Abstract-Well-logging tools employing tilted coil antennas have been proposed to provide directional (azimuth) information and improved estimates of anisotropy when compared to conventional tools that use horizontal coils. In this paper, we analyze the response of logging tools with tilted coil antennas in eccentric boreholes crossing cylindrical multilayered formations (earth formations with invasion zones) by two approaches. The first approach is based on a pseudoanalytic formulation, previously applied for concentric boreholes and extended here to include eccentric boreholes. The second approach is based on three-dimensional (3-D) numerical simulations using the finite-difference time-domain (FDTD) algorithm extended to cylindrical coordinates and incorporating cylindrical perfectly matched layers (PML). The results from the two formulations are compared for different formations, showing very good agreement.
This paper presents a complete overview of the electromagnetics (radiofrequency aspect) of MRI at low and high fields. Using analytical formulations, numerical modeling (computational electromagnetics), and ultrahigh field imaging experiments, the physics that impacts the electromagnetic quantities associated with MRI, namely (1) the transmit field, (2) receive field, and (3) total electromagnetic power absorption, is analyzed. The physical interpretation of the above-mentioned quantities is investigated by electromagnetic theory, to understand ‘What happens, in terms of electromagnetics, when operating at different static field strengths?’ Using experimental studies and numerical simulations, this paper also examines the physical and technological feasibilities by which all or any of these specified electromagnetic quantities can be manipulated through techniques such as B1 shimming (phased array excitation) and signal combination using a receive array in order to advance MRI at high field strengths. Pertinent to this subject and with highly coupled coils operating at 7 T, this paper also presents the first phantom work on B1 shimming without B1 measurements.
In ultrahigh field (UHF), human magnetic resonance imaging (MRI) concerns related to the homogeneity of the italicB1+ field [the radiofrequency (RF) magnetic field component responsible for the excitation of the spins] and the local/average specific absorption rate (SAR) are highly evident. In this work, through RF shimming techniques, a full-wave electromagnetic model that treats a coupled-RF coil and the load (an 18-tissue anatomically detailed human head model) as a single system is utilized to simultaneously (1) improve the homogeneity of italicB1+ field in various regions of interest across the volume of the human head and (2) minimize the total RF power deposition at 7 and 9.4 T. The numerical results illustrate that the italicB1+ field homogeneity (evaluated by the coefficient of variance) can be greatly improved in 3D slabs that vary in orientations and sizes, in the brain, and in the entire head volume without increasing the total RF power deposition in the head to exceed that obtained with quadrature excitation. The RF shimming simulation results are experimentally validated (by performing RF shimming without experimental B1 measurements) on a head-sized phantom using a 7-T human MRI scanner equipped with a transmit array excitation system. The SAR and associated temperature changes under quadrature and RF shimming excitation conditions are calculated and compared.
High-resolution (~0.22 mm) images are preferably acquired on whole-body 7T scanners to visualize minianatomic structures in human brain. They usually need long acquisition time (~12 min) in three-dimensional scans, even with both parallel imaging and partial Fourier samplings. The combined use of both fast imaging techniques, however, leads to occasionally visible undersampling artifacts. Spiral imaging has an advantage in acquisition efficiency over rectangular sampling, but its implementations are limited due to image blurring caused by a strong off-resonance effect at 7T. This study proposes a solution for minimizing image blurring while keeping spiral efficient. Image blurring at 7T was, first, quantitatively investigated using computer simulations and point-spread functions. A combined use of multishot spirals and ultrashort echo time acquisitions was then employed to minimize offresonance-induced image blurring. Experiments on phantoms and healthy subjects were performed on a whole-body 7T scanner to show the performance of the proposed method. The three-dimensional brain images of human subjects were obtained at echo time 5 1.18 ms, resolution 5 0.22mm (field of view 5 220mm, matrix size 5 1024), and in-plane spiral shots 5 128, using a home-developed ultrashort echo time sequence (acquisition-weighted stack of spirals). The total acquisition time for 60 partitions at pulse repetition time 5 100 ms was 12.8 min without use of parallel imaging and partial Fourier sampling. The blurring in these spiral images was minimized to a level comparable to that in gradient-echo images with rectangular acquisitions, while the spiral acquisition efficiency was maintained at eight. These images showed that spiral imaging at 7T was feasible. Magn Reson Med 63:543-552,
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