Biomedical Engineering Reference
In-Depth Information
of electrical currents directly or indirectly on the MRI signal
(98)
and task-related structural changes which may lead to alteration
of diffusion of water molecules
(99)
. Both of these emerging
fMRI approaches have potential to improve temporal resolution
because the signals are not delayed by hemodynamics. How-
ever, the greater challenge is to enhance the magnitude of the
non-hemodynamic effects (
<
1%) which are much smaller than
>
hemodynamic-based fMRI (
3%).
5. Future
The brain is mapped by several dynamic imaging methods sen-
sitive to a variety of events at the glutamatergic nerve terminal
(
Fig. 1.1
). These methods span not only wide ranges of spatial
and temporal resolutions (
Fig. 1.2
) where each technique has
obvious limitations, but also definite advantages. No
one
method
can cover the several orders of magnitude in temporal and spa-
tial resolutions and at the
same
time capture the many cellular
and vascular events. Any such promise
(100)
discounts potential
reachable insights, even at present, gained by combining different
techniques which complement each other
(101)
. The recent trend
to combine fMRI
(102-105)
or optical imaging
(106-109)
with
electrophysiology is in accord with this suggestion. Since energy
is the currency of trade between cellular events and the substrates
that those activities demand, dynamic functional imaging with
energetics may be a crucial direction to pursue in the future
(79)
.
In functional imaging, the qualitative way by which the base-
line signal is conveniently differenced away to reveal stimulation-
induced focal area(s) of interest ignored the high energy utiliza-
tion in the resting brain
(6)
for a long time. Recent awareness
(6, 110)
of the high baseline activity, which slowly varies in time,
has been investigated by multi-modal methods like fMRI
(111)
,
optical imaging
(112)
, and electrophysiology
(113)
. The anatom-
ical
(1)
and physiological
(2)
complexities of the brain, both at
rest and during activation, reward an extraordinary set of dynamic
imaging
(2,3,79)
and analytical
(114, 115)
tools.
Acknowledgements
This work was supported, in part, by NIH grants from NIMH
(R01 MH-067528), NIDCD (R01 DC-003710), and NINDS
(P30 NS-52519).
