Biomedical Engineering Reference
In-Depth Information
glucose oxidation, glutamate-GABA-glutamine cycling; see Refs.
(11,17,79)
for details. But this is a steady-state method where the
measured turnover times are in the order of about half an hour
(or slightly more). Recently, however, by dynamic nuclear polar-
ization (DNP), the MR detection sensitivity can be artificially
improved by orders of magnitude
(87)
. To date, this method
(using injection of hyperpolarized
13
Cor
15
N biomolecules) has
primarily been used to demonstrate perfusion through organs
with quite high temporal (within seconds) and spatial (below mm
range) resolutions. Although technical challenges remain
(88)
,
study of metabolism with
13
C and/or
19
F MRS in conjunction
with DNP is quite promising.
There are several hemodynamic-based functional MRI (fMRI)
methods that are sensitive to functional hyperemic events. The
blood oxygenation level dependent (BOLD) method
(89)
is the
most popular, in part, due to its simplicity. Both BOLD and
NIRS methods are sensitive to changing concentrations of oxyhe-
moglobin and deoxyhemoglobin (i.e., diamagnetic or red blood,
and paramagnetic or black blood, respectively). With NIRS,
both moieties are detected (by different absorption bands); with
BOLD, the magnetic properties of blood changing with oxy-
genation influences the tissue water signal (i.e., less paramagnetic
deoxyhemoglobin, higher MRI signal).
Just as hemoglobin is nature's endogenous MRI contrast
agent to measure blood oxygenation changes, blood-borne
exogenous MRI contrast agents are used to measure alterations
in blood volume with almost the same assumptions (i.e., more
paramagnetic agent, lower MRI signal)
(90)
. Principles of blood
flowmeasuredbyMRIaresimilartothatinPET.Following
magnetic labeling of arterial blood (water), subsequent dynamic
MRI maps reflect the degree to which the label has decayed
(by
4.3.2. MRI Methods
mixing
with
unlabeled
water),
thereby
reflecting
tissue
perfusion
(91)
.
Since its discovery, the biophysical understanding of BOLD
has improved significantly. Better specificity of the BOLD effect
at higher magnetic fields
(92)
allows changes in oxygen consump-
tion to be titrated out by combining the contributing changes in
blood flow and blood volume with the BOLD signal
(93)
.At
steady-state, the oxygen consumption predicted by BOLD cali-
bration has been shown to be in good agreement with oxygen
consumption measured by
13
CMRS
(94)
and which, in turn,
concurs with ensemble neuronal activity measured by extracellu-
lar recordings
(95)
. Since these multi-modal MRI methods can
be applied dynamically
(96)
, there is potential for transient ener-
getics with higher temporal resolution (200-500 ms)
(97)
.
Recently, there has been a surge in fMRI that is based on
non-hemodynamic events. Two prominent directions are effects
