Correlating brain structure with function using magnetic
resonance.
Magnetic resonance imaging (MRI) is a well known
and popular technique for non-invasive imaging of the
human body. Through use of a large magnetic field in
conjunction with pulses of radiowaves, MRI provides
high resolution images with spectacular clarity and
detail. A plethora of scan types are available on an
MRI system that can now allow visualization of brain
microstructure at sub-millimeter resolution with high
signal-to-noise ratio (SNR) and contrast-to-noise ratio
(CNR). A host of these specialized brain scans can
allow one to probe microvascular structure and tracts
of myelinated fibers. In addition sub-millimeter deposits
of iron and calcium can be observed, both of which
are linked to numerous brain pathologies. Diffusion
sensitized MRI, in combination with a post-processing
algorithm called tractography, can be used to visualize
brain microfiber connectivity. In parallel to development
of these advanced capabilities in structural imaging,
advances in the MRI system have also focused on the
probing of brain tissue function and metabolism. The
most popular and simplest example of this wide breadth
of applied MRI is functional MRI (fMRI), which probes
the increased oxy:deoxy haemoglobin ratio that results
when the brain is 'activated'. The fMRI technique allows
quantification and spatial localization of brain activation
during a host of simple to complex tasks. Furthermore,
in vivo nuclear magnetic resonance (NMR), better known
in the radiological world as magnetic resonance spectroscopy
(MRS) can also be performed using many types of MRI
systems. This technique allows one to directly probe
the biochemical and metabolic makeup of the brain.
Proton, or hydrogen (1H) MRS has become popular in
clinical brain scanning to aid in evaluating numerous
brain problems including hypoxia, tumour characterization,
and demyelination (to name only a few). In addition
this technique can be use to quantify some neurotransmitters
(e.g. GABA, glutamate, glutatmine), and even some pharmaceuticals
in brain tissue. Few realize, however, that this spectroscopic
method is not limited to probing only molecules with
1H nuclei. Any atomic nucleus having the quantum mechanical
property 'spin', which exists biologically at a large
enough and freely mobile concentration, can in theory
be imaged with multinuclear MRS. Some biologically
relevant nuclei include carbon (13C), fluorine (19F),
sodium (23Na), and phosphorous (31P). It should be
noted that these are all naturally occurring isotopes
and thus there is no need to inject any radioactivity.
With a host of available nuclei for MRS evaluation
of the brain the assessment of neurodegenerative diseases,
mood disorders, cancer therapies, and metabolic disorders,
to name a few, has become easier and safer. In
this talk I will present an overview of new structural
and MRS imaging technologies that allow the combined
assessment of both structure and function for the study
of healthy and diseased brain.