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xSPEN: A new single-shot magnetic resonance … A new single-shot magnetic resonance imaging...
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xSPEN: A new single-shot magnetic resonance imaging technique with exceptional resilience to field heterogeneities Zhiyong Zhang, Amir Seginer, Lucio Frydman Chemical Physics Department, Weizmann Institute of Science, Rehovot, Israel Central to MRI is Mansfield’s echo-planar imaging (EPI) proposition [1], a single-shot approach relying on oscillating gradients to rapidly scan the k-space. During recent years an alternative single-shot method has emerged, based on spatiotemporal encoding (SPEN) concepts [2]. SPEN provides similar acquisition durations, sensitivity and resolution as EPI, but with an enhanced immunity to B0 inhomogeneities and susceptibility distortions [3, 4]. Still, instances will arise where SPEN’s immunity is insufficient to deliver meaningful data; this study introduces an alternative modality that we denominate cross-term spatiotemporal encoding –xSPEN for short– with good sensitivity and remarkable resilience to field inhomogeneities and/or the presence of multiple chemical sites. xSPEN departs from conventional k-space scanning, and relies instead on performing a spatiotemporally encoding of the image being sought. Unlike hitherto proposed SPEN methods, however, this encoding does not utilize only a field gradient along the direction being probed, but adds an ancillary source of inhomogeneous frequency broadening; for instance that imposed by an orthogonal field gradient. Images along the “y” axis are thereby read out in xSPEN by applying a “z” gradient. Figure 1 illustrates one possible implementation of the resulting single-scan xSPEN sequence. This incorporates an oscillating ±Gx readout gradient sampling the kro-space of one of the axes, and a constant gradient Gz charged with both the slice selection and the reading out of the second (y-axis) in-plane dimension. Figure 1b illustrates how these oscillating and constant gradients transverse the resulting “hybrid” (kro-y) space. All that such sequence needs –and what was implemented in this study– to transform the resulting signals into xSPEN images, is splitting the resulting FID string into ±Gro-sampled segments, position these in their correct 2D space coordinates, subject these rearranged arrays to a 1D FT vs kro, and display in magnitude-mode the resulting matrix. Figure 2 shows ex-vivo rat brain experiments performed for varying degrees of field inhomogeneities on an Agilent 7T scanner. Compared in this figure are spin-echo (SE) multiscan acquisitions acting as “gold-standard”, against single-shot 2D images collected by SE-EPI, SPEN and the new xSPEN sequence. Notice that while degrading the external magnetic field homogeneity rapidly distorts EPI, and eventually also the SPEN images, xSPEN scans remain insensible to these field distortions. Moreover, other than for a FT along the readout dimension and a magnitude calculation of the ensuing matrix, no special processing, field-mapping or navigator scans are associated to the retrieval of these xSPEN images. Figure 3 compares xSPEN and SE-EPI head scans acquired on a human volunteer with non-magnetic, metallic dentures. While for clarity only three representative images are shown for each plane, full sets of axial (60 slices), sagittal (40 slices) and coronal images (60 slices) were collected with a 4 mm isotropic resolution. Analysis of these images reveals numerous differences between xSPEN and SE-EPI; most evident among these being the fact that whereas SE-EPI can generally target only the most homogeneous brain regions, xSPEN has no problems to deliver single-shot images for most of the head –regardless of whether containing water or fat. The consequences of the denture are also evident in SE-EPI, which become distorted in proximity to the metallic implants and for which fat suppression becomes inefficient. Likewise, susceptibility effects are clearly visible by SE-EPI’s distorted eyeballs and ears. By contrast, the xSPEN images are remarkably faithful, and affected by relatively small SNR penalties. These and other examples confirm the unusual resilience of this new single-shot xSPEN method to shifts and field heterogeneities. Fig. 1 (a) Single-scan xSPEN sequence incorporating an oscillating k-based gradient encoding along x and an xSPEN-based rasterization of y-profiles by a z gradient. (b) Sampling of the kx-y space delivered by the 2D xSPEN sequence in (a) FIG. 2 Ex-vivo rat brain 1D NMR spectra and 2D imaging results, collected for different sequences under increasingly degraded magnet field homogeneity. Notice the progressive distortions introduced by the field inhomogeneity in all images but the xSPEN one, as judged by the dotted squares added for ease of viewing
Transcript of xSPEN: A new single-shot magnetic resonance … A new single-shot magnetic resonance imaging...
xSPEN: A new single-shot magnetic resonance imaging technique with exceptional resilience to field heterogeneities Zhiyong Zhang, Amir Seginer, Lucio Frydman
Chemical Physics Department, Weizmann Institute of Science, Rehovot, Israel Central to MRI is Mansfield’s echo-planar imaging (EPI) proposition [1], a single-shot approach relying on oscillating gradients to rapidly scan the k-space. During recent years an alternative single-shot method has emerged, based on spatiotemporal encoding (SPEN) concepts [2]. SPEN provides similar acquisition durations, sensitivity and resolution as EPI, but with an enhanced immunity to B0 inhomogeneities and susceptibility distortions [3, 4]. Still, instances will arise where SPEN’s immunity is insufficient to deliver meaningful data; this study introduces an alternative modality that we denominate cross-term spatiotemporal encoding –xSPEN for short– with good sensitivity and remarkable resilience to field inhomogeneities and/or the presence of multiple chemical sites.
