5th EuropEan MolEcular IMagIng MEEtIng - ESMI

5th EuropEan MolEcular IMagIng MEEtIng – EMIM2010
Magnetic resonance imaging – from structure to molecular interactions
Rudin M. .
ETH and University of Zürich, Switzerland
rudin@biomed.ee.ethz.ch
Magnetic resonance imaging (MRI) yields images
characterized by high spatial resolution and
unsurpassed soft-tissue contrast. The latter is
the consequence of the fact that the MRI signals
depends on multiple parameters such a various
relaxation rates, water diffusivity, proton exchange
reactions, which are tissue specific. By choice of
experimental parameters contrast can be varied to
highlight structures of interest. Paramagnetic or
superparamagnetic contrast agents can be used to
further enhance specific structures. It is therefore
not surprising that today MRI has become one of the
most important modalities for structural imaging.
Yet MRI provides information beyond mere anatomy.
The analysis of dynamic signal changes - e.g. during
administration of a contrast agent - yields relevant
quantitative information on physiological processes
such as blood flow, vascular permeability, or renal
function. Probably the most important physiological
MRI application is functional MRI (fMRI), which
has become an indispensable tool for studying
brain function, or more precisely the hemodynamic
response to neuronal activity, under normal and
pathological conditions. fMRI measures local
hemodynamic changes in the CNS that are linked to
neural activity through neurovascular coupling.
More recently, MRI has tapped into the field of
molecular imaging. By coupling the MRI contrast
agent to a targeting moiety, molecular information
can be derived at high spatial resolution. Yet the
MRI molecular imaging approach suffers from two
limitations. MRI is insensitive due to the small
quantum energy involved in the process, typically
concentrations have to be in the micromolar range
to induced chances detectable by MRI. This can be
in part be accounted for by increasing the relaxivity
of MRI reporters by increase of the payload or by
using physiological amplification. The second
limitation is that MRI contrast agents are in
general bulky, which renders target accessibility
difficult. Straightforward molecular targets that
can be reached by MRI contrast agents are those
expressed on the endovascular side. On the other
hand the efficiency of MRI probes, similar to that
of fluorescent probes, can be modulated through
the molecular interaction with the target. This
potentially allows discriminating target-bound from
free floating probe, thereby enhancing the contrastto-background
ratio. An attractive application of
targeted MRI imaging are studies of cell trafficking.
As many cells are rather tolerant to the amount of
contrast agent they can carry, sensitivity appears
to be less an issue. In fact there are some reports,
that under favorable conditions single cells might be
detected.
The lecture will discuss the various aspects of MRI with
focus on molecular and cellular imaging applications.
EuropEan SocIEty for MolEcular IMagIng – ESMI
day1
Parallel Session 3: TECHNOLOGY