AAWSAP DIRD, Technological Approaches to Controlling External Devices, March 2010

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EMG
Instead of measuring nerve impulses and amplifying them, one can design a system
that monitors muscle movement in a hands-free approach to a mechanical interface,
essentially using the muscles as a biological amplifier of neural signals. One may close
the feedback loop using a heads-up display, monocle, or other visual device, or another
traditional sensory feedback. The advantage of such a design is that controls may be
actuated through a lightweight wireless system, effectively providing control stations
anywhere such a wireless system would work. Such a system could be used to untie a
pilot from the cockpit. Recent work has shown that performance of 1-2 bits/s is possible
with minimal training, about four times the current performance of a comparison EEG
forehead sensor (Reference 14).
MRI AND FMRI
Magnetic Resonance Imaging (MRI) works by a simple excitation and relaxation of spin
states. When molecules containing hydrogen are placed in a strong static magnetic
field, a small but detectable number of hydrogen protons align their intrinsic spins along
the direction of the external field. An applied radiofrequency (RF) pulse near the proton
resonant frequency, 42.6 MHz/Tesla or 128 MHz at 3 Tesla, knocks the spins
perpendicular to the field and the relaxation back to ground state releases RF energy in
patterns that can be reconstructed to show both composition and distribution of any
hydrogen-rich materiel. The resonant frequency is a direct function of the local
magnetic field defined by the Larmor relation: c.o = y B; where c.o is the frequency of
precession, B is the local magnetic field, and y is a constant of the material, 42.6
MHz/Tesla for bare protons as mentioned above.
Small perturbations to the static field will change the resonant frequency. By applying a
small gradient to the static field, for example 100 milliTesla/meter along the z-axis, and
limiting the bandwidth of the RF excitation signal to 8ro, one may select a slice of the
brain perpendicular to the z-axis for excitation to 82. A change in the gradient field will
change the position of the excited slice for the next excitation. Similar gradients in the
x and y directions can limit the excitation to a single small volume of brain tissue. In
current MRis, these gradient fields are produced with electromagnets, and the series of
time-dependent imaging gradient manipulations is called the scan sequence.
Free hydrogen (H) would produce a resonant signal slightly different than the bare
proton due to the local field changes induced by its valance electron. Hydrogen gas (H2)
would produce a still different frequency since the local field around each proton is
altered by the two shared electrons. Water molecules (H2O) contain two hydrogen
atoms and an entirely different "electron shield" than either H or H2 and thus shows still
another slightly different resonant frequency. Fat and other lipid molecules, important
cell structure building blocks, have long chains of hydrocarbons, and the resulting
ensemble of electron screening produces a wide peak that is substantially shifted 10
from that of water.
Brain gray matter and white matter have different macroscopic lipid content and are
thus able to be differentiated in an MRI scan. Different signals also arise in bone,
1° Frequency detection sensitivities in MRI are very good, and "substantial" here means about 3 parts per million.
The frequency shifts caused by imaging gradients ranges in the parts per thousand.
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