Image Processing Reference
In-Depth Information
etc.) are usually necessary to prevent the development of nuisance currents
that can have direct contact with subject's scalp. Simulations show the
safe power range that should be used for some coil, power, or sensor
configurations to comply with FDA guidelines [58].
Another concern is the impact of EEG electrodes on the quality of MR
images. The introduction of EEG equipment into the scanner can potentially
disturb the homogeneity of the magnetic field and distort the resulting MR
images [44,59]. Recent investigations show that such artifacts can be effectively
avoided [60] by using specially designed EEG equipment [56], specialized
geometries, and new “MR-safe” materials (carbon fiber, plastic) for the leads.
To test the influence of a given EEG system on f MRI data, a comparison of the
data collected with versus without the EEG system being present, should be
conducted. Analysis of such data usually demonstrates the same activation pat-
terns in two conditions [59], although a general decrease in f MRI SNR is
observed when EEG is present in the magnet. A correction to the brain matter
conductivities (which are used for forward E/MEG modeling) for the Hall effect
finds the following first-order correction to be negligible:
σ
=.×
41 10 8
σ
for
H
B
=.
[]
15
T
[61].
8.3.2
E XPERIMENTAL D ESIGN L IMITATIONS
There are two ways of avoiding the difficulties associated with collecting EEG
data in the magnet: (1) collect EEG and MRI data separately or (2) use an
experimental paradigm that can work around the potential contamination between
the two modalities. The decision between these two alternatives will depend on
the constraints associated with research goals and methodology. For example, if
an experiment can be repeated more than once with a high degree of reliability
of the data, separate E/MEG and fMRI acquisition may be appropriate [62-65].
In cases when simultaneous measurements are essential for the experimental
objective (e.g., cognitive experiments in which a subject's state might influence
the results, as in monitoring of spontaneous activity or sleep-state changes), one
of the following protocols can be chosen:
Triggered fMRI: Detected EEG activity of interest (epileptic discharge,
etc.) triggers MRI acquisition [66-69]. Due to the slowness of the HR,
relevant changes in the BOLD signal can be registered 4 to 8 sec after
the event. The EEG signal can settle quickly after the end of the
previous MRI block [56], so it is acquired without artifacts caused by
RF pulses or gradient fields that are present only during the MRI
acquisition block. Note that ballistocardiographic and motion-caused
artifacts still can be present and will require postprocessing in order
to be eliminated. Although this is an elegant solution and has been
used with some success in the localization of epileptic seizures, this
Search WWH ::




Custom Search