Evgeniya Kirilina1,2, Fakhereh Movahedian Attar1, Luke J. Edwards1, Kerrin J. Pine1, and Nikolaus Weiskopf1
1Department of Neurophysics, Max Planck Institute for Human Cognitive and Brain Sciences, Leipzig, Germany, 2Center for Cognitive Neuroscience Berlin, Free University Berlin, Berlin, Germany
Synopsis
Information about intracortical fibers and connectivity
can potentially be obtained using diffusion weighted imaging (DWI). However, in
vivo intracortical DWI requires extraordinarily high spatial resolution. We demonstrate
in vivo DWI imaging in the human occipital cortex with an isotropic resolution
of 800 μm enabled by a high-performance
300 mT/m gradient system and flexible high-sensitivity RF receive coil optimized
for cortical imaging. Robust
detection of intracortical features was achieved in a reasonable scanning time.
The described setup opens the exciting possibility to study intracortical
connectomics in humans in vivo.
Introduction
Intracortical connectivity is key to brain
function and is strongly supported by the intracortical radial and tangential
fibers. Radial fibers are primary efferent/afferent axons leaving/entering the
cortex that connect cortical areas with distant areas. Tangential fibers
facilitate local cortico-cortical connections, e.g., include collaterals. Diffusion
weighted imaging (DWI) has the potential to provide unique information about intracortical
fibers in humans as has been shown in post mortem tissue1–3. However, reliable in vivo mapping
of intracortical fibers remains an unsolved challenge4. Human cortex has a
thickness of only a few millimetres and a much lower fiber density than white
matter. Therefore, intracortical DWI requires submillimeter resolution5, high sensitivity and still
moderate diffusion weightings6. Here we combined the high-performance
300 mT/m gradient system with a high-sensitivity flexible RF receive coil7. We demonstrate the feasibility of high
quality DWI of occipital cortex with an isotropic resolution of 800 μm within a reasonable scan time.
Methods
Three participants (2 females, age 25-28 years) took part in a scanning
session on a Connectom scanner (3T, Siemens Healthineers, Erlangen, Germany)
equipped with a flexible RF receive coil placed tightly around the
participant’s back of the head7 (Fig. 1). The coil
consists of 23 loop elements with a diameter of 45 mm providing high
sensitivity near the elements, i.e., in the cortex. DWI were acquired with a single-shot 2D echo
planar imaging (EPI) sequence8–10: 800 μm isotropic resolution, 60 isotropically
distributed diffusion directions, ten b=0 images, two shells with b=800,1800
s/mm2, TE=66 ms, TR=8700 ms, bandwidth BW= 1200 Hz/Px, Field-of-View
(FOV) 206 mm x 84.5 mm, matrix 256 x 106, 62 oblique nearly axial slices, Partial
Fourier 5/8. A saturation band anterior to the occipital lobe was applied to minimize
potential fold-over artefacts due to the limited FOV. Two repetitions, each 20
min long, were acquired. In addition, a series of 23 b=0 images
was acquired with both the 23-channel surface flex coil and with the manufacturer’s
32 channel head coil in order to estimate image SNR enhancement provided by the
flex coil. Sensitivities of both coils were compared by estimating the temporal
Signal-To-Noise (tSNR) ratio across the b=0 images. tSNR was used as a proxy
for coil sensitivity, since at the resolution of 800 μm thermal noise is expected to dominate.
Data were analysed using the ExploreDTI11 package for diffusion analysis.
Images were corrected for participant motion and eddy current induced
distortions by co-registration to b=0 images. The diffusion tensors were
estimated using both shells and a robust tensor estimation algorithm12.
Results and Discussion
Maps of tSNR for b=0 images recorded with both
coils are shown in Fig. 2. The receive sensitivity of the flex coil was higher
than of the head coil in superficial brain regions (up to 20 mm depth from the skull
inner surface; ROI1: averaged tSNR 15±4 for flex coil and 8±2.5 for head coil), but decayed rapidly towards the center of the brain
(ROI2: average tSNR 3.6±1.3 for flex coil and 4.6±1.4 for head coil).
Results of DTI analysis for one representative
participant are shown in Figs. 2 and 3 for axial and coronal slices through the
occipital cortex, respectively. White matter tracts such as the optical
radiation were robustly detected. In addition, in the cortex the direction of the
principal eigenvectors corresponded to the cortical normal in many regions (see
inserts a,b,c in Figs. 3 and 4). This is in line with the expected orientation
of intracortical radial fibers, which dominates the DTI direction in deep
cortical layers1. The high acquisition resolution potentially
allows for cortical-depth specific analysis, as can be seen in Fig. 3a. Future
studies will apply advanced diffusion modelling that more appropriately capture
the complex intracortical fiber distributions than DTI.
Conclusions
We demonstrated that combining a high
performance gradient system and high sensitivity receive coil makes in vivo DWI
acquisition at 800 μm resolution feasible in a realistic scan time of 40 min. DTI results
were in line with dominant radial intracortical fibers in the occipital cortex.
This novel hardware setup can be readily combined with advanced acquisition
schemes
5, further improving the resolution
and precision.
The setup described here opens the exciting
possibility of studying intracortical connectomics in humans in vivo.
Acknowledgements
The
implementation of diffusion weighted single-shot 2D EPI sequence was received
from the University of Minnesota Center for Magnetic Resonance Research. The
research leading to these results has received funding from the European
Research Council under the European Union's Seventh Framework Programme
(FP7/2007-2013) / ERC grant agreement n° 616905. This project has also received
funding from the BMBF (01EW1711A & B) in the framework of ERA-NET NEURON.References
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