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Início
Agenda |
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- 17 de junho de 2015
16h30
Sala
Celeste -
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Michael Garwood
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University of
Minnesota, USA
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Frequency-
and Gradient-Modulated MRI: Imaging with Extreme Field
Inhomogeneity
MRI has evolved into an incredibly powerful tool due to
impressive innovations over the past 3-4 decades. However, the
general approach used to obtain MR images has not deviated
significantly from Lauterbur’s original MRI experiment (1).
That is, in state-of-the-art MRI, the pulse sequence used to
generate spatially-encoded signals for imaging requires two
basic elements: a sufficiently short RF pulse to excite all
resonance frequencies of interest simultaneously, followed by a
finite time during which magnetization is allowed to evolve in
the presence of a field gradient to encode their spatial
locations. This universal approach to MR imaging is optimal only
under stringent experimental conditions. Specifically, frequency
variation due to field inhomogeneity and magnetic susceptibility
must be limited to a small fraction of the Larmor frequency,
γB0. Although recent advances have improved the ability of
Fourier-based imaging to tolerate B0 inhomogeneity, these
improvements fall short of meeting the technical requirements
for human imaging using small and portable magnets. For example,
MRI of the human brain cannot be performed with the subject’s
body from the shoulder down placed outside the magnet bore. No
MRI technology has ever been conceived to accomplish this; that
is, until possibly recently. Namely, spatiotemporal encoding
(2-7) has the capability to go far beyond standard Fourier
imaging in terms of tolerating B0 inhomogeneity, allowing B0 to
vary by a significant fraction of the nominal field.
Improved tolerance to B0 inhomogeneity with
spatiotemporal-encoded MRI has already been demonstrated by us
and others. However, these improvements were limited because B0
compensation was restricted to one or possibly two spatial
directions simultaneously. Now, due to the recent invention of
3D frequency-swept excitation pulses in STEREO (7), in
combination with parallel transmit “beam-steering”,
multi-coil shimming, and simultaneous transmit and receive
electronics, it should be possible to fully compensate extreme
B0 inhomogeneity and thus accomplish MR imaging of brain using a
small and portable head-only magnet. STEREO, which stands for
steering resonance over the object (7), produces non- Fourier
based images by moving a localized resonant region along a
curved trajectory in space such that the excitation itself is
spatially selective and temporally sequential. In STEREO, the
gradients are modulated to produce a “spatial rapid passage”.
By producing a sweeping 3D-selective excitation in time,
temporal adjustments to the pulse can be made to compensate for
the spatial variation of B0 and B1 as the “resonant region”
traverses through space. Simultaneous RF transmission and signal
detection preserves ultrashort-T2* signals. This method of
achieving spatial isolation is particularly novel in MR imaging
and opens opportunities for numerous novel applications that
could not be considered with standard Fourier-based techniques.
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