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Abstract:
Introduction: Since high SNR is necessary for high-resolution FMRI it is
advantageous to place the coils close against the monkey’s head whenever
possible. To this end, we designed a flexible 4-channel receive-only phased
array that can be used on monkeys of different sizes as well as for different
cortical areas. In addition, the preamplifiers including control electronics are detachable and can serve a variety of prefabricated and phase-matched fourelement
arrays of different configurations.
Methods: A linear array of 4 circular coils of ~23 mm diameter with gaps of
~11 mm was sutured onto a soft plastic strip (Figure 1). The assembly having
in-line connectors was attached to phase-matched coaxial cables to modified
commercial high reflection coefficient preamplifiers (Stark Contrast
Inc., Erlangen, Germany) via cable traps. Detuning of the individual coils
during RF transmission was achieved using DC currents within the coaxial
cables and pin diode controlled notch filters within the array elements.
Experiments were done on anesthetized monkeys on a vertical 4.7T Bruker
Biospec running ParaVision 5. The array was positioned over the occipital
pole. RF transmission was done with a de-tunable ‘type D’ partial volume
coil. We obtained high-resolution FLASH (Figure 2) and FMRI data using
EPI with and without acceleration (GRAPPA). The stimulus was a full-field
rotating checkerboard. FLASH: resolution 100x100x1000 μm, TE 23 ms, TR
2000 ms, 1 average; FMRI: GE-EPI, resolution 500x500x2000 μm, TE 21 ms,
TR 750 ms, 8 segments with R = 1, or 4 segments with R = 2.
Results: The high-resolution FLASH anatomical images show intracortical
veins and the Gennari line in entire V1, including peripheral V1, which is located
deep in the brain. Using a dual-coil setup, it is only possible to observe
these features in parts of V1 that are close to the surface. The functional map
proofs that the increased coverage allows us to obtain activation in V1-V5 in
both hemispheres at high resolution (Figure 3). SNR was sufficient to also
allow accelerated FMRI at the same resolution.
Discussion: Compared to a dual-coil setup the phased array provides improved
SNR and coverage, which allows for high resolution anatomical imaging
and FMRI of the entire early visual cortex, including better performance
in deep brain areas.
