Mri

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BOLD?

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Blood-oxygen-level dependent

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What does fMRI measure?

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Haemodynamic response (change in blood flow).

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What is BOLD?

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The MRI contrast of blood deoxyhaemoglobin.

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Relationship between size of BOLD signal and strength of magnetic field.

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BOLD signal increases as the square of the MF strength (Tesla).

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How is BOLD acquired?

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Using contrast weighted by T1 or T2.

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How is BOLD computed?

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Ratio of oxyhaemoglobin with deoxyhaemoglobin.

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Effect size conversion in fMRI

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Converted to percentage of signal change.

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What is diffusion?

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Movement of particles from high to low concentration areas.

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How diffusion relates to entropy?

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Diffusion increases the entropy of a system.

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What causes Brownian motion?

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Brownian motion is caused by collision.

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What is (diffusion) anisotropy?

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Probability of displacement in certain directions is higher than in others.

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What is (diffusion) isotropy?

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Probability of displacement is equal in all direction.

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Diffusion in the absence of concentration gradient?

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Self-diffusion or tracer diffusion.

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Example of isotropic tissue in brain.

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CSF

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Tensor of rank 2 in R^n.

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Matrix of order n*n = n^2.

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Tensor of rank 3 in R^n.

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Array of order n*n*n = n^3.

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Tensor of rank 0 in R^n.

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Scalar.

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Which diffusion gradient is most important?

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Principal eigenvector of gradient tensor.

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Interpretation of eigenvalues from diffusion tensor.

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Amount of diffusion in the diffusion gradient given by corresponding eigenvector.

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Interpretation of eigenvectors from diffusion tensor.

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A diffusion gradient.

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Mean diffusivity?

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Trace of diffusion tensor.

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Properties of quantities based on diffusion tensor?

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Rotational invariant.

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What is tractography?

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Follow-up of local diffusion gradients.

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What are tensors in comparison to matrices?

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Generalization of vectors and matrices to any rank (i.e. more than 2).

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Another name for a square wave?

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A boxcar function.

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What is the effective imaging resolution in fMRI.

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About 3mm, as constrained by the size of capillaries.

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One major limitation of DTI.

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Fibres are not parallel (e.g. kissing fibres)

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What causes an increase in BOLD signal?

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Decrease of deoxyhemoglobin.

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What an increase in BOLD signal mean?

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Increase in oxygen consumption.

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What does increase in oxygen consumption mean?

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Increase in oxyhemoglobin and decrease in deoxyhemoglobin.

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Why fMRI focuses on deoxyhemolgobin?

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Causes greater dephasing than oxyhemoglobin.

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What does HRF stand for?

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Hemodynamic Response Function.

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Where does diffusion occur?

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Diffusion is mainly extracellular.

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Prewhitening?

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Removing unwanted autocorrelations from time series, prior to analysis.

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Fractional Anisotropy.

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Normalized standard deviation of eigenvalues (dimensionless quantity).

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Mean Diffusivity.

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Mean of Eigenvalues.
MD has a (diffusion) unit.