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Distortion correction in EPI using an extended PSF method with a reversed phase gradient approach.

In MH, Posnansky O, Beall EB, Lowe MJ, Speck O - PLoS ONE (2015)

Bottom Line: In echo-planar imaging (EPI), such as commonly used for functional MRI (fMRI) and diffusion-tensor imaging (DTI), compressed distortion is a more difficult challenge than local stretching as spatial information can be lost in strongly compressed areas.In addition, the effects are more severe at ultra-high field (UHF) such as 7T due to increased field inhomogeneity.Further we demonstrate that the extended PSF method with an improved weighted combination can recover local distortions and spatial information loss and be applied successfully not only to spin-echo EPI, but also to gradient-echo EPIs acquired with both PE directions to perform geometrically accurate image reconstruction.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Magnetic Resonance, Institute for Experimental Physics, Otto-von-Guericke University Magdeburg, Magdeburg, Germany.

ABSTRACT
In echo-planar imaging (EPI), such as commonly used for functional MRI (fMRI) and diffusion-tensor imaging (DTI), compressed distortion is a more difficult challenge than local stretching as spatial information can be lost in strongly compressed areas. In addition, the effects are more severe at ultra-high field (UHF) such as 7T due to increased field inhomogeneity. To resolve this problem, two EPIs with opposite phase-encoding (PE) polarity were acquired and combined after distortion correction. For distortion correction, a point spread function (PSF) mapping method was chosen due to its high correction accuracy and extended to perform distortion correction of both EPIs with opposite PE polarity thus reducing the PSF reference scan time. Because the amount of spatial information differs between the opposite PE datasets, the method was further extended to incorporate a weighted combination of the two distortion-corrected images to maximize the spatial information content of a final corrected image. The correction accuracy of the proposed method was evaluated in distortion-corrected data using both forward and reverse phase-encoded PSF reference data and compared with the reversed gradient approaches suggested previously. Further we demonstrate that the extended PSF method with an improved weighted combination can recover local distortions and spatial information loss and be applied successfully not only to spin-echo EPI, but also to gradient-echo EPIs acquired with both PE directions to perform geometrically accurate image reconstruction.

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Related in: MedlinePlus

The EPI-PSF kernel based weighting maps and the ratio map in GE- (I) and SE-EPIs (II): two weighting maps, Wf(s) (a) and Wr(s) (b), are calculated respectively from the forward and reverse EPI-PSF kernel using Eq. 6 and the ratio (c) is shown in logarithmic scale using the equation loge(Wf(s)/Wr(s)).
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pone.0116320.g005: The EPI-PSF kernel based weighting maps and the ratio map in GE- (I) and SE-EPIs (II): two weighting maps, Wf(s) (a) and Wr(s) (b), are calculated respectively from the forward and reverse EPI-PSF kernel using Eq. 6 and the ratio (c) is shown in logarithmic scale using the equation loge(Wf(s)/Wr(s)).

Mentions: Fig. 5 shows weighting maps calculated from the EPI-PSF kernels. Since the EPI-PSF kernel contains information about both blurring and shift of the PSF due to distortions, the correction procedure considers these effects in calculating the weighting maps. For example, image blurring occurs along the PE direction in the acquired EPI due to the use of a partial Fourier factor of 6/8 together with zero-filling for the image reconstruction. Since the PSF blurring is usually larger near structure boundaries, such as tissue/tissue, tissue/vessels, tissue/ventricles, or tissue/air boundaries in the brain, the locally measured weightings are similar in both maps, as shown in Fig. 5A and 5B. In contrast, weightings due to PSF shifts vary more smoothly (or on global scale) within the slice. Since the protocols were identical with exception of the reversed PE gradient in the EPI measurement, the effects of the PSF shift, but not blurring, are reversed on the weighting maps. Therefore, as shown in the weighting ratio image (Fig. 5C) in logarithmic scale, the effects of PSF shifts are mainly accounted for in the combination process. Compared to the reverse EPI image, the forward EPI image introduces 64 times higher contribution (loge8n = n×2.16, n = 2) to the combined image in the anterior regions (see arrows in Fig. 5) and lower contribution near ventricle regions when the combination process is performed.


