Biomedical Engineering Reference
In-Depth Information
The third study was carried out for two purposes: First, to
confirm that the observed signal loss is predominantly due to
the intravoxel dephasing resulting from the spatially incoherent
displacement of the gel rather than to the bulk displacement of
the wire itself (as the wire does not generate any MR signal);
and second, to demonstrate the high temporal resolution of the
LEI technique. In this experiment, the current intensity was set
at 500
μ
A and the current pulses were delayed with respect to the
positive lobes of the oscillating gradients by 0-15 cycles, result-
ing in an overlap of 15-0 cycles between the two, respectively
(
Fig. 14.1
, bottom line). As such, the Lorentz force induced
displacement remained identical throughout the study, while the
amount of loss of phase coherence and resulting signal decay
due to the incoherent displacement was systematically varied. The
study was conducted on phantom A with all other parameters
identical to those used in the first study. The image acquired with
no overlap between the current and the oscillating gradients (in
which no signal change should occur) served as the reference,
and, as such, was acquired using five averages to increase the SNR.
While the above experiments demonstrate LEI effect in a wire
model, similar phenomenon is observed in ionic conductions in
our recent work
(26)
, with LEI effect further amplified by the sur-
rounding water molecules. This solution model of the LEI effect
may be better suited to simulate the neural conductions in vivo.
4. In vivo
Experiments
The concept of using synchronized oscillating gradients to
increase the signal detectability from electrical currents was
extended to in vivo experiments. To minimize potential con-
founds from hemodynamic modulations or physiological noise
commonly seen in brain activation studies and ensure a precise
timing control on the stimuli, these experiments were performed
in the human median nerve by using electrical stimulation of
the wrist to induce intrinsic sensory compound nerve action
potentials.
Electrical stimulation of the median nerve was accomplished
with a high-impedance electrical current stimulator (Grass S12;
Grass-Telefactor, West Warwick, RI, USA) and two gold-plated
disk electrodes secured on the ventral and dorsal sides of the right
wrist directly over the median nerve (Note: Identical results were
obtained when both electrodes were placed on the ventral side
of the wrist directly over the median nerve). The current was
delivered through the filtered penetration panel via shielded and
twisted cables, with the shield grounded to the panel. All electri-
cal switches were installed outside the magnet room, effectively
isolating the electrical stimulation in the magnet room and thus
