Limits...
Correction of vibration artifacts in DTI using phase-encoding reversal (COVIPER).

Mohammadi S, Nagy Z, Hutton C, Josephs O, Weiskopf N - Magn Reson Med (2011)

Bottom Line: We refined the model of vibration-induced echo shifts, showing that asymmetric k-space coverage in widely used Partial Fourier acquisitions results in locally differing signal loss in images acquired with reversed phase encoding direction (blip-up/blip-down).COVIPER was validated against low vibration reference data, resulting in an error reduction of about 72% in fractional anisotropy maps.COVIPER can be combined with other corrections based on phase encoding reversal, providing a comprehensive correction for eddy currents, susceptibility-related distortions and vibration artifact reduction.

View Article: PubMed Central - PubMed

Affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, United Kingdom. siawoosh.mohammadi@ucl.ac.uk

Show MeSH

Related in: MedlinePlus

Schematic diagrams showing (a) vibration-induced rotational in-plane movement (r) of the brain-tissue, (b–d) the k-space coverage (black curve) and the echo (red dashed curve). The data are assumed to be sampled asymmetrically over the range [kmin, kmax] due to Partial Fourier imaging in phase-encoding (PE) direction. b: In the absence of motion, the echo center is at ky = 0. c: Vibration-induced rotational movement of the brain-tissue might shift the echo center towards the shorter k-space edge (here kmax) for a given PE direction (here blip-up) and lead to signal-loss. d: When reversing the polarity of the PE direction (blip-down), the acquisition window is shifted in such a way that short and long k-space edges are swapped. In this case, the echo center can be fully sampled and the signal will be preserved. Note that in other parts of the brain the echo center might be shifted towards the negative k-space edge, leading to signal-loss in the blip-down images. In this case, the above model is still valid but in a reversed manner. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3569871&req=5

fig01: Schematic diagrams showing (a) vibration-induced rotational in-plane movement (r) of the brain-tissue, (b–d) the k-space coverage (black curve) and the echo (red dashed curve). The data are assumed to be sampled asymmetrically over the range [kmin, kmax] due to Partial Fourier imaging in phase-encoding (PE) direction. b: In the absence of motion, the echo center is at ky = 0. c: Vibration-induced rotational movement of the brain-tissue might shift the echo center towards the shorter k-space edge (here kmax) for a given PE direction (here blip-up) and lead to signal-loss. d: When reversing the polarity of the PE direction (blip-down), the acquisition window is shifted in such a way that short and long k-space edges are swapped. In this case, the echo center can be fully sampled and the signal will be preserved. Note that in other parts of the brain the echo center might be shifted towards the negative k-space edge, leading to signal-loss in the blip-down images. In this case, the above model is still valid but in a reversed manner. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Mentions: Motion during the diffusion-weighting period may shift the echo center towards the edge of k-space in PE direction (Fig 1) and lead to loss of the DW signal, as has been demonstrated for linear rigid-body motion (45). For linear rigid-body motion the shift of the echo center Δk only depends on rotational but not on the translational movement (45, 46):1 where γ is the gyromagnetic ratio, Ω is the angular velocity vector, a dimensionless unit vector in the diffusion gradient direction and m1 is the first moment of the gradient. Note that the first order gradient moment depends on the gradient waveform and thus might vary between diffusion sequences (see e.g., Ref.47).


Correction of vibration artifacts in DTI using phase-encoding reversal (COVIPER).

Mohammadi S, Nagy Z, Hutton C, Josephs O, Weiskopf N - Magn Reson Med (2011)

Schematic diagrams showing (a) vibration-induced rotational in-plane movement (r) of the brain-tissue, (b–d) the k-space coverage (black curve) and the echo (red dashed curve). The data are assumed to be sampled asymmetrically over the range [kmin, kmax] due to Partial Fourier imaging in phase-encoding (PE) direction. b: In the absence of motion, the echo center is at ky = 0. c: Vibration-induced rotational movement of the brain-tissue might shift the echo center towards the shorter k-space edge (here kmax) for a given PE direction (here blip-up) and lead to signal-loss. d: When reversing the polarity of the PE direction (blip-down), the acquisition window is shifted in such a way that short and long k-space edges are swapped. In this case, the echo center can be fully sampled and the signal will be preserved. Note that in other parts of the brain the echo center might be shifted towards the negative k-space edge, leading to signal-loss in the blip-down images. In this case, the above model is still valid but in a reversed manner. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3569871&req=5

fig01: Schematic diagrams showing (a) vibration-induced rotational in-plane movement (r) of the brain-tissue, (b–d) the k-space coverage (black curve) and the echo (red dashed curve). The data are assumed to be sampled asymmetrically over the range [kmin, kmax] due to Partial Fourier imaging in phase-encoding (PE) direction. b: In the absence of motion, the echo center is at ky = 0. c: Vibration-induced rotational movement of the brain-tissue might shift the echo center towards the shorter k-space edge (here kmax) for a given PE direction (here blip-up) and lead to signal-loss. d: When reversing the polarity of the PE direction (blip-down), the acquisition window is shifted in such a way that short and long k-space edges are swapped. In this case, the echo center can be fully sampled and the signal will be preserved. Note that in other parts of the brain the echo center might be shifted towards the negative k-space edge, leading to signal-loss in the blip-down images. In this case, the above model is still valid but in a reversed manner. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Mentions: Motion during the diffusion-weighting period may shift the echo center towards the edge of k-space in PE direction (Fig 1) and lead to loss of the DW signal, as has been demonstrated for linear rigid-body motion (45). For linear rigid-body motion the shift of the echo center Δk only depends on rotational but not on the translational movement (45, 46):1 where γ is the gyromagnetic ratio, Ω is the angular velocity vector, a dimensionless unit vector in the diffusion gradient direction and m1 is the first moment of the gradient. Note that the first order gradient moment depends on the gradient waveform and thus might vary between diffusion sequences (see e.g., Ref.47).

Bottom Line: We refined the model of vibration-induced echo shifts, showing that asymmetric k-space coverage in widely used Partial Fourier acquisitions results in locally differing signal loss in images acquired with reversed phase encoding direction (blip-up/blip-down).COVIPER was validated against low vibration reference data, resulting in an error reduction of about 72% in fractional anisotropy maps.COVIPER can be combined with other corrections based on phase encoding reversal, providing a comprehensive correction for eddy currents, susceptibility-related distortions and vibration artifact reduction.

View Article: PubMed Central - PubMed

Affiliation: Wellcome Trust Centre for Neuroimaging, UCL Institute of Neurology, University College London, United Kingdom. siawoosh.mohammadi@ucl.ac.uk

Show MeSH
Related in: MedlinePlus