Synopsis
It has been shown the application of Mean-Shift
Clustering (MSC) to fMRI analysis can increase detection sensitivity in low
contrast to noise situations. In this study, MSC was utilized with a feature
space containing both temporal and spatial features to further increase its
detection power. If successful, the proposed technique can
improve detection in techniques that inherently have low CNR, such as non-proton
fMRI or non-BOLD fMRI.Introduction
It
has been shown the application of Mean-Shift Clustering (MSC) to fMRI analysis
can increase detection sensitivity in low contrast to noise (CNR) situations
(Ai et al. 2014). The feature space previously used with MSC was based purely on
spatial features of the data. In this study, MSC was utilized with a feature
space containing both temporal and spatial features to further increase its
detection power. The proposed MSC was applied to simulated and real fMRI data to
examine its sensitivity and specificity and compared to the commonly used
cluster analysis (CA). If successful, the proposed technique can improve detection
in techniques that inherently have low CNR, such as non-proton fMRI or non-BOLD
fMRI.
Methods
The
new feature space used with MSC for this study is the estimated mean Z values
surrounding a voxel and the signal power at the base frequency of the experimental
design. The mean Z values surrounding a voxel were used to take into
consideration neighboring effects while the power at base frequency takes into
consideration temporal characteristics. The MSC technique was evaluated with
(MSC+CA) and without additional CA applied. The techniques will be applied to a
statistical parametric image generated using cross correlation analysis (CCA). A
cluster size threshold of 4 voxels was used for the application CA when applied.
The
new feature space was tested on a simulated fMRI data sets (100 images, a
128x128 matrix) while simulating a block design with 2.5 off/on cycles and an
event related design with a stimulation every 4 images. Receiver Operating
Characteristic (ROC) curves were generated for evaluating the efficacy of the
technique at various CNRs (0.20, 0.40, 0.60, 0.80).
The
same MSC technique was also tested on real BOLD fMRI data acquired on a Siemens
3T Trio scanner (Siemens Medical Solutions, Erlangen, Germany) using electrical
stimulations delivered to the right median nerve with a Grass S48 stimulator (Grass
Technologies, USA). The stimulation intensity was adjusted to 15V above the
minimum required to obtain a thumb twitch. A block design with four and a half
off/on cycles was used with a randomized inter-stimulation interval (ISI) of
1.5-2.5 seconds with a length of 180 images. The volunteers were asked to stay
still and awake for the duration of the scan with no task actively performed
during the scan. The images were processed using Analysis of Functional NeuroImages
(AFNI) and custom written Matlab code.
Results
The
results indicate that the proposed MSC technique can improve activation detection
over CA based on the ROC curves (Figure 1) generated from simulated data, with
MSC+CA improving detections even more. Using a kernel size of 0.30 in both block
and event related designs, improvements can be seen over CA with the MSC
techniques in the lower CNR situations (0.20 and 0.40). Activations can be seen
when the same method is utilized on real BOLD fMRI data (Figure 2) with
detected activations being apparently larger when compared to CA at the same significance
level.
Discussion and Conclusion
The
results in the ROC curves with the simulated data using MSC show an improvement
in detection over CA in low CNR situations, even more so if using MSC+CA. Since
activation detection at high CNR scenarios typically does not present an issue
but does at low CNR scenarios, utilizing MSC with temporal characteristics can
improve activation detection in such situations. Analysis techniques that
target very low CNRs could be particularly useful for imaging studies utilizing
methods with inherently low CNRs, non-proton imaging or non-BOLD fMRI for
instance. MSC may also be able to shorten acquisition time required for
currently used BOLD fMRI methods by reducing the number of repetitions required.
Further optimization studies need to be performed on this technique, on kernel
sizes, different clustering techniques to use in conjunction, or different feature
spaces for example.
Acknowledgements
No acknowledgement found.References
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