xSPEN departs from conventional k-space scanning, and relies instead on performing a spatiotemporally encoding of the image being sought. Unlike hitherto proposed SPEN methods, however, this encoding does not utilize only a field gradient along the direction being probed, but adds an ancillary source of inhomogeneous frequency broadening; for instance that imposed by an orthogonal field gradient. Images along the “y” axis are thereby read out in xSPEN by applying a “z” gradient. Figure 1 illustrates one possible implementation of the resulting single-scan xSPEN sequence.
This incorporates an oscillating ±Gx readout gradient sampling the kro-space of one of the axes, and a constant gradient Gz charged with both the slice selection and the reading out of the second (y-axis) in-plane dimension. Figure 1b illustrates how
these oscillating and constant gradients transverse the resulting “hybrid” (kro-y) space. All that such sequence needs –and what was implemented in this study– to transform the resulting signals into xSPEN images, is splitting the resulting FID string into ±Gro-sampled segments, position these in their correct 2D space coordinates, subject these rearranged arrays to a 1D FT vs kro, and display in magnitude-mode the resulting matrix. Figure 2 shows ex-vivo rat brain experiments performed for varying degrees of field inhomogeneities on an Agilent 7T scanner. Compared in this figure are spin-echo (SE) multiscan acquisitions acting as “gold-standard”, against single-shot 2D images collected by SE-EPI, SPEN and the new xSPEN sequence. Notice that while degrading the external magnetic field homogeneity rapidly distorts EPI, and eventually also the SPEN images, xSPEN scans remain insensible to these field distortions. Moreover, other than for a FT along the readout dimension and a magnitude calculation of the ensuing matrix, no special processing, field-mapping or navigator scans are associated to the retrieval of these xSPEN images. Figure 3 compares xSPEN and SE-EPI head scans acquired on a human volunteer with non-magnetic, metallic dentures. While for clarity only three representative images are shown for each plane, full sets of axial (60 slices), sagittal (40 slices) and coronal images (60 slices) were collected with a 4 mm isotropic resolution. Analysis of these images reveals numerous differences between xSPEN and SE-EPI; most evident among these being the fact that whereas SE-EPI can generally target only the most homogeneous brain regions, xSPEN has no problems to deliver single-shot images for most of the head –regardless of whether containing water or fat. The consequences of the denture are also evident in SE-EPI, which become distorted in proximity to the metallic implants and for which fat suppression becomes inefficient. Likewise, susceptibility effects are clearly visible by SE-EPI’s distorted eyeballs and ears. By contrast, the xSPEN images are remarkably faithful, and affected by relatively small SNR penalties. These and other examples confirm the unusual resilience of this new single-shot xSPEN method to shifts and field heterogeneities.
Fig. 1 (a) Single-scan xSPEN sequence incorporating an oscillating k-based gradient encoding along x and an xSPEN-based rasterization of y-profiles by a z gradient. (b) Sampling of the kx-y space delivered by the 2D xSPEN sequence in (a)
FIG. 2 Ex-vivo rat brain 1D NMR spectra and 2D imaging results, collected for different sequences under increasingly degraded magnet field homogeneity. Notice the progressive distortions introduced by the field inhomogeneity in all images but the xSPEN one, as judged by the dotted squares added for ease of viewing
In summary, xSPEN can deliver single-scan MRI with good sensitivity and unprecedented resilience to field inhomogeneities. This ability could enable investigations that have hitherto escaped from MRI’s scope.
Acknowledgements: This work was funded by the Israel Science Foundation grant 795/13, by ERC-2014-PoC grant # 633888, by the Kimmel Institute of Magnetic Resonance, and by the generosity of the Perlman Family Foundation. References:
[1] Mansfield P. Multi-planar image-formation using NMR spin echoes. J Phys C: Solid State Phys 1977;10:L55-L58. [2] Shrot Y, Frydman L. Spatially encoded NMR and the acquisition of 2D magnetic resonance images within a single scan. J Magn Reson 2005;172:179-190. [3] Schmidt R, Frydman L. New spatiotemporal approaches for fully refocused, multislice ultrafast 2D MRI. Magn Reson Med 2014;71:711-722. [4] Tal A, Frydman L. Single-scan multidimensional magnetic resonance. Prog Nucl Magn Reson Spectrosc 2010;57:241-292.
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Fig. 3. Representative images from three-plane single-shot xSPEN and SE-EPI scans, involving whole head acquisitions on a volunteer with a non-ferromagnetic metallic denture on a 3T Siemens TrioTIM scanner. No fat suppression was used in the xSPEN experiments, while it was needed in SE-EPI.
