurements in complex biofilm-coated structures. Biotechnology and Bioengineering. Accepted Article, (2008)
6. B. S. Akpa, M. L. Johns, et al. (2008). Heteronuclear PFG-NMR methods for the sizing of emulsion droplets using natural abundance 13C. 49th Experimental NMR Conference, Asilomar, CA.
7. B. S. Akpa, O. Abdullah, et al. (2008). Anomalous diffusion expressed through fractional order differential operators in the Bloch-Torrey equation. Magnetic Resonance in Porous Media, Cambridge, MA.
8. B. S. Akpa, S. M. Matthews, et al. Study of miscible and immiscible flow in a microfluidic device using magnetic resonance imaging. Analytical Chemistry. 79, 6128-6134 (2007)
9. S. P. Sullivan, B. S. Akpa, et al. Simulation of miscible diffusive mixing in microchannels. Sensors and Actuators B. 123, 1142-1152 (2007)