Biomedical Engineering Reference
In-Depth Information
3.1. Non-fMRIStudies
Neuroimaging data of SWDs in the pre-fMRI era showed great
variability. Global increases in cerebral metabolism or blood flow
in human patients have been reported
(24, 29-32)
using sin-
gle photon emission computed tomography (SPECT) studies or
positron emission tomography (PET). At the same time, other
PET,
133
Xe clearance, transcranial Doppler, and near-infrared
spectroscopy (NIRS) studies of blood flow and metabolism have
shown an absence of change, focal change, and generalized
increases, decreases, and biphasic changes
(29, 33-43)
. One limi-
tation of these studies is that the time resolution of Tc99m single
photon emission computed tomography is approximately 30 s,
the time resolution of fluoro-2-deoxy-D-glucose PET is approxi-
mately 30 min and the time resolution of
133
Xe clearance is a few
minutes. These modalities may have trouble capturing the tran-
sient metabolic changes of absence seizures, which typically last
less than 10 seconds. SPECT, PET, and
133
Xe are likely to inte-
grate changes before, during, and after SWD episodes. Transcra-
nial Doppler and NIRS have higher time resolution, but these
methods lack sufficient spatial resolution. Animal studies have
shown similarly confusing results (e.g. increased metabolism and
decreased CBF during SWD in the same model)
(37, 44)
.
3.2. EEG-fMRI
In order to capture adequately the dynamic neuroenergetic
changes during brief absence seizures (less than 10 s), an imag-
ing modality must have two characteristics. First, simultaneous
EEG must be taken so that it is possible to distinguish interic-
tal (non-seizure) and ictal (seizure) images. Second, the modal-
ity must have sufficient temporal resolution to capture individual
seizure or SWD events while also having sufficient spatial resolu-
tion to distinguish brain regions. Using the above criteria, fMRI
is the most effective imaging modality currently in use to cap-
ture the complex dynamic changes in energy metabolisms and
blood flow of SWDs. Experiments utilizing fMRI and simultane-
ous EEG-fMRI
(45-47)
have begun to explore the relationship
between electrophysiology and neuroimaging changes associated
with SWDs in human and animal models .
4. Relation
Between fMRI and
Neuronal Activity
During
Spike-Wave
Our interest in reviewing fMRI studies of SWD reflects
(1)
a belief
that fMRI is the best modality to understand the neural activity
underlying SWD and
(2)
a desire to understand the metabolic
implications of the BOLD signal. “Neuronal activity” (
), which
includes presynaptic and postsynaptic membrane voltage changes
associated with neural signaling, consumes energy (ATP). A large
ν
