Biomedical Engineering Reference
In-Depth Information
removing any electrical interference with the MRI signal. The
stimuli consisted of a series of biphasic rectangular current pulses
with a duration of 1 ms and an amplitude ranging from 1.8 to 2.7
mA, which was set prior to each session to be just below the motor
threshold for finger movement in order to avoid motion arti-
facts (electromyography measurements were performed for con-
firmation). The stimulator was triggered by the MRI scanner to
ensure an accurate synchronization between the electrical stimu-
lation and the pulse sequence.
The studies were performed on the same 4 T whole-body
MRI scanner as used for the phantom experiments. All images
were acquired by using a surface coil designed in-house and a gra-
dient echo single-shot spiral imaging sequence with the following
parameters: Repetition time 2,000 ms, TE 35.5 ms, flip angle 80
◦
,
field-of-view 20 cm, matrix size 64
64, and three contiguous
axial slices (sagittal with respect to the forearm), which were each
15 mm thick and centered on the median nerve to ensure that the
section of the nerve between the wrist and the elbow would be
fully contained within one slice. A series of oscillating gradients
with an amplitude of 36 mT/m and a duration of 5 ms for each
lobe was applied along both axes orthogonal to the main mag-
netic field. Because the conduction time of the sensory activation
in the human median nerve between the wrist and the elbow is
about 4 ms
(28)
, this duration of 5 ms was chosen to ensure that
sensory nerve action potentials could propagate in that section of
the nerve within one lobe. High-resolution T
1
-weighted images
were also acquired at the same location for anatomical reference.
Four experiments were performed to examine the signal
changes during stimulation. To ensure sufficient loss of phase
coherence while limiting the
T
2
decay, a maximum of three cycles
of gradient oscillations and three synchronized electrical pulses
was used for the first experiment (Exp. 1,
Fig. 14.3a)
. To estab-
lish a graded effect, a second experiment was carried out with only
two cycles of gradient oscillations and two synchronized electri-
cal pulses (Exp. 2,
Fig. 14.3b)
. In addition, two control experi-
ments were performed that were identical to Exp. 1 but with the
electrical pulses delayed by 50 ms with respect to the oscillating
gradients (Exp. 3,
Fig. 14.3c
) or without oscillating gradients
(Exp. 4,
Fig. 14.3d)
.
The activation paradigm was a block design consisting of
seven alternating rest and stimulation periods, each lasting 20 sec-
onds, during which ten image volumes were acquired. During the
stimulation periods, electrical pulses were triggered to excite the
median nerve, whereas during the rest periods, no stimulation
was applied. Four runs were acquired for each experiment and
averaged to increase the SNR.
After each run, subject bulk motion was assessed by com-
puting the displacement of the image center-of-mass over time
slice-by-slice, and runs with an in-plane displacement exceeding
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