@phdthesis{Heidemann2008,
  author    = {Heidemann, Robin},
  title     = {Non-Cartesian Parallel Magnetic Resonance Imaging},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26893},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2008},
  abstract  = {Besides image contrast, imaging speed is probably the most important consideration in clinical magnetic resonance imaging (MRI). MR scanners currently operate at the limits of potential imaging speed, due to technical and physiological problems associated with rapidly switched gradient systems. Parallel imaging (parallel MRI or pMRI) is a method which allows one to significantly shorten the acquisition time of MR images without changing the contrast behavior of the underlying MR sequence. The accelerated image acquisition in pMRI is accomplished without relying on more powerful technical equipment or exceeding physiological boundaries. Because of these properties, pMRI is currently employed in many clinical routines, and the number of applications where pMRI can be used to accelerate imaging is increasing. However, there is also growing criticism of parallel imaging in certain applications. The primary reason for this is the intrinsic loss in the SNR due to the accelerated acquisition. In addition, other effects can also lead to a reduced image quality. Due to unavoidable inaccuracies in the pMRI reconstruction process, local and global errors may appear in the final reconstructed image. The local errors are visible as noise enhancement, while the global errors result in the so-called fold-over artifacts. The appearance and strength of these negative effects, and thus the image quality, depend upon different factors, such as the parallel imaging method chosen, specific parameters in the method, the sequence chosen, as well as specific sequence parameters. In general, it is not possible to optimize all of these parameters simultaneously for all applications. The application of parallel imaging in can lead to very pronounced image artifacts, i.e. parallel imaging can amplify errors. On the other hand, there are applications such as abdominal MR or MR angiography, in which parallel imaging does not reconstruct images robustly. Thus, the application of parallel imaging leads to errors. In general, the original euphoria surrounding parallel imaging in the clinic has been dampened by these problems. The reliability of the pMRI methods currently implemented is the main criticism. Furthermore, it has not been possible to significantly increase the maximum achievable acceleration with parallel imaging despite major technical advances. An acceleration factor of two is still standard in clinical routine, although the number of independent receiver channels available on most MR systems (which are a basic requirement for the application of pMRI) has increased by a factor of 3-6 in recent years. In this work, a novel and elegant method to address this problem has been demonstrated. The idea behind the work is to combine two methods in a synergistic way, namely non-Cartesian acquisition schemes and parallel imaging. The so-called non-Cartesian acquisition schemes have several advantages over standard Cartesian acquisitions, in that they are often faster and less sensitive to physiological noise. In addition, such acquisition schemes are very robust against fold-over artifacts even in the case of vast undersampling of k-space. Despite the advantages described above, non-Cartesian acquisition schemes are not commonly employed in clinical routines. A reason for that is the complicated reconstruction techniques which are required to convert the non-Cartesian data to a Cartesian grid before the fast Fourier transformation can be employed to arrive at the final MR image. Another reason is that Cartesian acquisitions are routinely accelerated with parallel imaging, which is not applicable for non-Cartesian MR acquisitions due to the long reconstruction times. This negates the speed advantage of non-Cartesian acquisition methods. Through the development of the methods presented in this thesis, reconstruction times for accelerated non-Cartesian acquisitions using parallel imaging now approach those of Cartesian images. In this work, the reliability of such methods has been demonstrated. In addition, it has been shown that higher acceleration factors can be achieved with such techniques than possible with Cartesian imaging. These properties of the techniques presented here lead the way for an implementation of such methods on MR scanners, and thus also offer the possibility for their use in clinical routine. This will lead to shorter examination times for patients as well as more reliable diagnoses.},
  subject      = {NMR-Bildgebung},
  language  = {en}
}