Biomedical Engineering Reference
In-Depth Information
by the oscillating gradients rather than the bulk displacement of
the nerve itself, since this displacement is present in both control
experiments, but is not synchronized with oscillating gradients. It
should thus be emphasized that the magnitude of the displace-
ment does not by itself determine the sensitivity of the tech-
nique, since, for a given displacement, the loss of phase coherence
can be independently amplified by using more, stronger, and/or
longer oscillating gradients, given sufficient SNR. Furthermore,
the fact that the loss of phase coherence can be amplified by the
oscillating gradients demonstrates that it is indeed caused by the
spatially incoherent displacement rather than the magnetic field
induced by the current. Finally, Experiments 3 and 4 also show
that there are no artifacts due to eddy currents induced in the
stimulation circuit by the oscillating gradients or electrical inter-
ference from the stimulation pulses, respectively, since no activa-
tion was detected when oscillating gradients or electrical pulses
were applied alone.
It can therefore be derived that Experiment 2 is equivalent to
Experiment 1 but with a one-cycle (i.e., 10 ms) temporal offset
between the electrical stimulation and the oscillating gradients,
since only electrical pulses that are synchronized with oscillating
gradients contribute to the observed activation. As such, the sig-
nificant difference between the results of Experiments 1 and 2
shows that our technique is sensitive to timing differences of the
stimulation paradigm on the order of milliseconds, thus demon-
strating its high temporal resolution.
Finally, to evaluate the test-retest reliability of our technique
for directly imaging neuroelectric activity in vivo in a healthy
median nerve, Experiment 1 was carried out in seven separate
sessions on the same subject under identical experimental con-
ditions to remove the dependence on subject and experimental
variability. Highly significant activation was consistently detected
along the median nerve in each session. The time course in the
activated regions averaged over the seven sessions (
Fig. 14.8e
)
highly resembles that obtained in a single session (
Fig. 14.8a
)
and shows a systematic signal decrease of (6. 3
2. 2)% during
the stimulation periods, confirming the consistency and robust-
ness of our technique.
±
6. Discussion
and Conclusions
The results presented here demonstrate the capability of the
LEI technique for imaging spatially incoherent yet temporally
synchronized electrical currents on the order of microamperes
with a temporal resolution on the order of milliseconds. Accord-
ingly, they provide a theoretical and experimental foundation
