Co-Planar Stereotaxic Atlas of the Human Brain—3-Dimensional Proportional System: An Approach to Cerebral Imaging, J. Talairach, P. Tournoux. Georg Thieme Verlag, New York (1988), 122 pp., 130 figs. DM 268

The sparsity which is implicit in MR images is exploited to significantly undersample k -space. Some MR images such as angiograms are already sparse in the pixel representation; other, more complicated images have a sparse representation in some transform domain–for example, in terms of spatial finite-differences or their wavelet coefficients. According to the recently developed mathematical theory of compressedsensing, images with a sparse representation can be recovered from randomly undersampled k -space data, provided an appropriate nonlinear recovery scheme is used. Intuitively, artifacts due to random undersampling add as noise-like interference. In the sparse transform domain the significant coefficients stand out above the interference. A nonlinear thresholding scheme can recover the sparse coefficients, effectively recovering the image itself. In this article, practical incoherent undersampling schemes are developed and analyzed by means of their aliasing interference. Incoherence is introduced by pseudo-random variable-density undersampling of phase-encodes. The reconstruction is performed by minimizing the 1 norm of a transformed image, subject to data

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Sparse MRI: The application of compressed sensing for rapid MR imaging.

The invention relates to a method of parallel imaging for obtaining images by means of magnetic resonance (MR). The method includes the simultaneous measurement of sets of MR singals by an array of receiver coils, and the reconstruction of individual receiver coil images from the sets of MR signals. In order to reduce the acquisition time, the distance between adjacent phase encoding lines in k-space is increased, compared to standard Fourier imaging, by a non-integer factor smaller than the number of receiver coils. This undersampling gives rise to aliasing artifacts in the individual receiver coil images. An unaliased final image with the same field of view as in standard Fourier imaging is formed from a combination of the individual receiver coil images whereby account is taken of the mutually different spatial sensitivities of the receiver coils at the positions of voxels which in the receiver coil images become superimposed by aliasing. This requires the solution of a linear equation by means of the generalised inverse of a sensitivity matrix. The reduction of the number of phase encoding lines by a non-integer factor compared to standard Fourier imaging provides that different numbers of voxels become superimposed (by aliasing) in different regions of the receiver coil images. This effect can be exploited to shift residual aliasing artifacts outside the area of interest.

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SENSE: Sensitivity Encoding for fast MRI

In this study, a novel partially parallel acquisition (PPA) method is presented which can be used to accelerate image acquisition using an RF coil array for spatial encoding. This technique, GeneRalized Autocalibrating Partially Parallel Acquisitions (GRAPPA) is an extension of both the PILS and VD-AUTO-SMASH reconstruction techniques. As in those previous methods, a detailed, highly accurate RF field map is not needed prior to reconstruction in GRAPPA. This information is obtained from several k-space lines which are acquired in addition to the normal image acquisition. As in PILS, the GRAPPA reconstruction algorithm provides unaliased images from each component coil prior to image combination. This results in even higher SNR and better image quality since the steps of image reconstruction and image combination are performed in separate steps. After introducing the GRAPPA technique, primary focus is given to issues related to the practical implementation of GRAPPA, including the reconstruction algorithm as well as analysis of SNR in the resulting images. Finally, in vivo GRAPPA images are shown which demonstrate the utility of the technique.

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Generalized autocalibrating partially parallel acquisitions (GRAPPA).

Functional magnetic resonance imaging (fMRI) is currently the mainstay of neuroimaging in cognitive neuroscience. Advances in scanner technology, image acquisition protocols, experimental design, and analysis methods promise to push forward fMRI from mere cartography to the true study of brain organization. However, fundamental questions concerning the interpretation of fMRI data abound, as the conclusions drawn often ignore the actual limitations of the methodology. Here I give an overview of the current state of fMRI, and draw on neuroimaging and physiological data to present the current understanding of the haemodynamic signals and the constraints they impose on neuroimaging data interpretation.

What we can do and what we cannot do with fMRI

To use high-permittivity materials (HPM) positioned near radiofrequency (RF) surface coils to manipulate transmit/receive field patterns. A large HPM pad was placed below the RF coil to extend the field of view (FOV). The resulting signal-to-noise ratio (SNR) was compared with that of other coil configurations covering the same FOV in simulations and experiments at 7 T. Transmit/receive efficiency was evaluated when HPM discs with or without a partial shield were positioned at a distance from the coil. Finally, we evaluated the increase in transmit homogeneity for a four-channel array with HPM discs interposed between adjacent coil elements. Various configurations of HPM increased SNR, transmit/receive efficiency, excitation/reception sensitivity overlap, and FOV when positioned near a surface coil. For a four-channel array driven in quadrature, shielded HPM discs enhanced the field below the discs as well as at the center of the sample as compared with other configurations with or without unshielded HPM discs. Strategically positioning HPM at a distance from a surface coil or array can increase the overlap between excitation/reception sensitivities, and extend the FOV of a single coil for reduction of the number of channels in an array while minimally affecting the SNR.