Distortion correction in EPI using an extended PSF method with a reversed phase gradient approach.

In MH, Posnansky O, Beall EB, Lowe MJ, Speck O - PLoS ONE (2015)

The EPI-PSF kernel based weighting maps and the ratio map in GE- (I) and SE-EPIs (II): two weighting maps, Wf(s) (a) and Wr(s) (b), are calculated respectively from the forward and reverse EPI-PSF kernel using Eq. 6 and the ratio (c) is shown in logarithmic scale using the equation loge(Wf(s)/Wr(s)).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4338274&req=5

pone.0116320.g005: The EPI-PSF kernel based weighting maps and the ratio map in GE- (I) and SE-EPIs (II): two weighting maps, Wf(s) (a) and Wr(s) (b), are calculated respectively from the forward and reverse EPI-PSF kernel using Eq. 6 and the ratio (c) is shown in logarithmic scale using the equation loge(Wf(s)/Wr(s)).
Mentions: Fig. 5 shows weighting maps calculated from the EPI-PSF kernels. Since the EPI-PSF kernel contains information about both blurring and shift of the PSF due to distortions, the correction procedure considers these effects in calculating the weighting maps. For example, image blurring occurs along the PE direction in the acquired EPI due to the use of a partial Fourier factor of 6/8 together with zero-filling for the image reconstruction. Since the PSF blurring is usually larger near structure boundaries, such as tissue/tissue, tissue/vessels, tissue/ventricles, or tissue/air boundaries in the brain, the locally measured weightings are similar in both maps, as shown in Fig. 5A and 5B. In contrast, weightings due to PSF shifts vary more smoothly (or on global scale) within the slice. Since the protocols were identical with exception of the reversed PE gradient in the EPI measurement, the effects of the PSF shift, but not blurring, are reversed on the weighting maps. Therefore, as shown in the weighting ratio image (Fig. 5C) in logarithmic scale, the effects of PSF shifts are mainly accounted for in the combination process. Compared to the reverse EPI image, the forward EPI image introduces 64 times higher contribution (loge8n = n×2.16, n = 2) to the combined image in the anterior regions (see arrows in Fig. 5) and lower contribution near ventricle regions when the combination process is performed.

Bottom Line: In echo-planar imaging (EPI), such as commonly used for functional MRI (fMRI) and diffusion-tensor imaging (DTI), compressed distortion is a more difficult challenge than local stretching as spatial information can be lost in strongly compressed areas.In addition, the effects are more severe at ultra-high field (UHF) such as 7T due to increased field inhomogeneity.Further we demonstrate that the extended PSF method with an improved weighted combination can recover local distortions and spatial information loss and be applied successfully not only to spin-echo EPI, but also to gradient-echo EPIs acquired with both PE directions to perform geometrically accurate image reconstruction.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Magnetic Resonance, Institute for Experimental Physics, Otto-von-Guericke University Magdeburg, Magdeburg, Germany.

ABSTRACT
In echo-planar imaging (EPI), such as commonly used for functional MRI (fMRI) and diffusion-tensor imaging (DTI), compressed distortion is a more difficult challenge than local stretching as spatial information can be lost in strongly compressed areas. In addition, the effects are more severe at ultra-high field (UHF) such as 7T due to increased field inhomogeneity. To resolve this problem, two EPIs with opposite phase-encoding (PE) polarity were acquired and combined after distortion correction. For distortion correction, a point spread function (PSF) mapping method was chosen due to its high correction accuracy and extended to perform distortion correction of both EPIs with opposite PE polarity thus reducing the PSF reference scan time. Because the amount of spatial information differs between the opposite PE datasets, the method was further extended to incorporate a weighted combination of the two distortion-corrected images to maximize the spatial information content of a final corrected image. The correction accuracy of the proposed method was evaluated in distortion-corrected data using both forward and reverse phase-encoded PSF reference data and compared with the reversed gradient approaches suggested previously. Further we demonstrate that the extended PSF method with an improved weighted combination can recover local distortions and spatial information loss and be applied successfully not only to spin-echo EPI, but also to gradient-echo EPIs acquired with both PE directions to perform geometrically accurate image reconstruction.

Show MeSH
Related in: MedlinePlus