Biomedical Engineering Reference
In-Depth Information
device is usually standardized with suspensions (e.g., latex spheres
undergoing Brownian motion), it is a relative method and can-
not easily provide absolute units of blood flow. LDF is a widely
used technique in laboratories because of its minimal invasive-
ness and relatively high sensitivity for stimulation-induced blood
flow/volume changes
(76)
. LDF has found popular applications
in the clinical environment (e.g., neonatal care
(77)
) because it is
quite simple to use. The LDF signal changes can be quite large
(10-100%) and peak faster than NIRS signals (within 5 s) after
stimulus onset.
4.3. Magnetic
Resonance
Magnetic resonance (MR) is based upon the quantum mechan-
ical properties of an atom's nucleus
(78)
. Nuclei that contain
odd numbers of protons or neutrons have an intrinsic mag-
netic moment. Some MR visible and biologically relevant non-
radioactive isotopes are
1
H,
13
C,
15
N,
17
O,
19
F,
23
Na, and
31
P.
Sensitivity of MR detection for a given isotope depends on the
physical intrinsic property of the isotope (i.e., the gyromagnetic
ratio) and the abundance of the isotope prescribed by nature (can
be enhanced by enrichment). The most sensitive and weak iso-
topes that are commonly used for in vivo MR experiments are
1
H
and
13
C, respectively.
1
H has four times stronger intrinsic sensi-
13
C. Also
1
His
tivity than
91% more naturally abundant than
13
C. Together, this makes
1
H orders of magnitude more sensitive
than
13
C.
MR imaging (MRI) is primarily used in diagnostic imaging
to map structure, but lately also function. MRI is predominantly
based on
1
H signal from water. MRI data have good spatial reso-
lution and the temporal resolution depends on the specific type of
function being measured (see below). MR spectroscopy (MRS) is
a powerful technique used to obtain rich chemical information
about a wide range of biomolecules (other than water) based
on their variable chemical shifts. MRS detection of an isotope
depends on the experimental purpose (see below). MRS data can
also be viewed in two or three dimensions but the images do not
appear as crisp as MRI data because the MRS voxels are much
larger. The lower spatial resolution of MRS (mm to cm) compared
to MRI (
∼
m to mm) is mainly because of the orders of magnitude
of lower concentration of biomolecules (mM range) compared to
water (
μ
>
10 M) being detected. Since a wide range of MR meth-
ods can be used for neurobiological studies
(79)
, the few that
meet the criteria of dynamic functional imaging in
Figure 1.2
are discussed.
In the past,
31
P MRS had been mainly used to detect high
energy phosphates (e.g., ATP, ADP) which lead to assessment of
tissue pH
(80)
. Since these signals do not change significantly
with modest functional challenges,
4.3.1. MRS Methods
31
P MRS found little use in
