Biomedical Engineering Reference
In-Depth Information
its propensity to become (para)magnetic upon introduction into
a magnetic field. It is analogous to polarization of insulators in an
electric field. The BOLD effect is a consequence of a dynamic pro-
cess of “microscopic” magnetic susceptibility changes occurring
as the concentration of oxygen-rich blood varies during a func-
tional challenge
(5, 6)
. However, interfaces between substances
with different magnetic susceptibilities lead to “macroscopic” sig-
nal loss near the boundary between media. In the rodent, regions
with such interfaces leading to signal degeneration include the
inferior temporal lobes adjacent to air-filled ear canals, the cau-
dal occipital lobes in which the tissues separating the cortex from
the cerebellum appear in the imaging plane, and the olfactory
bulb embedded in bony and cartilaginous tissue surrounded by
mucous membranes.
“Shimming” is a process of applying small corrective mag-
netic field gradients onto the main static magnetic field in order
to optimize field homogeneity in and around the sample. It is
a procedure to reduce structurally imposed signal loss, mainly
at the media interfaces, by correcting for gross susceptibility-
induced magnetic field inhomogeneities. Since the correction of
“macroscopic” magnetic field inhomogeneities becomes increas-
ingly important at higher fields, some automatic routines have
been developed
(45, 124-126)
.
For high sensitivity fMRI studies, it is crucial that the surface
coil be as close to the brain as possible. Therefore, it seems obvi-
ous that the removal of the scalp (skin, galea aponeurotica) and
periosteum should improve the signal intensity from the brain
since the whole brain moves closer to the coil. However, this
approach will also move the air-tissue (bone) interface closer to
the tissue of interest (brain), which in turn may lead to field inho-
mogeneities beyond the capabilities of the shim gradients. We
avoid the scalp removal procedure whenever possible, focusing
instead on careful positioning of the animal and shimming.
There is a special shimming problem in the case of olfactory
bulb imaging. Since the nasal cavity is very close to the olfac-
tory bulb, changes in the level of mucous secretion will affect
the local susceptibility, and thereby the shimming. A stable phys-
iological condition of the nasal epithelium during odor delivery
experiments is required not only to prevent these adverse mag-
netic inhomogeneity effects on the measured signal, but also to
ensure the odorant molecules' access to the epithelial odorant
receptors. A common choice of anesthetic in rodent fMRI studies,
α
-chloralose, causes thick, mucoid oral-nasal discharge
(127)
.For
odor stimulations, we used urethane - a long lasting anesthetic
with excellent analgesic properties - the general use of which
though must be discouraged for survival studies because of its
carcinogenic and mutagenic effects. In addition to the choice of
anesthetic, we carefully controlled the temperature of the incom-
