A gradient system is anisotropic if the impulse responses of at least two of the gradient channels, x, y, or z, differ from each other. Such an undesired condition may arise, for example, from differences between the gradient channels with respect to eddy currents or from unbalanced time delays in the electronic components. Depending on the degree of anisotropy, the actual gradient then deviates from the nominal, desired gradient under certain oblique orientations during the transient periods of gradient switching. The adverse consequence is degradation of image quality, such as distortion, ghosting, and blurring. In this paper, a theoretical analysis is given of the basic effects. Furthermore, the implications for the MRI process and possible correction methods are described. The effects of anisotropy are shown experimentally for echo-planar imaging and two-dimensional selective RF excitation with spiral gradient pulses.
Purpose:To study the feasibility of three-dimensional (3D) whole-body, head-to-toe, water/fat resolved MRI, using continuously moving table imaging technology.
Materials and Methods:Experiments were performed on nine healthy volunteers, acquiring 3D whole-body head-totoe data under continuous motion of the patient table. Two different approaches for water/fat separation have been studied. Results of a three-point chemical shift encoding and a spectral presaturation technique were compared with respect to image quality and performance. Furthermore, fast, low-resolution, whole-body water/fat imaging was performed in two minutes total scan time to derive patient-specific parameters such as the total water/fat ratio, the intraperitoneal/extraperitoneal fat ratio, and the body mass index (BMI).
Results:Good water/fat separation with decent image quality was obtained in all cases. The three-point chemical shift encoding approach was found to be more efficient with respect to signal-to-noise ratio (SNR) and acquisition time.
Conclusion:Whole-body water/fat sensitive MRI using continuous table motion is feasible and could be of interest for clinical practice. Some improvements of the method are desirable.
A combination of continuously moving table imaging and parallel imaging based on sensitivity encoding (SENSE) is presented. One specific geometry is considered, where the receiver array is fixed to the MR magnet and does not move with the table, which allows for head-to-toe imaging with a small total number of coils. Sensitivity maps are defined for the enlarged virtual field of view and are composed according to the k-space sampling scheme such that established parallel reconstruction techniques are applicable to good approximation. In vivo experiments show the feasibility of this approach, and simulations determine the application range. Three-dimensional head-to-toe imaging of volunteers is performed in 77 s with a SENSE reduction factor of 2 in a virtual field of view of 1800 ؋ 460 ؋ 100 mm
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