Application of k-space energy spectrum analysis to susceptibility field mapping and distortion correction in gradient-echo EPI

Harvard University, Cambridge, Massachusetts, United States
NeuroImage (Impact Factor: 6.36). 07/2006; 31(2):609-22. DOI: 10.1016/j.neuroimage.2005.12.022
Source: PubMed


Echo-planar imaging (EPI) is widely used in functional MRI studies. It is well known that EPI quality is usually degraded by geometric distortions, when there exist susceptibility field inhomogeneities. EPI distortions may be corrected if the field maps are available. It is possible to estimate the susceptibility field gradients from the phase reconstruction of a single-TE EPI image, after a successful phase-unwrapping procedure. However, in regions affected by pronounced field gradients, the phase-unwrapping of a single-TE image may fail, and therefore the estimated field maps may be incorrect. It has been reported that the field inhomogeneity may be calculated more reliably from T2*-weighted images corresponding to multiple TEs. However, the multi-TE MRI field mapping increases the scan time. Furthermore, the measured field maps may be invalid if the subject's position changes during dynamic scans. To overcome the limitations in conventional field mapping approaches, a novel k-space energy spectrum analysis algorithm is developed, which quantifies the spatially dependent echo-shifting effect and the susceptibility field gradients directly from the k-space data of single-TE gradient-echo EPI. Using the k-space energy spectrum analysis, susceptibility field gradients can be reliably measured without phase-unwrapping, and EPI distortions can be corrected without extra field mapping scans or pulse sequence modification. The reported technique can be used to retrospectively improve the image quality of the previously acquired EPI and functional MRI data, provided that the complex-domain k-space data are still available.

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Available from: Lawrence Panych, May 11, 2015
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    • "However, EPI has two main problems in image quality. First, due to the low acquisition bandwidth in the phase encoding dimension, the geometric distortion caused by field inhomogeneity and chemical shift effects is severe (Chen et al., 2006). Second, the length of acquisition window is confined to the T Ã 2 value of imaged tissue, which constrains the spatial resolution of EPI image. "
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    ABSTRACT: Spatiotemporally encoded (SPEN) single-shot MRI is an ultrafast MRI technique proposed recently, which utilizes quadratic rather than linear phase profile to extract the spatial information. Compared to the echo planar imaging (EPI), this technique has great advantages in resisting field inhomogeneity and chemical shift effects. Super-resolved (SR) reconstruction is adopted to compensate the inherent low resolution of SPEN images. Due to insufficient sampling rate, the SR image is challenged by aliasing artifacts and edge ghosts. The existing SR algorithms always compromise in spatial resolution to suppress these undesirable artifacts. In this paper, we proposed a novel SR algorithm termed super-resolved enhancing and edge deghosting (SEED). Different from artifacts suppression methods, our algorithm aims at exploiting the relationship between aliasing artifacts and real signal. Based on this relationship, the aliasing artifacts can be eliminated without spatial resolution loss. According to the trait of edge ghosts, finite differences and high-pass filter are employed to extract the prior knowledge of edge ghosts. By combining the prior knowledge with compressed sensing, our algorithm can efficiently reduce the edge ghosts. The robustness of SEED is demonstrated by experiments under various situations. The results indicate that the SEED can provide better spatial resolution compared to state-of-the-art SR reconstruction algorithms in SPEN MRI. Theoretical analysis and experimental results also show that the SR images reconstructed by SEED have better spatial resolution than the images obtained with conventional k-space encoding methods under similar experimental condition. Copyright © 2015 Elsevier B.V. All rights reserved.
    Full-text · Article · Apr 2015 · Medical image analysis
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    • "Only after those distortions have been corrected, the low-resolution functional information reflected by EPI can be co-registered accurately with the high-resolution structural information collected by multi-shot imaging methods. In the past twenty years, many techniques have been developed for EPI corrections [11] [12] [13] [14] [15] [16]. However, the aggravated distortions at high field often result in overlap, piling and dropout of signals due to limited field of view, strong local field gradients and severe deviation of the k-space trajectory, all of which drastically influence the accuracy of the corrected results. "
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    ABSTRACT: This paper presents a phase evolution rewinding algorithm for correcting the geometric and intensity distortions in single-shot spatiotemporally encoded (SPEN) MRI with acquisition of asymmetric self-refocused echo trains. Using the field map calculated from the phase distribution of the source image, the off-resonance induced phase errors are successfully rewound through deconvolution. The alias-free partial Fourier transform reconstruction helps improve the signal-to-noise ratio of the field maps and the output images. The effectiveness of the proposed algorithm was validated through 7T MRI experiments on a lemon, a water phantom, and in vivo rat head. SPEN imaging was evaluated using rapid acquisition by sequential excitation and refocusing (RASER) which produces uniform T2 weighting. The results indicate that the new technique can more robustly deal with the cases in which the images obtained with conventional single-shot spin-echo EPI are difficult to be restored due to serious field variations. Copyright © 2015 Elsevier Inc. All rights reserved.
    Full-text · Article · Feb 2015 · Journal of magnetic resonance (San Diego, Calif.: 1997)
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    • "c o m / l o c a t e / y n i m g local field inhomogeneity), the quality of the corrected results will be degraded (Chen and Wyrwicz, 2001; Wan et al., 1997; Weiskopf et al., 2005). Fourthly, the distortion correction methods will be invalid when the B 0 field inhomogeneity is so serious that folding has occurred in the spin-echo EPI images (Chiou et al., 2003; Holland et al., 2010; Knopp et al., 2009; Nam and Park, 2011; Nguyen et al., 2009; Truong et al., 2010; Weiskopf et al., 2005; Zaitsev et al., 2004; Zeng and Constable, 2002; Zeng et al., 2004), or when the k-space energy peaks are shifted completely outside the sampling window due to a very significant in-plane local field gradient in the gradient-echo EPI (Chen et al., 2006; Truong et al., 2010). "
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    ABSTRACT: Owing to its intrinsic characteristics, spatiotemporally encoded (SPEN) imaging is less sensitive to adverse effects due to field inhomogeneity in comparison with echo planar imaging, a feature highly desired for functional, diffusion, and real-time MRI. However, the quality of images obtained with SPEN MRI is still degraded by geometric distortions when field inhomogeneity exists. In this study, a single-shot biaxial SPEN (bi-SPEN) pulse sequence is implemented, utilizing a 90° and a 180° chirp pulse incorporated with two orthogonal gradients. A referenceless geometric-distortion correction based on the single-shot bi-SPEN sequence is then proposed. The distorted image acquired with the single-shot bi-SPEN sequence is corrected by iterative super-resolved reconstruction involving the field gradients estimated from a field map, which in turn is obtained from its own super-resolved data after a phase-unwrapping procedure without additional scans. In addition, the distortion correction method is applied to improve the quality of the multiple region-of-interest images obtained with single-shot bi-SPEN sequence.
    Full-text · Article · Jan 2015 · NeuroImage
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