Weiran Deng1, Michael Herbst1, and V. Andrew Stenger1
1University of Hawaii JABSOM, Honolulu, HI, United States
Synopsis
A subset of fMRI images is used to reconstruct a structural image at a higher resolution using a Super-Resolution (SR) reconstruction method. The subset of fMRI images are selected such that the translation and rotation between the shots are less than the pixel size and therefore useful for modeling the pixel characteristics. The preliminary results demonstrate the feasibility of reconstructing a structural image with susceptibility contrast from a subset of fMRI images.
Purpose
To reconstruct a High-Resolution
Susceptibility Weighted Image (SWI) from a subset of low-resolution functional
MRI (fMRI) data using Super-Resolution (SR) reconstruction.
Introduction
Subject head motion is
inevitable during fMRI scans. Even when the subject makes an effort to remain
still, sub-voxel involuntary motions are still impossible to control. However,
such motions are useful for reconstructing a high-resolution image using SR reconstruction
from the multiple low resolution fMRI images. There are existing studies that
use SR for DTI white matter fiber track visualization
1 and to
enhance imaging resolution in general
2. This abstract presents preliminary
results of a high-resolution SWI reconstructed from a subset of fMRI data. The
high-resolution image can then be used as a template over which the activation
from the low resolution scans can be overlaid.
Theory
Conceptually, a high-resolution
image can be reconstructed form a series of low-resolution images provided
these images have sub-pixel shifts with respect to each other. The spatial
characteristics of the voxel can be modelled and used to reconstruct an image
with a higher resolution. The observation model of the
i-th low-resolution fMRI image
yi, is the result of
applying a series of linear operators (
Di,
Fi,
Hi) and the addition of
a noise term
ni: $$y_{i}=D_{i}F_{i}H_{i}z+n_{i}$$, where
Di,
Fi,
and
Hi
represent the down-sampling, geometrical transformation, and blurring,
respectively. The reconstruction of the high-resolution image
z
is a classical linear inverse problem, which can be solved using methods such
as non-uniform interpolation, the frequency domain approach,
projection-onto-convex sets, or regularized methods
3.
Methods
A resting state fMRI scan was
performed on a 3T Siemens TimTrio scanner using a 32-channel head coil. The
sequence was a Simultaneous Multi-Slice EPI (TE/TR=30/1000ms, FOV=220mm, 64 2mm
slices, FA=55o). Motion parameters (translation and rotation) were estimate
using FSL’s FLIRT program
4. Sixteen volumes were selected from the
fMRI data and the SR reconstruction was permed using a MATLAB SR program
5.
A structure-adaptive normalized convolution approach was used for the SR
reconstruction at 2mm resolution
6. The resting state Independent
Component Analysis (ICA) analysis was performed using FSL’s MELODIC program
7.
Results
Figure 1 shows image translation
(top) and rotation during the fMRI scan. The black dashed box annotates the
time points of the 16 volumes selected for the SR reconstruction. Note that the
translation (less than 2mm pixel size) and rotation were small enough to be
useful for SR. Figure 2 shows a comparison of a LR image (a), an HR image (b)
interpolated from the LR image using a bilinear interpolation method, and an SR
image (c), respectively. A resting state activation map from the default mode
was overlaid on top of the bilinear interpolated image and the SR reconstructed
image and shown in Figures 3a and 3b, respectively.
Discussions
There are several ways to
improve the SR reconstruction presented here. The SR images were reconstructed
slice-by-slice from the LR fMRI data for computational ease. Better results may
be obtained if the SR reconstruction is performed on the whole 3D volume using
3D translation and rotation parameters. Second, the alignment from motion was
estimated using FLIRT. Because the SR reconstruction highly depends on the
accuracy of the motion parameters, the reconstruction can be improved using parameters
measured accurately using a MRI-compatible motion-tracking system
8.
Third, it is unclear if the variation of the voxels from BOLD hemodynamics
contributes to the quality of the SR reconstruction. A potential solution is to
use a remove hemodynamic trend before the SR reconstruction.
Conclusion
The preliminary results
presented here demonstrate the feasibility of deriving a high-resolution image
using a subset of fMRI images. Future work will investigate options to improve
the image quality of SR reconstruction.
Acknowledgements
Work supported by the NIH grants R01DA019912, R01EB011517, and K02DA020569.References
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