Biomedical Engineering Reference
In-Depth Information
did not take advantage of the strong main magnetic field and
high-power gradient systems of modern MRI scanners.
To boost the signal detectability, we propose a new MRI
technique termed Lorentz Effect Imaging (LEI)
(23, 24)
, which
uses magnetic field gradients to significantly amplify and detect
the Lorentz effect induced by spatially incoherent yet temporally
synchronized neural-range electrical activity in a strong magnetic
field. In the present work, we demonstrate its feasibility for imag-
ing electrical currents on the order of microamperes with a tem-
poral resolution on the order of milliseconds in gel phantoms and
in vivo
(24, 25)
.
2. Theory
The LEI technique relies on the well known Lorentz effect,
whereby a current-carrying conductor (or individual ions)
exposed to a magnetic field experiences a Lorentz force equal to
the cross product of the current vector (or electric charge) and the
magnetic field. If the conductor (or individual ions) is surrounded
by an elastic medium, this force induces a spatially incoherent dis-
placement of the elastic medium in adjacent regions, resulting in a
spatially incoherent displacement of the spins in these regions. In
the presence of a magnetic field gradient, these spins experience a
loss of phase coherence, which in turn results in a destructive sig-
nal summation within a voxel and, thus, a signal decay similar to
that seen in the transverse relaxation effect. This contrast mecha-
nism remains valid even for randomly oriented electrical current,
or ionic flows, within a voxel, as the resultant spatially incoherent
displacement of surrounding media (e.g. water molecules) still
leads to a destructive signal summation, as shown in our recent
manuscript
(26)
.
Since a magnetic field gradient also induces a loss of phase
coherence of the static spins, resulting in an unwanted signal
attenuation outside the regions of interest, balanced gradients
(with positive and negative lobes of the same amplitude and dura-
tion) need to be applied, so that the phase shifts experienced
by the static spins are rephased, as in diffusion-weighted imag-
ing. These gradients must then be synchronized with the current
such that it occurs only during either the positive or the nega-
tive lobe in order to preserve the phase shifts due to the Lorentz
force-induced displacement. Furthermore, multiple cycles of such
synchronized oscillating gradients can be used to greatly amplify
the loss of phase coherence due to the Lorentz effect, and there-
fore increase the sensitivity of the technique. In this work, we
use a gradient echo sequence with a series of oscillating gradients
applied between excitation and data acquisition (
Fig. 14.1
).
