Biomedical Engineering Reference
In-Depth Information
neuronal activity accompanied by an excessive decrease in CBF.
This causes reduced net oxygen levels and therefore a decrease
in pO
2
andinS(
Fig. 9.2C
)
(58, 59)
. However, as has recently
been shown in some brain regions during intense seizure activity,
it is also possible to observe BOLD fMRI decreases in the pres-
ence of increased
and CMRO2 if CBF does not change, or does
not increase sufficiently to meet metabolic demands, leading to
a decrease in pO
2
andinS(
Fig. 9.2D
)
(60, 61)
. Similarly, it is
possible for fMRI signals to decrease with little or
no change
in
neuronal activity if CBF is abnormally reduced (e.g., by vascular
steal or vasospasm).
Thus, fMRI signal change alone is not adequate to deter-
mine the underlying changes in neuronal activity in the imaged
system. While the relationship between the BOLD signal and
the underlying hemodynamics is being studied and characterized,
it is generally true, under
normal
physiological conditions, that
fMRI increases reflect increased neuronal activity and that fMRI
decreases reflect decreased neuronal activity
(56, 59)
. However,
additional study of these relationships is necessary to fully under-
stand these relationships under abnormal circumstances such
as those seen in epilepsy
(14, 15, 61, 62)
. Possible relationships
between fMRI decreases and neuronal activity during SWD will
be more fully discussed towards the end.
ν
5. Human fMRI
Studies of
Spike-Wave
Attempts to identify BOLD changes in humans associated with
SWD have yielded fascinating, though confusing, and sometimes
contradictory results. Human studies to date have investigated
patients with a broad spectrum of epileptic disorders, ages, and
treatments. These studies have attempted to localize those brain
regions associated with SWD while teasing apart the signifi-
cance and meaning of the BOLD signal increases and decreases
observed in their analyses. There remain many important unan-
swered questions regarding both human SWD as well as the phys-
iological and neurological underpinnings of the BOLD signal.
More extensive experimentation, in both human and animal mod-
els, is required. We will now discuss BOLD fMRI changes in spe-
cific anatomical regions during human SWD.
5.1. Cortical BOLD
Changes
The importance of the cortex in SWD generation and mainte-
nance has been widely shown in animal models
(2,18,23)
. Human
fMRI experiments have reported cortical BOLD signal change
to be predominately negative during SWD although positive
changes have also been documented. Negative BOLD changes in
the cortex during SWD have drawn the most intense scrutiny and
