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

Cardiovascular Magnetic Resonance (CVMR) imaging has proven to be of clinical value for non-invasive diagnostic imaging of cardiovascular diseases. CVMR requires rapid imaging; however, the speed of conventional MRI is fundamentally limited due to its sequential approach to image acquisition, in which data points are collected one after the other in the presence of sequentially-applied magnetic field gradients and radiofrequency pulses. Parallel MRI uses arrays of radiofrequency coils to acquire multiple data points simultaneously, and thereby to increase imaging speed and efficiency beyond the limits of purely gradient-based approaches. The resulting improvements in imaging speed can be used in various ways, including shortening long examinations, improving spatial resolution and anatomic coverage, improving temporal resolution, enhancing image quality, overcoming physiological constraints, detecting and correcting for physiologic motion, and streamlining work flow. Examples of these strategies will be provided in this review, after some of the fundamentals of parallel imaging methods now in use for cardiovascular MRI are outlined. The emphasis will rest upon basic principles and clinical state-of-the art cardiovascular MRI applications. In addition, practical aspects such as signal-to-noise ratio considerations, tailored parallel imaging protocols and potential artifacts will be discussed, and current trends and future directions will be explored.

Beschleunigung der kardiovaskulaeren MRT mittels paralleler Bildgebung: Grundlagen, praktische Aspekte, klinische Anwendungen und Perspektiven [Acceleration of cardiovascular MRI using parallel imaging: basic principles, practical considerations, clinical applications and future directions]

CHAPTER 3 – Advanced Cardiovascular Magnetic Resonance Imaging Techniques: Spiral, Radial, and Parallel Imaging

We investigated how high permittivity materials affect the S-parameters and transmit efficiency of a head-sized birdcage coil. We also studied whether re-tuning and matching are necessary for improved performance. While adjusting the S-parameters is possible during coil development and for prototype coils, it is often unfeasible to re-tune and match commercially manufactured coils, and poses a challenge after adding dielectric material to such coils. Our work demonstrates the change in the S-parameters and the transmit efficiency after placing a dielectric sleeve within a head-sized birdcage coil, and reports that coil performance can be improved after re-tuning and matching the coil in the presence of the dielectric material.

Effect of an annular sleeve of high permittivity material on resonant modes and homogeneity of a birdcage coil

An imaging system for imaging a portion(s) of an anatomical structure can be provided. For example, a source first arrangement can provide x-ray radiation, and a multi-hole collimator second arrangement can be provided in a path of the x-ray radiation, and can be configured to undersample the radiation beam which can be forwarded to the portion(s) of the anatomical structure. A third hardware arrangement can be configured to receive a further radiation from the portion(s) that can be based on the undersampled radiation.

System, method and computer accessible medium for modulating x-ray beam intensity

This chapter introduces the key conceptual underpinnings of magnetic resonance imaging (MRI) with an intuitive description of the underlying physics. The behavior of proton magnetization within an externally applied magnetic field is described, as is the use of radiofrequency magnetic field pulses to manipulate magnetization and create detectable signal. Determinants of image contrast are explored by describing the growth and disappearance of longitudinal and transverse magnetization due to the effects of the inherent tissue T1 and T2 relaxation rates during the repetition time and echo time. Spatial localization by magnetic field gradients is explained, including a conceptual description of k-space and Fourier transformation. These elements of signal creation, image contrast creation, and spatial localization are brought together as building blocks of the basic MRI pulse sequence. Other commonly used techniques are introduced, including diffusion weighted and susceptbility weighted imaging, motion compensation methods, and parallel imaging.

Daniel K. Sodickson

Papers

Beschleunigung der kardiovaskulaeren MRT mittels paralleler Bildgebung: Grundlagen, praktische Aspekte, klinische Anwendungen und Perspektiven [Acceleration of cardiovascular MRI using parallel imaging: basic principles, practical considerations, clinical applications and future directions]

CHAPTER 3 – Advanced Cardiovascular Magnetic Resonance Imaging Techniques: Spiral, Radial, and Parallel Imaging

Effect of an annular sleeve of high permittivity material on resonant modes and homogeneity of a birdcage coil

System, method and computer accessible medium for modulating x-ray beam intensity

Introductory Magnetic Resonance Imaging Physics