Measuring information gain for frequency-encoded super-resolution MRI

Mayer, Gregory S.; Vrscay, Edward R.

doi:10.1016/j.mri.2006.12.006

Cited by 19 publications

(10 citation statements)

References 36 publications

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“…Such proposals of spatial aliasing (or “hyperacuity”) are unlikely for several reasons. First, Fourier-based MRI is inherently band-limited in the phase-encode and frequency-encode dimensions, rendering similar “super-resolution” effects either impossible (Greenspan et al, 2002), or at best severely limited (Carmi et al 2006; Mayer and Vrscay, 2007). Second, an aliased signal can change dramatically with small shifts in relative phase.…”

Section: Discussionmentioning

confidence: 99%

Multiscale Pattern Analysis of Orientation-Selective Activity in the Primary Visual Cortex

Swisher

Gatenby²,

Gore³

et al. 2010

J. Neurosci.

188

208

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Although orientation columns are less than a millimeter in width, recent neuroimaging studies indicate that viewed orientations can be decoded from cortical activity patterns sampled at relatively coarse resolutions of several millimeters. One proposal is that these differential signals arise from random spatial irregularities in the columnar map. However, direct support for this hypothesis has yet to be obtained. Here, we used high-field, high-resolution functional magnetic resonance imaging (fMRI) and multivariate pattern analysis to determine the spatial scales at which orientation-selective information can be found in the primary visual cortex (V1) of cats and humans. We applied a multiscale pattern analysis approach in which fine-and coarse-scale signals were first removed by ideal spatial lowpass and highpass filters, and the residual activity patterns then analyzed by linear classifiers. Cat visual cortex, imaged at 0.3125 mm resolution, showed a strong orientation signal at the scale of individual columns. Nonetheless, reliable orientation bias could still be found at spatial scales of several millimeters. In the human visual cortex, imaged at 1 mm resolution, a majority of orientation information was found on scales of millimeters, with small contributions from global spatial biases exceeding ϳ1 cm. Our high-resolution imaging results demonstrate a reliable millimeters-scale orientation signal, likely emerging from irregular spatial arrangements of orientation columns and their supporting vasculature. fMRI pattern analysis methods are thus likely to be sensitive to signals originating from other irregular columnar structures elsewhere in the brain.

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Section: Discussionmentioning

confidence: 99%

Multiscale Pattern Analysis of Orientation-Selective Activity in the Primary Visual Cortex

Swisher

Gatenby²,

Gore³

et al. 2010

J. Neurosci.

188

208

View full text Add to dashboard Cite

show abstract

“…Mayer and Vrscay (2007) suggested that while super-resolution may be technically possible in the Phase-Encoding direction, it can at best contribute only a very limited amount of additional information. Therefore, band limitations of the imaging and reconstruction processes prevent or limit detection of sub-voxel supra-Nyquist signals.…”

Section: Discussionmentioning

confidence: 99%

Modeling and analysis of mechanisms underlying fMRI-based decoding of information conveyed in cortical columns

et al. 2011

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Multivariate machine learning algorithms applied to human functional MRI (fMRI) data can decode information conveyed by cortical columns, despite the voxel-size being large relative to the width of columns. Several mechanisms have been proposed to underlie decoding of stimulus orientation or the stimulated eye. These include: (I) aliasing of high spatial-frequency components, including the main frequency component of the columnar organization, (II) contributions from local irregularities in the columnar organization, (III) contributions from large-scale non-columnar organizations, (IV) functionally selective veins with biased draining regions, and (V) complex spatio-temporal filtering of neuronal activity by fMRI voxels. Here we sought to assess the plausibility of two of the suggested mechanisms: (I) aliasing and (II) local irregularities, using a naive model of BOLD as blurring and MRI voxel sampling. To this end, we formulated a mathematical model that encompasses both the processes of imaging ocular dominance (OD) columns and the subsequent linear classification analysis. Through numerical simulations of the model, we evaluated the distribution of functional differential contrasts that can be expected when considering the pattern of cortical columns, the hemodynamic point spread function, the voxel size, and the noise. We found that with data acquisition parameters used at 3 Tesla, sub-voxel supra-Nyquist frequencies, including frequencies near the main frequency of the OD organization (0.5 cycles per mm), cannot contribute to the differential contrast. The differential functional contrast of local origin is dominated by low-amplitude contributions from low frequencies, associated with irregularities of the cortical pattern. Realizations of the model with parameters that reflected best-case scenario and the reported BOLD point-spread at 3 Tesla (3.5 mm) predicted decoding performances lower than those that have been previously obtained at this magnetic field strength. We conclude that low frequency components that underlie local irregularities in the columnar organization are likely to play a role in decoding. We further expect that fMRI-based decoding relies, in part, on signal contributions from large-scale, non-columnar functional organizations, and from complex spatio-temporal filtering of neuronal activity by fMRI voxels, involving biased venous responses. Our model can potentially be used for evaluating and optimizing data-acquisition parameters for decoding information conveyed by cortical columns.

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“…Improvement in the visual appearance of images generated in this manner was felt to reflect an increase in the signal-to-noise ratio (SNR). In keeping with this, studies by other researchers in which the object of interest was spatially shifted, or the demodulation frequency was changed provided no additional information in the image spectrum (8). To generate a set of shifted images containing different information, Peled and Yeshurun (9,10) suggested FOV shifts by modulations of both the demodulation frequency and phase.…”

mentioning

confidence: 99%

MRI resolution enhancement: How useful are shifted images obtained by changing the demodulation frequency?

Tieng

Cowin

Reutens

et al. 2010

Magnetic Resonance in Med

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Super-resolution reconstruction is a process by which a set of different low resolution images of the same object are used to create an enhanced, higher resolution image of that object. Recently there has been debate amongst researchers whether it is possible to obtain in-plane image enhancement using a set of low resolution magnetic resonance images, acquired by making small, independent changes to the demodulation frequency. We show that shifted low-resolution images contain different information that can be used to obtain denser sampling, leading to image enhancement. We conclude this from specific phantom experiments, applying signal processing sampling theory and taking into consideration the relative sampling of the point spread function with respect to the location of signal sources. Furthermore, the maximum achievable resolution for Fourier encoded MRI data at a boundary or object feature is governed by the effective width of the point spread function or the Fourier pixel size determined by the extent of k-space; this is verified experimentally. Magn Reson Med 65:664-672,

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Measuring information gain for frequency-encoded super-resolution MRI

Cited by 19 publications

References 36 publications

Multiscale Pattern Analysis of Orientation-Selective Activity in the Primary Visual Cortex

Multiscale Pattern Analysis of Orientation-Selective Activity in the Primary Visual Cortex

Modeling and analysis of mechanisms underlying fMRI-based decoding of information conveyed in cortical columns

MRI resolution enhancement: How useful are shifted images obtained by changing the demodulation frequency?

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