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Manipulating transmit and receive sensitivities of radiofrequency surface coils using shielded and unshielded high-permittivity materials.

Recently, the concept of traveling wave NMR/MRI was introduced by Brunner et al. (Nature 2009;457:994–992), who demonstrated MR images acquired using radio frequency (RF) waves propagating down the bore of a MR scanner which acts as a waveguide. One of the significant limitations of this approach is that each bore has a specific cutoff frequency, which can be higher than most Larmor frequencies at the magnetic field strengths commonly in use for MR imaging and spectroscopy today. One can overcome this limitation by using a central conductor in the waveguide and thereby converting it in to a transmission line which has no cutoff frequency. Broadband propagation of waves through the sample thus becomes possible. NMR spectra and images with such an arrangement are presented and traveling wave behavior is demonstrated. In addition to facilitating NMR spectroscopy and imaging in smaller bores via traveling waves, this approach also allows one to perform multinuclear traveling wave experiments (an example of which is shown), and to study otherwise difficult-to-access samples in unusual geometries. © 2011 Wiley Periodicals, Inc. Concepts Magn Reson Part A 38: 253–267, 2011.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/cmr.a.20226

Cutoff-free traveling wave NMR

Purpose Transmit arrays are essential tools for various RF shimming or parallel excitation techniques at 7T. Here we present an array design with triangular coils to improve diversity in the B1 profiles in the longitudinal (z) direction and allow for next-nearest neighbor decoupling. Methods Two cylindrical 8-channel arrays having the same length and diameter, 1 of triangular coils and the other of rectangular coils, were constructed and compared in phantom imaging experiments using measures of excitation distribution for a variety of RF shim settings and geometry factor maps for different accelerations on different planes. Results Coupling between elements was -20 dB or better for all triangular coil pairs, but worse than -12 dB for several of the rectangular coil pairs. Both coils could produce adequate shims on a central transverse plane, but the same shim produced worse results off center for the triangular coil array than for the rectangular coil array. Compared to the rectangular coil array, the maximum geometry factor for the triangular coil array was reduced by a factor of 13.1 when using a 2-fold acceleration in the z-direction. Conclusion An array design with triangular coils provides effective decoupling mechanisms for nearest and next-nearest neighboring elements, as well as diversity in B1 profiles along the z-direction, although this also means that individual slices must be shimmed separately. This design is well suited for parallel transmit applications while also having high receive sensitivity.

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A highly decoupled transmit-receive array design with triangular elements at 7T.

Objectives:We utilized dynamic magnetic resonance imaging to visualize the pharynx and upper esophageal segment in normal, healthy subjects.Methods:A 3-T scanner with a 4-channel head coil and a dual-channel neck coil was used to obtain high-speed magnetic resonance images of subjects who were swallowing liquids and pudding. Ninety sequential images were acquired with a temporal resolution of 113 ms. Imaging was performed in axial planes at the levels of the oropharynx and the pharyngoesophageal segment. The images were then analyzed for variables related to alterations in the area of the pharynx and pharyngoesophageal segment during swallowing, as well as temporal measures related to these structures.Results:All subjects tolerated the study protocol without complaint. Changes in the area of the pharyngeal wall lumen and temporal measurements were consistent within and between subjects. The inter-rater and intra-rater reliabilities for the measurement tool were excellent.Conclusions:Dynamic magnetic reson...

Dynamic Magnetic Resonance Imaging of the Pharynx during Deglutition

Apparatus, method, and computer-accessible medium embodiments for a noninvasive mapping of electrical properties of tissues or materials. For example, it is possible to apply a plurality of stimulations to a target. It is possible to receive at least one signal from the target in response to the applied stimulations. Further, it is possible to process the at least one signal to determine electromagnetic-field-related quantities associated with the stimulations and the target response. Also, it is possible to supply the electromagnetic-field-related quantities to a system of equations relating these quantities to a plurality of electrical property values and residual field-related unknown values of the at least one target. It is also possible to determine a solution to the system of equations, including determining at least one electrical property of the at least one target.

Daniel K. Sodickson

Papers

Manipulating transmit and receive sensitivities of radiofrequency surface coils using shielded and unshielded high-permittivity materials.

Cutoff-free traveling wave NMR

A highly decoupled transmit-receive array design with triangular elements at 7T.

Dynamic Magnetic Resonance Imaging of the Pharynx during Deglutition

Apparatus, method and computer-accessible medium for noninvasive determination of electrical properties of tissues and materials